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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_6034_Библиотеки_им_академика_М_И_Перельмана.pdf
CHAPTER 38
Dynamic Stabilization in the Treatment of Low Back Pain Due to Degenerative Disorders
Dilip K. Sengupta
Several reports in the literature indicate that successful
fusion may fail to impro ve chronic low back pain in a significant number of patients (1,2). This has renewed the
interest in dynamic stabilization in the lumbosacral spine.
Essentially, dynamic stabilization means instrumentation
to control movement and load transmission through the
motion segment. The terms semirigid fixation, flexible
stabilization, and soft stabilization are apparently syn-
onymous with dynamic stabilization. Ho w ev er , there is an
essential difference between these terms. When the goal
of stabilization is to improve the rate and quality of fusion
without stress-shielding, the technique is called semirigid
fixation. In contrast, when the goal is to preserve a controlled motion it is usually described as soft stabilization
or flexible stabilization.
The following discussion elaborates the rationale,
techniques, and evolution of the various dynamic stabilization systems in the treatment of chronic low back
pain.
ROLE OF FUSION
The role of fusion in the treatment of degenerative low
back pain is a matter of debate. In a review of the
Cochrane databases for randomized controlled trials,
Gibson et al. (1) concluded that for degenerative lumbar
spondylosis there is no scientific evidence in favor of the
effectiveness of any form of surgical decompression or
fusion when compared with natural history, placebo, or
conservative treatment. In contrast, in a multicenter randomized controlled trial, the Swedish Lumbar Spine
Study Group reported that the outcome of fusion is significantly better than that of nonsurgical treatment. However, most authors agree that in a significant number of
patients successful fusion may not produce adequate pain
relief.
ETIOLOGY OF PERSISTENT BACK PAIN
AFTER SUCCESSFUL FUSION
Chronic low back pain due to disc degeneration is
believed to be generated by abnormal movement, abnormal load transmission, or both in the motion segment.
When conservative treatment fails, the traditional surgical treatment is fusion. Theoretically, fusion should
address both these mechanisms; load transmission is
direct from bone to bone and there should be no movement after fusion. Instrumentation may be added to
increase the fusion rate. Evolution of instrumentation
techniques during the last decade has improved fusion
rates to close to 95%. Unfortunately, this has not resulted
in an equivalent success in the functional outcome (2).
It is unclear why pain persists in some patients following a successful fusion. The possible explanations that
have been suggested by various authors may be summarized as follows:
1. Misinterpretation of pseudar throsis as solid fusion
2. Adjacent segment disease
3. Abnormal load transmission despite fusion
4. Abnormal sagittal balance.
It has been well recognized that radiologic assessment
of fusion is often unreliable (3–5). McAfee (6) reported
that many cases of failed back syndrome following successful fusion were in fact a misinterpretation of pseudarthrosis. He emphasized that presence of the “sentinel
sign”, bridging bone in front of the cages, is the only
definitive evidence of fusion. In a study involving 100
cases of failed back syndrome following apparent fusion,
three-dimensional computed tomography (CT) uncovered incomplete fusion in 17%, transitional syndrome in
13%, and pseudarthrosis in 6% of cases (3). A solid
fusion alters the biomechanics at the adjacent level,
resulting in increased mechanical demands (7). There
373
374 /SECTION V/SPECIFIC CLINICAL ENTITIES
have been reports of increased rates of adjacent-level
pathologic lesions after fusion, but these have not been
taken into account for the natural history of degenerative
changes (8,9). After solid fusion using metallic interbody
cages, the mechanical stress at the contact surface of the
end plate to the cages may be much higher than normal
and may generate pain (10). Studies of finite element
models have demonstrated that vertebral loads corresponding to certain activities may generate end-plate
stresses at the control surface with cages that approach
and exceed the failure stress for cortical bone (10,11).
Loss of sagittal balance and “flat-back syndrome” following lumbar fusion may increase the stress at the adjacent segment leading to persistent back pain (12). In a
review of 83 consecutive cases of spinal fusion, patients
with abnormal C7 plumb line or abnormal sacral inclination in the immediate postoperative radiographs were
found to have a much higher incidence of adjacent level
degeneration compared with patients with normal sagittal
balance (13). This has also been established in an in vivo
study in a sheep fusion model (14).
AL TERNA TIVE T O THE CONVENTIONAL
FUSION PROCEDURES
Because of failure of fusion to relieve back pain, alternative surgical approaches have been developed. These
may be broadly divided into three categories:
support. Without this anterior support, the construct is
expected to fail. Mochida et al. (19,20) described an ingenious method of using a Dacron ligament (Leeds Kieo
Ligament, Neoligaments, Ltd., Leeds, UK) for spinal
fusion, which was originally introduced for reconstruction of the anterior cruciate ligament. They described the
technique as syndesmoplasty in which each end of the
ligament is fed into a hole in the pedicle, crossed in a tunnel in the vertebral body, pulled out of the contralateral
pedicle, and tied around the spinous process of the inferior segment. Their results were comparable to that with
rigid fixation when the instability was small.
Artificial Disc or Nucleus Prosthesis
The goal of prosthetic replacement of the disc or the
nucleus is to preserve motion. The pain is controlled by
removing the pain generator, the diseased disc, and also
by uniform load transmission through the end plate and
the facet joints. The stabilization provided is indirect and
is dependent on tensioning the remaining annulus and the
ligamentous structure after insertion of the prosthesis.
The artif icial disc or nucleus differ from dynamic stabilization by prosthetic replacement of a section from the
motion segment.
Artificial disc and nucleus have been discussed in
Chapters 39 and 40.
1. Semirigid stabilization—to achieve fusion without
stress shielding
2. Artificial disc or nuclear prosthesis—to preserve
motion and to stabilize the segment, while replacing
a component of the motion segment
3. Dynamic stabilization—to preserve motion while
stabilizing the segment, without replacement of any
anatomic structure.
Semirigid Stabilization
As opposed to rigid stabilization, semirigid stabilization uses a somewhat flexible construct for fixation. The
goal is to achieve fusion of the motion segment. The
objective of semirigid implants is to avoid “stress shielding” which may discourage formation of the fusion mass,
lead to osteoporosis, and cause loosening of implants. In
vitro biomechanical studies showed semirigid devices
share load with the anterior bone graft or cage to promote
fusion (15,16).
Most authors describe semirigid rods between the
pedicle screws. Musha et al. (17) reported over 97% successful fusion rate using a semirigid system consisting of
rod and pedicle screws. Gertzbein et al. (18) reported
97% fusion rate using 4 mm threaded rod and polyaxial
pedicle screw. They suggested that the system should
only be used in conjunction with an anterior structural
Dynamic Stabilization
Due to the unpredictable outcomes of fusion procedures, spinal surgeons are showing an increasing interest
in dynamic stabilization procedures. The goal of dynamic
stabilization is to preserve motion while stabilizing the
motion segment. It may be used alone or in conjunction
with rigid stabilization, to “top off ” the proximal segment adjacent to fusion to prevent its accelerated degeneration.
The ideal mechanism of dynamic stabilization has not
been clearly defined in the literature. The issues of the
degrees of restriction of motion and how much disc
unloading is necessary have not been resolved. A clear
understanding of the cause of low back pain in disc
degeneration is needed before we may consider the ideal
mechanism of dynamic stabilization.
ETIOLOGY OF CHRONIC LOW BACK PAIN
DUE TO DISC DEGENERATION
Segmental Spinal Instability
The role of instability as a cause of chronic low back
pain is not well understood. Panjabi (21) suggested that
instability is a mechanical entity and is defined as a loss
of stiffness to a given load. Frymore and Krag (22)
echoed the same definition, as a loss of motion segment
CHAPTER 38/DYNAMIC STABILIZATION / 375
stiffness, such that application of force would produce
greater than normal displacement. This may result in
pain, progressive deformity, and neurologic deficit. However, biomechanical and radiological studies using open
magnetic resonance imaging (MRI) in flexion and extension has shown that segmental motion either does not
change significantly with the disc degeneration (23–25),
or may in fact decrease, except during earl y stages of disc
degeneration (26). Mulholland and Sengupta suggested
that the era of back pain due to a disorder called “instability” was based on interpretation of spinal instability in
a purely biomechanical sense, validated to some e xtent in
other joints like the knee and shoulder, and fusion was
seen as the appropriate solution (27).
In the clinical scenario of mechanical back pain, the
instability concept fails to explain two commonly
observed facts. First, patients with disc degeneration
often experience episodes of acute exacerbations superimposed on a mild to moderate degree of baseline symptoms. If there were any abnormal translation or instability
in the diseased segment, symptoms would be continuous.
It is difficult to understand that if instability or abnormal
movement is the cause of pain, why then is the acute pain
only periodic rather than continuous. Secondly, manipulation by chiropractors, at least in some patients, results
in dramatic relief of symptoms in an acute episode of low
back pain. If instability is the etiology of the pain, it
would not be reduced by manipulation. This also contradicts the notion that instability is the causative factor in
low back pain secondary to disc degeneration.
Back Pain: Movement or Load-Related?
The intervertebral disc has two important biomechanical functions; it must transmit load and it must allow a
controlled range of motion. This movement must not
compromise the adjacent neural elements. Following disc
degeneration, either the load transmission or the movement or both may become abnormal. The contribution of
each of these to back pain in disc degeneration is unclear .
Load Transmission through Normal and Degenerated
Discs
In the normal disc, the hygroscopic nature of the proteoglycan in the nucleus with intact annulus acts like an
inflated car tire and helps in uniform distribution of load
across the end plate. In a degenerated disc, the structure
of the nucleus changes to a nonhomogeneous mixture of
fragmented and condensed collagen, areas of fluid, and
on occasion, areas of gas. Isolated fragments of annulus
or end plate may add to the loose fragments inside the
disc (28). The nucleus becomes depressurized and an
increasingly larger load is transmitted through the annulus, which leads to splitting and inward folding of the
annulus (29). The central area of the end plate overlying
the depressurized nucleus now transmits lesser load, and
corresponding end-plate changes, such as destruction and
thinning of the trabeculae and thinning of the cartilaginous end plate (30,31), are noted in this area.
Mechanical Back Pain Related to Posture and Activity
The abnormal distribution of load across the disc space
following disc degeneration as explained previously may
causes baseline mild to moderate pain or discomfort. In the
degenerated disc the principal area of load transmission
becomes dependent on posture. In flexion, the anterior
annulus bears major component of the load , while in e xtension the posterior annulus bears the major component. The
abnormal high load transmission through the various areas
of the annulus with changes in posture may explain the
activity- and posture-related mechanical back pain.
Acute Episodes of Pain and “Stone in the Shoe”
Hypothesis
It seems most likely that the acute episodes of back pain
must be related to a movement of tissues within the disc. In
a degenerated disc, the fragments of nucleus, end-plate cartilage, or annulus may move under the end plate, and
become areas of high spot loading depending on their position within the disc. The best analogy for this theory is the
“stone in the shoe, ” a concept proposed by Mulholland (27).
When a stone moves under the heel, it causes high spot
loading and pain, similar to an acute exacerbation of back
pain. When the fragment shifts, the pain may subside.
Manipulation of the lumbar spine by a chiropractor may, on
occasion, dislodge the fragment from its weight-bearing
position, bringing an immediate relief of acute pain (27).
McNally and Adams (32) demonstrated the nature of
load distribution across the normal and degenerated disc.
Disc pressure profilometry studies in cadaver spine
shows that in the normal disc the load is evenly distributed, but in de generated disc the nucleus is depressurized,
higher load is transmitted near the peripheral annulus,
and there are irregular areas of high spot loading. A subsequent in vivo study established that abnormal pressure
profiles cor relate with abnormal discograms with positive pain provocation (33).
THE RATIONALE AND PRINCIPLES OF
DYNAMIC STABILIZATION
If the primary cause of back pain is abnormal load
transmission, the aim for treatment should be unloading
the disc. In particular, the abnormal high spot loading and
abnormal high load transmission through the annulus
should be prevented. Movement should be preserved
since the transport of nutrients and metabolites in the disc
is dependent on movement (34). However, any abnormal
376 /SECTION V/SPECIFIC CLINICAL ENTITIES
FIG. 38-1. The pressure tracing from the center of
the disc in cadaver lumbar spine during flexion and
extension movement. Normally (N) the pressure is
lowest during the early phase of extension and
rises both during flexion and extension. Following
Graf ligament application (G), the pressure was
raised at neutral position. Following application of
FASS system (described later) with moderate
degree of compression by the ligament (F-1), the
disc was unloaded during flexion but not during
extension.When the FASS system w as applied with
larger compression force by the ligament (F-2), the
system became rigid in flexion and unloaded the
disc further, indicating a possibility that such a system would eventually fail. It should also be noted
that with the FASS system there was very little
effect on disc unloading in extension.
range or direction of motion may secondarily cause areas
of spot loading, and therefore should be prevented.
The pertinent questions in the dynamic stabilization
therefore are (a) how much disc unloading; (b) ho w much
control of motion would be desirable; and (c) in the longterm, how can fatigue failure be prevented, in view of
constant movement of the stabilized segment. Pseudarthrosis may lead to failure of rigid implant, but a flexible implant should be able to accommodate the movement without failure.
The fatigue life of a dynamic stabilization system will
depend on two factors: (a) load-sharing property, and (b)
instant axis of rotation (IAR). The system should share the
load with the disc and facet joints uniformly throughout the
range of movement. Let us consider arbitrarily that the system should bear around 30% of the load and allow the
TABLE 38-1. Classification of the dynamic stabilization
devices in the treatment of low back pain, currently
described in the literature
I. Interspinous distraction devices
a. Minns silicone distraction device
b. Wallis system
c. X-stop
II. Interspinous ligament devices
a. Elastic ligament (Bronsard ligament across the
spinous processes)
b. Loop system
III. Ligaments across the pedicle screws
a. Graf ligament
b. Dynesis system
c. Fulcrum-assisted soft stabilization system (FASS)
IV. Semirigid metallic devices across the pedicle screws
a. Dynamic stabilization systems (DSS-I and DSS-II)
Modified from Sengupta DK. Dynamic stabilization devices
in the treatment of low back pain. Orthop Clin North Am
2003; 35(1):43–56, with permission.
remaining 70% of the load to be transmitted through the
disc and facet joints. If at any time during the range of
motion the implant system has to bear near 100% of the
load, and unload the disc fully, the system would eventually
fail (Fig. 38-1). Secondly, each spinal motion segment has
an optimum IAR, depending on the anatomy of the disc and
the facet joints. Similarly, every dynamic stabilization
device has an optimum IAR, which can be determined in
laboratory, when the system is implanted in two polyethylene blocks representing vertebral bodies, not connected by
the disc or facet joints. If there is a mismatch in the location
of the IAR of the motion segment and the device after
implantation, they will tend to fight against each other during motion. This will lead to abnormal high stress to the
device and the implant-bone junction, leading to the failure
of the instrumentation in the long run. The ideal dynamic
stabilization device should be a load-sharing device
throughout the range of motion, and have an IAR close to
that of the motion segment.
The various dynamic stabilization systems described in
the literature are all posterior implants (35–43). Most of
these devices aim at restriction of some motion but do not
describe the mechanism of action or extent of disc
unloading. Table 38-1 provides a classification of the currently described dynamic stabilization devices.
The Interspinous Distraction Devices
These are floating devices (i.e., not rigidly connected to
the vertebrae). This avoids the possibility of loosening, a
major concern for any implant that would have to survive
against motion. The primary indication for interspinous distraction devices is degenerative spinal stenosis with neurogenic claudication in an older adult patient. By causing distraction between the spinous processes at the stenotic
CHAPTER 38/DYNAMIC STABILIZATION / 377
Minns and Walsh (35) described silicone interspinous
spacers, which on biomechanical testing in the cadaver
spine showed unloading the disc and correcting sagittal
plane imbalance of the spine. No clinical application of
this system has since been described by the authors. A
titanium interspinous distraction device, X-Stop (Fig. 38-
2) has been described by Lindsey et al. (SFMT, Concord,
CA) (37). These are typically indicated for older adult
patients with spinal stenosis presenting with neurogenic
claudication. Senegas et al. (36) described an interspinous spacer made of polyetheretherketone (PEEK),
“the Wallis implant” (Fig. 38-3), which is held between
the spinous processes with Dacron tape. The addition of
the Dacron tape provides further restriction of motion.
The authors have advocated this system for treatment of
early disc degeneration.
FIG. 38-2. The X-Stop titanium interspinous distraction system (St. Francis Medical Technologies, Inc., Concord, CA).
(From Sengupta DK. Dynamic stabilization devices in the
treatment of low back pain. Orthop Clin North Am 2003;
35(1):43–56, with permission.)
segment, an interspinous distraction device unfolds the
buckled ligamentum flavum and posterior annulus, thereby
relieving the stenosis. The device holds the segment in relative flexion, a posture adopted by these patients to relieve
their symptoms. The posterior distraction may unload the
facet joint joints and the posterior part of the disc. Howe v er ,
kyphosis of the segment may increase the load in the anterior part of the disc and these devices may not be suitable
for primarily discogenic back pain.
The Interspinous Ligaments
These devices are applied directly to the vertebrae,
without using any metal anchorage. There is no rigid
component to share the load. These de vices do not unload
the disc or the facet joint. Their primary mechanism of
action is by limitation of the range of motion.
Caserta et al. (38) reported their experience of using
elastic ligament using alone or to supplement the segment
adjacent to fusion (Fig. 38-4). The authors have used the
FIG. 38-3. The W allis implant consists of polyetheretherketone
interspinous spacer, anchored in its place by wrapping two
woven Dacron ligaments around the spinous processes of the
adjacent vertebrae under tension. (From Sengupta DK.
Dynamic stabilization devices in the treatment of low back pain.
Orthop Clin Nor th Am 2003; 35(1):43–56, with permission.)
FIG. 38-4. The elastic interspinous ligament as described by
Caserta et al. Peroperative picture of L4-5 elastic stabilization
following rigid fixation of the L5-S1 segment.(From Caserta S,
La Maida GA, Misaggi B, et al. Elastic stabilization alone or
combined with rigid fusion in spinal surgery: a biomechanical
study and clinical experience based on 82 cases. Eur Spine J
2002;11 Suppl 2:S192–197, with permission.)
378 /SECTION V/SPECIFIC CLINICAL ENTITIES
FIG. 38-5. The Loop system (Spinology, Inc., Stillwater,
MN). (From Garner MD, Wolfe SJ, Kuslich SD. Development
and preclinical testing of a new tension-band device for the
spine: the Loop system. Eur Spine J 2002;11 Suppl
2:S186–191, with permission.)
system in 82 cases since 1994 and described encouraging
results. Unfortunately, their report does not describe any
detail of the implant material or the clinical results. Garner et al. (39) described a tension-band device, the Loop
System (Spinology, Inc., Stillwater, MN), which consists
of a braided polyethylene cable for stabilization of the
spine across the spinous processes. The polymer cable
provides high fatigue strength, in addition to tensile
strength similar to that of metallic cables (Fig. 38-5).
Ligaments across Pedicle Screws
These devices are designed to be anchored to the vertebral bodies through the interface of pedicle screws. A
fabric ligament connects the screw heads to restrict the
motion of the spinal segment. In addition to the ligament,
some of these devices may have a semirigid component.
The Graf ligament (Neoligaments, Leeds, UK), described by Henry Graf in 1989 (40), is the most commonly used device in this group (Fig. 38-6). This system
consists of a pair of Dacron ligaments applied to the pedicle screws with a predetermined compression force. It
immobilizes the spine in lordosis and locks the facet
joints into full extension. No biomechanical study on the
effect of this system has been published by the inventor.
The system restricts motion but does not unload the disc.
An independent biomechanical study shows that, in fact,
it increases the load in the posterior part of the disc and
FIG. 38-6. The Graf ligament system (Neoligaments, Leeds,
UK), applied between pedicle screws at L4-5 and L5-S1 segment in saw-bone. (From Mulholland RC, Sengupta DK.
Rationale, principles and experimental evaluation of the
concept of soft stabilization. Eur Spine J 2002;11[Suppl
2]:S198–205, with permission.)
the annulus (45). The Graf ligament has been used by
several independent surgeons in Europe and Asia, who
reported with clinical success comparable to that of
fusion (45–51). The proposed clinical indications include
back pain due to segmental instability, to supplement a
direct repair of low-grade spondylolisthesis, and in combination with fusion to stabilize an adjacent segment. In
the author’s experience the Graf ligament was found to be
most useful for stabilizing multisegment disc disease in
younger patients, where fusion has obvious disadvantages. One common complication with the Graf ligament
is postoperative leg pain due to narro wing of the foramen
or buckling of the posterior annulus secondary to hyperlordosis; a prophylactic decompression of the nerve roots
has been recommended in these situations. The initial
encouraging clinical results tend to deteriorate during
next 2 years of follow-up (45) and the long-term results
have been reported to be disappointing compared to
fusion (52). This is probably because the Graf ligament
increases the load in the posterior part of the disc and the
facet joints causing accelerated degeneration and also
that the ligament stretches over time and becomes ineffective (52).
The Dynesys (the dynamic neutralization system [Centerpulse Spine-Tech, Minneapolis, MN]) was described
by Gilles Dubois (41) in 1994. The system consists of
titanium alloy (Protasul 100) pedicle screws, polyester
(Sulene-PET) cords, and polycarbonaturethane (Sulene-
FIG. 38-7. The Dynesys system consists of pedicle screws,
connected with a fabric cord, passed through the cylindrical
spacers between the heads of the pedicle screws. (From
Mulholland RC, Sengupta DK. Rationale, principles and
experimental evaluation of the concept of soft stabilization.
Eur Spine J 2002;11[Suppl 2]:S198–205, with permission.)
PCU) cylindric spacers (Fig. 38-7). The pedicle screw
heads are connected by a cord under a given tension similar to the Graf ligament. The cord is threaded through the
hollow cylindric spacers between the pedicle screws,
which prevents e xcessive compression between the screw
heads by the cord. The stabilizing cord carries tensile
forces and the spacers resist compressive forces. The purpose of the system is to establish a mobile load transfer
and control motion of the segment in all planes.
The biomechanical testing of the Dynesys system is
mostly limited to fatigue testing of the whole construct and
determining biocompatibility of the nonmetallic components. In a recent biomechanical study on cadaver spine, the
Dynesys system was found to provide greater flexibility in
extension and rotation but similar stiffness in flexion and
lateral bending as compared to rigid fixation (53). There is
no data available for the load-sharing characteristics of the
spinal motion segments with the Dynesys system.
The initial clinical results in a multicenter trial have
been encouraging and the device was found to be safe
(41). However, unlike the Graf ligament, screw loosening
was observed in seven cases. Early surgical intervention
was needed in four cases, and late surgery was needed in
CHAPTER 38/DYNAMIC STABILIZATION / 379
five cases in the same segment and in seven cases for
adjacent segment disease.
The distraction between the pedicle screw heads by the
Dynesys system may force the segment into kyphosis and
increase the load in the anterior part of the disc. The spinal
extensor muscles may be able to restore the lordosis of the
segment by distracting the disc space and unloading the
disc. This will force the spacers to act as a load-bearing fulcrum. Therefore, the lordosis and load sharing by the plastic cylinder depends very critically on distraction produced
by the implant, and on the ability of the patient to achieve
lordosis with the extensor muscles (27).
The Fulcrum-Assisted Soft Stabilization (FASS) system
(Fig. 38-8A, B) was introduced (42) to address what was
perceived as disadvantage by the author of the Graf system. These include posterior compression leading to narrowing of the foramen and increased load over the posterior annulus.
In the FASS system a flexible fulcrum is placed
between the pedicle screws, in front of the ligament to
distract the posterior annulus. A fabric ligament, preferably of elastic material, placed posterior to the fulcrum,
applies a compressive force across the pedicle scre ws and
maintains lordosis. The fulcrum transforms this posterior
compression force into an anterior distraction force,
which distracts and unloads the disc, independent of muscle action. The degree of disc unloading depends on the
relative tension and compression produced by the fulcrum and the ligament. For a given distraction by the fulcrum, the higher the compressive force applied by the
ligament the greater would be the disc unloading. Laboratory experiments on spine models and cadaver spines
demonstrated that, as greater unloading of the disc was
achieved b y adjustment of the tension in the ligament and
the fulcrum, the system shared higher load, and the
motion segment lost flexibility (42). An undue stiffness
of the system may be unphysiologic and may cause early
loosening of the screws or implant failure. The other disadvantage of the FASS system was that the polytetrafluoroethylene (PTFE) fulcrum was solid and although flexible from side to side, was not compressible along its long
axis. The fabric ligament was also not elastic. The combination of such a fulcrum and ligament leads to a gross
limitation of flexion, but almost no limitation in extension. Consequently, disc unloading was greater in flexion
but very little in extension (Fig. 38-1). A second generation of the FASS system was tested, where the fulcrum
was made up of a compressible titanium spring, but the
ligament was the same. This spring-based FASS system
unloaded the disc and resisted flexion and extension more
uniformly throughout the range of motion (54). An ideal
FASS system should consist of a flexible as well as compressible fulcrum, and an elastic ligament that would not
creep significantly. Currently such a system is under
development.
380 /SECTION V/SPECIFIC CLINICAL ENTITIES
A
FIG. 38-8. A: The Fulcrum-Assisted Soft Stabilization (FASS) system. In this prototype, the fulcrum is made of flexible polytetrafluoroethylene and the ligament is made of an elastic fabric band containing polyurethane. B: The FASS system applied to a cadaver
spine for biomechanical testing. (From Sengupta DK.Dynamic stabilization devices in the treatment of low back pain. Orthop Clin
North Am 2003;35(1):43–56, with per mission.)
B
Semirigid Metallic Devices across the Pedicle Screws
Currently there is no semirigid metallic device for soft
stabilization without fusion available for clinical use.
There are a few such devices under development.
The Dynamic Stabilization System (DSS) system
(43) (Spinal Concepts, Inc., Austin, TX) is presently
being tested in the laboratory. This system consists of a
titanium spring connected to the vertebra with the pedicle screws. Two designs of the springs have been tested.
The DSS-I system (Fig. 38-9A) consists of a “C”shaped spring, 3 mm in cross-sectional diameter. The
DSS-II system (Fig. 38-9B) consists of an elliptical coil
spring of 3 to 4 mm in cross-sectional diameter. These
A
FIG. 38-9. The Dynamic Stabilization System (DSS) (Spinal Concepts, Inc., Austin, TX). A: DSS-I system consists of titanium spring in the shape of a “C”, the straight ends of which are attached to the vertebral body with the pedicle screws.The axis of rotation of this spring is located at the center of the curvature of the spring at C both during flexion and extension. B: DSS-II system consists of a titanium coil
spring. The axis of rotation of this spring lies in front of the coil spring, at C0 location in resting position,
moves forward in flexion to CF, and backward in extension to CE locations.
B
CHAPTER 38/DYNAMIC STABILIZATION / 381
systems may be applied to the motion segment, with an
appropriate degree of distraction and lordosis, to produce a mild disc unloading at resting position. The
stiffness of the spring limits the range of motion and
unloads the disc further during motion.
As explained earlier, the uniform disc unloading
throughout the range of motion will require the IAR of
the spring to lie close to that of the motion segment. The
axis of rotation of the DSS-I system is located close to
the center of the “C” (Fig. 38-9A). In the DSS-II system, the axis of rotation is located in front of the coil,
and moves forward and backward during flexion and
extension, respectively, resembling the translation of
IAR of a spinal motion segment (Fig. 38-9B). Therefore, the DSS-II system unloads the disc more uniformly during flexion-extension motion. This has been
established in a continuous record of disc pressure from
the center of the disc in cadaver lumbar spine, following application of the two spring systems (Fig. 38-10).
Normally, the disc pressure at the center of the disc is
lowest at the early phase of extension, and rises both in
flexion and in extension because the anterior part of the
disc is compressed in flexion and the posterior part in
extension. Biomechanical testing on cadaver spine
shows that the DSS-I system unloads the disc and
restricts motion favorably during flexion. However, in
extension the system forces the entire disc into distraction, resulting in greater restriction of motion and also
lowest disc pressure at full extension. This is because
the IAR of the DSS-I lies far behind that of the motion
segment, and it becomes a full load-bearing structure
toward the end of extension. This indicates that DSS-I
is more likely to experience fatigue failure or loosening.
The IAR of the DSS-II system translates like that of a
normal spinal motion segment. Therefore, application
of DSS-II system causes a more uniform disc unloading
and restriction of motion in flexion and extension (43)
(Fig. 38-10).
Semirigid metallic devices across the pedicle screws
have a unique advantage over the other dynamic stabilization devices. They may be applied with the initial pretension, to distract the disc, when disc unloading is
intended. Conversely, it may be applied with an elastic
compression force on the disc, when a fusion is intended.
Therefore DSS-I may be used in conjunction with an
interbody graft to keep the graft under compression, and
to resist the instability in extension caused by excision of
the annulus in anterior lumbar interbody fusion.
FIG. 38-10. The disc pressure tracing at the center of the disc in cadaver lumbar spine during flexionextension movement, with 10 Nm pure moment, in a 6° freedom spine tester. Normally the pressure
rises both in flexion and extension and is lowest during the early phase of extension. Following stabilization with DSS-II system the disc was partly unloaded both in flexion and extension, because of uniform load sharing with the disc. Following DSS-I stabilization the disc was partly unloaded in flexion, b ut
fully unloaded in extension, which indicates that the implant becomes a fully load-bearing structure in
extension, and therefore is more likely to experience fatigue failure or loosening.
382 /SECTION V/SPECIFIC CLINICAL ENTITIES
SOFT ST ABILIZATION AS AN ADJUNCT TO
DISC PROSTHESIS
Prosthetic disc replacement is an equivalent of a partial
joint replacement. In the presence of significant facet
joint arthritis, disc replacement may not relieve pain.
When radicular pain warrants decompression involving
partial facetectomy, prosthetic disc replacement may
destabilize the motion segment. A posterior dynamic stabilization system may add the necessary stability for disc
prosthesis to work in this situation. In effect, addition of
a posterior dynamic stabilization system may con vert disc
replacement into a total joint replacement.
SOFT STABILIZATION AND DISC REPAIR
If a favorable environment may be created in the
motion segment by unloading the disc and permitting
near normal motion, the disc may be able to repair itself.
Gene therapy in degenerati v e disc diseases, either by promoting enzymes to produce proteoglycans, or b y pre v enting enzymes like proteases that damage the disc, is an
emerging technology with much promise. Soft stabilization may further enhance the reparative process activated
by the gene therapy.
SUMMARY
In summary, dynamic stabilization appears to have an
important role in the treatment of the degenerative lumbar spine. Fusion of one or two motion segments does not
make a significant difference in the total range of motion
of the lumbar spine. However, preserving flexibility of a
motion segment may prevent adjacent segment disease.
Dynamic stabilization is more physiologic and may deliver a better clinical outcome in chronic low back pain
than fusion. Additionall y, it may permit disc replacement,
even when facet joints need to be excised. If a favorable
environment is created in the motion segment by unloading the disc and permitting motion by dynamic stabilization, the disc may be able to repair itself or may supplement reparative potential of gene therapy.
Despite all these bright prospects, a cautious approach
is recommended before accepting any new implant system. The implant for fusion onl y has to serve a temporary
stabilization until fusion takes place. Implant loosening is
not uncommon in the presence of pseudarthrosis. After
soft stabilization, the implant has to provide stability for
an indefinite period, and also stay anchored to the bone
despite allowing movement. This sounds like a daunting
task. This may only be possible if the dynamic stabilization device functions only as a load-sharing device
throughout the range of motion and does not become a
load-bearing structure at a certain range. To achieve uniform load sharing and disc unloading, the instant axis of
rotation of the implant has to lie close to that of the
motion segment. Any mismatch between the kinematics
of the implant system and the motion segment would
result in an early implant failure or loosening. Therefore,
the need for a strict bench test in the laboratory cannot be
overemphasized. The few dynamic stabilization systems
that have been used clinically have been reported to produce clinical outcomes comparable to that of fusion. No
prospective randomized controlled trial has been reported
yet, which is essential for the practice of evidence-based
medicine.
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