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Vol 10, No 4, July/August 2002
233
Dislocation is one of the most com-
mon and distressing early complica-
tions of total hip arthroplasty. The
reported incidence of dislocation
ranges from 0% to 10% after a prima-
ry arthroplasty and from 10% to 25%
after a revision arthroplasty. A wide
variety of predisposing causes and
associated factors have been suggest-
ed.
1,2
Pellicci et al
3
described the use
of a posterior approach and enhanced
soft-tissue repair in an attempt to
decrease the early incidence of dislo-
cation. Nonsurgical treatment of the
initial dislocation with a cast or brace
is successful in approximately two
thirds of patients. However, when
surgical treatment is required for
recurrent dislocation, satisfactory
results have been achieved in only
60% of hips using a wide variety of
techniques.
1
Additionally, the chance
of success is even less when a precise

etiology cannot be determined. It is
for these situations that constrained
components have been considered.
By definition, constrained total hip
arthroplasty components include a
mechanism that locks the prosthetic
femoral head into a polyethylene
acetabular component. A thorough
understanding of the design features
of constrained components in total
hip arthroplasty, indications for their
use, and results and complications is
essential for the effective application
of this technique.
Historical Perspective
The use of constrained total hip
arthroplasty components has been
limited.
4
Sivash first reported on his
constrained prosthesis in 1963 in
Moscow, at a conference on tubercu-
losis of bones and joints.
5
The Sivash
prosthesis was a locked one-piece
prosthesis, with the cup and head-
neck components fabricated as a
connected whole (Fig. 1). The first
components were fabricated of steel;

they were later modified to include
chrome-cobalt and titanium alloys.
The acetabular component was a
threaded hemisphere made of a tita-
nium alloy and was available in 51-,
57-, and 65-mm diameter sizes. The
femoral component had a chrome-
cobalt head welded onto a titanium-
alloy stem and was available in three
sizes: 14-, 16-, and 18-mm proximal
diameter. The articulating surface
was polyethylene. Fixation was
Dr. Lachiewicz is Professor, Department of
Orthopaedics, University of North Carolina at
Chapel Hill, Chapel Hill, NC. Dr. Kelley is
Associate Professor, Department of Orthopaedics,
University of North Carolina at Chapel Hill,
Chapel Hill.
One or more of the authors or the departments
with which they are affiliated have received
something of value from a commercial or other
party related directly or indirectly to the sub-
ject of this article.
Reprint requests: Dr. Lachiewicz, 242 Burnett-
Womack Building, CB 7055, Chapel Hill, NC
27599.
Copyright 2002 by the American Academy of
Orthopaedic Surgeons.
Abstract
The use of a constrained component may be appropriate for the surgical treatment of

recurrent dislocation due to soft-tissue insufficiency following a total hip arthroplasty.
Constrained components usually include a locking mechanism incorporated into the
polyethylene acetabular liner to keep the prosthetic femoral head in place. Two differ-
ent prosthetic designs are available and have been approved by the U.S. Food and
Drug Administration. The S-ROM constrained component uses additional polyeth-
ylene in the rim, which deforms to more fully capture the femoral head and then is
held in place by a metal locking ring. The Howmedica Osteonics constrained compo-
nent is a tripolar device; its bipolar component articulates with another polyethylene
liner. These constrained components transfer hip forces that would otherwise lead to
dislocation to the locking mechanism, the liner-shell interface, or the bone-prosthesis
interface. These forces may eventually contribute to failure of the component due to
loosening, dissociation, breakage, or recurrent dislocation. Studies of these compo-
nents show a failure rate of 4% to 29% at relatively short-term follow-up.
J Am Acad Orthop Surg 2002;10:233-238
The Use of Constrained Components
in Total Hip Arthroplasty
Paul F. Lachiewicz, MD, and Scott S. Kelley, MD
Perspectives on Modern Orthopaedics
either press-fit or cemented. After
appropriate reaming of both surfaces,
the femoral component was implant-
ed, followed by impaction or cement-
ing of the locked-on acetabular com-
ponent. Sivash reported its use in
200 cases, with 1- to 9-year follow-up
in 169 patients.
5
The most common
indications were ankylosing spondy-
litis in 107 patients and tuberculous

arthritis in 56 patients. Although
there was no detailed analysis of
results, Sivash reported that the pros-
thesis fractured in 13 hips.
A modified Sivash prosthesis
with specially designed rasps was
described in 1974.
6
A case report in
1981 described the successful use of
this modified prosthesis for recurrent
dislocation and anecdotally suggest-
ed that cerebral palsy, Parkinson’s
disease, and loss of hip muscula-
ture were indications for its use.
7
Although the prosthesis was used ex-
tensively in Europe,
8
it was used only
sporadically in the United States.
Bryan and Reeve
9
described a
case of a patient with recurrent dis-
location who was treated with this
device. Failure was eventually
caused by fatigue fracture of the con-
straining ring and severe polyethyl-
ene wear and metal-metal abrasion.

Koffman
10
reported the use of three
different designs of constrained com-
ponents (including the Sivash) in five
hips of four patients with spastic
cerebral palsy. The Sivash prosthesis
was implanted in the only ambulato-
ry patient and failed because of
dislocation and early acetabular
loosening.
Current Constrained
Designs
Two constrained total hip arthro-
plasty liner systems are presently
approved by the U.S. Food and
Drug Administration (FDA) and
have data published on results of
their use. They are the S-ROM con-
strained acetabular liner (Poly-Dial;
DePuy Orthopaedics Warsaw, IN)
and the Howmedica Osteonics con-
strained acetabular liner (Stryker
Howmedica Osteonics, Rutherford,
NJ). Other constrained liners have
been used in FDA trials or are in
development.
The S-ROM constrained acetabu-
lar liner has been available since
1987. It was marketed under a

Premarket Notification from the
FDA. To date, according to the com-
pany, more than 6,000 have been
implanted. This constrained acetab-
ular liner was designed for use with
S-ROM metal shells. The constraint
is derived from the addition of extra
polyethylene in the rim, which
deforms to more fully capture the
femoral head implant (Fig. 2). In
addition, a capture ring provides
increased constraint. The design of
this component allows the head to
dissociate from the liner before the
forces dislodge the acetabular shell
from the pelvic bone. Cameron
11
reported that the force required for
withdrawal of this component is 60
inch-pounds and that the metal
constraining ring increases the hold-
ing power to 300 inch-pounds.
Lombardi et al
12
reported that the
metal ring provided a constraining
force of more than 600 pounds with
a 32-mm head and 325 pounds with
a 28-mm head. However, they
found that the amount of leveraged

torque required to pry the femoral
head out of the liner was 150 lbs/in
2
.
The optimal amount of torque
required for removal of the femoral
head from a constrained acetabular
component is not known. The S-
ROM component, which is “dialed”
into the acetabular shell, is currently
available with an internal diameter
of 28 or 32 mm and with a standard
rim or a 10° elevated rim. The liner
is available to fit acetabular shells
with an outer diameter of 48 mm to
68 mm and is fabricated of cross-
linked polyethylene, with a mini-
mum thickness of 5 mm. The aver-
age arc of motion (when used with
Constrained Components in Total Hip Arthroplasty
Journal of the American Academy of Orthopaedic Surgeons
234
Figure 1 The one-piece Sivash constrained
component for total hip arthroplasty.
(Reprinted with permission from Amstutz
HC, Grigoris P: Metal on metal bearings in
hip arthroplasty. Clin Orthop 1996;[329
suppl]:S11-S34.)
Figure 2 The S-ROM constrained acetabu-
lar liner with locking ring and correspond-

ing uncemented metal shell. (Reprinted
with permission from Kaper BP, Bernini
PM: Failure of a constrained acetabular
prosthesis of a total hip arthroplasty: A
report of four cases. J Bone Joint Surg Am
1998;80:561-565.)
an S-ROM femoral component) is
reported to be 88° with a 28-mm
head and 98° with a 32-mm head.
12
This arc of motion is probably less
when an elevated rim liner is com-
bined with a “skirted” modular
femoral head component.
The Howmedica Osteonics con-
strained acetabular liner was intro-
duced as a custom component in
1988
13
and was marketed under a
Premarket Approval from the FDA,
until recently converted to a class II
device. This component is basically
a tripolar device (Fig. 3): a polyeth-
ylene inner liner is covered with a
polished cobalt-chrome shell; the
shell articulates with another poly-
ethylene liner (the outer bearing),
which is inserted into a standard
noncemented acetabular shell. The

inner liner accepts a 22-, 26-, or 28-
mm femoral head and has a locking
ring identical to the ring in a bipolar
prosthesis.
Some authors have suggested that
the constrained acetabular liner can
be cemented into a well-fixed acetab-
ular shell or into an acetabulum pre-
pared for cement fixation.
14
The
Howmedica Osteonics constrained
acetabular liner has been cemented
into both an acetabular shell of
another manufacturer and into an
acetabulum prepared for cement fix-
ation.
14
If the former technique is
used, the surgeon should carefully
consider preoperatively if this con-
strained liner will fit. The smallest
acetabular shell into which this liner
could be safely cemented is probably
52 mm. According to the manufac-
turer, the total range of motion is 72°
when it is used with 50- to 54-mm
outer acetabular shells, 82° with a 56-
mm shell, and 84° with 58- to 74-mm
shells. The two polyethylene articu-

lating surfaces have a thickness rang-
ing from 5.2 to 7.7 mm for the inner
bearing and from 4.3 to 10.4 mm for
the outer bearing. The polyethylene
thickness varies based on femoral
head size and acetabular shell diam-
eter. The pullout strengths of the
three segments of this tripolar liner
have not been reported.
Indications
The use of a constrained acetabular
component in total hip arthroplasty
is indicated for recurrent dislocation
of the hip due to soft-tissue insuffi-
ciency (capsular or abductor muscu-
lature) that is not amenable to repair
or augmentation. If the abductor
mechanism has been resected, then
reconstruction with a constrained
system may be required. Soft-tissue
laxity (not insufficiency) due to short-
ening of the prosthetic hip may be
treated by lengthening the femoral
neck and/or lateralizing the acetabu-
lar component or liner. Component
malposition, loosening, or wear
should be treated by revision of one
or both components. Dislocation re-
sulting from impingement of bone or
a “skirted” femoral head against an

elevated-rim acetabular liner should
not routinely be treated by a con-
strained component. Bone impinge-
ment can be treated by bone resection
and impingement of the femoral
head by revision of the head, liner,
or acetabular component. Acute
avulsion of a greater trochanteric
osteotomy or fracture of the greater
trochanter should be treated by sur-
gical repair and/or advancement.
However, recurrent dislocation due
to a chronic nonunion of the greater
trochanter, with severe and irrepara-
ble loss of abductor muscle function,
may be an indication for use of the
constrained component. Recurrent
dislocation associated with a large
mismatch between the femoral head
size and the outer acetabular compo-
nent diameter, as reported by Kelley
et al,
15
should be treated by revision
to a larger head and corresponding
acetabular liner, if possible.
Late (>1 to 2 years postopera-
tively) recurrent dislocation, which
may be associated with weight loss,
decrease in muscle mass, and/or

chronic disease (cancer, rheumatoid
arthritis) without component malpo-
sition, is extremely difficult to treat.
The constrained component may be
a reasonable option in patients with
these conditions. When late disloca-
tion is associated with an acute or
chronic infection, the treatment is
complex, must be individualized,
and may involve the use of a con-
strained component.
Contraindications for the use of
constrained components include
acute dislocation, dislocation due to
component loosening or malposi-
tion, insufficient acetabular bone
structure, acute infection, skeletal
immaturity, and neurologic spas-
Paul F. Lachiewicz, MD, and Scott S. Kelley, MD
Vol 10, No 4, July/August 2002
235
Outer bearing
(UHMWPE)
Inner bearing
shell (CoCr)
Acetabular shell
Inner bearing
(UHMWPE)
Bipolar retaining
ring (UHMWPE)

Figure 3 A, The Howmedica Osteonics constrained acetabular liner. B, Schematic showing
the tripolar nature. UHMWPE = ultra-high-molecular-weight polyethylene, CoCr = cobalt-
chrome. (Adapted with permission from Stryker Howmedica Osteonics, Rutherford, NJ.)
A B
ticity. Neurologic spasticity may
seem to be an attractive indication
for the use of this component, but
Root et al,
16
in reporting the results of
total hip arthroplasty without con-
strained components performed in
patients with cerebral palsy, found
that only 2 of 15 patients had a recur-
rent dislocation, and both had com-
ponent malposition.
The prophylactic use of con-
strained components in primary or
revision total hip arthroplasty is con-
troversial. Because good data are
lacking, constrained acetabular liners
should not be used routinely in these
situations. Larger femoral head
sizes, femoral necks with greater
length and offset, and/or elevated
rim liners are better choices.
Results and Complications
Theoretically, constrained acetabular
components should transfer the
forces that would otherwise lead to

dislocation to the locking mecha-
nism, the liner-shell interface,
or the bone-prosthesis (or bone-
cement) interface. If the hip center is
shifted laterally, which may occur
with either of the two available con-
strained components, these forces
may be increased. The reported
results of constrained components
have demonstrated four types of fail-
ure: loosening of the acetabular
component;
12
dissociation of the con-
strained liner from the shell (with
redislocation)
17,18
(Fig. 4); material
failure (breakage) or disengagement
of the constraining ring (with or
without redislocation)
17,19
(Fig. 5);
and dissociation of a modular
femoral head from its neck.
20
An
additional potential mode of failure
is excessive wear of a thin acetabu-
lar liner interface.

According to information avail-
able from the manufacturer, the S-
ROM constrained acetabular liner
has a low rate of dissociation-
dislocation in the more than 6,000
implanted since 1987. However, a
careful clinical review of these cases
has not been performed, and there
are relatively few published data on
the component.
Lombardi et al
12
reported a retro-
spective review of 57 S-ROM con-
strained acetabular liners implanted
in 55 patients. Six were used in
primary arthroplasties and 51 in
revision arthroplasties. Of these,
however, only 31 were done for dis-
location, and of those, 13 patients
had experienced multiple disloca-
tions (average, 2.7; range, 2 to 5). Al-
though the clinical follow-up period
for the entire groupwas relatively
short (mean, 30.2 months), two
patients experienced early definite
radiographic loosening of the ace-
tabular component with screw
breakage and migration. Five of 55
patients (9%) experienced failure due

to redislocation at a mean of 2.5
months (range, 1 to 9 months) post-
operatively. Three of these five
patients had undergone the proce-
dure because of recurrent disloca-
tion, and thus the failure rate of the
constrained component for this indi-
cation was 23% (3 of 13). Open
reduction was necessary when a dis-
location of this constrained compo-
nent occurred.
Anderson et al
17
reported the
results of S-ROM constrained
acetabular liners in 21 patients, 18 of
whom had experienced recurrent
dislocation. At a mean follow-up of
31 months (range, 24 to 64 months),
15 patients (71%) reported no fur-
ther dislocations. However, six
patients (29%) reported eight redis-
locations at a mean of 10 months
postoperatively (range, 1 to 30
months). In four cases, the polyeth-
ylene liner (still securely fixed to the
femoral head) was levered out of
the metal shell; in two failures, the
femoral head pulled out of the liner;
and in two other dislocations, the

metal retaining ring disengaged
from the polyethylene liner. In all
six patients with redislocations, the
preoperative diagnosis was recur-
rent dislocation, for a failure rate of
33%. However, no loosening of the
19 porous-coated acetabular compo-
nents was reported in this study.
Fisher and Kiley
18
reported two
cases of failure of the S-ROM com-
ponent. One occurred 9 months
postoperatively and was due to fail-
ure of the retaining ring and poly-
ethylene wear; the other occurred 5
months postoperatively, with both
loosening of the metal shell and
pullout of the polyethylene liner
from the shell following a traumatic
event. Of 51 hips in which the S-
ROM constrained component was
used, either for recurrent dislocation
or in extensive revisions, there were
5 failures—3 redislocations and 2
dissociations (10% failure)—and all
required open reduction or revision
of the component.
18
Of 12 patients managed with the

S-ROM component at their institu-
tion, Kaper and Bernini
19
reported
Constrained Components in Total Hip Arthroplasty
Journal of the American Academy of Orthopaedic Surgeons
236
Figure 4 Radiograph showing the S-ROM
constrained polyethylene component dislo-
cated from the metal shell. Probably there is
also loosening of the acetabular shell.
(Reprinted with permission from Anderson
MJ, Murray WR, Skinner HB: Constrained
acetabular components. J Arthroplasty
1994;9:17-23.)
failure in four. In two, the constrain-
ing ring had fractured, and in the
other two, the liner had pulled out of
the metal acetabular shell. Because
two of the failures involved an ele-
vated-rim constrained liner, these
authors suggested that the use of that
liner may contribute to a lever-out
mechanism.
McPherson et al
21
recently de-
scribed a new technique that resulted
in the successful closed reduction of a
dislocated S-ROM constrained liner

in three medically compromised
patients. With the patient under gen-
eral anesthesia and using fluoroscop-
ic guidance, the femoral head was
perched into the opening of the
acetabular component. With the leg
positioned in 40° of abduction and
30° of flexion, a minimum of three
people using a “bear hug” maneuver
of the hip and pelvis apply a contin-
ued axial compressive force for at
least 90 to 120 seconds, until an audi-
ble and palpable clunk of reduction
has occurred. No complications were
reported, but all three patients later
had revision or resection arthroplas-
ty. The advantage of this technique is
the ability to delay revision surgery
until conditions (the patient’s health
and the availability of equipment and
personnel) are more favorable.
Because of wide variation in series
size and in indications for surgery,
little information beyond anecdotal
case reports can be gleaned from the
four series reviewing the use of S-
ROM constrained components.
12,17-19
However, the reported rate of failure
or redislocation is high (9% to 33%).

There is even less published
experience with the Howmedica
Osteonics constrained acetabular
liner, in part because its use was ini-
tially restricted to two medical cen-
ters. Goetz et al
14
reported the use of
this acetabular liner for recurrent dis-
location in 56 hips. Forty-six con-
strained components were inserted
without cement, and 10 were inserted
with cement (four of these were
cemented into acetabular shells of
another manufacturer). The 38
patients (39 hips) still living at the
time of the report had been followed
for a mean of 5.3 years (range, 3 to 8
years), and the deceased 16 patients
had been followed for a mean of 2.3
years (range, 1 to 81 months). One
patient was lost to follow-up. Only
two patients (4%) experienced failure
described as “recurrent dislocation.”
However, in one patient, the acetabu-
lar shell (with screws) pulled out of
the pelvis, and in the other, the
cemented constrained component
dissociated from a well-fixed shell.
Seven hips (13%) required revision

surgery in the follow-up period,
including four for infection and one
for acetabular component loosening.
Radiographic analysis was per-
formed for 38 hips with a minimum
2-year follow-up. There was acetabu-
lar osteolysis in 2 of 27 hips (7%)
treated with a new acetabular shell
and a new constrained liner, both in-
serted without cement. There was
also definite loosening of 2 of 34
uncemented acetabular components
(6%) and 2 of 33 uncemented femoral
components (6%).
Goetz et al
14
emphasized that,
because the primary goal of these
revisions was a stable hip, the patients
and surgeons were willing to accept
the increased risk of polyethylene
wear, osteolysis, and component loos-
ening. It also should be emphasized
that these patients were predominant-
ly elderly, debilitated women with a
mean age of 71 years. There are no
published reports on the use of this
constrained component in younger or
active patients, in whom an even
higher rate of failure of fixation

would be expected.
Summary
Constrained components should be
used judiciously for the surgical
treatment of recurrent dislocation of
the hip. The ideal patient is an elder-
ly, low-demand patient with recur-
rent dislocation despite well-fixed
and properly positioned compo-
nents. The etiology of these
dislocations is usually soft-tissue
(capsule or musculature) insufficien-
cy around the prosthetic hip joint.
These components should be consid-
ered for use only when other options
are exhausted and only when bipolar
arthroplasty, resection arthroplasty,
or a constrained acetabular liner
remains. For the two presently avail-
able constrained hip components,
the rates of failure, including redislo-
cation, dissociation of the liner from
the acetabular shell, and loosening of
the acetabular shell, are reported to
be from 4% to 29% at short-term
follow-up. Based on the limited
published data regarding these con-
strained components, prophylactic
use of these components is not
presently recommended because of

the danger of excessive wear of thin
polyethylene, breakage, and acceler-
ated loosening of components.
Paul F. Lachiewicz, MD, and Scott S. Kelley, MD
Vol 10, No 4, July/August 2002
237
Figure 5 Radiograph showing the con-
straining ring displaced from the S-ROM
polyethylene component. (Reprinted with
permission from Anderson MJ, Murray
WR, Skinner HB: Constrained acetabular
components. J Arthroplasty 1994;9:17-23.)
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