During 1995–2005, a large number of dynamic stabilization devices have been introduced. The posterior interpedicular dynamic stabilization is primarily indicated for activity-related mechanical back pain. Once their safety and efficacy is established to achieve the primary goal, these devices may be more frequently indicated for stabilization of the adjacent segment. The interspinous distraction devices are primarily indicated for treatment of spinal stenosis with neurogenic claudication. The definition of spinal instability, rationale of dynamic stabilization, and interspinous distraction, as well as the available devices and their clinical experience, are discussed in this chapter.
Spinal instability is an abnormal motion, often accompanied by an increased neutral zone motion caused by ligament laxity, even when the total range of motion (ROM) may be diminished.
The most important biomechanical challenge for pedicle screw–based posterior dynamic stabilization is survival against fatigue failure because they are expected to work indefinitely allowing spinal motion.
The key to this survival is uniform reduction of motion, uniform unloading by load sharing with the existing disc and facet joints, and normal location of the instantaneous axis of rotation.
The biomechanical goals for posterior dynamic stabilization devices are motion preservation and load sharing.
Posterior Dynamic Stabilization
Primary indication: Treatment of activity related mechanical back pain
Secondary indications: Prevention of spinal instability
Iatrogenic instability after decompression
Stabilization of an adjacent motion segment
Supplement of total disc replacement
Contraindications for posterior dynamic stabilization:
Advanced degeneration with stiffness of the segment
Interspinous Distraction Device
Primary indication: Spinal stenosis with neurogenic claudication
Secondary indication: Foraminal stenosis with radicular symptom
Predominant axial back pain
Relative contraindication at L5-S1
Clinical application of posterior motion-sparing instrumentation in the treatment of mechanical low back pain was introduced by Henry Graf in 1989. The range of posterior motion-sparing devices introduced include: (1) semirigid rods to primarily achieve a better fusion without stress shielding; (2) posterior dynamic stabilization systems to primarily address mechanical back pain; and (3) finally, spinous process distraction devices with primary goal to address spinal stenosis with claudication, by indirect decompression. The semirigid rods do not truly intend to spare motion, and these will not be discussed further in this chapter. The posterior dynamic stabilization devices and the spinous process distraction devices are discussed separately because their primary goals are different.
POSTERIOR DYNAMIC STABILIZATION DEVICES
The recommendation for clinical application of posterior dynamic stabilization devices often extends beyond their primary indication. Most devices were introduced without defining the biomechanical basis of design rationale and mechanism of actions. The clinical applications of such devices were always combined with decompression, which ensures pain relief, masking any inadequacy of the device-related clinical benefit to relieve mechanical back pain. The reason behind such a plethora of dynamic stabilization devices is poor understanding of “spinal instability” as a cause of back pain.
Spinal Instability as a Cause of Chronic Low Back Pain
Panjabi has described spinal stability in vivo as a function of three subsystems: (1) the spinal column, (2) the spinal muscles, and (3) the neural control unit. The action of spinal muscles and the neural control may be important stabilizer in vivo, but it is the passive stability provided by the spinal column that constitutes the focus of surgical manipulation to treat back pain. Mulholland and Sengupta forwarded a hypothesis that, unlike in knee or shoulder joint, spinal instability in chronic low back pain does not mean increased motion, as commonly misunderstood, but indicates abnormal load distribution across the vertebral end plate. This, according to Mulholland and Sengupta, is the primary cause of mechanical back pain. This hypothesis was supported by a close association of abnormal disc pressure profile to positive discogram with pain provocation described by McNally et al. The instability in the spinal column as a result of disc or facet degeneration, or both, may be associated with an increased motion in the very early stage. By the time the mechanical back pain becomes severe, requiring surgical intervention, the range of motion (ROM) is often diminished. Regardless of increased or decreased ROM, spinal instability is always associated with an abnormal quality of motion. Panjabi redefined spinal instability as an abnormal motion often accompanied by an increased neutral zone (NZ) motion caused by ligament laxity, even when the ROM may be diminished. The apparent discrepancy between instability and stiffness was explained by Panjabi with an analogy of a marble rolling on a soup bowl ( Fig. 46-1 ). The abnormal load distribution (Mulholland) versus abnormal motion (Panjabi) theories of spinal instability may be interrelated, without any conflict between these two theories. An abnormal motion may cause abnormal load distribution, which, in turn, may cause pain. At the same time, it may be possible that an abnormal motion may not cause abnormal load distribution, and in such cases, it may not be associated with pain production. This may explain why many black discs are not associated with back pain!
Biomechanical Goals for Posterior Dynamic Stabilization Devices
Addition of a posterior dynamic stabilization device to an already degenerated motion segment is expected to reduce the flexibility of the motion segment. As much ROM as possible should be preserved, but any abnormal motion, which occurs during the NZ motion, should be restricted. Because NZ motion represents ligament laxity, all posterior dynamic stabilization systems, which generally increase the stiffness of the motion segment, will automatically reduce NZ regardless of their mechanism of action. This explains why most of the devices claim clinical success in relieving back pain, at least in the short term. When a motion segment is rendered unstable by a decompression procedure, it often introduces an abnormal increased translation; in such cases, the goal of posterior dynamic stabilization device should be to restore a normal range and quality of motion. Normally, it is unlikely that a dynamic stabilization device will increase the ROM of a degenerated segment, unless it induces a favorable biological reaction to the disc and facet joint by off-loading these structures. On rare occasion, however, it may be expected that if the device may restore the collapsed disc height by distraction, it may increase the ROM toward normal.
According to Mulholland and Sengupta, the mechanism of low back is an abnormal increased loading of the disc or facet joint, or both. The mechanism of pain relief with dynamic stabilization should, therefore, be unloading the disc and the facet joints by load sharing. How much load should be shared by the device is not clear, but the device should prevent any abnormal excessive loading of the disc and the facet joint. It is more important that the load sharing is uniform throughout the ROM and uniform in all directions of motion, allowing the disc and the facet joints to bear the remaining load at all stages of the motion. Normally, the disc pressure increases both in flexion and extension, and is lowest in neutral position. A posterior dynamic stabilization device should ideally permit the increase in the disc pressure in both flexion and extension but to a smaller magnitude because of load sharing. If the disc pressure does not increase at all, particularly in extension with introduction of the posterior motion-sparing device, it may indicate that the device is acting like a total load-bearing structure during extension, rather than as a load-sharing structure.
Design Rationale of the Posterior Dynamic Stabilization Devices
The four main design rationales for posterior dynamic stabilization devices focus on the following: (1) resistance to fatigue failure, (2) pedicle-to-pedicle distance excursion, (3) a safe and easy salvage procedure, and (4) compatibility with minimally invasive instrumentation.
The biggest challenge for a posterior dynamic stabilization device is to survive fatigue for an indefinite period despite allowing continued motion. It is well known that even a fusion rod fixation may eventually fail in fatigue should fusion fail to occur. Normally, the fatigue property of a fusion device is tested in an American Society for Testing and Materials (ASTM) standard spine model consisting of two plastic cubes as vertebral bodies, but nothing to represent anatomic structures such as the disc or the facet joints. Unfortunately, the fatigue property of a dynamic stabilization device may not be adequately tested in such spine model, because the devices are meant for sharing the load with the existing anatomic structures. It is, therefore, imperative to predict the fatigue failure property indirectly, by testing the effect of the device in motion restriction and load sharing. Uniform restriction of ROM and uniform load sharing throughout the ROM may indirectly predict fatigue resistance of a particular device. A mismatch in the location of the instantaneous axis of rotation (IAR) of the device and the motion segment is another predictor of fatigue failure of the device.
Nonmetallic devices may deform, soften, and creep to adapt to the kinematics of the motion segment, and survive fatigue better, at the cost of reduced efficacy over time. Is creep and softening of a nonmetallic device a disadvantage? One may argue that dynamic stabilization may stimulate a favorable biological response to repair the motion segment, and its subsequent creep or softening is truly an advantage, when its function is over! Conversely, metallic spring devices may retain their property over time but are more subject to fatigue failure, should there be any mismatch in the kinematics. Notably, fatigue failure of a nonmetallic device may not be recognized radiologically, unless there is screw loosening or breakage, but failure of a metallic device cannot hide!
Pedicle-to-Pedicle Distance Excursion
A normal pedicle-to-pedicle excursion during flexion-extension may be as large as 6 to 9 mm, less in lateral bending, and minimal in rotation ( Fig. 46-2 ). The device should permit a normal pedicle-to-pedicle distance excursion to permit normal ROM and ensure that the device should not act as a motion “stopper” in any direction. Unfortunately, only a few dynamic stabilization devices can accommodate such a large degree of flexibility.
Safe and Easy Salvage and Compatibility with Minimally Invasive Procedure
Screw design may not matter when a posterior dynamic stabilization device is used as a stand-alone device. But when used to supplement a fusion in the adjacent segment, it is easier if the device can be inserted using the regular pedicle screws for fusion rod. It further helps conversion to fusion as a salvage procedure for failed dynamic stabilization without the need for changing the screws. Because fatigue failure is a challenge, hydroxyapatite-coated screws may help to strengthen screw-bone interface.
Posterior dynamic stabilization devices share their activity with an unobstructed functioning of the existing anatomic structures, such as the disc, facet, ligaments, and muscles across the motion segment. It is therefore desirable that the device may be implanted with minimally invasive instrumentation to avoid damage to the anatomic structures such as facet capsule and ligaments, among others. This is particularly important when dynamic stabilization is performed without decompression or fusion in adjacent segment.
Posterior Dynamic Stabilization Devices
The list of posterior motion-sparing devices is growing continuously, and it is impossible to make a comprehensive listing or classification. The Isobar TTL (Scient’x, Maitland, Florida) is a semirigid metal rod with disc springs in the midsection acting as a dampener. The CD Horizon Legacy PEEK rod (Medtronic Sofamor Danek, Memphis, Tennessee) is another semirigid alternative to titanium fusion rods. Both of these devices are primarily indicated for achieving a solid fusion without stress shielding. These devices are not typical motion-preservation devices. The facet replacement devices such as TOPS (Total Posterior Arthroplasty System; Impliant Spine, Princeton, NJ), TFAS (Total Facet Arthroplasty System; Archus Orthopedics Inc., Redmond, Washington), or ARFS (Anatomic Facet Replacement System; Facet Solutions Inc., Logan, Utah) differ from the remaining dynamic stabilization devices by the fact that these are truly prosthetic devices, primarily indicated for replacement after excision of the facet joints. Their role may be to supplement total disc replacement (TDR) in presence of posterior joint disease, converting it to a total joint replacement. The pedicle screw–based posterior dynamic stabilization devices are designed to work together with the facet joint. Their recommendation for stabilization after facet joint excision, or to supplement TDR, may need appropriate biomechanical evaluation.
Most of the posterior interpedicular dynamic stabilization devices introduced in the United States for clinical application obtained Food and Drug Administration (FDA) approval under 510(k) as a fusion rod alternative. Their use as dynamic stabilization device without fusion represents off-label use only and is not recommended by the FDA.
Indications for Posterior Dynamic Stabilization
The primary indication for posterior dynamic stabilization is treatment of spinal instability:
Activity-related mechanical back pain in early stages of disc/facet degeneration (disc degeneration, facet degeneration, degenerative spondylolisthesis)
Secondary indications include prevention of spinal instability:
Iatrogenic instability after decompressive laminectomy/discectomy
Stabilization of a motion segment with early degeneration, adjacent to fusion
Supplement TDR to achieve total joint replacement
The primary goal of dynamic stabilization is treatment of mechanical back pain caused by spinal instability. Radicular pain or claudication pain can be adequately treated by decompression alone; the role of additional dynamic stabilization here is only to prevent instability and back pain. The evidence of clinical efficacy of a dynamic stabilization device can be established only by application of the device to treat mechanical back pain in absence of decompression. Once that is established, it may be recommended for application in conjunction with decompression procedure.
Classification of the Posterior Dynamic Stabilization Devices
Pedicle screw–based devices
Dynesys (Zimmer Spine Inc., Warsaw, IN)
Transition (Globus Medical Inc., Audubon, PA)
DSS-II Dynamic Stabilization System (Abbott Spine Inc., Austin, TX)
BioFlex (Bio-Spine Corp., Seoul, Korea)
AccuFlex (Globus Medical Inc.)
Stabilimax NZ (Applied Spine Technologies Inc., New Haven, CN)
Cosmic Posterior Dynamic System (Ulrich GmbH & Co, Ulm, Germany)
Hybrid devices (metallic component with plastic bumper)
Axient (Innovative Spinal Technologies, Mansfield, MA)
CD Horizon Agile (Medtronic Sofamor Danek)
NFlex (N Spine, Inc., San Diego, CA)
Clinical Experience with Posterior Dynamic Stabilization
The Graf ligament, described by Henry Graf in 1992, is one of the earliest dynamic stabilization devices and forms the basis of many other devices introduced subsequently. The device is still used in a few centers in both Europe and Asia, but in general, its use has declined. It consists of a circular Dacron ligament around the pedicle screw heads applied in compression, and thereby locking the facet joints. The surgical procedure is simple, and unlike fusion, it avoids exposure of the transverse processes or the need to harvest bone graft. Unfortunately, there is a high incidence of radicular symptoms secondary to either disc herniation or narrowing of the foramen as a result of compression applied between the pedicle screws. The compressive force may also have a deleterious effect on the facet joint and may lead to back pain.
Currently, the most extensively used posterior dynamic stabilization device is Dynesys (Zimmer Spine Inc., Warsaw, IN). Dynesys was evolved as an improvement over the Graf ligament, by incorporating a plastic cylinder (Sulene-PCU) around the cord to apply a distraction force unloading the facet joints. Dynesys is primarily a posterior distraction device, and intuitively one may expect that the device should act as an extension stop and, therefore, should have restricted extension rather than flexion ROM. In fact, in vivo ROM testing using MRI scan in upright position before and after application of Dynesys shows the extension is limited and flexion remains unaffected. In contrast, the biomechanical tests with Dynesys in cadaver spine show that the range of flexion is limited, apparently because the device holds the segment in nearly full flexion. The range of extension remains nearly normal, because the flexibility of the cylinder permits extension. The load-sharing studies on cadaver spine show that the disc unloading with Dynesys is ideal in flexion, where it acts as a partial load-sharing device, and unloads the disc by an equivalent extent. However, in extension, it becomes a total load-bearing structure, allowing no load to be transmitted through the disc. Another biomechanical study shows high stress at the pedicle screws after Dynesys instrumentation. This may explain why screw loosening has been so rare with Graf ligament but fairly common with Dynesys, as high as 17% in some clinical series. Fortunately, the plastic cylinder may soften in the body temperature in vivo and may creep over time, which may reduce the efficacy of the device in distracting the pedicle screws but may also protect it against fatigue failure.
Initial clinical results with Dynesys in the hands of the inventor of the device were comparable with those of fusion. However, more than 60% of their cases had spinal stenosis, and Dynesys was used in conjunction with decompression, making it difficult to evaluate whether the good outcome was secondary to Dynesys or decompression. Subsequent studies by Grob et al. found that stand-alone Dynesys produced a good outcome in only 39% of cases, compared with 69% when combined with decompression. Similar experience was expressed by other independent researchers as well.
Dynesys has been used for quite some time in the United States after it was approved by the FDA as a fusion device. An FDA-controlled Investigational Device Exemption clinical trial is in progress comparing the effect of Dynesys as a dynamic stabilization device, against fusion. Unfortunately, the inclusion criteria include patients with predominant leg pain, and cases with predominant back pain are being excluded. The clinical trial combines Dynesys together with decompression and does not study the effect of Dynesys alone. Therefore, even at the conclusion of this expensive clinical trial, this study is not expected to establish the clinical efficacy of Dynesys in the treatment of activity-related mechanical back pain, which is the main clinical indication for surgical treatment with dynamic stabilization.
Transition stabilization system (Globus Medical Inc.) is similar to Dynesys, consisting of a cylindrical polycarbonate urethane (PCU) spacer around a polyethylene tetraphthalate cord between the pedicle screws. It differs from Dynesys in three major design perspectives. First, the soft bumper at the end allows adequate excursion between the pedicle-to-pedicle distance with flexion-extension. The second design perspective is lordosis and distraction. The length of the spacer causes distraction and defines the unloading of the facet joint, whereas an active lordosis, produced at the junction cylinder and the screw head, ensures unloading of the disc. These two important mechanical properties permit uniform motion restriction in all directions and uniform load sharing through the ROM. The third design perspective is regular pedicle screws. Transition uses regular pedicle screws, which makes it easy to combine fusion at an adjacent segment, or conversion to fusion as a salvage procedure later, without changing screws. Transition is a preassembled, and pretensioned, top-loading, “drop and lock” implant that avoids the challenges of in situ assembly and may be inserted with a less invasive surgical approach.
DSS-II dynamic stabilization system (Abbott Spine Inc.), developed by the author, is one of the earliest titanium metallic springs used for dynamic stabilization. Its precursor, the DSS-I, was a C-shaped titanium spring, which on biomechanical tests showed normal flexion but an uneven restriction to extension and excessive unloading of the disc in extension, both indicating fatigue failure. Therefore, this system has never been used clinically. In contrast, the “α”-shaped titanium spring in DSS-II design produces distraction, as well as lordosis, and permits adequate pedicle-to-pedicle excursion and physiologic translation of the IAR of the motion segment. It allows uniform motion restriction and uniform load sharing throughout the ROM, the two essential biomechanical characteristics for survival of the implant against fatigue failure. The device has never been introduced in the United States, but a pilot clinical trial in a small group of patients (n = 19) was completed in Sao Paulo, Brazil. The inclusion criteria were patients who had mechanical back pain, with minimal or no leg symptom, and did not need decompression. The idea was to assess the clinical efficacy of the DSS-II system without the confounding effect of decompression. The clinical outcome was encouraging, but more importantly, no implant failure was observed in 2 to 3 years of follow-up.
Other all metal posterior dynamic stabilization devices described include BioFlex (BioSpine Corporation, Seoul, Korea), Stabilimax NZ (Applied Spine Technologies Inc.), and Cosmic Posterior Dynamic System (Ulrich GmbH & Co). The Hybrid devices, which include Axient (Innovative Spinal Technologies), CD Horizon Agile (Medtronic Sofamor Danek), and NFlex (N Spine, Inc.), incorporate metallic rod connected to a flexible segment, with a nonmetallic bumper to allow shock absorption, as well as some degree of pedicle-to-pedicle excursion. The obvious clinical advantage is that the device looks similar to a fusion rod and can be used with a regular pedicle screw. A full discussion of these devices is beyond the scope of this chapter.