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!

Analogy of a “marble on a soup bowl” for spinal instability, as described by Panjabi. A, Intact spine. The marble can move with little resistance across the neutral zone (NZ) but faces increasing resistance toward the elastic zone (EZ). B, Injured or unstable motion segment is represented by a flat bowl allowing the marble to move to and fro with little resistance; that is, increased NZ with no increase in EZ. C, An ideal stabilized segment represents a smaller cup that reduces primarily the NZ, with minimal reduction of EZ movement. D, Represents a stabilized segment, but too much stiffness is present, which is undesirable. E, Represents an uneven stabilization, with different in one direction, which is again undesirable.

Biomechanical Goals for Posterior Dynamic Stabilization Devices

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.

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.

A , The pedicle-to-pedicle distance from flexion to extension may be as large as 8 to 9 mm, and to preserve normal motion, (B) the dynamic stabilization (e.g., Transition; Globus Medical Inc., Audubon, Pa) may need to permit an excursion of the pedicle screw heads by the same magnitude.

(Part A adapted from Yue JJ, Bertagnoli R, McAfee PC, An HS, ed. Motion Preservation Surgery of the Spine – Advanced Techniques and Controversies. Philadelphia, PA: Saunders Elsevier; 2008. Printed with permission. Part B adapted from Sengupta DK. Dynamic stabilization. SpineLine. 2008;9(3):10–18. © North American Spine Society. Used with permission.)

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

• A.
  

  Pedicle screw–based devices

  
  
  
  
  • a.
    

    Nonmetallic devices

    
    
    
    
    • i.
      

      Dynesys (Zimmer Spine Inc., Warsaw, IN)

      
      
    • ii.
      

      Transition (Globus Medical Inc., Audubon, PA)

    
    
    
    
  • b.
    

    Metallic devices

    
    
    
    
    • i.
      

      DSS-II Dynamic Stabilization System (Abbott Spine Inc., Austin, TX)

      
      
    • ii.
      

      BioFlex (Bio-Spine Corp., Seoul, Korea)

      
      
    • iii.
      

      AccuFlex (Globus Medical Inc.)

      
      
    • iv.
      

      Stabilimax NZ (Applied Spine Technologies Inc., New Haven, CN)

      
      
    • v.
      

      Cosmic Posterior Dynamic System (Ulrich GmbH & Co, Ulm, Germany)

    
    
    
    
  • c.
    

    Hybrid devices (metallic component with plastic bumper)

    
    
    
    
    • i.
      

      Axient (Innovative Spinal Technologies, Mansfield, MA)

      
      
    • ii.
      

      CD Horizon Agile (Medtronic Sofamor Danek)

      
      
    • iii.
      

      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.