Functional Lumbar Artificial Nucleus Replacement: The DASCOR System

Functional Lumbar Artificial Nucleus
Replacement: The DASCOR System

John Emery Sherman and Bruce Randall Bowman

Principles and Expectations of Nucleus Replacements

Surgical Approaches

Post Operative Results

Design Implications

Clinical Assessments

The DASCOR Disc Arthroplasty System

Conclusions

The intervertebral disk consists of two primary structures: the nucleus pulposus and the annulus fibrosus. Annular tearing and disruption are associated with decreased proteoglycan synthesis, with dehydration of the nucleus leading to degeneration of the disk. If this becomes symptomatic, traditional surgical treatment consisting of diskectomy or arthrodesis can be performed. Acceptable clinical outcomes have been reported with arthrodesis, using many surgical approaches.^1,2 Eliminating motion of the functional spinal motion segment, however, is clearly a nonphysiological approach to the symptomatic spinal motion segment. Alternatives to arthrodesis have been offered by total disk arthroplasty (TDA), with devices such as the Charité Artificial Disc (DePuy Spine, Raynham, MA) and ProDisc Total Disc Replacement (Synthes, Inc., West Chester PA).^3–5 Two metal-on-metal prostheses have recently concluded clinical enrollment in U.S. Investigational Device Exemption (IDE) studies. However, total disk replacement requires an extensive surgical exposure with mobilization of the great vessels, near complete resection of the nucleus and annulus, and fixation to the bony end plates. Additionally, revision of TDA is likely to be extremely difficult technically.

Potentially, to address the clinical situation of degenerative disk disease, replacement of only the dehydrated nucleus in a less surgically invasive approach is appealing.^6–9 Disk nucleus replacement presents different anatomical, biochemical, and biomechanical challenges than TDA. The goal of either treatment is to relieve pain, restore disk kinematic function, restore or maintain disk height, limit progression of adjacent level disease, and improve long-term outcomes. This chapter addresses some of the preclinical research and early clinical results of disk arthroplasty by replacing the nucleus pulposus with an in situ curable polymer injected and contained in an expandable balloon.

Principles and Expectations of Nucleus Replacements

Biomechanically, normal disk structure allows both stability and flexibility within each individual spinal motion segment. Normal load carried by the nucleus diminishes with increasing load on the annulus as the disk degenerates. Increased load will accelerate annular incompetence with diminished disk height. This may lead to bulging or herniation of the nucleus pulposus with nerve root compression or, with time, spinal stenosis. Diskogenic low back pain is another clinical manifestation of this process. Increased stimulation of nociceptors within the outer aspect of the annulus and vertebral end plates secondary to the increased loads likely contributes to the underlying pain syndrome of diskogenic back pain.10,11 If a diskectomy is performed for herniated nucleus pulposus, this can further disrupt the load-sharing function of the nucleus, with increasing compressive load requirements on the annulus and facet joints accompanied by an increase in the degenerative cascade.¹²

Whereas a biological replacement of the nucleus is appealing, a more practical biomechanical solution could achieve these goals. Mechanical replacement of the disk nucleus could be performed to reestablish the normal load on the nucleus and annulus and restore disk height. This would help to restore the overall normal disk function of stability as well as allow motion.^13,14 There may be an advantage for the remaining nucleus or implant to hydrate, mimicking the biophysical function of diurnal variation in normal disks.⁸ Theoretically, less adjacent level disease than seen with other treatment methods could be realized if normal height and function of the disk could be restored. Ideally, greater long-term clinical outcomes would be achieved.

The ideal artificial nucleus has several basic requirements.¹³ Multiple surgical techniques should be possible for any given nucleus replacement procedure based on specific patient indications. Ideally, the procedure should involve a small annular incision, decreasing the risk of migration of the implant, even when large annular defects are present. The implant should have good conformity to the superior and inferior end plates, improving the loading characteristics. The procedure should allow for a minimally invasive or percutaneous-type technique to be implemented and at the same time be technically practical. In the face of a postoperative clinical failure, the revision strategy should be safe and reliable and should allow for different surgical solutions, such as total disk arthroplasty or arthrodesis.

Surgical Approaches

Current surgical approaches to the lumbar disk include posterior laminotomy, anterior retroperitoneal, trans-psoatic, extraforaminal, open, and percutaneous. Each approach has its own clinical indications and advantages. The technique of prosthetic implantation should be adaptable to the clinical need to address the patient’s pathology. If the patient has a typical posterolateral extruded herniation requiring excision, the prosthetic nucleus would ideally be implanted by a posterior laminotomy approach while limiting further disruption of the annular defect. Anatomically, the area for placement is more constrained in the posterior or percutaneous application than in the anterior retroperitoneal. This limitation makes the nuclectomy as well as implantation more difficult. It is difficult to envision that a preformed nucleus replacement device of a fixed geometry would prove to be successful as a stand-alone device because placement requires a commensurate annular opening. If this were inserted without bony fixation, it is quite likely that the preformed device would extrude through the annulotomy defect. The annulotomy defect created for placement of a device or a preexisting large annular defect may be associated with extrusion. Certain prostheses may have the additional requirement of repair of the annular defect for them to be safely applied.

Postoperative Results

Revision surgery of a nucleus replacement prosthesis will be driven by the surgical approach of the index procedure. Epidural scarring during repeat posterior surgery will be challenging. Although technically demanding, revision could be accomplished by an experienced surgeon without undue patient morbidity. It may require the prosthesis to be fragmented within the disk space to avoid undo neural retraction during explantation. Repeat anterior surgery may require mobilization of the great vessels. However, if the initial retroperitoneal approach did not require mobilization of the great vessels, such as would occur with a small annulotomy, this revision could be accomplished with less risk than after a total disk arthroplasty. The potential for catastrophic bleeding in this approach needs to be considered.15,16 At some point in time, clinical failure may occur and whether a disk prosthesis could remain in situ as an anterior spacer while pain relief is achieved by adding a posterior fusion or other posterior stabilization remains unknown.

Restoration of the nuclear load is a basic principle of nucleoplasty.^8,13 This load is created by the interface of the nucleus implant with the vertebral end plates and annulus. A device with greater conformity to the end plates with large surface area coverage should result in more balanced, even loading. This likely will result in less bony reaction and postoperative modic changes. This is best accomplished by forming the implant in situ or allowing a preformed device to deform in situ under the influence of load, as with certain hydrogel prostheses.

Design Implications

There are additional biomechanical and biomaterial issues that arise with respect to artificial nucleus prostheses. Implants utilized to achieve an arthrodesis need to withstand load without failure until an arthrodesis is solidly achieved.17 At that time the device becomes partially off-loaded as the fusion mass begins to share the load. However, when the goal is to maintain motion, there will be ongoing, continued load and shear across the device. This creates unique biomechanical requirements for the nucleus prosthesis. The ideal nucleus replacement should have a similar modulus of elasticity as the intact nucleus. A prosthesis with a modulus that is too low will have a high incidence of extrusion and may not reestablish normal biomechanical loading. A nucleus replacement with a relatively high modulus of elasticity would be associated with increased load on the end plates and could lead to subsidence or other untoward effects. Fernström reported this issue with the use of the stainless steel ball endoprosthesis.¹⁸ In this study, he demonstrated that 88% of the implants were found to have loss of vertebral disk height over time.

The interface between the implant and the end plates is critically influenced not only by the modulus and contouring of the device but by the contact area and by the coefficient of friction between the device and the end plates. The goal of minimal end plate wear would be enhanced if the ideal prosthesis distributed the forces evenly over a large percentage of the surface area of the end plates. Likewise, a lower coefficient of friction between the implant and the end plates would be expected to lead to less end plate or prosthetic wear. An additional benefit of improved implant end plate conformity is likely a diminished risk of implant extrusion.

Clinical Assessments

Nucleus replacement devices will require clinical studies with standard outcome measures, including visual analog pain scales, Oswestry disability scores, and the short form (SF)-36. The patients will need to be followed for some time to assess the long-term clinical outcome and potential late complications. In addition to clinical evaluation of patients, it will be equally important to follow individuals with appropriate imaging. Because end plate changes are most readily seen on magnetic resonance imaging (MRI), this documentation should be included in the routine follow-up. It is likely that the U.S. IDE studies will be randomized against total disk arthroplasty. Unfortunately, the MRI implant device artifact associated with total disk replacement is such that postoperative MRI will not allow for this comparison in prospective studies. Whether postoperative Modic changes correlate with any clinical outcomes will be an interesting question. Ideally, an implanted prosthesis should not lead to MRI observations of end plate changes.

One of the most challenging components of clinically studying a nucleus replacement prosthesis is the proper selection of the comparative control group. Because the nucleus replacement would ideally be placed in a less invasive fashion than TDA or arthrodesis, the control group should not inherently have an experimental bias of dramatically greater surgical morbidity. Anterior stand-alone interbody cages can potentially be implanted with less morbidity than, for example, arthrodesis of both the anterior column and the posterior elements (a so-called 360 degree fusion). In earlier disk degeneration with a relatively tall disk space, reliability and clinical outcomes of stand-alone interbody cages are somewhat questionable. In the United States, randomization against a device not approved by the Food and Drug Administration (FDA) is not likely to receive approval. Currently the Charité total disk arthroplasty device (DePuy Spine, Raynham, MA) is commercially available. However, it is still early in its clinical application in the United States, and long-term outcomes in well-controlled populations are not available. Thus randomization against a total disk arthroplasty device could also be problematic.

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