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Adjustable wing
C H A P T E R 5 6 e Role of Dynamic Stabilization and the Aging Spine
375
Tissue expander
Oval spacer
F IG UR E 5 6- 1 X-Stop device interspinous spacer (Kyphon).
Fixed wing
F IG UR E 5 6- 3 The Diam interspinous stabilizer (Medtronic).
F IG UR E 5 6- 2 The Wallis device interspinous spacer (Zimmer Spine).
degenerative discs, or both. It also restores disc height and foraminal volume.
It is indicated in moderate to severe spinal stenosis, degenerative spondylolisthesis, and mild to moderate degenerative scoliosis. ExtenSure is currently
undergoing clinical evaluation and is not FDA approved.
In-Space (Synthes)
In-Space is a laterally placed PEEK cylindrical device, secured by deployable wings (Figure 56-6). It is intended to stop segmental extension and to
distract the symptomatic interspinous space. In doing so, maintenance of
foraminal height, opening of the area of the spinal canal, reduction of stress
on the facets, and relieving of pressure on the posterior annulus results. It
is indicated in lumbar spinal stenosis, disc protrusions with discogenic low
back pain, facet syndrome due to face osteoarthritis, degenerative spondylolisthesis up to grade 1, and degenerative disc disease. In-Space is not FDA
approved.
Superion (Vertiflex)
Superion is a titanium alloy interspinous device with deployable wings
(Figure 56-7). It is designed for percutaneous implantation in the treatment
of moderate degenerative lumbar stenosis at one or two levels. Superion is
currently under clinical investigation in the United States and is not FDA
approved.
F IG UR E 5 6- 4 The Coflex device (Paradigm).
Facet Devices
The facet poses unique challenges. The anatomy and functional interrelationships are complex. The 3-D sliding synovial articulation is difficult
to replicate. Physiologically, the facets have nociceptive and proprioceptive input that is vital to the proper function of the motion segment. The
resulting chemical and mechanical pain generation is difficult to treat surgically. Kinematically complex, facet joints engage in coupled shear and
sliding, as well as rotational load sharing, all of which can be specific to
certain levels.
This results in a very complex continuum of disease. Painful inflammation, osteoarthritis, stenosis, abnormal loading, and total failure of the
functional spinal unit can all originate with facet disease. These challenges
are not easily overcome, addressed, or surgically treated. Facet arthroplasty, a
rapidly evolving subspecialty in motion preservation, strives to address these
issues in the most ergonomic manner.
376
P A R T V I I Surgical Treatment Modalities: Lumbar Spine
F IG UR E 5 6- 5 The ExtenSure interspinous spacer (NuVasive).
F IG UR E 5 6- 6 The In-Space device (Synthes).
The overarching goal is reduction of pain and the return of function.
The design rationales for the facet-based devices are to clinically correlate
intensity of intervention to severity of disease, to allow or restore more physiological loading, to allow or restore the physiological center of rotation, and
to control the range of motion of the motion segment.
The emerging technologies are thoughtfully engineered. They are well
studied and modeled as the result of extensive research and analysis. The
facet-based devices vary from resurfacing, through augmentation and partial
replacement, to total replacement. These devices are designed to complement residual kinematics of the motion segment, or to duplicate native
motion in total replacement.
Zyre (Quantum Orthopedics)
Zyre is an interpositional arthroplasty device (Figure 56-8). It consists of a
cobalt chromium intraarticular spacer, through which passes a PET cord
with chromium retainers. It is minimally invasive, requires no bone resection, maintains capsular integrity, and can be implanted with or without
decompression. Indications include painful degeneration of the facet with
failed CMM. Advantages include minimal disruption of anatomy and
F IG UR E 5 6- 7 The Superion interspinous device (Vertiflex).
F IG UR E 56 - 8 The Zyre interpositional arthroplasty device (Quantum
Orthopedics).
multiple revision options. This concept is early technology with a paucity of
clinical data. Zyre is not FDA approved.
Fenix (Gerraspine AG)
Fenix is a cobalt chromium facet joint resurfacing device. It has superior and inferior components secured with a translaminar locking screw
(Figure 56-9). It is designed to eliminate the painful components, resurface the facet joint, preserve supporting structure and anatomy, and allow
or restore physiologic motion. Surgery entails removal of capsule and intraarticular cartilage, with or without decompression. Indications include significant facet disease and subarticular stenosis. Fenix has had limited clinical
use and is not FDA approved.
Anatomic Facet Replacement System (Facet Solutions)
Anatomic Facet Replacement System (AFRS) is a total facet replacement
device composed of cobalt-chromium-molybdenum articular surfaces that
uses conventional pedicle screw fixation (Figure 56-10). It is designed to
reproduce facet anatomy and preserve or restore natural lumbar biomechanics. Surgical implantation is through a midline approach and entails total
facetectomy. Indications include osteoarthritis of the facets causing stenosis, and low grade degenerative spondylolisthesis. Fixation with standard
pedicle screws allows ready revision options. AFRS I is in clinical trial in the
United States and is not FDA approved.
Total Facet Arthroplasty System (Archus)
Total Facet Arthroplasty System (TFAS) is a total facet replacement device
composed of cobalt-chromium articular surfaces and titanium alloy cross
arm assembly (Figure 56-11). It comprises paired cephalad bearings and
paired caudal housings in a “ball in cup” motion-constraining configuration.
These are supported by the titanium cross arm assembly with cemented
pedicle post fixation. In situ modular assembly allows for precise adjustment
to individual anatomy. Proposed indications include degenerative disease of
the facets, facet instability, up to grade I spondylolisthesis with neurological
impairment, central or lateral spinal stenosis, at L3-L4 or L4-L5. TFAS is
being clinically evaluated in the United States and is not FDA approved.
C H A P T E R 5 6 e Role of Dynamic Stabilization and the Aging Spine
377
C
D
B
F IG UR E 5 6- 9 The Fenix facet joint resurfacing device (Getraspine AG).
A
A
C
B
F IG UR E 5 6- 1 0 Anatomic Facet Replacement System (Facet Solutions).
Total Posterior System (Impliant)
Total Posterior System (TOPS) is composed of opposing titanium plates
with interlocking PCU. It is fixed to the spine with polyaxial pedicle screws
(Figure 56-12). The entirety of the posterior elements is totally replaced.
Motion is restored and constrained in all planes via polymeric dampening. Utilizing the same surgical technique as standard posterior fusion, it
recreates the normal biomechanics of the spine, allowing full physiologic
range of motion. Surgery occurs through a midline approach and laterally
placed pedicle screws. Total facetectomy and removal of posterior elements
is necessary. Implantation requires a precise jig assembly. Indications include
moderate to severe spinal stenosis. TOPS is being clinically evaluated in the
United States and is not FDA approved.
Pedicle-Based Dynamic Rods
The pedicle-based devices offer the most secure fixation to the spine and
thus the greatest opportunity to control motion. Influence is exerted on
facets, posterior ligaments, and posterior disc complex. Implantation can be
unloaded utilizing a neutral position of the spine, resulting in passive control of motion, or implantation can be loaded utilizing a more distracted
position of the spine, resulting in more dynamic load sharing. These devices
may be stand-alone or placed in conjunction with decompression or as an
adjunct to fusion. There are many devices, each with unique characteristics.
This results in a broad range of control exerted, and thus coverage over a
wide spectrum of potential spine pathologies. All share the common goal
of alleviating back and leg pain using more flexible constructs and materials
to stabilize the spine while preserving anatomical structures.
F IG UR E 5 6- 11 Total Facet Arthroplasty System (Archus).
N-Hance (Synthes)
N-Hance is a flexible posterior stabilizing device that is composed of a collar
of paired PCU spacers with interposed titanium ring and end caps (Figure
56-13). This unit slides over the tapered core of a 6-mm titanium rod. The
construct provides elongation, compression, and angulation. N-Hance is
510K approved by the FDA as an adjunct to fusion.
Stabilimax NZ (Applied Spine)
Stabilimax NZ is an investigational posterior stabilizing system that utilizes a dual spring configuration to confer physiologic motion parameters
(Figure 56-14). It is intended to provide maximum stabilization to the
spine, decreasing the abnormal motion that causes pain, while maintaining
physiologic motion.
It is designed to provide stabilization of the lumbar spine in patients
receiving decompression surgery for the treatment of clinically symptomatic
central or lateral spinal stenosis. Stabilimax NZ is currently being clinically
evaluated and is not FDA approved.
Dynesys (Zimmer Spine)
The Dynesys Dynamic Stabilization System is a posteriorly placed pediclebased device (Figure 56-15). It is composed of titanium alloy screws, an
interposed PCU spacer, and a through-passed PET cord. During implantation of the device, the cord is placed in tension and the spacer is placed in
378
P A R T V I I Surgical Treatment Modalities: Lumbar Spine
F IG UR E 5 6- 1 4 The Stabilimax NZ investigational posterior stabilizing
system (Applied Spine).
F IG UR E 5 6- 12 Total Posterior System (Impliant).
F IG UR E 5 6- 13 The N-Hance flexible posterior stabilizing device
(Synthes).
F IG UR E 5 6- 15 The Dynesys Dynamic Stabilization System (Zimmer
Spine).
Dynamic TTL-Rod (Scient’x)
The Dynamic TTL-Rod is composed of titanium rods with an interposed
damper (Figure 56-16). The damper is a series of washers contained within
a bell housing. This configuration results in a posterior rod with micro
motion of 2 mm. It is designed to stabilize the spinal segment in semirigid
fashion with reduced forces at the bone screw interface. The Dynamic
TTL-Rod is 510K approved as an adjunct to fusion.
compression. These opposing vectors of force act through the pedicle screws
to dynamically neutralize the spinal segment in flexion, extension, and at
rest. The system is 510K approved as an adjunct to fusion. In prospective
OUS clinical trial, dynamic neutralization proved to be a safe and effective
alternative (to fusion) in the treatment of unstable lumbar conditions.
3
Dynesys is currently in FDA panel discussion regarding approval as a
nonfusion dynamic stabilization system.
CD Horizon Legacy Peek Rod System (Medtronic)
The CD Horizon Legacy Peek Rod System is a pedicle-based, posterior
rod device (Figure 56-17). It is composed of standard polyaxial pedicle
screws attached to a peek rod. It is designed to provide semirigid fixation
that closely replicates the natural load distribution of the lumbar spine for
patients who undergo spinal fusion surgery. The CD Horizon Legacy Peek
Rod System is 510K approved as an adjunct to fusion.
C H A P T E R 5 6 e Role of Dynamic Stabilization and the Aging Spine
F IG UR E 5 6- 16 The Dynamic TTL-Rod (Scient’x).
379
F IG UR E 5 6- 18 The DSS Spine Stabilization System (Paradigm).
synovial deterioration, and bony overgrowth. The metabolic and structural
deterioration of bone ultimately results in osteopenia, osteoporosis, and
mechanical insufficiency. These degenerative processes , individually and in
combination, result in a broad spectrum of spinal conditions, which vary in
severity, clinical manifestation, anatomical location, and intensity of appropriate intervention. For each of these conditions a dynamic solution can be
considered. As this is still a new and evolving field, most proposed interventions are intuitive with a paucity of supporting clinical data.
F IG UR E 5 6 -1 7 The CD Horizon Legacy Peek Rod System (Medtronic).
DSS Spine Stabilization System (Paradigm)
The DSS Spine Stabilization System is a pedicle-based posterior coupler
device (Figure 56-18). It is an entirely modular system composed of titanium monoaxial pedicle screws, upon which are placed washers and spherical spacers. This allows polyaxial orientation of the couplers. The couplers
are made of titanium. The configuration of outer spiral cut housing and
inner shaft with a ball-in-socket piston confers motion is all directions with
stoppage within physiologic ranges. The hemispherical screw interfaces
allow further polyaxial implantation. It is designed to allow physiologic
motion within an overall reduced range and to restrict the neutral zone. The
DSS Spine Stabilization System is 510K approved as an adjunct to fusion.
Dynabolt (VertiFlex)
Dynabolt is a pedicle screw–based flexible posterior rod. It allows for full
range of motion and can be delivered percutaneously. It is designed to
reduce bone–screw interface stresses and to enable load sharing through
the spinal column during motion. Dynabolt is 510K approved as an
adjunct to fusion.
CLINICAL APPLICATION
The aging spine demonstrates characteristic degeneration that is unique
depending on anatomical location. The ligaments progress from desiccation and loss of elasticity, through inflammation, to end-stage hypertrophy
and calcification. The facets suffer inflammation, capsular insufficiency,
Ligament
Mild ligamentous disease, beyond simple inflammation, may result initially
in ligamentous laxity. This state contributes to mild instability. In addition,
thickening or buckling of specific ligaments lining the spinal canal can contribute to spinal stenosis. The interspinous spacers overall may have clinical
efficacy in such cases. X-Stop has demonstrated efficacy in these patients.
The dynamic rods may also be used but represent a more aggressive solution. The relative invasiveness of pedicle fixation in such cases is overkill in
many instances. The facet replacement devices likewise entail more disruption of anatomy than can be recommended for otherwise mild disease.
Moderate ligamentous laxity can result in more significant instability.
The abnormal motion, characterized by an increased neutral zone, can
result in back pain. A greater degree of central canal stenosis can also be
seen. Surgical treatment often entails direct decompression with concomitant worsening of instability. The interspinous devices could be considered
in these instances, but for the most part might lack sufficient influence of
translation to be effective. A more robust interlaminar device such as Coflex
may be used in selected cases. The dynamic rods have much more utility in
this patient population. Pedicle-based devices that control the neutral zone
include DSS and Stabilimax. Following more extensive laminectomy and
partial facetectomy or with greater instability, more rigid devices may be
needed. Dynesys, CD Horizon Peek, and Dynamic TTL have sufficient
rigidity as to be useful in up to grade 1 spondylolisthesis.
More severe ligamentous dysfunction can result in degenerative spondylolisthesis, more severe central canal stenosis, lateral or subarticular stenosis, and secondary disc failure. The stiffer pedicle-based devices could
be considered here. Devices to be considered include Dynesys, Isobar, and
PEEK rods. Efficacy is exceeded with greater than grade 1 spondylolisthesis,
pars defect, and greater than 50% facetectomy. Fusion with traditional rigid
fixation technology is then warranted.
Facet
Mild capsular failure and intractable synovial inflammation may result in
facet pain. Facet resurfacing technology, such as Zyre may be efficacious.
380
P A R T V I I Surgical Treatment Modalities: Lumbar Spine
Moderate capsular failure and synovial dysfunction also occurs and
results in significant pain and disability. Synovial cysts with lateral compression, facet overgrowth with subarticular stenosis, and degenerative subluxation of facets could be treated with facet reconstruction. Fenix and similar
technologies may be indicated in such cases.
Severe facet disease results in more severe stenosis, and facet failure. This
probably requires facet replacement. Applicable technologies include TFAS,
Facet Solutions, and Tops.
Canal
Moderate lumbar stenosis, beyond the mild stenosis seen with ligamentous
buckling or involution, requires surgical decompression. The accompanying
mild instability may be treated with interlaminar devices such as Coflex and
Vertiflex.
Moderately severe stenosis with more accompanying instability may be
treated with the stiffer pedicle-based devices such as Dynesys, PEEK rods,
or Isobar.
Osteopenia
Bone quality is the overarching consideration for any device implantation.
Osteopenia and osteoporosis are typically contraindications to most device
usage. Dynamic stabilization technologies in general have reduced forces
at the bone–screw interface when compared to traditional fixation techniques. This may result in a more suitable construct for stabilization in such
instances of poor bone quality.
CONCLUSION
The aging spine is less adaptive to, and less forgiving of, less than ideally
applied solutions to pathological conditions. Hence there is an even greater
need for thoughtfulness of approach, broad spectrum of choice, and proper
selection of technique.
Posterior dynamic stabilization of the lumbar spine offers promising
solutions to a variety of challenging clinical scenarios. In most instances,
inadequate clinical data exist to fully evaluate appropriateness and efficacy
in specific pathological entities. Rigid clinical science is needed to support
what is presently clinical intuition. Only through patient trials and physician
experience will the proper use of and indication for these devices become
clear. Also, further evolution of proper diagnostic techniques that specifically evaluate the dynamic elements of spine pathology will be needed to
guide proper selection of technology.
References
1. Anderson, et al., J. Neurosurg. 4 (6) (2006 Jun) 463–471.
2. S.C. Park, et al., J. Korean. Neurosurg. Soc. 46 (4) (2009 Oct) 292–299.
3. T.M. Stoll, et al., Eur. Spine J. (11 Suppl 2) (2002 Oct) S170–S178.
4. Neurosurg. Rev. 32 (3) (2009 Jul) 335–341. discussion 341–2.
5. Zucherman, et al., Eur. Spine J. 13 (1) (2004 Feb) 22–31.
6. Spine 30 (12) (2005 Jun 15) 1351–1358, 2004.
Pedicle Screw Fixation in
the Aging Spine
Hajeer Sabet and Frank M. Phillips
57
k e y p o i n t s
When performing reconstructive spinal surgery in the elderly patient, the
surgeon must consider the fragility of osteoporotic bone, the stability of the
spine, and the potential failure mechanisms of any applied instrumentation.
Increasing pedicle screw length, diameter, or both can be the first line in
improving pedicle screw construct rigidity.
Undertapping pedicle screws can achieve increased insertional torque and
screw pullout strength.
Triangulation of screws increases the overall pullout strength of the construct
and provides higher resistance against loads perpendicular to the pedicle
screws.
A twofold to threefold increase in screw pullout can be achieved with the
use of polymethylmethacrylate injected into the vertebral body around the
screws.
INTRODUCTION
The number of people with osteoporosis is expected to rise with the increasing longevity of the population, so spine surgeons must appreciate the impact
of osteoporosis on the management of spinal disorders in the elderly. Older
patients desire to remain active and are reluctant to accept disability and
deformity as an inevitable consequence of aging. These patient expectations
coupled with advances in spinal surgical techniques have resulted in more
spinal procedures being performed on the elderly. The spinal surgeon may be
required to treat direct sequelae of osteoporosis in the form of painful spinal
fractures or resultant deformity, or may be required to consider osteoporosis
as it relates to spinal reconstruction in the older patient. Regardless of any
surgical decisions in the osteoporotic patient, the spine surgeon must ensure
that the patient is being appropriately medically treated for osteoporosis.
As larger reconstructive spine surgeries are performed on older patients,
the ability of the osteoporotic spine to support spinal implants must be considered. The selection of spinal instrumentation must take into account the
fragility of osteoporotic bone, the stability of the spine, and the likely failure mechanisms of any applied instrumentation. The preoperative workup
should include evaluation for the severity of osteoporosis, which might
impact the surgeon’s choice of reconstruction techniques.
Posterior instrumentation is most commonly applied to the osteoporotic
spine in an effort to stabilize the spine and promote fusion after decompression of neural elements. In this situation, the anterior column is typically
intact and no frank instability exists, so that posterior instrumentation alone
is often adequate. Surgery primarily for deformity correction in the elderly
is challenging and infrequently indicated. Posterior instrumentation may be
used to correct spinal deformity; however, if the deforming forces exceed the
stability of the implant–bone interface, posterior construct failure will occur.
In current clinical practice, the large majority of posterior instrumentation spinal surgeries involve pedicle screw instrumentation. In the
osteoporotic spine, the weak link in the instrumentation construct is the
implant–bone interface. The majority of instrumentation failures involve
screw loosening and pull-out, which may lead to failure of fusion or the
development of recurrent or de novo deformity. Posterior thoracolumbar
instrumentation failure has been shown to correlate with bone mineral density (BMD).
plate with cyclical flexion–extension loading are directly related to BMD
and may occur even at physiologic loads in the osteoporotic spine.
biomechanical study, Soshi and colleagues
fixation should be avoided in patients with a BMD less than 0.3 g/cm
At the time of pedicle screw insertion, the surgeon may recognize poor
screw purchase in osteoporotic bone because of the low insertion torque
required to advance the screw. Insertion torque not only correlates with
BMD and screw pull-out, but also predicts early screw failure.
screw purchase is recognized intraoperatively, the surgeon should attempt
to salvage the situation rather than rely on inadequate fixation to achieve the
goals of instrumentation.
1-3
Screw pull-out and also cutout through the adjacent end-
1-3
2
concluded that pedicle screw
4-6
If poor
In a
2
.
PEDICLE SCREWS IN THE OSTEOPOROTIC SPINE
Screw Placement
The surgeon may consider increasing the length or diameter of the pedicle
screw in an attempt to improve the screw purchase in bone (Table 57-1).
Increasing screw length does increase screw pull-out strength, although
this effect may be less pronounced in osteoporotic bone.
cal screws in the lumbar spine is limited because of risk of vascular injury.
However, in the sacrum, a bicortical screw can be placed safely and improves
pull-out strength.
bral body cortex intraoperatively may affect the surgeon’s ability to safely
place longer screws, since screws extending beyond the anterior vertebral
body may predispose to vascular injury. At the sacrum, bicortical purchase
may be safely accomplished with medially directed pedicle screws with a low
risk of vascular injury. Increasing screw diameter will also increase pull-out
7,11-13
strength
limit the screw diameter. In the osteoporotic spine, when the screw diameter
exceeds 70% of the pedicle diameter, a risk of pedicle fracture is created.
Directing pedicle screws toward the stronger subchondral bone adjacent
to the vertebral body endplate will improve pull-out resistance.
sacrum, optimal screw purchase is achieved by directing the screws toward
the disc space anteriorly or through the sacral promontory.
TA BL E 57 -1 Pedicle S crew Size Rel ationsh ip
Screw Size 6.0 5.0
Screw outer diameter (mm) 6.0 5.0
Screw minor diameter (mm) 4.8 3.8
Tap minor diameter (mm) 4.75 3.75
9,10
The inability to accurately gauge the anterior verte-
; however, the dimensions of the pedicle being cannulated may
7,8
Use of bicorti-
15,16
17-19
14
In the
381
382
F IG UR E 5 7 -1 Triangulation of pedicle screws with a cross plate.
(Redrawn from P. Richard, M.D. Schlenk, M.D. Todd Stewart, et al., The biomechanics of iatrogenic spinal destabilization and implant failure, Neurosurg
Focus 15(3), 2003.)
P A R T V I I Surgical Treatment Modalities: Lumbar Spine
Another strategy to improve stability of the pedicle screw construct in
osteoporotic bone is to distribute forces by increasing the number of fixation points to the spine by including additional levels in the construct. The
advantages of this approach must be weighed against the risks and morbidity associated with the additional level surgery as well as the potential
long-term consequences of a fusion spanning additional levels. The surgeon
may also augment the pedicle screw construct with offset sublaminar hooks,
which are well suited for use in the osteoporotic spine because they rely on
the relatively unaffected cortical laminar bone for fixation.
cal studies have supported the ability of supplemental sublaminar hooks to
increase the rigidity and pull-out strength of pedicle screw constructs.
1,20
Biomechani-
21,22
Convergence of pedicle screws with a triangulation effect can substantially increase the overall pull-out strength of the construct and provides
higher resistance against loads perpendicular to the pedicle screw (Figure
23
57-1).
Triangulation of pedicle screws increased pullout strength by
143% over single pedicle screws.
23
Bilateral triangulated pedicle screws
allow the screws to, in effect, hold all of the bone between the screws rather
than just the bone within the threads of the individual screws. Ruland and
colleagues23 suggested that for triangulated screws to fail simultaneously, a
transverse fracture through the vertebral body at the level of the tips of the
pedicle screw had to occur. Kilincer and colleagues
24
demonstrated there was
no biomechanical benefit to converging pedicle screws more than 60 degrees.
The ability to triangulate pedicle screws is impacted by local bony anatomy. Larger diameter pedicles at levels where the pedicles natural converge
(for example L5) allow for medial angulation of the screws. Smaller diameter or deformed pedicles where pedicle morphology is more straight ahead
(for example T12) are more challenging for placement of convergent screws.
Undertapping Pedicle Screws
In osteoporotic bone, loss of fixation at the bone–screw interface is the primary mode of failure for screws. The preparation technique of the bone–
implant interface is important for optimal screw purchase. Typically the path
for the pedicle screw is tapped before screw placement. In osteoporotic bone,
a tap with a diameter smaller than that of the pedicle screw is recommended
in order to conserve cancellous bone, which is compacted around the screw
heads thereby increasing screw stability. Carmouche and colleagues
formed a cadaveric pullout resistance study comparing tapping, undertapping, and no-tapping techniques. The authors reported that same-size
tapping of lumbar and thoracic pedicle screws decreased pullout resistance
when compared to undertapping or no-tapping. Kuklo and colleagues
reported a 93% increase in insertional torque when undertapping thoracic
screws by 1 mm when compared to line-to-line tapping. Halvorson found
in a cadaveric model that screw insertion technique did not affect pullout
resistance with normal bone density (BMD > 1 g/cm
2).27
In osteoporotic
bone, however, there was a marked benefit to undertapping by 1 mm.
25
27,28
per-
26
F IG UR E 5 7- 2 Articulated cross-link.
Transverse Connectors
Transverse connectors, also known as “cross-links”, serve to link together and
add rigidity to two screw–rod constructs (Figure 57-2). The cross-link does
not directly effect fixation at the screw-bone interface, but instead augments
stability of the overall construct that can indirectly facilitate fixation by
minimizing micro motion. Biomechanical testing has confirmed the ability
of cross-links to increase torsional and lateral stability in an unstable burst
fracture model. An additive effect to stability with the application of one and
then two cross-links was reported.
29
Transverse connectors had little effect
on flexion-extension, lateral bending or tensile rod stress. Longer constructs
such as those used to treat spinal deformity have also been tested with crosslinks. Kuklo and colleagues
30
showed that in long pedicle screw–rod constructs, cross-links increased predominantly axial rotational stability, with
the effect enhanced by the addition of a second cross-link (additional 15%).
Location of the cross-link within the longer constructs did not significantly
impact stability.
Disadvantages of cross-links include breakage
31
and hardware prominence because these are the most dorsally placed elements in the instrumentation construct. The dorsal cross-link prominence may lead to localized
discomfort and at times the formation of an overlying bursa. Additionally,
connectors can theoretically add to instrumentation crowding, thus reducing available bone surface area for fusion.
Bone Cement
The bone–screw interface also may be improved by injecting polymethyl-
methacrylate (PMMA) bone cement into the pedicle around the pedicle
screw. A twofold to threefold increase in screw pullout has not been demonstrated with the use of PMMA injected into the vertebral body through
a cannulated pedicle.
pedicle has not been shown to significantly increase the pullout strength.
Possible risks of this technique include cement extravasation outside of the
vertebra, with potential for leakage into the spinal canal or neural foramina.
Other cements such as hydroxyapatite cement, calcium phosphate, and carbonated apatite have also been shown to enhance the screw–bone interface
and increase pedicle screw pullout strength.
reported that the failure modes seen with PMMA and calcium phosphate
cement differed in pullout tests. With PMMA augmentation, pedicle fracture occurred at or near the junction with the vertebral body in 80% (25 of
30) of the samples. In contrast, failure of calcium phosphate augmentation
occurred at the cement–screw interface in 80% (24 of 30) of the samples. In
an in vivo animal model of pedicle screw augmentation, injectable calcium
sulfate cement was shown to significantly improve the immediate pullout
strength of pedicle screw fixation, and this effect was maintained even after
the calcium sulfate cement had been absorbed completely.
Kiner and colleagues
ing that larger diameter pedicle screws increased construct rigidity greater
than did cement augmentation. Cement augmentation of screws has been
used in patients with osteoporosis and metastatic spinal tumors undergoing spinal instrumentation with acceptable clinical results and low rates of
instrumentation failure.
2,8
Increasing the amount of PMMA injected into the
13,33,34
Moore and colleagues33
35
36
recently reported a biomechanical study suggest-
37-39
Interestingly
32
C H A P T E R 5 7 Pedicle Screw Fixation in the Aging Spine
F IG UR E 5 7- 3 Biomet expandable pedicle screw.
Expandable Screws
Recently, expandable pedicle screws have been designed that can pass
through a fixed pedicle size and then expand in situ in the cancellous bone
of the vertebral body to improve screw fixation. These screws are similar to
those used in drywall. These allow for greater bone contact at the screw tip
with no increase in pedicle insertion diameter or screw length. These screws
may be particularly beneficial in the osteoporotic patient.
Various designs of expanding pedicle screws are available. In one design,
the pedicle screw is cannulated to accept an expansion peg. The distal two
thirds of the screw is split lengthwise by two perpendicular slots to form
four anterior fins when expanded. An expansion peg (a smaller-gauge
screw) is threaded into the inner core of the pedicle screw. As the expansion
peg advances into the slotted portion of the screw, it spreads and opens up
the slotted tip of the screw, creating fins. Withdrawal of the expansion peg
collapses the fins, allowing for removal of the screw (Figure 57-3).
Ngu and colleagues
40
examined the load-to-failure strength of an
expandable screw design (Omega-21, Biomet, Warsaw, Ind.). The expandable-screw pullout strength (391 N) was significantly stronger than a standard pedicle screw l (145 N) reflecting a 170% increase in pullout strength.
It should be noted in this study that cemented augmented pedicle fixation
had an even higher pullout resistance (599 N, or 284% increase). Cook and
associates
41
found expandable screws to have an approximate 50% increase
in pullout strength compared with conventional pedicle screws.
In a clinical study of 145 patients that received expandable screw fixation, 21 of these patients were osteoporotic. Of these 21 patients, 18 (86%)
went on to have solid fusion, and 20 (95%) had expandable screws intact at 2
to 5 years. Expandable screw breakage occurred in 3% of all patients studied,
with osteoporotic patients demonstrating a higher screw breakage rate of
5% (1/21 cases, 5/97 screws). Broken expandable screws were difficult to
remove. Screw breakage most frequently occurred at the level of the prongs.
CONCLUSION
With the aging of today’s population, the spine surgeon must appreciate the
effects of osteoporosis on the spine. The risk–benefit ratio of spinal surgery
in the elderly osteoporotic patient must be cautiously weighed by both the
surgeon and patient. The surgeon must understand the limitations of spinal
instrumentation in the osteoporotic spine and should consider strategies to
reduce the likelihood of construct failure.
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The Role for Biologics in the Aging Spine
David A. Essig, Christopher P. Miller, and Jonathan N. Grauer
58
k e y p o i n t s
Define the current biologic options for treatment of degenerative conditions
of the spine.
Define the role for bone morphogenetic proteins (BMPs) in degenerative
conditions of the spine.
Identify bone graft substitutes and their properties.
Discuss other potential application of biologics in the aging spine.
INTRODUCTION
Currently, there are over 36 million people over the age of 65 years in the
United States. This number is projected to increase to 71 million (nearly
20%) by 2030.
related diseases, such as degenerative disorders of the spine. The elderly are
also living a more active lifestyle than at any other time, and thus the demand
for addressing issues in this population will only increase over time. However, treating degenerative conditions of the aging spine poses particularly
challenging situations to surgeons. Not only must the spinal surgeon address
the spinal pathology, but these patients often have serious comorbidities that
may affect the treatment options.
cians will have to weigh the additional risks of operative treatment against
the benefits of reducing disabling pain and improving quality of life.
Spinal fusion is commonly considered in this population to address
degeneration, deformity, and/or stabilize a decompressed segment. Although
instrumentation is frequently used for initial stability, fusion is a biological
event in which solid bridging bone forms between adjacent vertebrae.
Currently, iliac crest autologous bone (autograft) remains the gold standard bone graft material for achieving spinal fusion in all ages. It is an excellent choice because it is the only bone graft option that provides all of the
components necessary for arthrodesis: osteoconductive matrix, osteoinductive proteins, and osteogenic cells. However, there are significant problems
with using autograft including the limited supply of available bone and subjecting the patient to a secondary invasive procedure to harvest the autologous bone which is, in itself, associated with potential morbidity, such as
infection, fracture, and intractable pain.
relevant for the elderly population who, because of their medical comorbidities, often have longer recovery times and more complications following
surgeries of any kind. As a result, there has been significant work to develop
materials to supplement or even replace iliac crest bone graft in the hope of
minimizing surgical morbidities while ensuring that the treatment objectives are still achieved.
There have been few studies examining the use of biologics specifically
in older patients. What has been done has focused primarily on complications as opposed to health outcomes and fusion success. Despite the
limited information, older patients would intuitively seem to be an ideal
target population for bone graft alternatives because of their poorer quality iliac crest bone and their higher risk for graft-associated complications.
Although the roles of such biologics are still being defined, they provide
1
As the population ages there will be a similar increase in age-
2,3
Therefore, patients, families, and physi-
4,5
This consideration is particularly
384
an exciting adjunct or alternative treatment for spinal conditions in the
aging patient population. This chapter will discuss some of these biologics
designed to stimulate a successful spinal arthrodesis and their use in the
elderly population.
BONE MORPHOGENETIC PROTEINS
Bone morphogenetic proteins (BMPs), members of the transforming
growth factor-beta superfamily, have gradually become better understood
since their initial identification by Marshall Urist
tion by binding to the cell membrane of undifferentiated mesenchymal type
of cells to promote the induction of bone formation. Although more than
12 BMPs have been identified, only a few have been explored for potential
clinical use.
Two BMP molecules have been approved for use in humans. Recombinant human bone morphogenetic protein-2 (rhBMP-2) is currently
approved, in conjunction with a collagen sponge and threaded intervertebral cage, for the treatment of degenerative lumbar spine disease. Recombinant human bone morphogenetic protein-7 (rhBMP-7), also referred to as
osteogenic protein-1 (OP-1), currently has a humanitarian device exemption (HDE) status for posterolateral lumbar fusions in challenging fusion
environments.
In a prospective, randomized trial comparing anterior lumbar interbody
fusion with either rhBMP-2 implanted on a collagen sponge in a tapered
fusion cage or autogenous iliac crest bone graft, there was a 94.5% fusion
rate in the BMP group versus an 88.7% fusion rate in the control group, as
determined radiographically.
control group experienced adverse events related to their bone grafting procedures, and 32% reported persistent pain at the donor site at the time of final
follow-up. Back, leg, and neurologic pain scores improved in both groups to
a similar extent. Nonetheless, this procedure is less commonly considered
than posterior procedures in the aged population because of the increase
in complication rate and recovery time associated with anterior approaches.
Posterior lumbar fusion with BMP has significant challenges including
limited surface area for healing, distractive forces, and the large gap between
transverse processes needing to be bridged. Furthermore, studies have demonstrated the need for a bulking agent in posterior procedures. One recent retrospective study comparing instrumented posterolateral fusion with rhBMP-2
versus iliac crest autograft demonstrated equivalent fusion masses between
the two groups at 2-year follow-up.
group compared to 11.1% in the control group. No significant differences in
the bulking agent used (local bone, allograft bone, demineralized bone matrix,
and ceramic) were noted. Demonstrated outcomes when rhBMP-7 (OP-1)
is used are comparable to those of autograft when used in uninstrumented
posterolateral fusion for the treatment of degenerative spondylolisthesis.
Recently Glassman and colleagues11 reported on the utility of rhBMP-2
with an absorbable collagen sponge (ACS) compared to autograft in an elderly
population (older than 60 years of age). They assessed the clinical, radiographic, and economic outcomes at 2-year follow-up for 102 patients treated
by posterolateral lumbar fusion with iliac crest autograft versus rhBMP-2/
ACS. They found no increased rates of complications due to the rhBMP-2
8
Moreover, 5.9% of the subjects in the autograft
9
Nonunion rate was 6.6% in the BMP
6
in the 1960s.7 They func-
10