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CHAPTER 6/SPINAL INSTRUMENTATION / 73
continuously increasing, promoting the likelihood that
decreased resorption would occur. The other solution is
the use of dynamized fixation devices, which will be discussed later in this chapter.
The ability of a hinged pedicle screw-rod fixation
(dynamized) device to transmit more loads across the stabilized segment compared with its rigid equivalent system was predicted using the FE models (70). In general,
the hinged screw device allowed for slightly larger axial
displacements of L3, while it maintained flexion rotational stability similar to the rigid screw device (Table 6-
3). Slightly larger axial displacements may be suff icient
enough to increase the load through the graft since the
stiffness of the disc was increased by replacing it (shown
as the “nucleus” in the tables) with a cancellous, cortical,
or titanium interbody device to simulate the fusion mass
in the model (Table 6-4).
The work of Goel et al. described above neglects the
effect of muscle forces on the construct mechanics.
Rohlmann et al. developed a set of FE models of the lumbar segment to address such issues (66). The diameters of
the longitudinal rod of the fixator were also varied to be
3, 5, 7, and 10 mm in the model, and the forces of the
trunk muscles were simulated. The diameter of the longitudinal rod strongly affected the fixator loads but hardly
influenced the stresses in the vertebral end plates. The
stresses in the bridged discs were strongly reduced. However, the internal fixator had only a minor influence on
the stresses in the annulus fibrosus and the pressure in the
nucleus pulposus of the adjacent discs. These results support the cadaver-based motion data of Gwon et al.
described in an earlier section (37).
FE modeling coupled with adaptive bone remodeling
algorithms has been used to investigate temporal changes
associated with interbody fusion devices. Grosland et al.
(50) have predicted the change in bone density distribution after implantation of the BAK device (Fig. 6-3). The
major findings include hypertrophy of bone directly in
the load train (directly overlying and underlying the
implant) and lateral atrophy secondary to the relatively
high stiffness of the implant. The model also predicted
that bone grows into and around the larger holes in the
implant, resulting in sound fixation of the device. Further
insight into the biomechanics of the cages using the FE
models was provided in an earlier section of this chapter.
TABLE 6-3. Axial displacement and angular rotation of L3
with respect to L4 for the 800 N axial compression
Axial displacement (mm) Rotation (degrees)
Graft Rigid Hinged Rigid Hinged
Cancellous 0.258 0.274 0.407 0.335
Cortical 0.134 0.137 0.177 0.127
Titanium 0.132 0.135 0.174 0.126
TABLE 6-4. Loads transferred through the “nucleus” and
the device for the 800 N axial compression
Rigid Hinged
Graft “Nucleus” Device “Nucleus” Device
Cancellous 712.4 87.6 767.9 32.1
Cortical 741.2 58.8 773.5 26.5
Titanium 742.5 57.5 774.3 25.7
Obviously the value of FE modeling is that mapping of
the osseous, ligamentous, and instrumentation stresses
and strains can be obtained in a relativel y inexpensi ve and
time-efficient manner. Predictions of temporal changes
of bone in response to the implantation of a device have
yielded important data. Also, design perturbations can be
quickly assessed as to their relative advantages (and disadvantages).
MORE RECENT FUSION INITIATIVES
The preceding review clearly shows that a large number of fusion enhancement instrumentation is available to
surgeons. However, none of the instrumentation is totally
satisfactory in its performance and there is room to
improve the rate of fusion success, if fusion is the goal.
Naturally, alternative fusion approaches (mechanical,
biological) are currently being pursued.
The rigidity of a spinal fixation device and its ability to
share load with the fusion mass is considered essential for
the fusion to occur. If the load transferred through the
fusion mass, how e v er , is increased without sacrificing the
rigidity of the construct, a more favorable environment
for fusion may be created. To achieve this objective, posterior as well as anterior “dynamized” systems have been
designed (70–72). One such posterior system consists of
rods and pedicle screws and has a hinged connection
between the screw head and shaft compared with the rigid
screws (73,74) (Fig. 6-4A). Another example of the
dynamized antero-lateral compression device (ALC
[DePuy Spine, Inc., Raynham, MA]) is shown in Figure
6-4B. Load-displacement tests were performed to assess
the efficacy of these devices in stabilizing a severally
destabilized spinal segment. The hinged and rigid posterior systems provided significant stability across the L2L4 segment in flexion, extension, and lateral bending as
compared with the intact case (p < 0.05). The stabilities
imparted by the hinged-type and its alternative rigid
devices were of similar magnitudes (Fig. 6-4A) (71). The
ALC dynamized and rigid anterior systems also provided
significant stability across the L3-L5 segment in flexion,
extension, and lateral bending (p <. 05). The stability
imparted by the Dynamized ALC and its alternate rigid
system did not differ significantly (Fig. 6-4B) (72).
Anterior bone graft in combination with posterior
instrumentation has been shown to provide superior sup-
74 /SECTION I/BASIC SCIENCE
A
B
FIG. 6-4. The two different types of dynamized systems used in a cadaver model to assess their stability characteristics. The data were compared with the corresponding “rigid” systems. A: Posterior system. B: Anterior system. (From Scifert J, Sairyo K, Goel VK, et al.Stability analysis of an enhanced load
sharing posterior fixation device and its equivalent conventional device in a calf spine model. Spine
1999;24:2206–2213 and Hitchon PW, Goel VK, Rogge T, et al. Biomechanical studies of a dynamized
anterior thoracolumbar implant. Spine 2000;25(3):306–309.)
port because the graft is in line with axial loads and posterior elements are left intact. However, employing posterior instrumentation with anterior grafting requires execution of two surgical procedures. Furthermore, use of a
posterior approach to place an interbody graft requires
considerable compromise of the posterior elements,
although it reduces the surgery time. It would be advantageous to minimize surgical labor and structural damage
caused by graft insertion into the disc space through a
posterior approach. This issue has been addressed by
preparing an interbody bone graft using morselized bone
(73,74). This device consists of a gauze bag of Dacron
inserted into the disc space, f illed with morselized bone,
and tied shut (Fig. 6-5). Testing in vitro measured the
rotations of each vertebral level of mechanically loaded
cadaver lumbar spines, both in intact and several experimental conditions. With the tension band alone, motion
was restored to the intact case, except in extension where
it was reduced (Fig. 6-5). With the graft implant, motion
was restored to intact in all of the loading modes, except
in flexion where it was reduced. With the tension band
and graft, motion was again restored to intact except in
flexion and extension where it was reduced. The in vitro
results suggest that a tension band increases stability in
extension, while the bag device alone seems to provide
increased stability in flexion. The implanted bag f illed
CHAPTER 6/SPINAL INSTRUMENTATION / 75
B
FIG. 6-5. A: The bag system developed by Spineology, Inc. (Maplewood, MN). B: The increases and
decreases in motion with respect to intact segment
for bag alone and bag and band are also shown.
(From Dooris AP. Experimental and theoretical
investigations into the effects of artificial disc
implantation on the lumbar spine [PhD dissertation].
A
Iowa City, IA: University of Iowa; 2001.)
with morselized bone in combination with a posterior
tension band, restores intact stif fness. P ostc yclic results in
axial compression suggest that the morselized bone in the
bone-only specimens either consolidates or extrudes from
the cavity despite confinement. Motion restoration or
reduction as tested here is relevant both to graft incorporation and segment biomechanics. The posterior interbody grafting method using morselized bone is amenable
to orthoscopy. It produces an interbody graft without an
anterior surgical approach. In addition, this technique
greatly reduces surgical exposure with minimal blood
loss and no facet compromise. This technique would be a
viable alternative to current 360° techniques pending animal tests and clinical trials.
Bone grafting is used to augment bone healing and
provide stability after spinal surgery. Autologous bone
graft is limited in quantity and unfortunately associated
with increased surgical time and donor-site morbidity.
Recent research has provided insight into methods that
may modulate the bone healing process at the cellular
level in addition to reversing the effects of symptomatic
disc degeneration which is a potentially disabling condition, managed frequently with various fusion procedures.
Alternatives to autologous bone graft include allograft
bone, demineralized bone matrix, recombinant growth
factors, and synthetic implants. Each of these alternatives
could possibly be combined with autologous bone marrow or various growth factors. Although none of the
presently available substitutes provides all three of the
fundamental properties of autograft bone (osteogeneticity, osteoconductivity, and osteoinductivity), there are a
number of situations in which they have proven clinically
useful. A literature review indicates that alternatives to
autogenous bone grafting find their greatest appeal when
autograft bone is limited in supply or when acceptable
rates of fusion may be achieved with these substitutes
(75). For example, bone morphogenetic proteins have
been shown to induce bone formation and repair (75).
Relatively little research has been undertaken to investigate the efficacy of osteoconductive protein 1 (OP-1) in
the aforementioned stated role (75,76). Grauer et al. performed single-level intertransverse process lumbar
fusions at L5-L6 in 31 New Zealand White rabbits (76).
These were divided into three study groups: autograft,
carrier alone, and carrier with OP-1. The animals were
euthanized 5 weeks after surgery. Five (63%) of the eight
in the autograft group had fusion detected by manual palpation, none (0%) of the eight in the carrier-alone group
had fusion, and all eight (100%) in the OP-1 group had
fusion. Biomechanical testing results correlated well with
those of manual palpation. Histologically, autograft specimens were predominantly fibrocartilage, OP-1 specimens were predominantly maturing bone, and carrieralone specimens did not show significant bone
formation. OP-1 was found to reliably induce solid intertransverse process fusion in a rabbit model at 5 weeks.
Smoking interferes with the success of posterolateral
lumbar fusion and a group of authors from the aforementioned investigation extended their study to review the
effect of using OP-1 to enhance the fusion process in
patients who smoke (77). OP-1 was able to overcome the
inhibitory effects of nicotine in a rabbit posterolateral
spine fusion model, and to induce bony fusion reliably at
5 weeks.
Magin et al. undertook a study to determine whether
the use of recombinant human (rh) OP-1 or HA would
improve on the intercorporal fusion achieved by interbody autologous bone grafting in a sheep model (78).
Vertebral fusion quality was examined by plain radiograph at 4-week intervals, by scintigraphy at 3 and 6
76 /SECTION I/BASIC SCIENCE
months, and by computed tomography scan, magnetic
resonance imaging, biomechanical testing, and histologic
evaluation. All examination methods demonstrated superior fusion after administration of rhOP-1, with radiologic fusion apparent at 4 months. Autologous bone
grafts eventually produced bony healing in most cases,
albeit of a lower quality than with rhOP-1. HA use led
only to the formation of a tight pseudoarthrosis. The
results indicated that rhOP-1 use was an appropriate
method for improving interbody fusion in the sheep
spine. In addition to offering the potential for improved
bone healing, rhOP-1 use may permit less invasive
surgery such as transpedicular fusion and the use of
cages. In another similar study, Sandhu et al. investigated
the efficacy of recombinant human bone morphogenetic
protein 2 (rhBMP-2)-collagen composite in comparison
with autograft to enhance spinal interbody fusion. Comparisons were drawn from temporal radiographic and
end-point biomechanical and histologic data (79). Twelve
sheep underwent single-level anterior lumbar interbody
fusion performed with a cylindrical fenestrated titanium
interbody fusion device (INTER FIX, Medtronic
Sofamor Danek, Inc., Memphis, TN). The device was
filled either with rhBMP-2-collagen (n < 6) or autogenous iliac crest bone graft (n < 6). Radiographs revealed
a bony bridge anterior to the cage in five of six rhBMP2-treated animals, whereas it was present only in one of
five in the autogenous bone graft group. Segments treated
with rhBMP-2 were 20% stiffer in flexion than autografttreated segments at 6 months. All six in the rhBMP-2
group and two of six in the autograft group showed complete fusion. There w as a significantly higher rate of bony
continuity observed at the fenestrations of the rhBMP-2
group. Three times more cage fenestrations in the
rhBMP-2 group demonstrated “all-bone” when compared
with the autograft group (p < .001). Fur ther, the scar tissue in and around the autograft-treated cages was 16-fold
more (p < .01) than that seen for rhBMP-2-treated cages.
The study demonstrated that rhBMP-2 can lead to earlier
radiologic fusion and a more consistent increased stiffness of the segments when compared with autograft in
sheep anterior lumbar interbody fusion. Furthermore, a
three times higher histologic fusion rate was attainable
with significantly reduced fibrous tissue around the
implant when rhBMP-2 is used.
NONFUSION TREA TMENT AL TERN ATIVES
Various methods have been employed in the characterization of device effectiveness for which spinal fusion is
indicated. Because of the nonphysiologic nature of fusing the spinal segments that are supposed to provide
motion/flexibility, adjacent-level degeneration, and other
complications associated with the fusion process, alternatives to fusion have been proposed.
Nucleus Replacements
Ray Nucleus
In 1988 Ray presented a prosthetic nuclear replacement consisting of flexible woven filaments (Dacron)
surrounding an internal semipermeable polyethylene
membranous sac filled with hyaluronic acid and a
thixotropic agent (i.e., a hydrogel) (73,80). As a nucleus
replacement, the implant can be inserted similar to a thoracolumbar interbody fusion device, either posteriorly or
transversely. Two are inserted per disc level in a partly
collapsed and dehydrated state, but would swell due to
the strongly hygroscopic properties of the hyaluronic acid
constituent. The designer expects the implant to swell
enough to distract the segment while retain enough flexibility to allow a normal range of motion. An option is to
include therapeutic agents in the gel that would be
released by water flow in and out of the prosthesis
according to external pressures.
Recent reports on biomechanical tests of the device
show that it can produce some degree of stabilization and
distraction (73,80). Loads of 7.5 Nm and 200 N axial
were applied to six L4-L5 specimens. Nucleotomized
spines increased rotations by 12% to 18% depending on
load orientation, but implanted spines (implant placed
transversely) showed a change of −12% to +2% from the
intact with substantial reductions in neutral zone. Up to 2
mm of disc height was recovered by insertion. The
implant, however, was implanted and tested in its nonhydrated form. The biomechanics of the h ydrated prosthesis
may vary considerably from its desiccated form.
In situ Curable Prosthetic Intervertebral Nucleus
Device
The prosthetic intervertebral nucleus (PIN) device
(Disc Dynamics, Inc., Minnetonka, MN) consists of a
compliant balloon connected to a catheter (Fig. 6-6)
(73,74). This is inserted and liquid polymer injected into
the balloon under controlled pressure inflating the balloon, filling the cavity, and distracting the intervertebral
disc. Within 5 minutes the polymer is cured. Five freshfrozen osteoligamentous three-segment human lumbar
spines, screened for abnormal radiograph and low bone
density, were used for the biomechanical study. The
spines were tested under four conditions: intact, denucleated, implanted, and fatigued. Fatiguing was produced by
cyclic loading from 250 to 750 N at 2 Hz for at least
100,000 cycles. Nucleotomy was performed through a
5.5 mm trephine hole in the right middle lateral side of
the annulus. The device was placed in the nuclear cavity
as described earlier. Following biomechanical tests, these
specimens were radiographed and dissected to determine
any structural damage inflicted during testing. Middle
segment rotations generally increased with discectomy
CHAPTER 6/SPINAL INSTRUMENTATION / 77
FIG. 6-6. In situ curable prosthetic intervertebral
nucleus (PIN) device being developed by Disc Dynamics, Inc., Minnetonka, MN. (From Dooris AP. Experimental and theoretical investigations into the effects of ar tificial disc implantation on the lumbar spine [PhD
dissertation]. Iowa City, IA: University of Iowa; 2001.)
but were restored to the normal intact range with implantation. After fatiguing, rotations across the implanted
segment increased. However, these were not more than,
and often less than the intact adjacent segments. During
polymer injection under compressive load, the segment
distracted as much as +1.8 mm (average) at the disc center as determined by the surrounding gauges. Over 1.6
mm was maintained during polymer cure with compression. The immediate goals of a disc replacement system
are to restore disc height and provide segment mobility
without causing instability. This study showed that the
PIN device could reverse the destabilizing effects of a
nucleotomy and restore normal segment stiffness. Significant increases in disc height can also be achieved.
Implanting the majority of disc replacement systems
requires significant annulus removal. This device
requires minimal surgical compromise and has the potential to be performed arthroscopically.
Artificial Disc
One of the most recent developments for nonfusion
treatment alternatives is replacement of the intervertebral
disc. The goal of this treatment alternativ e is to restore the
original mechanical function of the resected disc (81).
One of the stipulations of artif icial disc replacement is
that the remaining osseous spinal and paraspinal soft tissue components are not compromised by pathologic
changes. Bao et al. (82) have classified the designs of
total disc replacements into four categories: (a) low-friction sliding surface; (b) spring and hinge systems; (c)
contained fluid-filled chambers; and (d) discs of r ubber
and other elastomers. The former tw o designs seek to take
advantage of the inherently high fatigue characteristics
that all-metal designs afford. The latter two designs
attempt to incorporate some of the viscoelastic and compliant properties that are exhibited by the normal, healthy
intervertebral disc. The disc must be able to maintain its
mechanical integrity to approximately 85 million cycles;
consist of biocompatible materials; exist entirely within
the normal disc space and maintain physiologic disc
height; restore normal kinematic motion wherein the axes
of each motion, especially sagittal plane motion, are correctly replicated; duplicate the intact disc stiffness in all
three planes of rotation and compression; provide immediate and long-term fixation to bone; and , finally, provide
fail-safe mechanisms so that if an individual component
of the design fails, catastrophic failure is not immediately
imminent and does not lead to peri-implant soft tissue
damage. This is certainly one of the greatest design challenges that bioengineers have encountered to date. In the
following paragraphs, some of the methods are discussed
that are being employed in an attempt to meet this rigorous challenge.
One of the available studies reviews iterative design of
the artificial disc replacement based on measured biomechanical properties. Lee et al. (83,84) looked at incorporating three different polymers into their prosthetic intervertebral disc design and tried to represent the separate
components (annulus fibrosis and nucleus) of the normal
disc in varying proportion. They loaded their designs
under 800 N axial compression and in compression-torsion out to 5°. The results indicated that discs fabricated
from homogeneous materials exhibited isotropy that
could not replicate the anisotropic behavior of the normal
human disc. Thus, 12 layers of fiber reinforcement were
incorporated in an attempt to mimic the actual annulus
fibrosis. This method did result in more closely approxi-
78 /SECTION I/BASIC SCIENCE
FIG. 6-7. The intact finite element model of a ligamentous segment was modified to simulate the balland-socket type artificial disc implant. (From Dooris AP. Experimental and theoretical investigations into
the effects of artificial disc implantation on the lumbar spine [PhD dissertation]. Iowa City, IA:University
of Iowa; 2001.)
mating the mechanical properties of the normal disc.
Through this method of redesign and testing, the authors
claim that eventually “a disc prosthesis that has mechanical properties comparable to the natural disc could be
manufactured” (83, 84).
Another artificial intervertebral disc has been developed, and its intrinsic biomechanical properties, bioacti vity, and effectiveness as a total disc replacement were
evaluated in vitro and in vivo (85). The artificial interver-
tebral disc consists of a triaxial three-dimensional fabric
(3-DF) woven with a UHMWPE f iber, and spray-coated
with bioactive ceramics on the disc surface. The arrangement of weave properties was designed to produce
mechanical behavior nearly equivalent to the natural
intervertebral disc. Total intervertebral disc replacement
at L2-L3 and L4-L5 was performed using a 3-DF disc
with or without internal f ixation in a sheep lumbar spine
model. The segmental biomechanics and interface histology were evaluated after surgery at 4 and 6 months. The
tensile-compressive and torsional properties of prototype
3-DF were nearly equivalent to those of human lumbar
disc. The lumbar segments replaced with the 3-DF disc
alone showed a signif icant decrease of flexion-extension
range of motion to 28% of control values as well as partial bony fusion at 6 months. However, the use of temporary fixation provided a nearly physiologic mobility of
the spinal segment after implant remo v al as well as excellent bone-disc fusion at 6 months. An artificial intervertebral disc using 3D-F demonstrated excellent in vitro
and in vivo performance in both biomechanics and inter-
face histology. There is a potential for future clinical
application.
FE analyses have also been recruited in an effort to
perturb design with an eye toward optimizing the
mechanical behavior of artificial discs. Langrana et al.
(83) generated an FE model that examined the effect of
orientation of the synthetic disc fiber layers, number of
fiber layers, and the order of the reinforcing layers.
Dooris et al. modified a previously validated intact FE
model to create models implanted with a ball-and-socket
FIG. 6-8. The intact finite element model of a ligamentous segment was modified to simulate the slipcore–type artificial disc implant. (From Dooris AP.
Experimental and theoretical investigations into the
effects of artificial disc implantation on the lumbar
spine [PhD dissertation]. Iowa City, IA: University of
Iowa; 2001.)
CHAPTER 6/SPINAL INSTRUMENTATION / 79
FIG. 6-9. Predicted rotations for the balland-socket and slip-core disc designs as
compared to the intact case. (From
Dooris AP. Experimental and theoretical
investigations into the effects of ar tificial
disc implantation on the lumbar spine
[PhD dissertation]. Iowa City, IA: University of Iowa; 2001.)
and slip-core–type artificial disc models through an anterior approach (73,86) (Figs. 6-7, 6-8). To study surgical
variables, small and large windows were cut into the
annulus, and the implants were placed anteriorly and posteriorly within the disc space. The anterior longitudinal
ligament was also restored. Models were subjected to
either 800 N axial compression force alone or to a combination of 10 Nm flexion-extension moments and 400 N
axial preload. Implanted model predictions were compared with those of the intact model. The predicted rotations for the two-disc implanted models were in agreement with the experimental data (73).
For the ball-and-socket design disc facet loads were
more sensitive to the anteroposterior location of the artificial disc than to the amount of annulus removed. Under
800 N axial compression, implanted models with an anteriorly placed artificial disc exhibited facet loads 2.5 times
greater than loads observed with the intact model,
whereas posteriorly implanted models predicted no facet
loads in compression. Implanted models with a posteriorly placed disc exhibited greater flexibility than the
intact and implanted models with anteriorly placed discs.
Restoration of the anterior longitudinal ligament reduced
pedicle stresses, facet loads, and extension rotation to
nearly intact levels. The models suggest that, by altering
placement of the artificial disc in the anteroposterior
direction, a surgeon can modulate motion-segment flexural stiffness and posterior load sharing, even though the
specific disc replacement design has no inherent rotational stiffness. The motion data, as expected, differed
between the two disc designs (ball and socket, and slip
core) and as compared to the intact disc as well (Fig. 6-
9). Similar changes were observed for the loads on the
facets (Fig. 6-10).
FIG. 6-10. Predicted facet loads for ball-and sock et and slip-core disc designs as compared to the intact
case. (From Dooris AP. Experimental and theoretical investigations into the effects of artificial disc
implantation on the lumbar spine [PhD dissertation]. Iowa City, IA: University of Iowa; 2001.)
80 /SECTION I/BASIC SCIENCE
The experimentally validated FE models of the intact
and disc-implanted L3-L5 segments revealed that both of
these devices do not restore motion as well as loads
across facets back to the intact case. (These designs
restore the intact biomechanics in a limited sense.) These
differences are not only due to the size of the implants but
the inherent design differences. Ball-and-socket design
has a more “fixed” center of rotation as compared to the
slip-core design in which the center of rotation (COR)
undergoes a wider variation. A further complicating factor is the location of the disc within the annular space
itself, a parameter under the control of the surgeon. Thus,
it will be difficult to restore biomechanics of the segment
back to normal using such designs. Only clinical followup studies will provide the effects of such variations on
the changes in spinal structures as a function of time (8).
MORE RECENT AND FUTURE INITIATIVES
Although many of the well-accepted investigation
techniques and devices have been discussed herein, other
techniques for the stabilization/fusion of the spine and
nonfusion approaches are currently being investigated.
These concepts are likely to play a significant role in the
future and are discussed in the following sections.
Vertebroplasty
A citation of the review article by Garfin et al. is the
most appropriate way to introduce this topic for further
discussion. Painful vertebral osteoporotic compression
fractures lead to significant morbidity and mortality (87).
This relates to pulmonary dysfunction, eating disorders
(nutritional deficits), pain, loss of independence, and
mental status change (related to pain and medications).
Medications to treat osteoporosis (primarily antiresorptive) do not effectively treat the pain or the fracture, and
require over 1 year to reduce the degree of osteoporosis.
Kyphoplasty and vertebroplasty are new techniques that
help decrease the pain and improve function in fractured
vertebrae.
Vertebroplasty is the percutaneous injection of PMMA
cement into the vertebral body. While PMMA has high
mechanical strength, it heals fast and thus requires only a
short handling time. Other potential problems of using
PMMA injection may include damage to surrounding tissues by a high polymerization temperature or by the nonreacted toxic monomer, and the lack of long-term biocompatibility. Bone mineral cements, such as calcium
carbonate and CaP cements, have longer working time
and low thermal effect. The y are also biodegradable w hile
having a good mechanical strength. However, the viscosity of injectable mineral cements is high, and the infiltration of these cements into vertebral body has been questioned. Recently, the infiltration properties of a CaP
cement have been significantly improved, which is ideal
for the transpedicular injection to the vertebral bodies for
vertebroplasty or augmentation of osteoporotic vertebral
body strength. Little is known, however, about the biomechanics of this treatment. Various authors have evaluated the biomechanical efficacy of this procedure by
comparing it with respect to the intact, the axial strength
of the vertebral body following fracture and injection of
the cement, and the corresponding load-displacement
behavior of the constructs.
Lim et al. evaluated the compression strength of
human vertebral bodies injected with new CaP cement
with improved infiltration properties before compression
fracture and also for vertebroplasty in comparison with
PMMA injection (88). The bone mineral densities of 30
vertebral bodies (T2-L1) were measured using DEXA.
Ten control specimens were compressed at a loading rate
of 15 mm per minute to 50% of their original height. The
other specimens had 6 mL of PMMA (n < 10) or the new
CaP (n < 10) cement injected through the bilateral pedicle approach before being loaded in compression. Additionally, after the control specimens had been compressed, they were injected with either CaP (n < 5) or
PMMA (n < 5) cement using the same technique, to simulate vertebroplasty. Loading experiments were repeated
with the displacement control of 50% vertebral height.
Load to failure was compared among groups and analyzed using analysis of v ariance. Mean bone mineral densities of all five groups were similar and ranged from
0.56 to 0.89 g/cm2. The size of the v ertebral body and the
amount of cement injected were similar in all groups.
Load to failure values for PMMA, the new CaP, and vertebroplasty PMMA were significantly greater than that of
controls. Load to failure of the vertebroplasty CaP group
was higher than the control group but not statistically significant. The mean stiffness of the vertebroplasty CaP
group was significantly smaller than control, PMMA,
and the new CaP groups. The mean height gains after
injection of the new CaP and PMMA cements for vertebroplasty were minimal (3.56% and 2.01%, respectiv el y).
Results of this study demonstrated that the new CaP
cement can be injected and infiltrates easily into the vertebral body. It was also found that injection of the new
CaP cement can improve the strength of a fractured vertebral body to at least the level of its intact strength. Thus,
the new CaP cement may be a good alternati ve to PMMA
cement for vertebroplasty, although further in vitro, in
vivo animal, and clinical studies should be done. Furthermore, the new CaP may be more effective in augmenting
the strength of osteoporotic vertebral bodies, and for preventing compression fractures considering our biomechanical testing data and the known potential for
biodegradability of the new CaP cement. Belkof et al.
found that the injection of either Orthocomp (Orthovita,
Malvern, PA) or Simplex P (Howmedica, Inc., Allendale,
NJ) resulted in vertebral body strengths that were significantly greater than initial strength values (89). Vertebral
CHAPTER 6/SPINAL INSTRUMENTATION / 81
bodies augmented with Orthocomp recovered their initial
stiffness; and, vertebral bodies augmented with Simplex
P were significantly less stiff than they were in their initial condition. However, these biomechanical results have
yet to be substantiated in clinical studies.
Previous biomechanical studies have shown that injections of 8 to 10 mL of cement during vertebroplasty
restore or increase vertebral body strength and stiffness;
however, the dose-response association between cement
volume and restoration of strength and stiffness is
unknown. Belkof et al. (89) investigated the association
between the volume of cement injected during percutaneous vertebroplasty and the restoration of strength and
stiffness in osteoporotic vertebral bodies. Two investigational cements were studied: Orthocomp and Simplex 20
(Simplex P with 20% by weight barium sulfate). Compression fractures were experimentally created in 144
vertebral bodies (T6-L5) obtained from 12 osteoporotic
spines harvested from female cadavers. After initial
strength and stiffness were determined, the vertebral bodies were stabilized using bipedicular injections of cement
totaling 2, 4, 6, or 8 mL and recompressed, from which
posttreatment strength and stiffness were measured.
Strength and stiffness were considered restored when
posttreatment values were not significantly different from
initial values. Strength was restored for all regions when
2 mL of either cement was injected. To restore stiffness
with Orthocomp, the thoracic and thoracolumbar regions
required 4 mL, but the lumbar region required 6 mL. To
restore stiffness with Simplex 20, the thoracic and lumbar
regions required 4 mL, but the thoracolumbar region
required 8 mL. These data provide guidance on the
cement volumes needed to restore biomechanical
integrity to compressed osteoporotic vertebral bodies.
Liebschner et al. undertook an FE-based biomechanical
study to provide a theoretical frame work for understanding
and optimizing the biomechanics of vertebroplasty, especially the effects of volume and distribution of bone
cement on stiffness reco very of the vertebral body, just like
the preceding experimental study (90). An experimentally
calibrated, anatomically accurate FE model of an older
adult L1 vertebral body was developed. Damage was simulated in each element based on empirical measurements
in response to a uniform compressive load. After virtual
vertebroplasty (bone cement filling range of 1 to 7 cm3) on
the damaged model, the resulting compressive stiffness of
the vertebral body was computed for various spatial distributions of the filling material and different loading conditions. Vertebral stiffness recovery after vertebroplasty was
strongly influenced by the volume fraction of the
implanted cement. Only a small amount of bone cement
(14% fill or 3.5 cm3) was necessary to restore stiffness of
the damaged vertebral body to the pre-damaged value. Use
of a 30% fill increased stiffness by more than 50% compared with the pre-damaged value. Whereas the unipedicular distributions exhibited a comparative stiffness to the
bipedicular or posterolateral cases, it showed a medial-lateral bending motion (“toggle”) toward the untreated side
when a uniform compressive pressure load was applied.
Only a small amount of bone cement (15% volume fraction) is needed to restore stiffness to pre-damage levels,
and greater filling can result in substantial increase in stiffness well bey ond the intact le vel. Such overfilling also renders the system more sensitive to the placement of the
cement because asymmetric distributions with large fills
can promote single-sided load transfer and thus toggle.
These results suggest that large fill volumes may not be the
most biomechanically optimal configuration, and an
improvement might be achieved by use of lower cement
volume with symmetric placement. These theoretical findings support the experimental observations described in
the preceding paragraph, except these authors did not analyze the relationship between cement type and volume
needed to restore strength.
Hitchon et al. compared the stabilizing effects of the
HA product with PMMA in an experimental compression
fracture of L-1 (91). No significant difference between
the HA– and PMMA–cemented-fixated spines was
demonstrated in flexion, extension, left lateral bending,
or right and left axial rotation. The only difference
between the two cements was encountered before and
after fatiguing in right lateral bending (p < .05). The
results of this study suggest that the same angular rigidity can be achieved b y using either HA or PMMA. This is
of particular interest because HA is osteoconductive,
undergoes remodeling, and is not exothermic.
According to Garfin et al. both vertebroplasty and
kyphoplasty have had a very high acceptance and use rate
(87,92). There is 95% improvement in pain and significant
improvement in function following treatment by either of
these percutaneous techniques. Kyphoplasty improves
height of the fractured vertebra, and improves kyphosis by
over 50%, if performed within 3 months from the onset of
the fracture (onset of pain). There is some height improvement, though not as marked, along with 95% clinical
improvement, if the procedure is performed after 3
months. Complications occur with both and relate to
cement leakage in both, and cement emboli with vertebroplasty. Kyphoplasty and vertebroplasty are safe and effective, and have a useful role in the treatment of painful
osteoporotic vertebral compression fractures that do not
respond to conventional treatments. Kyphoplasty offers the
additional advantage of realigning the spinal column and
regaining height of the fractured vertebra, which may help
decrease the pulmonary, gastrointestinal, and early morbidity consequences related to these fractures. Both procedures are technically demanding.
Bioartificial Disc
The rapidly advancing field of tissue engineering
opens new possibilities to solving spine problems. By
82 /SECTION I/BASIC SCIENCE
seeding and growing intervertebral disc cells, it could be
possible to grow a new bioartificial disc to be implanted
into the spine. Studies are in progress at a number of centers, including our own (93).
The FE model studies can be used to simulate even the
smallest of systems. For example, Baer et al. have developed an FE model to study the cell micromechanical
environment in the intervertebral disc (94). Hopefully,
this approach can be used to investigate the effects of various spinal instrumentations at the cellular level.
CONCLUSION
The prevalence of spinal fusion and stabilization procedures is continuously increasing. This chapter has presented many of the contemporary biomechanical issues
germane to stabilization and fusion of the spine. Because
of the wide variety of devices available, various testing
protocols have been developed in an attempt to describe
the mechanical aspects of these devices. Many in vitro
studies are performed during the earlier stages of implant
development to characterize and optimize the mechanical
behavior of the device. In addition, the investigations
reveal comparati ve advantages (and disadvantages) of the
newer designs to existing hardware. Subsequent in vivo
testing, specifically animal models, provides data on the
performance of the device in a dynamic physiologic environment. All of the testing, in vitro and in vivo, helps to
build confidence that the instrumentation is safe for clinical trial. The biomechanical testing and evaluation of
spinal fusion and stability has produced an extensive
knowledge base that has allowed for the design, development, and implementation of various devices. Future biomechanical work is required to produce newer devices
and optimize existing ones, with an eye toward reducing
the rates of nonfusion and pseudarthrosis. In addition,
novel devices and treatments that seek to restore normal
spinal function and loading patterns without fusion continue to necessitate advances in biomechanical methods.
These are the primary challenges that need to be incorporated in future biomechanical investigations. Finally,
one has to gain understanding of the effects of devices at
the cellular level and one must undertake outcome assessment studies to see if the use of instrumentation is warranted for the enhancement of the fusion process.
ACKNOWLEDGMENTS
This chapter is based on the work sponsored b y various
funding agencies over the last 20 years. Part of the literature review and writing of the actual manuscript w as done
while the senior author (Goel) was a Visiting Professor at
the Department of Electronic and Computer Engineering,
Tokyo Denki University, Ishizaka, Hatoyama-machi,
Hiki-Gun, Saitama 350-0394, Japan.
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