Ebook Dynamic reconstruction of the spine: Part 2

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Dynamic Stabilization for Revision of Lumbar Spinal Pseudarthrosis

29 Dynamic Stabilization for Revision of Lumbar Spinal
Pseudarthrosis with Transition
Paul C. McAfee, Liana Chotikul, Erin M. Shucosky, and Jordan McAfee

29.1 Introduction
With the Agile and N-Hance spinal devices being withdrawn
from the market and not being available for sale in the United
States, the practical application of dynamic stabilization is limited. Furthermore, the prospective randomized Food and Drug
Administration (FDA) study of the Dynesys Dynamic Stabilization
System (Zimmer Spine, Minneapolis, MN) was not approved. This
leaves the Transition Stabilization System (Globus Medical, Inc.,
Audubon, PA) as the main remaining instrumentation system
available for clinical use in the treatment of spinal pseudarthrosis.
It was felt in the original design that the use of a bumper of polycarbonate urethane (PCU) would be effective in damping the
force of correction transitioning from the rigid titanium rod portion of an instrumentation system to the uninstrumented mobile
portion of the spine outside the vertebral levels of surgery.1 There
were three indications for using the Transition: (1) topping off,
(2) hybrid cases with one or two levels of solid rod combined
with one or more levels of PCU, and (3) load sharing between the
posterior dynamic rod and an anterior poly ether ketone (PEEK)
spacer at the same vertebral level. We reviewed a series of 85
consecutive cases treated as one of these three indications in the
setting of revision surgery.2 Spinal fusion revision should be more
reliably achieved with the use of a dynamic instrumentation system that allows more of the load to be sequentially borne by the
spinal fusion. If 100% of the load is absorbed by the spinal instrumentation the pedicle screws bear higher cantilever bending
stresses, there are higher strains on the solid metal rods, and a
higher incidence of pedicle screw breakage can be expected. The

focus of this study of 85 consecutive patients was to take a discrete unarguable definition of failure—reoperation—and apply
dynamic, shock-absorbing instrumentation in the setting of
prior failure of lumbar fusion, thereby to determine whether the
reoperation rate is more favorable than with conventional instrumentation. All of the 85 cases were treated on label in strict
accordance with the FDA-approved indicated labeling of the
Transition instrumentation system.

29.2 Definitions of Successful
Clinical Fusion
All cases in this series were Lenke grade D in appearance on
radiographs. All patients had mechanical back pain, fulfilled the
criterion of disability for a minimum of 6 months preoperatively,
and had over 2 mm of angular motion of preoperative flexionextension radiographs.
In a review of 56 cases of isthmic spondylolisthesis, treated
with uninstrumented posterolateral fusion using autogenous
iliac crest bone graft, Lenke et al classified the different grades
of fusion from radiographs as follows:
(a) Definitely solid (50%)—large, solid, trabeculated bilateral
fusion masses

(b) Possibly solid (18%)—unilateral large fusion mass with
contralateral small fusion mass
(c) Probably not solid (11%)—small, thin fusion masses
bilaterally
(d) Definitely not solid (21%)—graft resorption bilaterally or
fusion mass with obvious bilateral pseudarthrosis

29.2.1 Clinical Experience
Eighty-five patients were treated with Transition segmental
spinal pedicle screw instrumentation for pseudarthrosis. Thirtythree of the prior surgical procedures had used spinal instrumentation, which had to be removed at the start of the revision

procedure. The indications for the secondary procedure were
pseudarthrosis (85 cases), recurrent herniated disc (14 cases),
recurrent lumbar spinal stenosis (22 cases), adjacent segment
disease (2 cases), and progression of the patient’s deformity—
scoliosis, spondylolisthesis, or retrolisthesis (19 cases)3
(▶ Table 29.1).

29.2.2 Transition Surgical Technique
and Design Features
Transition is a pedicle screw–based system consisting of a PCU
bumper and spacer to provide a soft stop at the limits of spinal
flexion and extension. The amount of flexibility is determined
by the total amount of PCU in the bumper and the spacers,
which can be of various lengths, from 20 to 42 mm. The Transition system uses a semirigid rod that provides translational
stability to horizontal shear forces because the spools overlap
and encompass the sleeves. The unique feature of the system is
that it provides for angulation of the adjacent pedicle screws
and variation of the interpedicular distance throughout flexion
and extension. The spools can be selected in a straight configuration, lordotic, or extralordotic so that the overall construct
lordosis can be varied from straight to 40 degrees of global
lumbar lordosis from L1 to S1. The polyethylene terephthalate
(PET) cord has greater than 3,100 tensile strength and pullout
strength of over 1,000 N where it attaches to the end of the titanium rod. In fatigue testing to 5 million cycles the Transition
rods did not fail with shear stresses, whereas two other competitive FDA-approved dynamic systems failed at a mean of

Table 29.1 Predominant indications (much overlap)
Indication

Number of patients

Pseudarthrosis

85

Recurrent herniated nucleus pulposus

14

Recurrent lumbar spinal stenosis

22

Adjacent segment disease

2

Deformity progression
(scoliosis, spondylolisthesis, retro)

19

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Restoration of the Lumbar Motion Segment

Fig. 29.1 (a) Bench-top laboratory testing reproduced the mechanism of failure for the AGILE system (Medtronic) using polycarbonate urethane (PCU)
and a cable to provide dynamic posterior stabilization. The laboratory failure is shown on the left, demonstrating cable breakage with 3 mm of

anterior-posterior shear during fatigue loading. This was predictive of clinical failure shown on the right, with cable breakage and dry frictional wear
particulate around the pseudarthrosis mass. (b) This is a fractured Agile rod–cable bilaterally in a 32-year-old man 12 months after the original surgery
elsewhere demonstrating pseudarthrosis. The mechanism of failure is identical to that predicted in (a). Notice that on the lateral radiograph the L4
vertebra is retrodisplaced 5 mm posterior relative to the L5 vertebra, indicating that the Agile device was not able to withstand posterior shear. (c) This
was successfully revised with repair of the pseudarthrosis with autologous bone graft and replacement with the Transition system (Globus Medical)
from L4 to S1. Notice that the posterior shear forces are stabilized by the instrumentation by means of titanium spools, which overlap and encompass
the polycarbonate urethane (PCU) spacers. (d) At 3-years post–revision surgery the flexion-extension radiographs document stability. The is 8 degrees
of angular motion and 8 mm of compression-elongation of the PCU spacers. There is no longer any posterior translation on the extension radiograph.

12,685 cycles (Agile) (▶ Fig. 29.1) and 13,465 cycles (N-Hance)
(▶ Fig. 29.2).
The goals of a compressible semirigid fusion system are
threefold4–7: (1) to optimize load sharing between the anterior
and posterior spinal columns to re-create the natural 80:20 balance or load distribution. Static titanium and stainless steel pedicle screw systems increase the load on the posterior column
well beyond 80%; (2) to promote more uniform loading of an
interbody spacer (as does the Transition) by allowing the pedicle-to-pedicle or interpedicular distance to compress as the
spine is loaded in an axial direction. PEEK rods and static rods
that do not vary the interpedicular distance keep the middle
column at a fixed length and cannot provide for compression
loading of an interbody spacer. Only the anterior column can
compress an interbody spacer with a static rod. Transition is
designed to allow telescoping of the middle column to the same
extent as the anterior column; therefore the interbody spacer is

208

loaded uniformly across the vertebral end plates; (3) to minimize stress at the bone–screw interface. Because the PCU rods
absorb some of the loads, there is less stress at the bone–screw
interface. The incidence of screw loosening and breakage should
be reduced, particularly in challenging revision procedures

with documented pseudarthrosis and windshield-wiper-type
failure with less compliant static instrumentation systems8,9
(▶ Fig. 29.3).

29.2.3 Patient Demographics
The mean age of the 85 patients was 54.5 years (range, 31–81
years). The mean estimated blood loss of the surgery was 806 mL
(range, 200–2,900 mL), including the revision decompression,
fusion, and Transition segmental instrumentation system. The
mean length of surgery was 147 minutes (range, 85–297 min).
The mean length of hospital stay was 3.54 days (range, 2–5 d).


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Dynamic Stabilization for Revision of Lumbar Spinal Pseudarthrosis

Fig. 29.2 (a) On the left is the failure of the N-Spine (N-Hance, Synthes, Inc.) instrumentation system in the laboratory, and on the right is shown a
clinical failure. Notice that cyclical bench-top testing with 3 mm of shear was predictive of instrumentation failure at the welds between the largerdiameter 5.5 mm rods and the step-down in diameter at the location of a welded collar. The polycarbonate urethane (PCU) failure is secondary in both
the in vivo and the in vitro situations. (b) A 66-year-old woman presented from elsewhere general with failure of both N-Spine rods at the weld marks
13 months postoperatively following an attempted L4 to S1 fusion. (c) Two years following revision to the Transition system (Globus Medical) from L4
to S1 with repair of pseudarthrosis she demonstrates a radiographic Lenke A fusion, and she is asymptomatic.

Fig. 29.3 (a) A 55-year-old woman presented
with severe mechanical back pain 4 months
following facet screw placement from L4 to S1.
The anteroposterior and lateral radiographs
demonstrate windshield-wiper toggling of the
facet screws at all levels, indicative of screw
loosening. (b) Two years postoperatively she had

relief of mechanical pain with repair of pseudarthrosis and insertion of the Transition system
(Globus Medical) improving from a visual analog
scale score of 7 down to 2. (c) Lateral radiograph
after revising the multilevel pseudarthrosis with
Transition segmental instrumentation from L3 to
S1. The patient’s clinical symptoms of instability
resolved.

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All 85 patients presented to our institution with a symptomatic
pseudarthrosis and/or instrumentation failure. The mean visual
analog scale (VAS) score for self-assessment of pain disability was
6.59 (range, 10–1). Thirty-three of the patients had prior surgical
instrumentation with a wide variety of methods and biomaterials
—Agile, N-Hance, Dynesys, Expedium Spine System (DePuy Spine,
Inc., Raynham, MA), CD Horizon, Harrington rods, Quantum rods,
Revere 4.5 Stabilization System (Globus Medical, Inc., Audubon,
PA), Silhouette Spinal Fixation System (Zimmer Spine, Minneapolis, MN), ISOLA/VSP Spine System (DePuy Spine, Inc., Raynham,
MA), VersiLok, Moss-Miami Spinal System (DePuy Spine, Inc.,
Raynham, MA), and TSRH Spinal System Pedicle Screw (Medtronic, Minneapolis, MN) instrumentation systems.
The Transition segmental instrumentation required 15 single-level fusion procedures, 44 two-level repairs of pseudarthroses, 20 three-level repairs, and 6 procedures that were
four or more levels. The autograft was always placed bilaterally
in the intertransverse or posterolateral location, and it was
placed the entire length of the instrumentation, including
across the PCU spacer portion of the Transition rods. This is the

on-label approved use consistent with the FDA clearance for the
Transition product.
▶ Table 29.2 lists the risk factors10–17 for pseudarthrosis in
the 85 patients—all 85 by definition presented with failure of a
prior fusion procedure 85/85 (100%); multilevel spinal fusions,
68/85 (80%); cigarette smoking, 18/85 (21.2%); body mass index
over 30 (41.2%); and adult-onset (type 2) diabetes mellitus
(11.8%).

29.2.4 Clinical Results
Reoperation Rate
Four of the 85 patients (4.7%) experienced device-related
adverse events. Three had broken porous ingrowth hydroxyapatite (HA) coated pedicle screws within the 2-year minimum
follow-up time period. There was one patient who had a broken
spool pedicle screw connector and a broken screw noted
8 months postoperatively. One patient each had reoperations
for an epidural hematoma, a misplaced pedicle screw, and a
total lumbar interbody fusion (TLIF) spacer that migrated into
the spinal canal. Therefore, in this revision series of 85 patients
there were a total of 7/85 (8.24%) reoperations within 2 years
of the index procedure.
There were no permanent iatrogenic neurological adverse
events for any of the 85 patients. One patient (case 36) developed
a postoperative epidural hematoma, which was successfully
decompressed with a reoperation 10 days after the index Transition procedure. The Transition rod on the symptomatic side was
removed during the exploration to improve visualization of the
symptomatic nerve roots. A Transition rod was reinserted following the decompression, and there was no long-term effect on the

Table 29.2 Risk factors for pseudarthrosis
Risk factor

Incidence

Multilevel spinal fusion

68/85 = 80%

Cigarette smoking
Body mass index > 40
Fusion performed as a revision procedure

210

85/85 = 100%

patient’s clinical outcome 24 months following the hematoma
decompression.
The VAS self-assessment of back pain improved from a mean
of 6.59 (range 10 to 1) down to a mean of 3.61 (range 10 to 1).
There was a mean of 25.2% improvement for the 85 patients.
Kelly reported a minimum clinically significant difference
(MCSD) in VAS pain score of 12 mm (95%: confidence interval
[CI] 9–15 mm). Therefore we used 15 mm as the MCSD to be
conservative and capture all the patients within the 95% CI.
For the total cohort of 85 patients there were 53 patients
who experienced 15 points or more improvement in VAS at
24 months or more follow-up. There were 23 patients whose
VAS was unchanged (0 to 14 mm). There were nine patients
who judged their pain as being worse, including the seven
patients who required repeat surgery at some point within the

24-month follow-up interval. In summary, in this challenging
group of salvage procedures there were 53 of 85 consecutive
patients (62.4%) with pseudarthrosis and/or instrumentation
failure who met or exceeded the MCSD for chronic lumbar
pain at 24 months or more follow-up after the Transition
reconstructive procedures.18

Fusion Assessment
The follow-up anteroposterior and lateral radiographs were
assessed according to the Lenke classification.19 To aid as a historical control reference point the percentage Lenke obtained in
his original series is shown in parenthesis:
(a) Definitely solid, 25/85 patients = 29.4% (Lenke, 50%)—solid
big trabeculated bilateral fusion masses
(b) Possibly solid, 17/85 patients = 20% (Lenke, 18%)—unilateral
large fusion mass with contralateral small fusion mass
(c) Probably not solid, 14/85 patients = 16.5% (Lenke, 11%)—
small, thin fusion masses bilaterally
(d) Definitely not solid, 29/85 patients = 34.1% (Lenke, 21%)—
graft resorption bilaterally or fusion mass with obvious
bilateral pseudarthrosis.
It is important to remember that the Lenke series consisted of
single-level fusions and instrumentation, whereas the Transition revision series largely consisted of multilevel procedures.

Optimum Rigidity for Lumbar
Instrumentation and Fusion Mass
It is beyond the scope of this chapter to define optimum rigidity. The optimum rigidity of spinal instrumentation for achieving a successful lumbar fusion has been the subject of decades
of spinal research.20–23
A published symposium attempted to resolve a critical discrepancy on the “criteria of successful arthrodesis following
interbody spinal fusions”—eight spinal experts presented their
views, particularly in terms of motion in flexion-extension

lateral radiographs and computed tomographic (CT) imaging.4
The authors concluded that a large difference of opinion exists
regarding the definition of successful spinal fusion. Most
experts believed that, even in the presence of solid fusion,
some degree of motion may be perceived in flexion-extension
lateral radiographs, which may vary from 1 to 5 degrees. It was
also generally believed that the absence of motion does not


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Dynamic Stabilization for Revision of Lumbar Spinal Pseudarthrosis
guarantee solid fusion, most possibly because rigid instrumentation may still hold the segment rigid, masking an underlying
pseudarthrosis. Furthermore, there are differences in patient
cooperation and effort with regard to the amount of flexion
and extension due to pain. The only convincing evidence of
fusion that was unequivocally agreed on was finding a bony
continuity on reexploration and no perceived motion against
applied force after removal of implants. Most authors agreed
that clinical success may be different from technical success.
Although bony arthrodesis is the objective of surgery, it is probably unnecessary to prove bony arthrodesis in patients who
have had a successful clinical result. Therefore it may be possible that a stable pseudarthrosis may achieve clinical success,
despite the failure of solid fusion. Based on the difference of
opinion as to what constitutes successful spinal fusion in the
published reports, it appears that there may be gradations of
successful spinal fusion, with varying degrees of rigidity
(▶ Fig. 29.4).
McAfee made an interesting observation,12 that even in the
presence of a well-consolidated posterolateral fusion, intervertebral motion may still be demonstrated following anterior
disc space clearance. This phenomenon is due to springing

through the pedicles, which may also permit a small amount of
movement through the posterior complex in the presence of a
well-consolidated interbody fusion. Weatherley et al presented
five cases with definitive posterolateral fusion with persistent
motion between the vertebral bodies.4 These clinical observations are not surprising from a study of the mechanical properties of bone.12 Bone is not a stiff structure like steel; it has a
degree of elasticity that increases its toughness, which has been
reported in some detail.
With the background of the prior discussion the authors concede the limitations of any investigation of the success of spinal
fusion. We have used the Lenke classification due to its widespread application and accept its limitations as being somewhat
subjective and somewhat arbitrary, and it requires compartmentalization of bone healing, which is a continuous process,
into a discrete classification of four categories.

Reoperation Rates of Historical Studies
Unlike the definition of successful lumbar fusion, there is universal agreement that a reoperation or a return to the operating
theater is a failure of the index procedure. So how does the
reoperation rate of 7/85 (8.24%) within 2 years of the index
procedure in a revision pseudarthrosis series match up in the
literature? In all large series the outcomes following the multioperated spine are less favorable than primary procedures.
Ten of the patients in this series had undergone three or more
spinal procedures prior to the index Transition procedure.24,25
Bago et al21 reported a fairly representative series of 133
patients with Cotrel Dubousset instrumentation, 22% of whom
needed a total of 28 additional operations and 21% required
implant removal. Pihlajämaki et al22 reported the complication
rates and pseudarthrosis incidence in a consecutive series of
102 patients treated with pedicle screw and rod fixation for
nontraumatic disorders. There were 75 multilevel and 27 single-level fusions. Forty-six patients had at least one further
operation for one or several complications (38.3%), including
20 fusion procedures for nonunions (19.6%). Brook et al23

Fig. 29.4 (a) Topping off. A 68-year-old man sustained an acute
lumbar vertebral body fracture simply by bending forward due to the
stress concentration of the pedicle screws at the upper instrumented
vertebra. (b) This is the typical appearance at the upper level of long
Transition system (Globus Medical) constructs with a supplemental
total lumbar interbody fusion. There were no cases of failure at the
upper instrumented vertebrae in any of the 85 cases in this series,
including 20 three-level cases and 6 cases requiring four or more
instrumented levels. (c,d) This is a more recently treated case
indicating our currently preferred instrumentation method, which uses
a cobalt-chromium (CoCr) 5.5 mm rod for adults that is connected to
the Transition system at the upper vertebral levels. This technique
provides the increased rigidity of the cobalt chrome instrumentation
such that there are actually two transitions in rod stiffness provided
along the construct—from CoCr to titanium (Ti) and from Ti to
polycarbonate urethane (PCU). The goal is to provide a less abrupt
change in modulus between long instrumented constructs and the
normally mobile spinal segments above.

performed an analysis of a database of 24,882 cases performed
in Washington State hospitals. Their follow-up was longer than
that for the Transition series, up to 11 years with most of the
reoperations occurring in the first 2 years. The cumulative incidence of reoperations was 19% with a higher reoperation rate
of 21.5% following fusion procedures and 18.8% following
decompression alone. After fusion surgery 62.5% of reoperations were associated with a diagnosis suggesting device complication or pseudarthrosis. Javalkar et al24 studied 335 patients
who underwent surgical treatment of lumbar spinal stenosis
with 63 undergoing spinal instrumentation and fusion, and 21
were reoperations. The overall reoperation rate was 44/335
(13%), with 50 reoperations performed in 44 patients. The risk
of reoperation hazard ratio (HR) was highest in patients with

prior instrumentation (HR 1.7, 95% CI 0.684–4.640). There
are no reports in the literature that study identical patient

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Restoration of the Lumbar Motion Segment
indications, have identical numbers of vertebral levels, and have
the same patient demographics as the Transition cohort of 85
patients described earlier. However, the studies show a consistent pattern of more reoperations than do conventional primary
fusion studies. With a reoperation range of between 13 and
38.3%, the Transition incidence of reoperations at 8.24% seems
encouraging enough to recommend more rigorous, carefully
controlled studies.

29.3 Summary
Two prospective randomized trials are under way as a result of
the findings of this study. (1) A prospective randomized controlled trial to compare Transition with titanium segmental
pedicle screw instrumentation one- and two-level fusions with
and without interbody spacers. This is part of an FDA postmarket surveillance study required of all products in the posterior
dynamic instrumentation field.25 (2) Headed by Berven et al,26
a prospective randomized controlled trial is currently enrolling
for treating and preventing the progression of proximal junctional kyphosis at the cephalad end of long scoliosis constructs
(four levels or more). It is hoped that the use of a compressible
rod will help to reduce the stress concentration that occurs at
the cephalad end of rigid static instrumentation constructs
(▶ Fig. 29.4).

29.4 References
[1] Durrani A, Jain V, Desai R et al. Could junctional problems at the end of a long
construct be addressed by providing a graduated reduction in stiffness? A
biomechanical investigation. Spine 2012; 37: E16–E22
[2] Kelly AM. The minimum clinically significant difference in visual analogue
scale pain score does not differ with severity of pain. Emerg Med J 2001; 18:
205–207
[3] McAfee PC, Boden SD, Brantigan JW et al. Symposium: a critical discrepancya criteria of successful arthrodesis following interbody spinal fusions. Spine
2001; 26: 320–334
[4] Weatherley CR, Prickett CF, O’Brien JP. Discogenic pain persisting despite solid
posterior fusion. J Bone Joint Surg Br 1986; 68: 142–143
[5] Currey JD. The many adaptations of bone. J Biomech 2003; 36: 1487–1495
[6] Deckey JE, Court C, Bradford DS. Loss of sagittal plane correction after
removal of spinal implants. Spine 2000; 25: 2453–2460
[7] Johnston CE II Welch RD, Baker KJ, Ashman RB. Effect of spinal construct stiffness on short segment fusion mass incorporation. Spine 1995; 20: 2400–2407
[8] Asher MA, Carson WL, Hardacker JW, Lark RG, Lai SM. The effect of arthrodesis, implant stiffness, and time on the canine lumbar spine. J Spinal Disord
Tech 2007; 20: 549–559

212

[9] Kotani Y, Cunningham BW, Cappuccino A, Kaneda K, McAfee PC. The role of
spinal instrumentation in augmenting lumbar posterolateral fusion. Spine
1996; 21: 278–287
[10] Craven TG, Carson WL, Asher MA, Robinson RG. The effects of implant stiffness on the bypassed bone mineral density and facet fusion stiffness of the
canine spine. Spine 1994; 19: 1664–1673
[11] Shirado O, Zdeblick TA, McAfee PC, Cunningham BW, DeGroot H, Warden KE.
Quantitative histologic study of the influence of anterior spinal instrumentation and biodegradable polymer on lumbar interbody fusion after corpectomy. A canine model. Spine 1992; 17: 795–803
[12] McAfee PC, Farey ID, Sutterlin CE, Gurr KR, Warden KE, Cunningham BW. The
effect of spinal implant rigidity on vertebral bone density. A canine model.
Spine 1991; 16 Suppl: S190–S197

[13] McAfee PC, Farey ID, Sutterlin CE, Gurr KR, Warden KE, Cunningham BW.
1989 Volvo Award in basic science: device-related osteoporosis with spinal
instrumentation. Spine 1989; 14: 919–926
[14] Saphier PS, Arginteanu MS, Moore FM, Steinberger AA, Camins MB. Stressshielding compared with load-sharing anterior cervical plate fixation: a clinical and radiographic prospective analysis of 50 patients. J Neurosurg Spine
2007; 6: 391–397
[15] Heggeness MH, Esses SI. Classification of pseudarthroses of the lumbar spine.
Spine 1991; 16 Suppl: S449–S454
[16] Kleiner JB, Odom JA Jr Moore MR, Wilson NA, Huffer WE. The effect of instrumentation on human spinal fusion mass. Spine 1995; 20: 90–97
[17] McAfee PC, Regan JJ, Farey ID, Gurr KR, Warden KE. The biomechanical and
histomorphometric properties of anterior lumbar fusions: a canine model.
J Spinal Disord 1988; 1: 101–110
[18] Hilibrand AS, Robbins M. Adjacent segment degeneration and adjacent
segment disease: the consequences of spinal fusion? Spine J 2004; 4 Suppl:
190S–194S
[19] Lenke LG, Bridwell KH, Bullis D, Betz RR, Baldus C, Schoenecker PL. Results of
in situ fusion for isthmic spondylolisthesis. J Spinal Disord 1992; 5: 433–442
[20] Sengupta DK. Dynamic stabilization system. In: Yue JJ, Bertagnoli R, McAfee
PC, An HS, eds. Motion Preservation Surgery of the Spine—Advanced Techniques and Controversies Philadelphia, PA: Saunders Elsevier: 2008:472–475
[21] Bago J, Ramirez M, Pellise F, Villanueva C. Survivorship analysis of CotrelDubousset instrumentation in idiopathic scoliosis. Eur Spine J 2003; 12: 435–
439
[22] Pihlajämaki H, Myllynen P, Böstman O. Complications of transpedicular
lumbosacral fixation for non-traumatic disorders. J Bone Joint Surg Br 1997;
79: 183–189
[23] Martin BI, Mirza SK, Comstock BA, Gray DT, Kreuter W, Deyo RA. Reoperation
rates following lumbar spine surgery and the influence of spinal fusion procedures. Spine 2007; 32: 382–387
[24] Javalkar V, Cardenas R, Tawfik TA et al. Reoperations after surgery for lumbar
spinal stenosis. World Neurosurg 2011; 75: 737–742
[25] TRANSITION Stabilization System 510K PreMarket notification FDA K073439.US
Food and Drug administration posting of Post-market Surveillance Studies.
/>[26] Berven, Sig. Proximal junctional kyphosis (PJK) following long instrumented

spinal fusion: the effect of implant selection. Clinical Trials.gov identifier:
NCT01441999. />

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Nonfusion Stabilization of the Degenerated Lumbar Spine with Cosmic

30 Nonfusion Stabilization of the Degenerated Lumbar
Spine with Cosmic
Archibald von Strempel
The degeneration of the lumbar motion segment starts with a
height loss of the disc caused by water loss of the nucleus pulposus. The facet joints lose their congruence, which may cause
a consecutive spondylarthritis.1 The fibers of the annulus fibrosus and the vertebral column ligaments lose tension so that a
structural loosening occurs, complete with increased rotation
instability.2,3,4 To compensate for the instability, hypertrophy of
the ligamentum flavum as well as the facet joints occurs very
frequently, which may lead to a reduction of the cross-sectional
surface of the central as well as the lateral spinal canal. At the
same time, the motion segment may lose its original position,
and scoliosis, flat back, rotation, and rotation sliding may
develop. Further along the course of degeneration, lateral and
frontal spondylophytes up to and including syndesmophytes
may form, which in turn may lead to spontaneous stiffening of
the segment.
The complaints depend on the stage of the vertebral column
degeneration. In the first phase, with a reduction of the height
of the vertebral disc and loss of the congruence of the facet
joints, chronically recurrent low back pain may occur that
increases under load stress. When the stenosis of the spinal
canal increases, additional symptoms may occur in one or both

legs, with the indications of a claudicatio spinalis. If spontaneous ankylosis of the segment occurs before a symptomatic
spinal channel stenosis occurs, the frequency and intensity of
low back pain decreases.
Leg symptoms can be caused by the compression of the neural structures, which results from a narrow spinal canal or
recessus or neural foramen. As a rule, adequate decompression
of the neural structures leads to good clinical success. The etiology of low back pain is less clear; also, clinical success does not
occur in the same measure as the fusion rate of a spondylodesis.
What is certain is that the instability in the motion segment
caused by the vertebral disc shrinkage is a trigger for the frequency of low back pain. Nonphysiologic movements that have
become possible due to the vertebral disc shrinkage lead to
shifting of the nucleus pulposus or nucleus pulposus fragments
within the vertebral disc, with the vertebral disc itself experiencing increased ingrowth of pain-conducting nerve ends as a
result of the degeneration.5 This increased innervation of the
degenerative vertebral disc is also responsible for the “memory
pain” in the discography.6
The term instability used in this context has been better
defined by Panjabi as a “clinical instability” that leads to a pathological movement capability and to pain, deformities, and neurologic failures.7
Operative treatment of the symptomatic lumbar vertebral
column degeneration has thus far consisted of stabilizing, correcting, and adequately decompressing the diseased segments,
always in connection with a spondylodesis. In recent years, the
various forms of the spondylodesis (anterior and posterior lumbar interbody fusion [ALIF and PLIF], total lumbar interbody
fusion [TLIF], and posterior lumbar fusion [PLF]) were discussed

intensively because 360-degree fusions were believed to provide the greatest clinical success rate.
This theory was disproved in a prospective, randomized,
double-blind study. The clinical results were independent of
the selected fusion form. Complications naturally increased
with increased surgical effort (360-degree fusion). In this study,
pseudarthroses did not have any influence on the clinical
result.8

A possible disadvantage of spondylodesis in the treatment of
degenerative lumbar vertebral column diseases is the increased
risk of accelerating degenerative processes in the neighboring
segment. Following a spondylodesis, 16.5% symptomatic vertebral disc degenerations after 5 years were expected in the
neighboring segment, and 36.1% after 10 years.9 Obviously
there is a lower risk for spondylodeses without the use of pedicle screw systems.10 It must be questioned whether the risk for
the adjacent segment increases with the rigidity of the spondylodesis, which would above all concern the currently favored
360-degree fusions that use a cage in combination with a pedicle screw system.11,12,13
This does not appear to apply to spondylodeses that were
performed for the correction of extended deformities. Thus,
even after more than 20 years following a Harrington spondylodesis, low back pain was found in only 13% of cases.14

30.1 Indications for Fusion
30.1.1 Why Are Spondylodeses
Performed in the Treatment of
Degenerative Lumbar Vertebral
Column Diseases?
Until recently, there were few alternatives to a standard spondylodesis. One important reason is that the surgery of the degenerative lumbar vertebral column is a relatively young chapter in
spine surgery. The techniques of correction and fusion, so successful in scoliosis surgery, were transferred to this new area of
spine surgery, increasing in usage since the early 1980s. An essential task of the spondylodesis consists of protecting the implants
used against failure (dislocation, breakage).

30.1.2 When Is a Correction Necessary?
In contrast to the treatment of adolescent scoliosis, where the
correction of the deformity is also the objective of the treatment, there are not very many indications for correction of the
degenerative lumbar vertebral column that actually serve the
direct objective of the operation with pain release and restoration of neurological functions.
Positional deformities in the sagittal and frontal planes that
in total do not lead to a loss of the body vertical plumb line
need not be corrected. This concerns most lateral deviations.

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Therefore, the correction of a degenerative lumbar scoliosis is
necessary only in exceptional cases.
The reduction of the vertebral disc always leads to flattening
of the lumbar vertebral column, which also does not need to be
corrected as long as the patient assumes an upright, wellbalanced posture. True and degenerative olisthies at an adult
age are not usually progressive. Stabilization and decompression
without correction lead to the objective of the treatment.
Therefore, it is not meaningful to transfer the principles of scoliosis surgery noncritically to the surgery of the degenerative
lumbar vertebral column.

30.1.3 When Is a Fusion Necessary?
A fusion is required when corrections (mostly in the sagittal
plane) are necessary to treat pain.

30.1.4 When Will It Be Possible to Do
Without a Fusion?
The precondition is a dynamic implant that does not require
the protection of a spondylodesis. It must be possible to achieve
the treatment objective (pain release, restoration of the neurologic function) without correction. The stabilization does not
need to include more than three segments.

30.2 The Cosmic Implant System
A posterior nonfusion implant system, which can do without

protection by a spondylodesis, should not have any rigid characteristics. However, to be able to control instabilities effectively, the system must also feature stable characteristics.
The Cosmic Posterior Dynamic System (Ulrich Medical, Ulm,
Germany) is a stable, nonrigid implant. Stability is ensured by
the 6.25 mm rod, and nonrigidity is assured by the hinged
screw head. The screw features a hinged joint between the
head and threaded part, which causes the load to be shared
between the implant system and the anterior vertebral column
(▶ Fig. 30.1). Laboratory tests show that Cosmic allows the same

Fig. 30.1 Cosmic Posterior Dynamic System screw (Ulrich Medical).

214

rotation stability as a healthy motion segment.15 In a cyclic
loading test with 0.3 to 3.0 kN/1 Hz, we did not find an implant
breakage or any debris after 10 million cycles.16 Because Cosmic
is used like a stability endoprosthesis, the bone healing of the
pedicle screws is of major importance. For this reason, the
threaded part of the screw is coated with Bonit. Bonit (Dot
GmbH, Rostock, Germany) is the second generation of bioactive
calcium phosphate coatings on implants. In 1995, it was originally used for the first time in oral surgery for dental
implants.17 In the area of vertebral column surgery, a study on
the use of a first-generation bioactive calcium phosphate coating on Schanz screws found significantly improved fixation of
the coated screws in comparison with uncoated screws.18 Thus
the screw is introduced transpedicularly into the vertebral
body, in a manner similar to that for an orthopedic endoprosthesis. To achieve a sufficient press-fit, the pedicle is widened
by drilling to 3.2 mm maximum, but only along ~50% of the
screw. The screw has a self-tapping thread so that the tapping
instrument is needed only in cases of extremely hard spongiosa.
To prevent early loosening of the screw, the screw must not be

manipulated in any major way. Before being implanted, the
rods must be prebent such that they can be connected without
any problems to the screw heads.
After the screw heads have been connected to the longitudinal rods, there remains only a micromobility in the hinges,
which, without rod connections, are caudally and cranially
mobile by ~20 degrees (▶ Fig. 30.2).
Due to its good rotation stability, Cosmic is used for purely
discogenic pain conditions and is also combined with conventional laminectomy or even facetectomy. A transverse stabilizer
is used for a monosegmental application in combination with
a laminectomy. For two- or three-segmental applications, no
transverse stabilizer is used.
The screws are implanted either by means of a conventional
midline approach (point of entry lateral to the facet joint, angle
~15 degrees horizontal to the sagittal plane) or by means of the
more laterally situated Wiltse access (a somewhat ventrally
located point of entry close to the base of the transverse continuations, angle 20–25 degrees horizontal to the sagittal plane)
(▶ Fig. 30.3). A purely sagittal implantation direction is not recommended because this will lead to parallel positioning of the
hinges and thus to increased mobility in the sagittal plane.

Fig. 30.2 (a) Flexion-extension view, (b) 1-year postoperatively.


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Nonfusion Stabilization of the Degenerated Lumbar Spine with Cosmic
facet joints is not taken into consideration. In addition, low back
pain and deformities may increase as an expression of the
increased clinical instability. For this reason, we always carry
out an additional stabilization with Cosmic (▶ Fig. 30.4a-c).

30.3.2 Chronically Recurring Low Back
Pain in the Case of Discogenic Pain and
Facet Syndrome

Fig. 30.3 Angle of screw direction in the horizontal plan 15 to
25 degrees.

Before the rod is implanted, the correct positioning of the
patient will be rechecked by means of lordosis that is as physiologic as possible. To avoid any early loosening, correction forces
must not be applied to the screw.

30.3 Indications for Dynamic
Stabilization with Cosmic
30.3.1 Symptomatic Lumbar Stenosis
(Claudicatio Spinalis)
Stand-alone decompression of the spinal channel carries the
risk of a recurrence of spinal narrowed because the instability
that led to the hypertrophy of the ligamentum flavum and the

Degenerated disc disease is present if, in the magnetic resonance imaging (MRI) scan, vertebral disc dehydration with
height loss and positive Modic signs is detected. If there are
further changed vertebral discs (black discs), we carry out an
additional discography. A positive memory pain confirms the
suspicion of symptomatic vertebral disc degeneration.19 In the
case of a facet syndrome, we carry out a diagnostic local anesthesia under X-ray control, using 2 mL local anesthetic, respectively. If the pain subsides for some hours, the suspected
diagnosis is confirmed. In such cases we carry out the Cosmic
stabilization using a paraspinous transmuscular approach
according to Wiltse20 (▶ Fig. 30.5a-c).

30.3.3 Recurrent Disc Herniation

In the case of a second recurrence of a disc herniation we carry
out a stabilization with Cosmic in addition to the nerve root
decompression.

30.3.4 In Combination with a
Spondylodesis
Cosmic can also be used if, in addition to the nonfusion stabilization, there is an indication of a spondylodesis in one or two
segments; for example, if there is a spondylolisthesis with a
clear shift in the functional X-rays and, in a further segment, a
symptomatic vertebral disc degeneration. In addition to the

Fig. 30.4 (a) Spinal stenosis, (b) pseudospondylolisthesis, (c) decompression and stabilization with the Cosmic Posterior Dynamic System (Ulrich
Medical).

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Restoration of the Lumbar Motion Segment

Fig. 30.5 (a) Degenerated disc disease (the “flat tire” disc), (b) positive Modic sign, (c) contrast computed tomographic scan.

Cosmic stabilization in situ, a posterolateral fusion is set up
within the area of the spondylolisthesis. A laminectomy or facetectomy is performed if there is an indication for this purpose
(▶ Fig. 30.6).

30.3.5 Extension of an Existing
Spondylodesis in the Case
of Painful Adjacent Level

Degeneration
Typically, in the case of a rigid 360-degree spondylodesis with
cage and pedicle screw rod or pedicle screw plate fixation,
there is a risk for development of a painful connection
instability. In these cases, we remove the pedicle screw rod or
plate system and stabilize the adjacent segment with Cosmic

Fig. 30.6 Unstable spondylolisthesis vera, stenosis L5–S1, degenerative
disc disease L4–L5. Cosmic Posterior Dynamic System screws (Ulrich
Medical) L4–S1, laminectomy L5, posterior fusion L5–S1.

216

together with a decompression, if indicated. We fill up the
existing pedicle drilling holes with bone chips and use a 7 mm
revision screw for this purpose (▶ Fig. 30.7).

30.4 Contraindications
Cosmic should be used for a maximum of only three segments. If corrections are necessary, to influence the complaints of the patient (as already stated, in almost all cases of
a degenerative deformity this is not indicated), a spondylodesis must be provided in addition to the Cosmic instrumentation. Such a case exists, for example, for a postfusion kyphosis
where a correction such as a closing wedge-osteotomy is
necessary to treat the pain. In adulthood, in the case of a
spondylolisthesis vera, there is as a rule no significant shift in
the lateral functional X-rays. If this is the case, Cosmic is used
in combination with a posterolateral fusion in situ. In cases of
greater instability, as typically found in younger adults or
youths, we carry out a posterior partial repositioning with
Cosmic in combination with a posterolateral and interbody
spondylodesis.

Fig. 30.7 Cosmic Posterior Dynamic System screws (Ulrich Medical)
L3–L5, (a) laminectomy L3, (b) preexisting fusion L4–L5.


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Nonfusion Stabilization of the Degenerated Lumbar Spine with Cosmic

30.4.1 Stabilizations Extending beyond
Three Segments
In the case of the degenerative lumbar vertebral column,
extended instrumentations should in principle be avoided. If
this is impossible, Cosmic may be used beyond three segments
only in combination with a posterolateral spondylodesis. If, for
example, there is a degenerative kyphoscoliosis with a loss of
balance in the sagittal plane, where there is a necessity to correct the kyphotic part, longer extended instrumentations are
required as a rule. Here, in the correction area, Cosmic can be
used with a posterolateral spondylodesis, and in the further
cranial segments in the nonfusion technology.

30.5 Surgical Technique
Under general anesthesia, the patient is positioned in the knee–
chest position with the hip joints flexed to 90 degrees to avoid
any pressure on the abdomen. The lordosis is radiologically
controlled and can be increased by lifting the leg section of the
operating table. For pure stabilizations, two skin cuts are made
paraspinally, each one positioned 4 cm lateral to the processus
spinalis vertebrae. The fascia thoracolumbalis is split, and a finger is used to prepare the muscular system between the multi-

fidus and longissimus, until the transverse processes can be felt.

The screw implantation is effected under lateral imager control
(C-arm). In the case of a monosegmental stabilization, only
closed screws are used (▶ Fig. 30.8a–c); two- or three-segmental stabilizations use closed screws at the caudal end of the
instrumentation and, in all other respects, open screws
(▶ Fig. 30.9). In the sacrum we recommend a bicortical screw
implantation if the screw length is 50 mm or shorter.
In the case of a monosegmental instrumentation, a straight
rod is implanted, and in the case of two- and three-segmental
instrumentations, the rod will first be bent in accordance with
the profile. With open screws, the rod is fixed by a clamp to the
screw base; the cap is then put on. The rod and screw base feature a thread for guaranteeing a high degree of rotation stability
between the rod and screw (▶ Fig. 30.10). Fixation is then
effected by a small grub screw that is tightened by a torque of 6
Nm. Lateral decompressions can also be performed from this
access point. If a laminectomy is necessary, we carry out a midline incision from which either (1) the muscular system is prepared beyond the joint continuations to carry out screw
implantation and decompression from the same access point,
or (2) the screws are set, using the technique already described,
followed by the economical preparation of the spinal muscular
system from the processus spinalis vertebrae and the vertebral
laminae to a medial position relative to the facet joints to make

Fig. 30.8 (a) Paraspinal approach L5–S1. (b) Single-level stabilization, anteroposterior and (c) lateral view at 2-year follow-up.

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Restoration of the Lumbar Motion Segment
low-molecular heparin for 2 weeks postoperatively. External

support is not used.

30.6 Results

Fig. 30.9 Three-level stabilization at 2-year follow-up.

the decompression. This latter procedure reduces blood loss
and damage to the spinal muscles. In the case of pure stabilizations, the patient will be mobilized on the first postoperative
day. In the case of a conventional midline approach with
decompression, such mobilization will be effected on the second day. The Redon drainage is removed on the first day. For
infection prophylaxis, a dose of cephalosporin is given before
the skin incision. The thrombosis prophylaxis is performed with

Fig. 30.10 Cosmic Posterior Dynamic System screws and rod (Ulrich
Medical).

218

From January 2002 to June 2005, 203 patients were operated in
Feldkirch, Austria. For 96 patients there was a 12-month followup course available, and for 38 patients of these 96 there was a
24-month follow-up course available. Preoperatively, there was
conventional X-ray imaging for all patients, anteroposterior (AP)
and lateral, in a standing position, and additionally, except where
this was not possible (agoraphobia). a computed tomographic
(CT) scan. The complaints were documented according to results
from the visual analog scale (VAS, a 10-part analog pain scale
from 0 = no pain, to 10 = unbearable pain), and the Oswestry
Disability Index (ODI) score.
Three months, 12 months, and 24 months postoperatively
conventional X-rays were performed again, AP and lateral, in a

standing position, and the clinical results documented by the
pain scale and the ODI score. The X-ray images were studied
for implant fractures, screw loosening, or screw dislocations.
A screw loosening is defined as a loosening seam around the
screw without any dislocation having occurred.
Of these 96 patients, 51 were female (53%) and 45 were male
(47%) with the following age distribution:
● 30–40 years: 3 patients
● 41–50 years: 8 patients
● 51–60 years: 30 patients
● 61–70 years: 19 patients
● 71–80 years: 31 patients
● 81–90 years: 5 patients
Four additional patients could not be examined for their 2-year
check-up because three patients had died (the death having had
no connection to the operation), and one patient had moved.
Fifty-one patients were stabilized in one segment, 35 patients
in two segments, and 10 patients in three segments. In total, 494
screws, 192 longitudinal rods, and 23 transverse stabilizers were
implanted.
The clinical results were compared with those from 75
patients with a follow-up of at least 24 months who, for the
same indications, had been treated with the Segmental Spinal
Correction System (SSCS; Ulrich Medical, Ulm, Germany), which
also contains a jointed head pedicle screw but without a coating, and with a conventional posterolateral fusion. The SSCS has
been used since 1989.
In both groups, the indications were comparable: symptomatic lumbar stenosis, painful olisthies, painful osteochondroses, painful spondylarthroses, recurring vertebral disc prolapses, and discogenic pain.
The average age in the nonfusion group was 67.2 years, and
in the fusion group 55.9 years. The reason for the increased age
of the nonfusion group is that, during the first year, we predominantly used the nonfusion technique for the treatment of

older patients to minimize the surgery trauma. With increasing
experience we then used the nonfusion technique in the case of
middle-aged adult patients.
In the nonfusion group, the VAS scores were 5.7 preoperatively and 2.9 postoperatively, and in the fusion group the
scores were 5.8 preoperatively and 3.4.


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Nonfusion Stabilization of the Degenerated Lumbar Spine with Cosmic
The ODI score in the nonfusion group was 25.4 points (50.8%)
preoperatively and 17.0 points (34%) postoperatively. In the
fusion group, the Oswestry activity score was 23.7 points
(47.4%) preoperatively and 14.7 points (29.4%) postoperatively.
The hospital stay in the nonfusion group was 7.4 days (6–
18 days), and in the fusion group 16.9 days (9–36 days).
The surgery time (skin to skin) in the nonfusion group was
118.8 minutes (62–200 minutes), and in the fusion group
172.4 minutes (120–215 minutes). Perioperatively, a total of
0.60 U of packed red blood cells were transfused (0–4 U), and,
in the fusion group, 2.96 U of eryconcentrate (0–6 U) were
transfused on average.
In the nonfusion group, revisions were performed in the case
of four patients (4.2% of 96 patients) and, in the fusion group,
revisions were performed in the case of six patients (8.0% of
75 patients). The revisions were caused by wound infections
(one case in the nonfusion group as well as three cases in the
fusion group), twice by symptomatic loosening of a screw in the
nonfusion group, once by a screw break-off in the nonfusion
group, and a total of three times due to a pseudarthrosis in connection with an implant fracture or implant loosening in the

fusion group.
In the nonfusion group, a total of two broken screws were
found in two patients, and in five patients a total of 10 screws
with loosening edges (2.4% of 494 implanted screws) were found.
In total, seven patients were affected; of these, three patients
developed symptoms causing a revision to be performed.
In the case of the revision, the loosened or broken screws
were removed, the pedicle bore holes were filled with bone
chips (similar to matchsticks) from the spinous process, and
7 mm Cosmic revision screws were implanted. There have been
no new occurrences of renewed loosening or screw fractures in
these patients. In one case, a rod fracture occurred without any
symptoms. There was no record of any screw dislocations or
the fracture of a transverse stabilizer. All implant failures
observed thus far occurred within the first year.
In June 2005 a multicenter study was started in cooperation
with six international spine centers. So far, 215 patients have
been documented; and for 100 of these 215, there was a 3month follow-up available, and for 58 there was a 12-month
follow-up. After 3 months no implant failures were found by
this study, and after 12 months a screw fracture was found that
led to a revision, as well as six loosening seams around the
screws that, however, remained without symptoms and so far
did not cause any revision. No screw dislocation was observed.

30.7 Discussion
Degenerative diseases of the lumbar vertebral column represent their own nosological entity. They have been treated primarily in accordance with the principles of deformities surgery
and traumatology.
To achieve optimum correction of existing deformities, rigid
implants have been used that provide three-dimensional
correction if at all possible. Fusion of individual segments of

the degenerative lumbar vertebral column may cause painful
connection instabilities (this applies in particular to the rigid
360-degree fusions), which increasingly casts doubt over
the use of such techniques for the treatment of degenerative
diseases.21–33 The postoperative sagittal profile of the lumbar

spine did not have any influence on the development of
adjacent instabilities.34
In addition, in the case of older patients, the bone quality frequently precludes any corrections, including secure fixation of
rigid implants. Some patients at an advanced age also show
additional secondary diseases that cause increased perioperative complications with more invasive operations on the vertebral column.
Fusion must also be questioned as the gold standard for the
treatment of chronic pain within the area of the degenerative
lumbar vertebral column because a 100% spondylodesis is not
the equivalent of a 100% clinical success rate.35,36
The significance of patient selection is justifiably regarded as
a decisive criterion for achieving a good clinical result.37 For this
reason it is not surprising that there is a search for alternative
operative techniques that prevent any fusion.38 But what may
be surprising is that it took so long to question fusion as the
gold standard. However, for some time already, there have been
individual efforts to develop alternative solutions in relation
to the fusion concept. The Graf band is possibly the first pedicle
screw–supported nonfusion system for the treatment of
painful degenerative instabilities of the lumbar vertebral column. Biomechanically, it increases the use of dorsal tension
chords and reduces painful movements in the facet joints and
also in the disc. There are some reports of excellent clinical
success.39–43
What remains disadvantageous is surely the missing rotational stability and the risk of early failure of the cable. The
Dynesys Dynamic Stabilization System (Zimmer Spine, Inc.,

Minneapolis, MN) represents a further development of the Graf
system. The band is provided with a plastic sleeve, and the band
is tensioned against this sleeve. This causes increased stability
but also an increased load on the interface between the vertebral bone and screw, which may cause loosening.44 In relation
to the rotational forces, the Dynesys system does not show any
stability comparable to that of an intact vertebral column.45
The clinical reports that have been published on the Dynesys
system are mostly positive.46,47 In combination with decompressions, these two systems, Dynesys and the Graf, are used
somewhat more rarely because even partial removal of the facet
increases the rotational instability.48
Other nonfusion techniques not based on pedicle screws stabilize the motion segment by spreading the processus spinalis
vertebrae, thereby expanding the spinal channel. The indications
are limited to light spinal narrowing and facet syndrome. Interspinous spreaders can be implanted minimally invasively. At this
time, major clinical studies are not yet available.
In contrast to the aforementioned posterior nonfusion
systems, Cosmic is used for symptomatic spinal stenosis, in
combination with decompressions, as well as in the case of
purely discogenic or facet joint–related pain.
The hinged screw provides a sufficient degree of dynamization and load sharing between the implant and the vertebral
column and prevents any rotation and translation instability.
The rotation stability corresponds to that shown by an intact
lumbar vertebral column.15
Because nonfusion implants act like stability prostheses and
must last permanently without the protection of a fusion, the
Cosmic screw was additionally coated with Bonit to ensure
better anchoring in the vertebral bone.18 The clinical results

219

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Restoration of the Lumbar Motion Segment
found so far, when compared with conventional fusions, are
equally good. The perioperative trauma was much lower. The
careful transmuscular (between the musculus multifidus and
musculus longissimus) access to the pedicles may further
decrease the operative trauma.
Even when using Cosmic, careful selection of patients is the
precondition for clinical success.
The radiological complex implant-related complications are
in the lower range of those specified in the literature with
regard to rigid implants in combination with a fusion. Between
2.5 and 15% screw fractures are specified.49,50
Radiological loosening seams, as documented in the present
study, are not considered in the literature, unless screw dislocations are noted. In fusion surgery as well as in nonfusion surgery, the meaning of implant-related complications cannot
always be equated with a clinical failure. In those cases where a
patient again develops pain after experiencing temporary relief
and where an implant-related complication can be radiologically detected, the revision is recommended in all cases. In principle, when using a nonfusion implant system, there is the
option to carry out a conventional fusion in addition to the
replacement of implants. The three patients revised in the present study due to symptomatic implant problems again received
Cosmic revision screws without fusion.
So far, the results found with the Cosmic system are very
encouraging. However, additional long-term observations are
necessary. For this reason we started an international multicenter study in June 2004, which is Internet based.

30.8 Summary
Posterior nonfusion stabilizations represent an alternative to
spondylodesis in the treatment of painful degenerative diseases
of the lumbar spine.

Cosmic is a dynamic nonfusion pedicle screw–rod system for
the stabilization of the lumbar vertebral column. The hinged
pedicle screw provides for the load being shared between the
implant and the vertebral column and allows a high degree of
stability in relation to the rotational forces.
This report covers the clinical and radiological results of 96
patients with a follow-up of 12 to 24 months. The clinical
results were compared with those from 75 patients who had
been treated with hinged screws and a conventional posterolateral fusion. In both groups, the indications were comparable:
symptomatic spinal stenosis, discogenic pain, facet syndrome,
and postdiscectomy syndrome. In both groups, the ODI score
and the VAS showed a good improvement of the symptoms
without any significant differences.
The perioperative morbidity in the nonfusion group was
significantly lower. In the nonfusion group, with 494 screws
implanted, two broken screws were found in two patients.
In the case of five patients, 10 screws with radiological loosening were found. From these seven patients, three again
developed symptoms that led to a revision. In the fusion
group, three pseudarthroses with screw fracture were found
that were also revised. All implant-related revisions occurred
within the first year.
The early- and medium-term results found so far with the
Cosmic system are very encouraging. Additional long-term
observations are necessary.

220

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[23] Kumar MN, Jacquot F, Hall H. Long-term follow-up of functional outcomes
and radiographic changes at adjacent levels following lumbar spine fusion for
degenerative disc disease. Eur Spine J 2001; 10: 309–313
[24] Gillet P. The fate of the adjacent motion segments after lumbar fusion. J Spinal
Disord Tech 2003; 16: 338–345
[25] Etebar S, Cahill DW. Risk factors for adjacent-segment failure following

lumbar fixation with rigid instrumentation for degenerative instability. J
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[26] Eck JC, Humphreys SC, Hodges SD. Adjacent-segment degeneration after lumbar fusion: a review of clinical, biomechanical, and radiologic studies. Am J
Orthop 1999; 28: 336–340
[27] Chou WY, Hsu CJ, Chang WN, Wong CY. Adjacent segment degeneration after
lumbar spinal posterolateral fusion with instrumentation in elderly patients.
Arch Orthop Trauma Surg 2002; 122: 39–43
[28] Booth KC, Bridwell KH, Eisenberg BA, Baldus CR, Lenke LG. Minimum 5-year
results of degenerative spondylolisthesis treated with decompression and
instrumented posterior fusion. Spine 1999; 24: 1721–1727
[29] Axelsson P, Johnsson R, Strömqvist B. The spondylolytic vertebra and its adjacent segment. Mobility measured before and after posterolateral fusion.
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[30] Sudo H, Oda I, Abumi K et al. In vitro biomechanical effects of reconstruction
on adjacent motion segment: comparison of aligned/kyphotic posterolateral
fusion with aligned posterior lumbar interbody fusion/posterolateral fusion.
J Neurosurg 2003; 99 Suppl: 221–228
[31] Okuda S, Iwasaki M, Miyauchi A, Aono H, Morita M, Yamamoto T. Risk factors
for adjacent segment degeneration after PLIF. Spine 2004; 29: 1535–1540
[32] Hilibrand AS, Robbins M. Adjacent segment degeneration and adjacent segment disease: the consequences of spinal fusion? Spine J 2004; 4 Suppl:
190S–194S
[33] Wenger M, Sapio N, Markwalder TM. Long-term outcome in 132 consecutive
patients after posterior internal fixation and fusion for Grade I and II isthmic
spondylolisthesis. J Neurosurg Spine 2005; 2: 289–297
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alignment on the development of adjacent instability. J Spinal Disord Tech

2004; 17: 353–357
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lumbar spondylodesis with the “Variable Screw Placement Spinal Fixation
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[36] Tunturi T, Kataja M, Keski-Nisula L et al. Posterior fusion of the lumbosacral
spine. Evaluation of the operative results and the factors influencing them.
Acta Orthop Scand 1979; 50: 415–425
[37] Fritzell P. Fusion as treatment for chronic low back pain—existing evidence,
the scientific frontier and research strategies. Eur Spine J 2005; 14: 519–520
[38] Huang RC, Wright TM, Panjabi MM, Lipman JD. Biomechanics of nonfusion
implants. Orthop Clin North Am 2005; 36: 271–280

[39] Sengupta DK, Mulholland RC. Fulcrum assisted soft stabilization system: a
new concept in the surgical treatment of degenerative low back pain. Spine
2005; 30: 1019–1029, discussion 1030
[40] Kanayama M, Hashimoto T, Shigenobu K et al. Adjacent-segment morbidity
after Graf ligamentoplasty compared with posterolateral lumbar fusion.
J Neurosurg 2001; 95 Suppl: 5–10
[41] Kanayama M, Hashimoto T, Shigenobu K. Rationale, biomechanics, and surgical indications for Graf ligamentoplasty. Orthop Clin North Am 2005; 36:
373–377
[42] Kanayama M, Hashimoto T, Shigenobu K, Oha F, Ishida T, Yamane S. Nonfusion surgery for degenerative spondylolisthesis using artificial ligament
stabilization: surgical indication and clinical results. Spine 2005; 30: 588–
592
[43] Brechbühler D, Markwalder TM, Braun M. Surgical results after soft system
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results. Acta Neurochir (Wien) 1998; 140: 521–525
[44] Schwarzenbach O, Berlemann U, Stoll TM, Dubois G. Posterior dynamic stabilization systems: DYNESYS. Orthop Clin North Am 2005; 36: 363–372
[45] Schmoelz W, Huber JF, Nydegger T, Dipl-Ing , Claes L, Wilke HJ. Dynamic stabilization of the lumbar spine and its effects on adjacent segments: an in vitro
experiment. J Spinal Disord Tech 2003; 16: 418–423
[46] Putzier M, Schneider SV, Funk J, Perka C. Application of a dynamic pedicle

screw system (DYNESYS) for lumbar segmental degenerations - comparison
of clinical and radiological results for different indications [in German]. Z
Orthop Ihre Grenzgeb 2004; 142: 166–173
[47] Stoll TM, Dubois G, Schwarzenbach O. The dynamic neutralization system for
the spine: a multi-center study of a novel non-fusion system. Eur Spine J
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31 Minimally Invasive Posterior Dynamic
Stabilization System
Luiz Pimenta, Roberto Diaz, and Dilip K. Sengupta
The principles of dynamic stabilization in the treatment of
activity-related chronic mechanical back pain have been
described by one of the authors (DKS) in Chapter 31 of this
book. An ideal dynamic stabilization system should prevent
abnormal high-load transmission through the disc and the facet
joints by sharing the load while permitting normal motion.1,2

Load sharing is the principal mechanism of pain relief.3,4 However, load sharing and motion preservation both need further
clarification.5,6

principles: disc unloading by around 30%, minimal restriction
of motion, and uniform load sharing throughout the ROM.
The surgical principles dictating the design of the DSS were
minimally invasive implantation and easy salvage in case of
device failure.

31.1 How Much Load Should Be
Shared and How Much Motion
Restricted?

The predecessor of the DSS design was the Fulcrum Assisted
Soft Stabilization (FASS) (Neoligaments, Leeds, United Kingdom) described by Sengupta and Mulholland.10 The aim was to
stabilize the spine and cause minimal restriction of motion,
maintain a normal lordotic posture of the spine, and unload the
disc. Biomechanical tests showed that, although the FASS system could unload the disc with minimal restriction of motion,
the disc unloading and motion restriction were uneven. The
FASS system produced restriction of flexion and too much
unloading of the disc in flexion, whereas extension was
unaffected. This predicted a possibility of excessive load being
shared by the device in flexion, possibly resulting in early
fatigue failure of the device or the implant–bone junction.10
The author (DKS) subsequently designed a C-shaped titanium
spring device described as the Dynamic Stabilization System-I
(DSS-I)11 (▶ Fig. 31.1). Biomechanical tests with this device on
cadaver spine showed a more favorable load-deformation curve
and a uniform restriction of flexion and extension ROM by
around 30% of normal range. This was important for fatigue

resistance. An uneven restriction of motion in any particular
direction would indicate excessive load sharing of the device in
that direction of motion, which can lead to an early fatigue failure. However, when the effect of DSS-I was tested on cadaver
spine, the disc pressure changes appeared to be unfavorable.
The device unloaded the disc by ~30% in flexion, which was
desirable, but in extension a complete unloading of the disc created a negative disc pressure at the maximum extension.

To maintain the nutrition and normal health of the disc and the
articular cartilage of the facet joints, it is important that they
bear a normal load and that motion is preserved. The device,
therefore, should share only a partial load and allow a near
normal load to be transmitted through the disc and the facet
joints. Restriction of motion is not the primary mechanism of
pain relief, nor is it desirable.7 However, to achieve load sharing,
the device almost invariably causes some restriction of motion.
How much load should be shared and how much motion
should be permitted for ideal pain relief have never been determined. Perhaps these two parameters vary among individuals
and also among motion segments in the same individual. It
may be hypothesized that around a 20–40% load may be shared
by the device, permitting the rest of the load to be transmitted
through the facet joints and the disc. Motion restriction should
be minimal if at all. However, it is more important that the
device prevent any abnormal, sharp load across the disc and
the facet joints throughout the range of motion (ROM), and that
any abnormal quality of motion be prevented as well.1,2

31.2 How Will the Device Survive
Fatigue Failure in the Presence of
Continued Motion?
Even a rigid fusion rod may fail if the segment does not fuse.

How can the dynamic stabilization device survive fatigue failure
in the presence of continued motion? The dynamic stabilization
systems are inherently flexible and are designed to survive
fatigue within a certain ROM and load. If at any stage during the
ROM the device has to share a much higher load (say > 60% of
the total load, when the device is meant to share only 30%) the
device may undergo fatigue failure or loosening at the implant–
bone junction. It is therefore important that load sharing be
uniform throughout the ROM.8,9
The development of the Dynamic Stabilization System (DSS)
(currently under development by the author [DKS], in collaboration with Abbott-Spine, Austin, TX) is based on these three

222

31.3 The Design Rationale of the
Dynamic Stabilization System

Fig. 31.1 The first-generation Dynamic Stabilization System (DSS-I)
was a C-shaped titanium spring made of a 4 mm diameter springgrade titanium. The ends were thickened to 6 mm or fitted with a
6 mm diameter bushing for easy fixation to the regular pedicle screws,
which can take a 6 mm rod.


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Minimally Invasive Posterior Dynamic Stabilization System

Fig. 31.2 (a) The load-deformation curve in
cadaver spine experiments with the firstgeneration Dynamic Stabilization System (DSS-I)
shows around 30% of restriction of motion in

both flexion and extension, compared with an
uninstrumented segment. (b) A continuous
tracing of the disc pressure at the center of the
disc space shows that the pressure rises during
both flexion and extension and is lowest at the
early phase of extension. Following stabilization
with the DSS-I, the final pressure at the end of
flexion was reduced by around 30%, but during
extension the disc pressure was continuously
decreasing and was the lowest at full extension.
This indicates that the DSS-I was sharing 70% of
the disc load in flexion but bearing nearly the full
load during extension, leaving almost no load to
be transmitted through the disc.

Normally, the disc pressure rises in both flexion and extension
and is lowest at the early phase of extension (▶ Fig. 31.2). A progressively increasing negative disc pressure with DSS-I indicated that the device shared increasing load toward maximum
extension, which was not reflected in the load-deformation
curve. Such excessive load sharing in extension would increase
the possibility of an early device failure, and therefore this
design was never implanted clinically (▶ Fig. 31.2).
The DSS-II was designed (▶ Fig. 31.3) to improve on the uniform load sharing during both flexion and extension motions.
This consists of an α-shaped coil made of a 4 mm diameter
spring grade titanium rod with an outer diameter of 25 mm.
The coiled section is elliptical rather than circular, so that the
instantaneous axis of rotation of the device simulates a normal
motion segment to ensure more uniform load sharing as mentioned earlier.8,9 The DSS-II device is attached to the pedicle
screws, with a polyaxial screw head (PathFinder system; Abbott
Spine, Inc. Austin, TX) designed to accept a 4 mm rod, as
opposed to the normal 6 mm rod used for fusion. The biomechanical tests on cadaver spine with the DSS-II system

showed that the ROM was uniformly restricted by 30% in flexion-extension and lateral bending, and by 20% in rotation, compared with normal. A continuous pressure profile tracing from
the center of the disc showed that the disc pressure was going

Fig. 31.3 The second-generation Dynamic Stabilization System (DSS-II)
was an α-shaped titanium coil made of a 4 mm diameter spring-grade
titanium. This device can be attached to the pedicle screws (PathFinder
system; Abbott Spine, Inc., Austin, TX), which can take 4 mm diameter
rods.

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Restoration of the Lumbar Motion Segment

Fig. 31.4 (a) The load–deformation curve in
cadaver spine experiments with the secondgeneration Dynamic Stabilization System (DSSII) shows around 30% of restriction of motion in
both flexion and extension, compared with an
uninstrumented segment. (b) A continuous
tracing of the disc pressure at the center of the
disc space shows that in an intact spine the
pressure rises during both flexion and extension
and is lowest at the early phase of extension.
Following stabilization with the DSS-II, the final
pressure at the end of both flexion and extension
was reduced by around 25%, indicating a uniform
unloading of the disc at both of these two
motions.

up in both flexion and extension as in the intact spine, but the
peak pressure at full flexion and extension was reduced by 25%.
This indicated that the DSS-II system uniformly unloaded the
disc by sharing ~25% of the load off the disc at full flexion and
extension. At the neutral position, the disc pressure remained
unchanged, indicating that the DSS-II did not unload the disc at
the neutral position (▶ Fig. 31.4). Clinically, mechanical back
pain due to degenerative instability is mostly experienced
toward terminal ranges of flexion and extension. It was therefore expected that the DSS-II should be able to prevent such
activity-related mechanical back pain. The disc pressure changes
during lateral bending and rotation were minor even in the
intact spine and therefore were not repeated with the DSS.
Fatigue testing in the laboratory showed that the DSS-II could
survive 10 million cycles of motion with ±7.5 degrees of flexion
and extension motion. It was expected that, because the DSS
would produce 30% limitation of ROM, the expected ROM of the
motion segment after stabilization with the DSS would not
exceed this range. More importantly, the pressure studies indicated that the DSS-II system was not expected to share more
than 25% of the load off the disc during the entire range of flexion and extension. The bench tests therefore indicated a reasonable assurance that the DSS-II would survive a fatigue failure.
It cannot be overemphasized that there is no ideal bench test
available to simulate the complex motion of the device in vivo,

224

which could predict a true fatigue life of the device after
implantation.

31.4 Clinical Study with the
Dynamic Stabilization System
(DSS-II)

Following the biomechanical studies, a pilot clinical study was
undertaken to test the clinical efficacy and safety of the DSS-II
device. The clinical study was performed in only one center
(Santa Rita Hospital, São Paulo, Brazil), and all the operations
were performed by one surgeon (Luiz Pimenta). The inclusion
criteria were activity-related mechanical back pain for a duration
of more than 6 months, associated with radiological evidence of
a single level of disc degeneration (X-ray, magnetic resonance
imaging [MRI], and positive discography) and failure of conservative treatment. Cases with previous failed discectomy and facet
joint degeneration not causing significant stenosis were also
included. Exclusion criteria included severe disc degeneration
with disc collapse exceeding 50% of normal height, spondylolisthesis exceeding grade II, and significant osteoporosis. More
importantly, patients requiring any other concomitant surgery,
such as decompression or discectomy at any level or fusion of an


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Minimally Invasive Posterior Dynamic Stabilization System
adjacent segment, were excluded because these procedures
might produce clinical improvement or deterioration, which may
confound the outcome of dynamic stabilization. Patients with a
body weight of more than 90 kg were excluded because only one
size of the DSS-II system was used, which may not have been
adequate for oversized patients. During 2002 through 2004, 19
cases were enrolled, of which 16 (7 M, 9 F) completed a minimum 1-year follow-up. Mean age was 52 years (range, 42–58).
Primary diagnosis included disc degeneration (9), disc degeneration with degenerative spondylolisthesis (4), failed nucleoplasty
(2), and failed disc replacement (1). Operated segments included
a single level in 14 cases (L4–L5 in 6, L5–S1 in 8) and two levels
in 2 cases (L4–S1). Preoperative discograms were performed in

all patients who had not failed a previous surgery.

lordosis of the instrumented segment in all the cases. Flexionextension radiographs showed mean 7.5 degrees of motion
(68% of the motion observed in the proximal adjacent segment)
at the operated segment. More importantly, the quality of flexion and extension was comparable to that of a normal motion
segment (i.e., the anterior edges of the end plate opposing each
other in flexion and posterior edges in extension) (▶ Fig. 31.6).
This indicated that the device was sharing the load in both flexion and extension, allowing the remaining load to be transmitted through the disc. No radiographic loosening or implant
breakage was seen. A postoperative MRI scan was obtained at
6 months in two cases, one of which showed increased signal
intensity in the T2-weighted MRI scan in the operated disc.12

31.7 Discussion
31.5 Surgical Technique for
Stabilization with the Minimally
Invasive Dynamic Stabilization
System
The patients were positioned prone on rolls to create the ideal
lumbar lordosis, and this was checked by lateral fluoroscopy.
The pedicle screws were inserted by percutaneous technique,
using fluoroscopy, over guide pins and progressive dilators.
Pedicle screws on the same side were inserted through the
same longitudinal skin incision ~3 cm long. The track between
the two pedicle screws was created by splitting the erector spinae muscles with a dissector. Each screw head was connected
to a slotted extension tube projecting outside the skin. Once
satisfactory screw positioning was confirmed, the DSS spring
was slid down the slot in the extension tubes of the pedicle
screws to sit into the screw heads. The coil of the DSS device
was maintained in the sagittal plane, and the locking nuts were
tightened to the screw heads. No distraction or compression

was applied. The procedure was repeated on the other side. The
average operative time was 90 minutes, and average blood loss
was less than 50 mL The patients were up and ambulatory
within a couple of hours after surgery and usually discharged
home the same day (▶ Fig. 31.5).

31.6 Clinical Outcome
The morbidity was very low, and most cases could appreciate
pain relief on the day of surgery. At the 1-year follow-up, the
mean visual analog scale (VAS) score was reduced from 7.3 to
3.5 and the mean Oswestry Disability Index (ODI) score was
reduced from 65 to 27. Fourteen of the 16 cases improved significantly from the preoperative pain and functional status.
Twelve returned to their original jobs. Of the three patients who
failed to experience significant improvement, one had a previous failed nuclear prosthesis (PDN, Raymedica, Inc., Minneapolis, MN), and two others had two previous discectomies and a
50% collapse of disc height. In this small study group there was
one case with persistent pain following disc replacement,
and another case with failed PDN, and both these cases had significant pain relief following DSS stabilization. Postoperative
standing lateral radiography showed well-maintained lumbar

The initial clinical results are encouraging. Most cases experienced significant pain relief, and there was no complication.
The operative procedure is fairly simple and morbidity is low.
The most important observation was that there was no implant
failure. Considering these implants have to survive in the presence of continued motion, device failure in the form of breakage
of the spring or screw loosening remains a matter of great concern. As mentioned earlier, the laboratory data regarding
fatigue are inadequate because cyclical motion in the laboratory
on a polyethylene spine model from the American Society for
Testing and Materials (ASTM) does not replicate the complexity
of the motion in vivo, and also because the influence of the anatomical structures of the motion segment (i.e., the disc and the
facet joints) cannot be re-created in the laboratory. The uniform
load sharing with the disc and the DSS device, as predicted by a

normal quality of flexion and extension, supports the hypothesis that the kinematics of the DSS is complementary to that of
the intact motion segment. Therefore, the influence of the disc
and the facet joints may not adversely influence the fatigue life
of the DSS. Only long-term follow-up will deliver the true
answer. Even if the DSS implant fails, it may be salvaged easily
by revision or converting it to a fusion.
Although the present pilot clinical study did not enroll any
patient who required concomitant surgery such as decompression or fusion, it does not preclude the use of the DSS
device as a concomitant procedure to supplement a discectomy
or decompression or fusion of an adjacent segment. The purpose of this clinical outcome study was to assess the efficacy of
DSS without the confounding effect of other surgical procedures, particularly decompressive laminectomy, which has a
high success rate in relieving leg pain secondary to spinal stenosis. In the previously reported clinical study with Dynesys,13
68% of cases had a concomitant decompression, making it difficult to assess whether the good result was due to decompression or to stabilization. Once the clinical efficacy of DSS is well
established, the indications may be extended to include prophylactic stabilization of a segment adjacent to a fusion level,
or to prevent instability following decompressive laminectomy.
The design of the DSS has been laboratory tested to match the
kinematics of a normal lumbar motion segment. The kinematics
may change after disc or nuclear replacement. As long as the
kinematics remain reasonably close to that of a normal motion
segment, the DSS may be a reasonable salvage operation for
failed disc or nuclear replacement cases. Salvage of such cases

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Restoration of the Lumbar Motion Segment

Fig. 31.5 (a) Surgical technique for the second-generation Dynamic Stabilization System (DSS-II) instrumentation. (b) The pedicle screws are inserted

by a minimally invasive technique over guide pins, under X-ray control. (c) Two slotted extension tubes, attached to the pedicle screw heads, project
through the skin incision. (d) The DSS-II system is inserted along the slots of the outriggers into the pedicle screw heads. (e) Final X-ray after bilateral
instrumentation. (f) The coil of the DSS-II system lies in the sagittal plane, well under the skin level, and does not stay prominent under the skin.
(g) Postoperative anteroposterior and lateral radiographs after DSS-II instrumentation.

with posterior fusion may not be easy in the presence of a prosthesis in the disc space. However, ideally, the DSS should be
tested together with such disc prostheses in the laboratory to
observe whether they have complementary kinematics before
the DSS may be recommended as a salvage procedure following

226

disc replacement. The presence of a significant facet joint arthrosis remains a contraindication to disc replacement. If the kinematic property of the DSS appears to be truly complementary to
the disc prosthesis, the two procedures may be performed
together as an elective procedure for a degenerative disc as well


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Minimally Invasive Posterior Dynamic Stabilization System
the results of the initial study it may be recommended that
the DSS device can be used even in conjunction with decompressive laminectomy and to stabilize segments adjacent to
fusion to prevent degeneration. However, several questions
remain unanswered. Although the 4 mm device is appropriate
in patients with average build, and at the L4–L5 or L5–S1 levels,
what is the ideal size of the device for overweight or underweight patients? How many segments can be safely stabilized?
Can it be used in conjunction with a disc or nuclear prosthesis
in front? A long-term study will be required to answer the
questions of device failure with fatigue and any effect on repair
or regeneration of the degenerated disc following stabilization.

31.9 References

Fig. 31.6 (a) Normally, the anterior edges of the vertebral bodies come
closer to each other during flexion and the posterior edges come
closure in extension, indicating that the center of rotation at these two
stages of motion lies behind an anterior margin of the disc space in
flexion and in front of the posterior margin of the disc space in
extension. (b) This normal kinematic pattern of motion is also
observed following second-generation Dynamic Stabilization System
instrumentation.

as for facet joint disease, making it a truly total joint replacement as opposed to total disc replacement, which is equivalent
to only a partial joint replacement.

31.8 Summary
This pilot clinical study showed that the present design of the
DSS appears to be safe and effective. The implantation is minimally invasive and postoperative morbidity is minimal. From

[1] Sengupta DK. Dynamic stabilization devices in the treatment of low back
pain. Orthop Clin North Am 2004; 35: 43–56
[2] Sengupta DK. Dynamic stabilization in the treatment of low back pain due
to degenerative disorders. In: Herkowitz HN, ed. The Lumbar Spine. Vol 1.
3rd ed. Philadelphia: Lippincott Williams and Wilkins; 2004:373–383
[3] McNally DS, Adams MA. Internal intervertebral disc mechanics as revealed by
stress profilometry. Spine 1992; 17: 66–73
[4] McNally DS, Shackleford IM, Goodship AE, Mulholland RC. In vivo stress measurement can predict pain on discography. Spine 1996; 21: 2580–2587
[5] Panjabi MM. Clinical spinal instability and low back pain. J Electromyogr
Kinesiol 2003; 13: 371–379
[6] Panjabi MM. The stabilizing system of the spine, II: Neutral zone and

instability hypothesis. J Spinal Disord 1992; 5: 390–396, discussion 397
[7] Mulholland RC, Sengupta DK. Rationale, principles and experimental evaluation of the concept of soft stabilization. Eur Spine J 2002; 11 Suppl 2: S198–
S205
[8] Sengupta DK, Demetropoulos CK, Herkowitz HN, et al. Instantaneous axis of
rotation and its clinical importance in a healthy lumbar functional spinal
unit. Roundtables in Spinal Biomechanics. St. Louis: Quality Medical Publishing; 2004:1:2–12
[9] Sengupta DK, Demetropoulos CK, Herkowitz HN, Mulholland RC. Instant
centre of rotation and intradiscal pressure study to identify load-sharing
property of dynamic stabilization devices in the lumbar spine without fusion:
a biomechanical study in cadaver spine. Paper presented at: World Spine II;
2003; Chicago, IL
[10] Sengupta DK, Mulholland RC. Fulcrum assisted soft stabilization system: a
new concept in the surgical treatment of degenerative low back pain. Spine
2005; 30: 1019–1029, discussion 1030
[11] Sengupta DK, Demetropoulos CK, Herkowitz HN, Hochschuler S, Mulholland
RC. Loads sharing characteristics of two novel soft stabilization devices in the
lumbar motion segments: a biomechanical study in cadaver spine. Paper presented at: Spine Arthroplasty Society Annual Conference; 2003; Scottsdale,
AZ
[12] Sengupta DK, Pimenta L, Mulholland RC. Prospective clinical trial of soft
stabilization with Dynamic Stabilization System (DSS) in the treatment
of chronic low back pain: results of minimum one-year follow-up. Paper
presented at: Spine Arthroplasty Society Annual Meeting; 2005; New York,
NY
[13] Stoll TM, Dubois G, Schwarzenbach O. The dynamic neutralization system for
the spine: a multi-center study of a novel non-fusion system. Eur Spine J
2002; 11 Suppl 2: S170–S178

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Restoration of the Lumbar Motion Segment

32 Clinical Results of IDE Trial of Dynesys for Dynamic
Stabilization
Alex Ha and Dilip K. Sengupta

32.1 Introduction

32.2.2 Study Design

The Dynesys Dynamic Stabilization System (Zimmer Spine,
Minneapolis, MN) was introduced in the United States in
2004 and was cleared for marketing when indicated for posterior fixation as an adjunct to fusion (through the Food and
Drug Administration [FDA] 510(k) process in K031511,
K045365, K060638, K071879, and K073347). In 2009, the
FDA controlled investigational device exemption (IDE) clinical trial completed the comparison between Dynesys, as a
stand-alone posterior dynamic stabilization device against
instrumented fusion, and the posterolateral fusion (PLF)
using pedicle screws and rods. The reader is referred to an
excerpt from the FDA Executive Summary for Zimmer
Spine’s Dynesys Dynamic Stabilization System submitted to
the Orthopedic and Rehabilitation Devices Panel of the FDA
on November 4, 2009.1

The sponsor provided data from the multicenter, prospective,
randomized, concurrently controlled, nonblinded, noninferiority
trial of the Dynesys spinal system compared with posterolateral
spinal fusion using autogenous bone with a posterior pedicle

screw system (Silhouette Spinal Fixation System) in patients with
radiculopathy and degenerative spondylolisthesis or retrolisthesis
(up to grade I), spinal stenosis, or other stenosing lesion as
defined in the study inclusion/exclusion criteria outlined following here. The original study design was approved for 399 patients
at 30 sites (266 Dynesys, 133 Silhouette); however, the final
study design sample size (after revisions to the statistical plan
including specification of a 15% noninferiority margin) was based
on 184 Dynesys and 92 Silhouette patients.
Patients (n = 368) were randomized and implanted at 26 clinical centers in the United States by 70 surgeons. After one Dynesys patient was excluded for three-level treatment, 367 were
evaluated, for a total of 253 Dynesys and 114 Silhouette
patients. An additional 28 nonrandomized, Dynesys patients
were studied as initial cases (one per active center) and are
referred to as the Dynesys training cohort throughout this summary. The first surgery was performed on March 3, 2003.
Individual patient success (i.e., overall success) was determined at 24 months and was defined as a composite end point.
A patient was considered a success if all of the following criteria
were met:
● Leg pain—improvement of at least 20 mm on a 100 mm VAS
from baseline level at 24 months
● Function—improvement of at least 15 points on the ODI (version 2) graded on a 100-point scale at 24 months as compared
with baseline
● Maintenance or improvement in four neurological assessments (motor function, sensory function, reflexes, and
straight leg raise) with no new permanent neurologic deficits
as compared with baseline at 24 months
● Absence of major complications, defined as major blood
vessel injury, neurologic damage, or nerve root injury, over
the first 24 postoperative months
● Freedom from additional surgical intervention, defined as
revision, reoperation, removal, or supplemental fixation/
fusion at the affected level, over the first 24 postoperative
months

32.2 Description of the Dynesys
FDA IDE Study
32.2.1 Purpose
The objective of this FDA IDE study was to assess the safety
and efficacy of the Dynesys spinal system for patients requiring one- or contiguous two-level posterior spinal stabilization
of the lumbar and/or sacral spine following decompression
in surgical treatment for degenerative back and leg pain. The
outcome with Dynesys stabilization was compared with a
posterior lateral spinal fusion (PLF) procedure using autogenous bone with a rigid polyaxial posterior spinal fixation
system (Silhouette Spinal Fixation System, Zimmer Spine,
Minneapolis, MN).
All patients were evaluated for major complications and
additional surgical intervention for 24 months to ensure the
primary safety objectives. Their neurologic assessments after
the same time period showed neurological success. The leg
pain success, based on patient pain reported on a 100 mm
visual analog scale (VAS) at 24 months and functional success, based on results from the Oswestry Disability Index
(ODI) at 24 months, were used to demonstrate the primary
efficacy objectives. Although the IDE study instructs using
the Dynesys following decompression, the Indications for
Use and study inclusion/exclusion criteria specified that
patients may require decompression at the levels considered
for treatment, and not all patients in the clinical study had a
concomitant decompression.

228

There were no radiographic end points included in the evaluation of overall success.
The study was considered a success if the overall success rate

for Dynesys was determined to be noninferior as compared
with the overall success rate for Silhouette.


| 20.02.15 - 13:27

Clinical Results of IDE Trial of Dynesys for Dynamic Stabilization

Inclusion/Exclusion Criteria

●

Inclusion Criteria
●

Patients having degenerative spondylolisthesis or
retrolisthesis (up to grade I)

And/or
Patients having lateral or central spinal stenosis or other
stenosing lesion as diagnosed byradiculopathic signs,
neurogenic claudication, or imaging studies
● Candidate for single-level or contiguous two-level PLF
between L1–S1
● Patients having a predominant component of leg rather than
back symptoms; symptoms include pain, muscle weakness,
and/or sensation abnormality as evidenced by patient history
and
● diagnostic studies.
● Patients may require decompression at the levels considered

for treatment
● Preoperative leg pain score ≥ 40 mm on a 100 mm VAS;
● Leg pain unresponsive to conservative (nonsurgical)
management for a minimum of 3 months;
● Preoperative ODI score ≥ 30 indicating at least moderate
disability
● Skeletally mature individual between ages 20 and 80
● Willing and able to comply with study requirements, including willing and able to sign a study-specific, institutional
review board (IRB)-approved informed consent form, complete necessary study paperwork, and return for required
follow-up visits
●

Exclusion Criteria
●

●

●

●

●

●

●

●

●

●

●

Primary diagnosis of discogenic back pain at affected levels
as evidenced by a larger back than leg pain component;
In the event of multilevel pathology a discogram should be
considered
Patients with leg pain due to etiologies other than those
already listed, such as trauma, peripheral vascular disease,
and neuropathy should be excluded
Degenerative scoliosis > 10 degrees at the affected motion
segment;
Supplemental interbody column support (e.g., bone graft,
spacers, or fusion cages) is planned at the affected level(s)
Greater than grade I spondylolisthesis or retrolisthesis at the
affected level(s);
Radiculopathic signs from more than two contiguous or two
noncontiguous vertebral body segment(s)
Previous lumbar fusion attempt(s), previous total facetectomy or trauma at the affected level(s);
Gross obesity defined as exceeding ideal weight by greater
than 40%;
Active local or systemic infection
Advanced osteoporosis as evidenced by plain film radiographs or history of fractures and confirmed by dual energy
X-ray absorptiometry (DEXA) scan of less than -2 t for the
age group. All women over 50 and men over 60 should have
a DEXA scan to confirm adequate bone density;
Receiving immunosuppressive or long-term steroid therapy

●

●
●

●
●
●

●

●

●
●
●
●

Active hepatitis (viral or serum) or positive for human immunodeficieny virus (HIV), renal failure, systemic lupus erythematosus, or any other significant medical conditions that
would substantially increase the risk of surgery
Documented history of titanium alloy, polyethylene tetraphthalate (PET) or polycarbonate urethane (PCU) allergy, or
intolerance
Active malignancy or other significant medical comorbidities
Current chemical dependency or significant emotional and/
or psychosocial disturbance that may impact treatment outcome or study participation as evidenced by three or more
positive Waddell signs
Pregnancy
Incarceration
Severe muscular, neural, or vascular diseases that endanger
the spinal column
Missing bone structures, due to severely deformed anatomy

or congenital anomalies, which make good anchorage of the
implant impossible
All concomitant diseases that can jeopardize the functioning
and success of the patient;
Vertebral fractures
Treatment of the thoracic and cervical spine
Severely deformed anatomy due to congenital anomalies
Paralysis

32.2.3 Postoperative Care
The potential investigators for this trial suggested that the subjects would be drawn from a population appropriate for fusion
and that the standard of postoperative care was established for
this procedure in the pre-IDE discussions. Therefore, there was
no specific postoperative regimen expected in the protocol for
the Dynesys spinal system IDE.

32.2.4 Evaluations
Patients were evaluated preoperatively (within 2 months of
surgery), intraoperatively, and postoperatively at 3 weeks, 3, 6,
12, and 24 months, and annually until the last patient was seen
for the 24-month evaluation. Over the course of the trial,
adverse events and complications were evaluated. The primary
and secondary clinical and radiographic outcome parameters
were evaluated at each evaluation time point. The initial data
from the 24 months of follow-up determined the success of the
experiment.

32.2.5 Adverse Events
Any unwanted change from a patient’s original condition, especially the worsening of a preexisting condition or symptom,
regardless of etiology, was defined as an adverse event. An

adverse event was also recorded when a patient had an
unscheduled visit as a result of pain or neurological or functional symptoms. All complication was then further classified
as a surgery-related complication, a device-related complication, or other.

229


| 20.02.15 - 13:27

Restoration of the Lumbar Motion Segment
All adverse events reported in the original Clinical Summary
Report dated December 21, 2007, underwent review and were
classified by an independent data safety monitoring board
(DSMB) with respect to the severity (complication or observation) and the relatedness of each event.

32.2.6 Assessments
Primary Study Assessments
1.
2.
3.
4.
5.

VAS leg pain
ODI
Neurological status
Major complications
Additional surgical intervention

Secondary Study Assessments

1.
2.
3.
4.
5.

VAS back pain
SF-12 Health Survey
VAS iliac crest pain
Patient satisfaction
Prolo economic and functional assessment

Segmental Stability without Fusion
(Dynesys Group Only)
Radiographic success in the Dynesys group was defined as
meeting the definition of segmental stability without meeting
the definition of fusion.
Segmental stability was defined as angular motion
● < 15 degrees at L1–L2, L2–L3, or L3–L4 or
● < 20 degrees at L4–L5 or
● < 25 degrees at L5–S1 and
● Translational motion < 4.5 mm

Primary Study End Point/Success Criteria
The primary study end point (individual patient success) was
determined at 24 months and was defined as a composite end
point. A patient was considered a success if all of the following
criteria were met:
● Leg pain—improvement of at least 20 mm on a 100 mm VAS
from baseline level at 24 months

● Function—improvement of at least 15 points on the ODI version 2 graded on a 100-point scale at 24 months as compared
to baseline
● Maintenance or improvement in four neurological assessments (motor function, sensory function, reflexes, and
straight leg raise) with no new permanent neurological deficits as compared to baseline at 24 months
● Absence of major complications defined as major blood vessel
injury, neurological damage, or nerve root injury over the first
24 postoperative months
● Freedom from additional surgical intervention defined as
revision, reoperation, removal, or supplemental fixation/
fusion at the affected level over the first 24 postoperative
months
There were no radiographic end points included in the evaluation of overall success for either the Dynesys or the Silhouette.
The study was considered a success if the overall success rate
for Dynesys was determined to be superior compared with the
overall success rate for Silhouette. The original protocol indicated a 10% noninferiority margin (delta), which was subsequently modified to 15%; however, the sponsor also included
analyses using the 10% noninferiority margin in its premarket
approval (PMA) based on an understanding that FDA would be
using the 10% delta in the primary safety and effectiveness
analyses.

Statistical Analysis Plan
Fusion (Silhouette Group Only)

Randomization and Blinding

Radiographic success in the fusion group was defined as meeting the definition of fusion.
Fusion was defined as
● clear evidence of bridging bone and
● translational motion < 3 mm and
● angular motion < 5 degrees

The randomization was in a 2:1 ratio of investigational to control patients and was blocked by study center with an initial
block of size 3, followed by 7 blocks of length 6, followed by a
block of length 3, and finally a block size of 6.
An important limitation of the study design and an unaddressed source of bias is the lack of patient and surgeon blinding. However, blinding was not achievable in this study due to
the nature of the two devices. Although the patients were
blinded until after surgery, the lack of blinding for the remainder
of the study could have led to reporting bias among patients and
investigators, potentially in favor of the investigational treatment,
or perhaps against the control treatment. The patient-reported
outcomes may have been tainted as a result.

The magnitude of angular and translational motion and the
presence of clear evidence of bridging bone were separately
determined from available radiographs by two independent
primary radiologists (with a third to adjudicate in instances of
disagreement regarding bridging bone, rotation, and translation
at the index level).
● Postoperative medication use: The use of narcotic analgesics,
nonnarcotic analgesics, nonsteroidal anti-inflammatory,
steroidal anti-inflammatory, and “other” medications
within each time of postoperative clinical assessment was
assessed.
● Works status: The proportion of patients working, not working due to back disability, and not working for reasons other
than back disability were evaluated at each postoperative
time point for each treatment group.

230

Patient Accounting

The IDE study database was initially closed on September 20,
2007, but was reopened for additional data entry on November
6, 2008, in order to respond to deficiencies from FDA. It was
again closed on March 13, 2009.
Four hundred sixty-seven patients received identifiers after
the randomized controlled clinical trial. The study device was


| 20.02.15 - 13:28

Clinical Results of IDE Trial of Dynesys for Dynamic Stabilization
Table 32.1 Reasons for the withdrawals after randomization in each
treatment group
Reason

Dynesys

Silhouette

Insurance denial

5

1

Patient decision

8

16

Physician decision

5

3

Did not meet inclusion/exclusion criteria

2

2

Enrollment closed after randomization

1

0

Total

21

22

implanted in 368 of the patients. However, one patient was
excluded from the experiment because his Dynesys arm was a
three-level construct, which resulted in a total of 367 randomized, treated patients (253 Dynesys, 114 Silhouette) at 26 sites.
In addition, 28 patients were nonrandomized and received the
investigational device as the initial implant at 28 sites and are

referred to as the Dynesys training cohort throughout this summary. Study IDs were also given to 71 patients who were not
treated. Of these, 24 patients in the aforementioned group were
screen failures and were never randomized. Three patients
were eligible but withdrew prior to randomization. One patient
was never randomized because the appeal for randomization
happened after enrollment in the trial. Hence, 411 patients
were randomized (275 Dynesys, 136 Silhouette). However,
43 were not implanted (21 in the Dynesys arm and 22 in the
Silhouette arm) (▶ Table 32.1). Seventy percent of Silhouette
patients and 74.6% of Dynesys patients have complete 24month primary end point data available, with 57.3% and 56%
within the protocol-defined study windows in each treatment
group, respectively. Considering all known data, there are a
number of patients for whom primary end point success/
failure can be determined even without complete primary end
point data because they are a known failure for one of the
required components. Specifically, primary end point determinations can be made for 217/252 (86.1%) of Dynesys patients
and 89/113 (78.8%) of Silhouette patients with the denominators referring to the number of theoretical patients minus
the one death prior to 24 months in each study group. Some
radiographic data are available for 76.4% of Dynesys patients
and 68% of Silhouette patients, and complete radiographic data
are available for 73% of Dynesys patients and 67% of Silhouette
patients.

Demographics and Preoperative Characteristics
The investigational and control groups are comparable in
demographic and baseline characteristics, except for history of
smoking, for which the average number of years is significantly
less in the Dynesys group than in the Silhouette group.

32.3 Results

The results from this clinical study report a 52.1% overall clinical success rate in Dynesys patients as compared with a
40.4% overall clinical success rate in Silhouette patients and
a 62.5% overall clinical success rate in Dynesys training
patients. The overall clinical success rates drop to 47.0% for
Dynesys and 37.5% for Silhouette if only patients in the protocol-defined windows are considered. In addition, only 74.6%
of Silhouette patients are considered fused at 24 months
(▶ Table 32.2).
A comparison was done of the overall clinical success outcomes for the whole study cohort to overall success outcomes
for the following subgroups of patients:
● Patients treated at one level and patients treated at two
levels
● Patients with the different radiographic indications for treatment (i.e., central stenosis, instability, lateral stenosis)
● Patients who were treated with a concomitant decompression
and those who were not
In summary, Dynesys and Silhouette patients experienced differential results depending on the primary indication and
whether treatment was at one or two levels. For those patients
with instability or lateral stenosis as the primary radiographic
indication, Dynesys patients performed notably better. Success
rates for instability indication were 64.6% for Dynesys and
41.5% for Silhouette; success rates for lateral stenosis were
50.7% for Dynesys and 29.2% for Silhouette. However, for central
stenosis the treatment difference was reversed, with Silhouette
patients performing better than Dynesys patients. Success rates
for central stenosis were 37.3% for Dynesys and 54.5% for
Silhouette. Although such an analysis was not prespecified,
there was a nominally statistically significant interaction for
treatment group by primary indication. This statistical result
indicates that data should be considered separately for the
subgroups defined by primary indication. While there was not

Table 32.2 Overall clinical success and components of overall clinical success at 24 months (analysis windows) primary outcome
Silhouette

Difference
(90% confidence
interval)

Fisher’s exact
test p-value
(left tail)

Dynesys training

Dynesys
VAS leg pain success

151/173 (87%)

51/70 (73%)

N/A

0.01

19/22 (86%)

ODI success

133/175 (76%)

49/70 (70%)

N/A

0.34

19/23 (83%)

Neurological success
(maintain/improve)

157/171 (92%)

58/69 (84%)

N/A

0.10

19/21 (90%)

No major complication

251/252 (99.6%)

112/113 (99.1%)

N/A

0.53

28/28 (100%)

No secondary surgery

231/253 (91%)

102/114 (89%)

N/A

0.56

27/28 (96%)

Overall clinical success

113/217 (52.1%)

36/89 (40.4%)

11.6% (1.4 – 21.8%)

0.98

15/24 (62.5%)

Abbreviations: N/A, not applicable; ODI, Oswestry Disability Index; VAS, visual analog scale.

231