Posterior Dynamic Stabilisation Devices

The concept of the dynamic stabilisation of the lumbar spine was initiated from the dissatisfaction with the outcomes of fusion surgery for the treatment of back pain. Open surgical intervention has traditionally been directed at decompressive procedures to relieve neural element compression and/or fusion techniques to stabilise the spinal column. Back pain is caused by altered spinal movement and loading from the degenerative changes in the disc and facet joints. It has been shown that disc degeneration causes dramatic changes in pressure distribution associated with pain1. Therefore, surgeons have sought an alternative treatment for degenerative back pain by allowing some motion but restoring natural loading of the disc. However, the mechanisms of pain related to the load transfer in the spine are not well understood.

This new category of dynamic stabilisation devices focuses on the concept of maintaining or restoring intervertebral motion in a controlled fashion. The principle of dynamic stabilisation consists in both increasing the stiffness of the intervertebral segment and limiting the amplitude of mobility to stop the otherwise inexorable course of degenerative disc disease, and possibly, in some cases, to foster the healing of the least severe lesions. The demand for an ideal dynamic stabilisation system has increased for younger patients with multisegment disc degeneration, where adjacent segment disease may be more likely to happen following fusion in the long-term follow-up. The interest in dynamic stabilisation has grown with indications like topping off an adjacent segment to fusion or stabilising a segment following decompression. Their popularity is based more on lack of satisfaction with conventional spinal fusion rather than proven superiority. These devices can be put into three general categories:

Indications
Although the indications for dynamic stabilisation are still being identified, there are several disorders in which the devices are expected to play a role.

Controlled Motion in the Iatrogenically Destabilised Spine
Although minimal access procedures such as microdiscectomy require very little bone removal, the treatment of lumbar spinal stenosis accompanied by lateral recess stenosis and superior facet encroachment can result in significant facet joint resection. One possible role of PDS devices would be to limit and control motion after a potentially destabilising laminectomy, avoiding the need for arthrodesis and reducing the likelihood of iatrogenic destabilisation.

Increased Anterior Load Sharing to Augment Interbody Fusion
Interbody fusion techniques have become highly popular over the last two decades. Although pedicle screw/rod stabilisation is typically used as an adjunct in these operations, concerns have arisen surrounding the highly rigid nature of these constructs. Stress shielding of the interbody graft is believed to be implicated in a certain percentage of pseudarthroses.(125) Thus, these devices may play a role in limiting the extremes of motion, which could result in graft displacement, while allowing for maximal anterior load sharing and fusion.

Protection and Restoration of Degenerated Facet Joints and Intervertebral Discs
These devices may be able to shield the disc and facet joint structures from motion that is destructive, potentially allowing for a reduction in local inflammatory processes or permitting self-repair mechanisms.

In Combination with Anterior Motion Preservation for 360° Circumferential Motion Segment Reconstruction
One of the major drawbacks of total disc arthroplasty is that facet disease remains a contraindication. In fact, it is believed that disc arthroplasty may accelerate facet degeneration. Using dynamic stabilisation technology, circumferential reconstruction of all the mobile units in a spinal segment becomes possible.

Adaptation of Stabilisation Techniques to the Aging Spine
Current spinal stabilisation devices (pedicle screws and rods or transfacet screws) provide a high degree of rigidity. Although this can be desirable, the use of these devices in the treatment of patients with osteopenia or osteoporosis can result in catastrophic destruction of host bone at metallic interfaces. The application of “softer stabilisation” techniques may be more desirable in these settings, reducing the likelihood of construct failure.

Prevention of Fusion-Related Sequelae
Loss of spinal motion due to fusion can result in a number of sequelae, including accelerated adjacent-level degeneration, fusion into states of malalignment, and pseudarthrosis. With respect to adjacent-level disease, it is believed that the elimination of mobility can overload adjacent segments and lead to accelerated degeneration and arthrosis. This can result in axial pain, deformity and kyphosis, and neurological compression.(126) Furthermore, loss of lumbar lordosis as a result of poorly contoured or excessive distraction of instrumentation can lead to flatback deformity or fixed sagittal imbalance.

Interspinous Spacer Devices
By keeping the spine in a rather flexed position, the interspinous devices increase the total canal and foraminal size, decompressing the spinal cord and the nerve roots responsible for neurogenic claudication². This type of device allows neural decompression with only a minimal amount of tissue resection, making the procedure less invasive. The devices are intended to be implanted without a laminectomy and function through indirect decompression, thus avoiding the risk of epidural scarring and cerebrospinal fluid leakage. Furthermore, these devices limit extension of the spine, unload the facet joint, and relieve the pain attributed to facet disease.

Typically, these systems provide dynamic, or “soft” stabilisation by providing a posterior tension band. The Minns device was the first “soft” interspinous spacer to be reported³. The implant was fashioned out of silicone into the shape of a dumbbell. They were made in various sizes with the central diameter ranging from 8 to 15mm. The implants were found to prevent the approximation of the spinous processes when the vertebral bodies were subjected to an axial loading force. The spinous process deflection increased with the increasing diameter of the implant. However, despite the promising in vitro results no further development or clinical application has been published to date.

The Wallis System
Wallis System (Abbott Spine), was developed by Sénégas in 1986⁴ and has the longest history. The device's original design was a titanium block that was inserted between adjacent processes and held in place with a flat Dacron cord or ribbon wrapped around the spinous process above and below the block. A second generation of Wallis implants was developed by change in the material from titanium to PEEK, a strong, plastic like polymer that has more elasticity. An important aspect of the Wallis implant is in its design and material, which minimizes the need for bone resection and avoids constraint on bone. It has notches that fit the physiological shape of the spine. The clinical trials of the first-generation implant provided evidence that the interspinous system of dynamic stabilisation is efficient against low-back pain due to degenerative instability and free of serious complications.

The authors recommend the Wallis system for lumbar disc disease in the following indications: (i) discectomy for massive herniated disc leading to substantial loss of disc material, (ii) a second discectomy for recurrence of herniated disc, (iii) discectomy for herniation of a transitional disc with sacralization of L5, (iv) degenerative disc disease at a level adjacent to a previous fusion, and (v) isolated Modic I lesion leading to chronic low-back pain.

The X STOP Device
This device (St. Francis Medical Technologies, Inc.) is an oval titanium metal spacer designed to fit between two adjacent lumbar spinous processes and achieve indirect decompression. Several clinical studies have been performed and have shown the efficacy of the X STOP device in treating neurogenic claudication secondary to lumbar stenosis. The implant procedure for this device can be done after induction of either local or general anesthesia. After a small midline incision, the paraspinal muscles are lifted subperiosteally from the spinous processes while all midline structures are left intact. The X STOP device is then placed between the spinous processes. The supraspinous ligament is carefully protected. Starting just posterior to the lamina, a dilator is placed between the spinous processes. A sizing instrument is then inserted between the spinous processes and expanded until the supraspinous ligament is taut. Using a gauge on the sizing instrument, the X STOP device is sized then inserted. Although the implant is not rigidly attached to the osseous anatomy, it is restricted from migrating posteriorly by the supraspinous ligament, anteriorly by the lamina, cranially and caudally by the spinous processes, and laterally by the device's wings on each side⁵. In randomized controlled prospective multicenter trials and retrospective studies, investigators reported a better outcome with X STOP compared with conservative management⁵.

The DIAM System
The DIAM Spinal Stabilization System (Medtronic Sofamor Danek) is a soft interspinous spacer. The core is made of silicone, which is covered by a polyethylene coating. The surgical technique consists of identifying the interspinous space, removing the remnants of the interspinous ligament down to the ligamentum flavum, and using a distracter of the spinous processes to facilitate the insertion of the device. It is secured in place with two laces, one around the spinous process above, and another around the one below. Biomechanical tests have demonstrated that a posterior soft shock-absorbing implant in the lumbar spine is able to reduce intradiscal pressure, retighten posterior elements of the vertebral bodies, and reduce rotatory dislocation6.

The Coflex, ExtendSure, and CoRoent Devices
Another device that is used in Europe is the Coflex (Paradigm Spine). It is a U-shaped metallic device that is inserted between the spinous processes. As with other interspinous devices, this one is designed to increase the cross-sectional diameter of the stenotic canal in patients suffering from neurogenic claudication. However, very little information is available in the literature on the biomechanics and efficacy of this product.

2. Posterior dynamic stabilisation systems
Devices in this category can best be described as an internal brace - allowing controlled movement of the affected segment of the spine. Most of these devices tend to be derived from the pedicle screw and rod constructs (used in spinal fusion surgery) of the 1980s and 1990s. Pedicle-based dynamic devices were first designed to stabilise the abnormal segment and to unload degenerated discs and facet joints, while maintaining the same level of normal motion. By unloading the pressure on the degenerated disc and facets, pedicle-based dynamic devices have the potential to reduce pain associated with these anatomical structures.

Graf ligament
One of the earliest of the pedicle screw based devices was the Graf ligament which was reported on in 1992⁷. This system was developed in Europe and uses braided polyester cables looped around the screws to provide stability while allowing motion.

The product was conceived to immobilize the lumbar spine in lordosis; alter the load bearing on the anulus and endplate; compress the posterior anulus, resulting in fewer anular tears; splint the motion segment, allowing healing of damaged.

There have been several published reports on the clinical results of this device, and the outcomes have been inconsistent^8,9. The variation in results may be due to differences in the patient populations operated on and/or in the outcome assessments used tissue to occur; and relax over time, allowing some return to movement.

Dynesys
Another system using pedicle screws and cords is the Dynesys, manufactured by Zimmer Spine. This device also incorporates a plastic spacer over the cords. The device has been used as a dynamic stabilisation device in Europe with mixed results reported¹⁰.

The FDA clearance for the Dynesys system is limited to use as an adjunct to spinal fusion of the thoracic, lumbar and sacral spine for degenerative spondylolisthesis with neurologic impairment, and for a prior failed spinal fusion (pseudoarthrosis). When used as a pedicle screw fixation system, the Dynesys Spinal System is indicated for use in patients who are receiving fusion of the lumbar or sacral spine with autogenous graft only, and who are having the device removed after development of a solid fusion mass.

Several clinical studies have reported on the efficacy of the Dynesys system as a posterior stabilising device. Prospective and retrospective studies have shown better outcome with Dynesys than with conservative treatment alone. However, those results were comparable to conventional rigid fixation¹¹, and a 17 to 19% rate of failure was reported in the literature^11,12.

The AccuFlex, PEEK, and Isobar Rods. Other semirigid rods being used in the US include the AccuFlex (Globus Medical, Inc.), Medtronic's PEEK rod, and Scient'X's Isobar. To date, only limited clinical and biomechanical data are available on these devices.

3. Total Facet Replacement Systems
Until recently, the focus on the intervertebral disc has resulted in an underestimation of the facet joints as a potential pain generator, and little is known about the pathogenesis of facet disease. The contemporary literature focuses on anterior column failure as leading to posterior element degeneration, with subsequent stenosis and segmental instability. However, facetogenic pain can occur in the absence of or combined with significant disc degeneration. For this reason, total facet replacement and arthroplasty may be a potential treatment analogous to total disc arthroplasty for discogenic pain.

References

1. Patwardhan AG, Havey RM, Meade KP et al. A follower load increases the load-carrying capacity of the lumbar spine in compression. Spine 1999;24:1003-9.
2. Lindsey DP, Swanson KE, Fuchs P, Hsu KY, Zucherman JF, Yerby SA: The effects of an interspinous implant on the kinematics of the instrumented and adjacent levels in the lumbar spine. Spine 28: 2192-2197, 2003
3. Minns RJ, Walsh WK. Preliminary design and experimental studies of a novel soft implant for correcting sagittal plane instability in the lumbar spine. Spine 1997;22:1819-25130 Stoll TM, Gilles Dubois G, Schwarzenbach O. The dynamic neutralization system for the spine: A multi-center study of a novel non-fusion system. Eur Spine J. 2002;11(Suppl 2):S170-8.
4. Senegas J: Mechanical supplementation by non-rigid fixation in degenerative intervertebral lumbar segments: the Wallis system. Eur Spine J 11 Suppl 2: S164-S169, 2002
5. Zucherman JF, Hsu KY, Hartjen CA, Mehalic TF, Implicito DA, Martin MJ, et al: A multicenter, prospective, randomized trial evaluating the X STOP interspinous process decompression system for the treatment of neurogenic intermittent claudication: two-year follow-up results. Spine 30: 1351-1358, 2005
6. Mariottini A, Pieri S, Giachi S, Carangelo B, Zalaffi A, Muzii FV, et al: Preliminary results of a soft novel lumbar intervertebral prothesis (DIAM) in the degenerative spinal pathology. Acta Neurochir Suppl 92: 129-131, 2005
7. Graf H. Lumbar instability. Surgical treatment without fusion. Rachis 1992;412:123-37. Madan S, Boeree NR. Outcome of the Graf ligamentoplasty procedure compared with anterior lumbar interbody fusion with the Hartshill horseshoe cage. Eur Spine J. 2003;12:361-8.
8. Hadlow S, Fagan, AB, Glas H, et al. The Graf ligamentoplasty procedure: Comparison with posterolateral fusion in the management of low back pain. Spine. 1998;23:1172-9.
9. Grob D, Benini A, Junge A, Anne F, Mannion AF. Clinical experience with the Dynesys semirigid fixation system for the lumbar spine: surgical and patient-oriented outcome in 50 cases after an average of 2 years. Spine. 2005;30:324-31.
10. Schnake KJ, Schaeren S, Jeanneret B: Dynamic stabilisation in addition to decompression for lumbar spinal stenosis with degenerative spondylolisthesis. Spine 31: 442-449, 2006
11. Grob D, Benini A, Junge A, Mannion AF: Clinical experience with the Dynesys semirigid fixation system for the lumbar spine: surgical and patient-oriented outcome in 50 cases after an average of 2 years. Spine 30: 324-331, 2005

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