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Vol 8, No 1, January/February 2000
3
Of the various bone-graft materials
available, autologous bone graft is
the standard for arthrodesis of the
spine. The relative superiority of
autologous bone grafting is due to
its osteoinductive, as well as its
osteoconductive and osteogenic,
properties. Autologous bone graft
is thought to contain growth fac-
tors such as bone morphogenetic
proteins (BMPs).
1
These proteins
have been shown to induce bone
formation through endochondral
mechanisms, leading to their possi-
ble use in isolated form to achieve
spinal fusion and to repair long-
bone fractures.
2,3
In 1979, Urist et al
4
first showed
that proteins with bone morpho-
genetic properties could be extracted
from animal cortical bone by diges-
tion of the demineralized cortical
bone matrix with bacterial collage-
nase and solubilization of the

digest in a neutral mixture of salt
and ethylene glycol. The BMP ex-
tracted in this manner was found
not to be species-specific; that is,
BMP extracted from a rabbit in-
duced new bone in rats and there-
fore might do so in man as well.
Bauer and Urist
5
then isolated
human BMP (by using a 4M guani-
dine hydrochloride solution) that
was capable of inducing bone for-
mation in the thigh muscles of
athymic nude mice.
Since that time, more BMP
types, better isolation techniques,
and the advent of recombinant
cloning techniques have made the
use of BMP in the clinical setting a
reality. Currently, the use of BMP
in humans is restricted to trials
evaluating its use in achieving
spinal arthrodesis
6
and in treating
nonunions and other difficult
problems. Possible future uses for
BMPs include enhancement or re-
placement of autologous bone

graft, treatment of delayed union
or nonunion, compensation for
patient factors such as nicotine
use, facilitation of spinal or recon-
structive arthrodeses, supplemen-
tation of biologic ingrowth, and
management of osteonecrosis and
certain pathologic osteopenias.
7
Additionally, current basic science
research is evaluating the efficacy
in animal models of a single dose
of recombinant human BMP-2
(rhBMP-2) versus the use of mar-
row cells genetically transformed
to produce BMP-2.
Dr. Zlotolow is Clinical Research Fellow,
Department of Orthopaedic Surgery, The
Rothman Institute at Thomas Jefferson
University, Philadelphia. Dr. Vaccaro is
Associate Professor of Orthopaedic Surgery, The
Rothman Institute. Mr. Salamon is Research
Assistant, Thomas Jefferson University. Dr.
Albert is Associate Professor of Orthopaedic
Surgery, The Rothman Institute.
Reprint requests: Dr. Vaccaro, The Rothman
Institute, 925 Chestnut Street, Philadelphia,
PA 19107.
Copyright 2000 by the American Academy of
Orthopaedic Surgeons.

Abstract
The attainment of a stable arthrodesis is critical to the successful management
of some types of spinal disorders. Autologous iliac-crest bone graft has been the
most commonly utilized substance associated with predictable healing in spinal
fusion applications. Although alternative graft substances exist, these have not
been shown to be as uniformly effective in achieving spinal fusion. Because of
the morbidity associated with bone autograft harvest, there is increasing inter-
est in alternative graft substances and especially in the osteoinductive abilities
of bone morphogenetic proteins (BMPs). Several animal models have demon-
strated that BMP-containing allograft or synthetic carrier medium is as effective
as or superior to autograft bone in promoting spinal fusion. Furthermore, the
limited number of human trials utilizing BMPs to treat nonunions in the
appendicular skeleton indicate that the results found in animal models will be
reproducible in the clinical setting.
J Am Acad Orthop Surg 2000;8:3-9
The Role of Human Bone Morphogenetic
Proteins in Spinal Fusion
Dan A. Zlotolow, MD, Alexander R. Vaccaro, MD, Michael L. Salamon, and Todd J. Albert, MD
Perspectives on Modern Orthopaedics
Classification
The BMPs, with the exception of
BMP-1, are grouped as a family
within the transforming growth
factor β (TGF-β) superfamily of
dimeric, disulfide cross-linked
growth and differentiation factors
(Table 1). Although BMPs have
been studied primarily for their
osteoinductive properties, they
may also be found in extraskeletal

tissues, where they function to reg-
ulate the development of other
organ systems.
8
The sole criterion
for BMP classification is the induc-
tion of ectopic bone formation in a
standard in vivo rodent assay sys-
tem. Individual BMPs are grouped
on the basis of their amino acid
sequences and molecular structural
components.
The BMP family encompasses
more than 12 proteins, 9 of which
have demonstrated an ability to
induce ectopic bone formation in
an in vivo assay system.
9
The
BMPs used for the initial basic and
clinical research endeavors were
originally extracted from deminer-
alized human cortical matrix. A
number of extraction schemes have
been developed, but all are com-
plex and do not reliably yield
usable quantities of BMP, as only 1
to 2 µg of BMPs are present in a
kilogram of cortical bone. Further-
more, the presence of contaminants

in these extracts is of some concern
in this era of blood-transmissible
infections.
7
The application of recombinant
DNA technology in the manufac-
turing of genetically engineered
BMPs as early as 1988
2
has allowed
for a virtually limitless supply of a
few rhBMPs (e.g., rhBMP-2, -4, and
-6,Genetic Institute, Cambridge,
Mass; OP-1 rhBMP-7, Stryker Bio-
tech, Natick, Mass) for basic re-
search and clinical trial applica-
tions. Despite this, the mechanism
of action and the role of each of the
seemingly redundant BMPs remain
controversial. What little is known
about these proteins has been stud-
ied mostly in in vitro assay systems
and in clinical trials in animals.
Osteoinductive Properties
Analogous to the cascade theory of
bone formation witnessed in em-
bryonic endochondral ossification
and fracture callus formation,
BMPs induce bone formation in a
stepwise fashion. The sequence

includes chemotaxis of progenitor
cells, proliferation of mesenchymal
cells, differentiation of chondro-
cytes, calcification of cartilage
matrix, angiogenesis and vascular
invasion, bone differentiation and
mineralization, and bone remodel-
ing and marrow differentiation.
Once formed, the bone seems to
function as typical endochondral
bone, responsive to both internal
and external stimuli.
8
The BMPs may themselves act in
a stepwise fashion. The presence of
one BMP may induce the expres-
sion of other BMPs. Moreover, two
different BMPs may join to form
disulfide-linked heterodimers, and
these heterodimers may bind and
activate BMP receptors with greater
affinity than that of BMP homo-
dimers. The combinations of BMP-2
with BMP-7 and BMP-2 with BMP-6
have been shown to be five- to ten-
fold more potent in inducing carti-
lage and bone formation than BMP-2
alone.
10
This, along with the dis-

covery of a joint BMP-2ÐBMP-4 re-
ceptor, suggests that BMPs may
function as heterodimers in vivo.
11
Alternatively, BMP-2 and BMP-4
may induce endochondral bone for-
mation, while BMP-6 may preferen-
tially induce direct membranous
bone formation. There is evidence
that BMP-7 stimulates mRNA levels
of BMP-6 while decreasing mRNA
levels of BMP-2 and BMP-4,
12
and
that BMP-2 stimulates BMP-3 and
BMP-4 mRNA levels.
13
Boden et al
14
found that physiologic levels of the
glucocorticoid triamcinolone ace-
tonide promoted osteoblast differen-
tiation from fetal calvarial cells by
inducing BMP-6 production. The
effects of the glucocorticoid could be
blocked by BMP-6 antisense DNA
(antisense DNA blocks sister-strand
mRNA translation), indicating that
BMP-6 mRNA translation to protein
is a necessary step downstream of

glucocorticoid production in the
osteoinductive pathway. Further-
more, BMP-6 was found to be 2 to
2.5 times more potent than either
BMP-2 or BMP-4, and to not be as
potentiated by a glucocorticoid infu-
sion as the other two.
15
The cascade of bone osteoinduc-
tion may be initiated by a BMP,
leading to the controlled expres-
sion of other BMPs, which may
then work synergistically to stimu-
late bone formation. In sponta-
neous bone formation, the gluco-
corticoid may be the principal or
early signaling molecule, and may
initiate a signal amplification cas-
cade beginning with BMP-6. Due
to its function early in the cascade,
BMP-6 appears to be the ideal
osteoinductive protein for investi-
gation in clinical trials.
Segmental-Defect Models
The potential of BMPs was initially
investigated by utilizing segmental-
defect animal models. In 1982,
Takagi and Urist
16
showed that

extracted bovine BMP could be
Human Bone Morphogenetic Proteins in Spinal Fusion
Journal of the American Academy of Orthopaedic Surgeons
4
Table 1
Family Groups Within the TGF-β
Superfamily
TGF-β family
Inhibin/activin family
MŸllerian inhibiting substance
family
Decapentaplegic BMP family
used to heal large femoral diaphy-
seal defects in rats. The authors
used an omega pin to distract the
two ends of the femur and to block
the migration of osteoinductive and
osteoconductive elements within
the marrow. Although a variety of
graft materials were evaluated,
including variable doses of BMP
without a carrier, only the defects
treated with BMP and autologous
marrow healed 100% of the time.
The earliest experimental studies
of rhBMP-2 also involved iatrogeni-
cally produced bone defects in an
animal model. Yasko et al
17
found

that rhBMP-2 combined with bone
marrow in a rat segmental femoral-
defect model produced union at a
rate of 100%, three times superior to
the rates achieved with autogenous
cancellous bone graft. Similar
bone-defect studies have been con-
ducted in rabbit tibia and ulna,
sheep femur, and canine spine and
mandible. Toriumi et al
18
showed
that rhBMP-2 could effectively in-
duce bone formation in the man-
dible, which embryologically follows
the intramembranous pathway, un-
like previous studies that focused
on endochondrally derived bones.
Early results in human clinical trials
for tibial nonunions suggest out-
comes equivalent to those obtained
with use of autologous bone graft.
Posterior Lumbar Spine
Fusion
Posterior lumbar fusions in humans,
unlike anterior fusions, may not be
loaded in compression and therefore
occur less predictably. Clinically im-
proving the rate of posterior fusion
may require use of an osteoinduc-

tive substance such as autograft or
BMP. At the present time, the only
human BMPs tested in clinical ani-
mal spine fusion models have been
rhBMP-2 and rhBMP-7. In addition
to investigating the efficacy of BMPs,
studies of these substances have pro-
vided information on the optimal
delivery system or carrier, potential
complications, and the effects of
dosage manipulation on fusion rate
and success.
Early Results
As early as 1989, Lovell et al
19
evaluated the effect of extracted BMP
on experimental posterior inter-
vertebral spine fusions in mature
mongrel dogs. Radiologic and histo-
morphometric evaluation showed
that the fusion levels augmented
with BMP had two to three times
more new-bone formation than con-
trol levels. Fusion occurred at 71% of
the levels treated with the BMP but
only 14% of the control levels. How-
ever, the polylactic-acid polymer car-
rier utilized was not resorbed com-
pletely, suggesting that a better
carrier material needs to be found.

Schimandle et al
20
demonstrated
higher fusion tensile strength and
stiffness in rabbit posterior lumbar-
spine fusions supplemented with
rhBMP-2 as compared with those in
animals that received autogenous
iliac-crest bone graft. All animals
treated with rhBMP-2 demonstrated
solid fusion, as evaluated by manual
palpation and radiographic exami-
nation, compared with only 42% of
the autograft animals. In another
study, performed in a posterolateral
intertransverse-process model in
rabbits, rhBMP-2 was shown to
reverse the inhibitory effects of the
nonsteroidal anti-inflammatory
drug ketorolac on fusion rate.
21
Dose Response
Using rhBMP-2 with a collagen
carrier in a canine spinal fusion
model, David et al
22
showed a dose-
dependent osteoinductive effect,
with a 100% clinical and radio-
graphic fusion rate. Using a canine

fusion model, Sandhu et al
23
dem-
onstrated that rhBMP-2, delivered
at a dose of 2,300 µg in a porous
polylactic-acid polymer, was supe-
rior to autologous iliac-crest bone
graft in achieving a single-level
lumbar arthrodesis. In a later
study, this group investigated the
dosage level at which no further
osteoinduction was achieved. Re-
combinant human BMP-2 was
implanted at multiple doses, in-
cluding 58, 115, 230, 460, and 920
µg. Histologically, abundant bone
formation occurred in all specimens
containing rhBMP-2 by 3 months.
24
The data from this study and the
one using 2,300 µg of rhBMP-2
showed no mechanical, radiographic,
or histologic variations in the quality
of intertransverse-process fusion re-
sulting from a 40-fold increase in
rhBMP-2 dosage (58 µg to 2,300 µg).
The rhBMP-2 dosage require-
ment for successful fusion was also
investigated by Boden et al
25

in a
primate model of laparoscopic
anterior lumbar interbody arthro-
desis in which a titanium-threaded
interbody fusion cage was used.
Before insertion, the cages were
soaked in either high-dose (1,500
µg/mL) or low-dose (750 µg/mL)
rhBMP-2. Although bone formation
and spinal fusion were achieved at
both doses tested, the higher dose
produced a more rapid fusion re-
sponse. In humans, the optimal
dosage based on the optimal carrier
remains to be determined, although
dose-response studies have been
carried out in nonspinal locations
in humans.
Carrier Medium
The role of the carrier medium is
to allow the BMP molecules to be
applied in an easily reproducible
localized fashion and to prevent
rapid uncontrolled diffusion into the
surrounding tissues. The ideal car-
rier medium would be biocompatible,
completely resorbable, structurally
stable, and easy to manipulate. A
variety of media, including biologic
substances, polymers of different

types, and even titanium sponges,
have been investigated in the ap-
pendicular skeleton.
Dan A. Zlotolow, MD, et al
Vol 8, No 1, January/February 2000
5
Sheehan et al
26
examined various
carrier media in posterior lumbar
spine fusions in an adult female ca-
nine model. Using the same surgi-
cal approach, the authors compared
four different treatments: autoge-
nous iliac-crest bone alone, bovine
type I collagen gel and autogenous
iliac-crest bone, type I collagen gel
combined with autogenous iliac-
crest bone and rhBMP-2, and con-
trol (sham) without an implant.
There was considerably more bone
formation at the sites containing
rhBMP-2 than at the sites contain-
ing autogenous iliac-crest bone
graft either alone or combined with
the collagen gel carrier. Biome-
chanical testing of the explants
demonstrated superior strength of
the rhBMP-2 fusion sites.
Fischgrund et al

27
evaluated the
augmentation of autograft by using
rhBMP-2 combined with various car-
rier media in a canine lumbar spine
fusion model. The carrier media
included a collagen ÒsandwichÓ
made of collagen sheets, collagen
ÒmorselsÓ (collagen sheets cut into
small pieces), open-pore polylactic
acid, and a polylactic acidÐglycolic
acid sponge sandwich, with auto-
graft alone or in combination with
rhBMP-2 as controls. Greater in-
creases in bone fusion mass were
recorded at all levels that involved
rhBMP-2 as compared with levels
containing autograft alone. In addi-
tion, carriers that were combined
with morselized bone graft offered
easier technical handling and appli-
cation during the operative proce-
dure. Use of the polylactic acidÐ
glycolic acid sponge sandwich as a
carrier was associated with a greater
incidence of voids within the fusion
mass compared with the use of colla-
gen sandwich, collagen morsels,
open-pore polylactic acid, or auto-
graft with rhBMP-2 alone. Ulti-

mately, no important difference in
the efficacy of the various carrier
media could be determined from this
study. However, the addition of
rhBMP-2 to autograft enhanced the
volume and maturity of the resulting
fusion mass.
The most recent studies suggest
that another approach to the use of
BMP-2 in spinal fusions may be pos-
sible. Rather than utilizing a single
dose at the time of surgery, geneti-
cally transformed marrow cells
were implanted, which produced
continuously generated quantities
of BMP-2. Boden et al
28
transplanted
marrow cells in rats with cDNA for
an osteoinductive protein and had a
100% fusion rate, compared with 0%
for controls. Similar results were
observed with adenoviral vectorÐ
transformed, BMP-2Ðproducing
marrow cells in a rat fusion model;
the effects were comparable to those
obtained with implanted rhBMP-2.
29
Decortication and
Minimally Invasive

Techniques
The value of host-bone decortica-
tion was studied by Sandhu et al
30
in a dog intertransverse-process
fusion model in which rhBMP-2
was used. The argument for the
necessity of decortication to achieve
fusion is that it unmasks marrow
elements, osteoinductive proteins,
inflammatory cells, the local blood
supply (including the initial hema-
toma), and osteogenic cells at the
fusion site. In that study, there was
no statistical difference in the clini-
cal and radiographic fusion rates
between decorticated and non-
decorticated fusion sites. Further-
more, as the dosage of rhBMP-2
was increased, there was little his-
tologic discrimination between
fusions in decorticated spines and
those in nondecorticated spines.
Boden et al
31
demonstrated the
feasibility, efficacy, and safety of a
minimally invasive application of
rhBMP-2 delivered in a collagen
sponge carrier in both a rabbit and

a nonhuman primate (rhesus mon-
key) intertransverse-process model.
This technique minimizes the mor-
bidity of paraspinal muscle dener-
vation and devascularization seen
with open intertransverse-process
fusion techniques while providing
effective fusion.
In a subsequent study, Boden et
al
32
again utilized the minimally
invasive video-assisted lateral inter-
transverse-process approach in rhe-
sus monkeys with a different carrier.
Instead of a collagen carrier, which
was criticized for its nonrigidity and
its lack of predictability of rhBMP
dose delivery, the researchers used
a rectangular-block ceramic carrier
made of hydroxyapatite (60%) and
tricalcium phosphate (40%) with 9
mg of rhBMP-2. At follow-up, the
intertransverse processes in all five
monkeys had fused solidly, com-
pared with only three of four mon-
keys in which a collagen carrier had
been utilized.
Anterior Interbody Fusion
Results of successful interbody

fusions were reported by Hecht et
al,
33
who studied the application of
rhBMP-2 delivery by means of an ab-
sorbable collagen sponge carrier
within a freeze-dried cortical-dowel
allograft in a nonhuman primate
(rhesus macaque) model (Fig. 1).
The rates of new-bone formation
and ultimate fusion success were
superior with the use of rhBMP-2
compared with autogenous cancel-
lous iliac-crest graft and freeze-
dried cortical-dowel allograft (Fig. 2).
Successful fusion was achieved in
all three animals in the rhBMP-2
group, whereas two of the three
control animals had pseudarthro-
ses (Fig. 3).
The benefits of rhBMP-2 in stim-
ulating a successful fusion response
have also been demonstrated re-
cently by Zdeblick et al
34
in a goat
model. In that study, animals treated
with cervical interbody cages filled
Human Bone Morphogenetic Proteins in Spinal Fusion
Journal of the American Academy of Orthopaedic Surgeons

6
with rhBMP-2 in a collagen sponge
demonstrated more predictable
bone growth than those treated
with local bone alone.
Human Studies
Johnson et al
35
were among the first
to use BMP in human clinical stud-
ies, evaluating the role of BMPs in
the treatment of femoral nonunions.
Twelve patients with an average of
4.3 surgical procedures each for
intractable femoral nonunions were
treated with internal fixation and
extracted human BMP implants.
All went on to have a successful
union at an average of 5 months.
Johnson and Urist
36
evaluated 15
patients with posttraumatic atrophic
femoral nonunions treated with a
one-stage lengthening procedure
(mean lengthening, 2.8 cm) involv-
ing the use of an implant of allo-
geneic antigen-extracted autolyzed
human bone perfused with par-
tially purified hBMP. Fourteen pa-

tients healed primarily with no
negative side effects from the allo-
geneic graft material, such as infec-
tion, allergic reaction, or tissue re-
jection.
Recently, Muschler et al
37
re-
ported on the first US prospective
human clinical trial of rhBMP-7, in
which they evaluated its efficacy
when coupled to a collagen carrier in
the treatment of complicated tibial
nonunions. In that study, both groups
were treated with a reamed tibial
nail, preparation of the nonunion
site, and placement of autogenous
bone graft or the rhBMP-7 device.
The researchers found no clinically
relevant difference in outcomes with
regard to pain, return to full weight-
bearing status, and avoidance of
surgery between the rhBMP-7 group
and the autogenous iliac-crest bone
grafting group. Unfortunately, the
study design did not utilize a control
group with no graft material; there-
fore, the effect of rhBMP-7 relative to
that of the surgical procedure alone
could not be discerned.

An unpublished Food and Drug
Administration pilot study utilizing
Dan A. Zlotolow, MD, et al
Vol 8, No 1, January/February 2000
7
Figure 1 Computed tomographic scans of the L5-S1 interspace in primates, obtained 3 months after attempted anterior interbody fusion
with allograft. Axial (A) and sagittal (B) reconstructions of a control animal that received a cortical-dowel allograft with iliac-crest auto-
graft placed inside the dowel. Minimal bone formation is demonstrated within the center of the dowel, with minimal incorporation of the
cortical allograft. C and D, Images of an animal that received a cortical-dowel allograft with a collagen sponge containing rhBMP-2 placed
within the dowel. Extensive bone formation is depicted within the center of the dowel, as well as extensive incorporation of the allograft
with fusion at the L5-S1 interspace. (Courtesy of Jeffrey S. Fischgrund, MD, Southfield, Mich.)
A B
C D
Figure 2 A, Histologic analysis 6 months after attempted L5-S1 fusion with cortical allo-
graft and cancellous autograft within the autograft in a rhesus monkey. The allograft is
still visible, and fibrous tissue is noted at the site of attempted fusion (Mallory azan, origi-
nal magnification ×14). B, Similar histologic analysis of tissue from another animal that
received rhBMP-2 within the cortical allograft. At the 6-month evaluation, solid fusion is
demonstrated across the interspace, with normal trabecular bone. Note complete incorpo-
ration of the allograft with preservation of the space available for the exiting spinal nerves.
(Courtesy of Jeffrey S. Fischgrund, MD, Southfield, Mich.)
A B
rhBMP-2 with anterior spinal cages
in the lumbar spine has reached the
1-year follow-up period. Prelimi-
nary results were presented at the
1998 meeting of the North Ameri-
can Spine Society and the 1999
meeting of the American Academy
of Orthopaedic Surgeons. The re-

searchers demonstrated 100% heal-
ing by 6 months in 11 patients who
received rhBMP-2 and collagen
without any autograft.
6
Another
human clinical trial with allograft
bone dowels used as rhBMP-2 car-
riers is in progress.
Human clinical trials of the use of
rhBMP-7 are taking place in Aus-
tralia, Sweden, and Denmark, with a
special focus on utilizing rhBMP-7
in lieu of autograft bone for various
operative procedures in the spine.
The first such study of the use of
rhBMP-7 in the United States is al-
ready under way.
Summary
Although the in vivo role of each
BMP is unclear, the ability of
BMPs to stimulate bone formation
is no longer in question. Various
studies, most recently human clin-
ical trials, have demonstrated that
BMPs not only can potentiate heal-
ing after autologous bone grafting
but also may be able to replace
that procedure. It is unlikely,
however, that BMPs will replace

rigid fixation in spine surgery,
except in cases of minimal instabil-
ity. The use of BMPs in a small
number of human trials suggests
that rhBMPs may be safe for use in
human subjects, although further
investigation is necessary before
widespread use can be sanctioned.
Currently, the costs of BMPs (an
estimated $3,000 to $5,000 per
dose) limit their use to experimen-
tal studies and selected cases of
nonunion or those with a high
probability of nonunion. However,
as this technology matures, the cost
will likely drop precipitously and
allow BMPs to be used in other
aspects of orthopaedic surgery. In
theory, BMPs will be as inexpen-
sive to produce as recombinant
human insulin or any recombinant
vaccine and will likely be as widely
used. The results of the work that
has been done with rhBMPs are
encouraging, suggesting that the
conventional techniques of spinal
surgery may change dramatically
in the not too distant future.
Human Bone Morphogenetic Proteins in Spinal Fusion
Journal of the American Academy of Orthopaedic Surgeons

8
A B
Figure 3 A, Microradiograph demonstrating persistence of a cortical allograft 3 months
after attempted L5-S1 fusion in a rhesus monkey. B, Microradiograph of another animal
that received rhBMP-2 within the cortical allograft. Note solid fusion across the interspace,
with trabecular bone formation. (Courtesy of Jeffrey S. Fischgrund, MD, Southfield, Mich.)
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Dan A. Zlotolow, MD, et al
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