Use of Nucleus Replacement for the Treatment of Degenerative Disorders of the Lumbar Spine

CHAPTER PREVIEW

CHAPTER SYNOPSIS:

Symptomatic degenerative disc disease (DDD) represents a significant socioeconomic burden on today’s society. In the past, the only alternative was discectomy with or without fusion, with inherent disadvantages. Disc arthroplasty in the form of nucleus replacement was developed to bridge the gap between these two treatment mechanisms and to provide the surgeon and patient an additional treatment option. The technology is diverse, consisting of elastomers and nonelastomers, with the clinical evidence suggesting that it is a clinically viable alternative in the treatment continuum of DDD.

IMPORTANT POINTS:

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Indicated for patients with single-level DDD with or without leg pain and with mild-to-moderate disc height loss
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Contraindicated for patients with end plate abnormalities, more than grade 1 or lytic spondylolisthesis, facet arthrosis, and incompetent annulus.
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Leaves other surgical options open such as total disc replacement and fusion.
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May be implanted via all traditional surgical approaches: anterolateral, lateral, and posterior.

CLINICAL/SURGICAL PEARLS:

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Nucleus replacement has the advantage over discectomy of maintaining disc height, which will prevent instability.
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When compared with fusion, nucleus replacement offers the advantage of preserving segmental motion, which may preserve adjacent discs.
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When compared with total disc replacement, the advantages of nucleus replacement includes being less invasive, non–bridge-burning, less revision risk, and being compatible with all traditional surgical approaches including a posterior approach.

CLINICAL/SURGICAL PITFALLS:

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The main challenge for nucleus replacement technology has been how to minimize the risk for implant extrusion.
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Because nucleus replacement devices preserve most surrounding tissues and rely on their concerted functions to restore the normal segment functions, the structure and functions of these surrounding tissues have to be grossly maintained.
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If a disc height has reduced to less than 5 mm, it suggests late-stage DDD, making distraction and subsequent proper load sharing difficult.

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Low back pain of discogenic origin represents a significant worldwide socioeconomic burden. In the past, the surgical treatment options were limited to discectomy with or without fusion. Surgeons and researchers alike have long understood the shortcomings of both these procedures, because they adversely affect the biologic and biomechanical functions of the intervertebral disc, leading to poor long-term clinical outcomes. Unlike total joints such as the hip and knee where arthroplasty is the primary treatment mechanism for degenerative changes, in the spine, fusion has been the primary treatment option for chronic low back pain of discogenic origin. Intuitively, one would like to preserve or maintain motion over fusion, and in the last 20 years, a paradigm shift has taken place with the development of disc arthroplasty, allowing the surgeon, and more importantly the patient, a safer and more effective treatment option. Although the history of disc arthroplasty can be traced to more than 40 years ago, it is only now becoming a clinically viable option for the treatment of discogenic back pain. It is hoped that with the advent of disc arthroplasty, a significant decrease in the health and socioeconomic burdens as a consequence of degenerative disorders of the lumbar spine can be accomplished. This may be achieved by treating the patient earlier in the degenerative cascade by bridging the gap between discectomy and while overcoming the clinical shortcomings of these treatment modalities.

Disc arthroplasty consists of two categories: total disc replacement and nucleus replacement. The goals of these two arthroplasty treatments are the same: relieving discogenic back pain through removing the pain source and restoring or maintaining motion segment function. However, nucleus replacement is advantageously different from total disc replacement in several aspects, while also having several distinct advantages over discectomy and fusion. Nucleus replacement technology can bridge the gap between discectomy and total disc replacement, while still preserving total disc replacement and fusion as future surgical options. The principle behind nucleus replacement technology is that, by preserving most of the existing tissues and replacing all or part of the degenerated nucleus with a synthetic nucleus, the stability and mobility of the spinal segment is preserved, and the symptoms of low back pain caused by degenerative disc disease (DDD) are relieved. In contrast with total disc replacement, instead of removing the entire disc, a full or partial nucleotomy is performed, while preserving the annulus and maintaining the integrity of the end plates. The surgical procedure is also far less invasive and can be minimally invasive, unlike total disc replacement. The indications for nucleus replacement are similar to those for total disc replacement in many aspects, with the main contraindications being large annular defects and a disc height less than 5 mm. For this reason, nucleus replacement should be performed only in mild-to-moderate cases of DDD, and can serve as an adjunct to a partial discectomy. Although the primary goal is pain relief, full or partial replacement of the nucleus can also maintain or increase the disc height and allow for the restoration of segmental motion, unlike discectomy and arthrodesis, respectively. Therefore, the advantages of nucleus replacement technology can make for a safer and more effective treatment option over the treatment mechanisms that are currently offered.

The current nucleus replacement technology platform is varied, and an efficient way of classification can be by material, which then dictates the design. The majority of the technology is composed of a wide variety of polymers, including elastomers, either hydrogels or nonhydrogels, which can be preformed or in situ cured, and nonelastomers, such as thermoplastics, pyrolytic carbon, and metal . In addition, the material and design selected may dictate the surgical approach, because it is desirable to have the benefit of being implanted minimally invasively from all approaches—anterolateral, lateral, and posterior. The ability to address pathology from a posterior approach represents a distinct advantage over the current total disc technology, which, in general, can be implanted only via a lateral or traditional anterior retroperitoneal approach. Even with the newly developed posterior approach–based total disc replacements, nucleus replacement is still a less invasive method in this aspect. Although clinical data are lacking from large, randomized, prospective studies, the nonrandomized clinical data for nucleus replacement devices available so far appear to be promising, with the main issues being extrusion and long-term biocompatibility and biodurability. The technology is, therefore, diverse, and the actual benefits may be dependent on this diversity, with the choice of material and design potentially affecting the clinical performance of the device.

SCOPE OF THE PROBLEM

Back pain is a major worldwide health and socioeconomic burden. In the United States alone, back pain has a prevalence rate of 60% to 90% and is second only to the common cold as a reason for a physician visit, with the total costs associated with back pain ranging from $100 to $200 billion annually. In Europe, the lifetime prevalence rate of back pain has been estimated at 59% to 90%, and in any one year, the incidence rate of back pain is reported to be approximately 5% of the population. Most episodes of back pain are short. However, many have a recurrent course with further acute episodes affecting 20% to 44% of patients within 1 year in the working population, and lifetime recurrence rates are up to 85%. Low back pain of discogenic origin resulting from DDD is the most common form of back pain. If nonsurgical treatments do not bring relief after a period of time and radiographic imaging indicates nerve compression with or without axial back pain that is consistent with the historical, physical, and neurologic findings, and/or when the segment becomes unstable, surgical treatment is often necessary.

Pain from DDD may occur at any stage of the degenerative continuum: from a simple annular tear to complete disc degeneration, deformity, instability, and compromise of the neural elements. Research has shown that biochemical, histologic, metabolic, and functional changes contribute to the degenerative disorders that result in the substantial economic burden on today’s society. The genesis of these degenerative changes can occur early in childhood, and they result from decreased proteoglycan content in the nucleus and annulus, a reduction of blood supply to the vertebral end plate, and a corresponding decrease in matrix synthesis. The consequences are functional changes that result in a reduction in the hydrostatic pressure caused by a decrease in the water-binding capability of the nucleus, and consequently a stiffer and weaker annulus. The disc then begins to lose disc height and the normal load sharing that occurs is compromised, placing an increasing biomechanical demand on the annulus with a consequential imbalance in the stress distribution across the disc space. Instability of the motion segment manifested as anterior or posterior subluxation now ensues as tension in the annulus is diminished. As the annulus bears an increasing load, circumferential, radial, or peripheral rim tears can occur, resulting in protrusion, extrusion, or sequestered nucleus material. Compression of the nerve elements occurs, leading to neurologic compromise from ischemia, neurotoxicity, or both, with axial back pain occurring via stimulation of the highly innervated posterior annulus. If left untreated, a continued loss of disc height results, leading to peripheral osteophyte formation, facet arthrosis, and increased stiffness. At this point, axial back pain may lessen; however, the potential for neurologic compromise increases because of central and foraminal stenosis.

DDD is multifactorial in nature and presents itself as a difficult and challenging problem for the spinal surgeon to diagnose and treat. In the past, the selection of treatments for degenerative disorders was limited, consisting of only decompression with or without fusion. These two treatment modalities are not ideal for relieving the symptoms brought on by the degenerative cascade and, in many instances, advance this continuum. Although the short-term outcomes for discectomy can be superior to conservative care, the long-term outcomes are far from ideal. Whether the procedure is traditional or minimally invasive, ultimately a discectomy results in a loss of disc height, further destabilizes the motion segment, and predisposes the patient to multiple reoperations for recurrent herniations, contralateral herniations, or inadequate pain relief caused by spinal stenosis or facet pain. Ultimately, arthrodesis is performed as a last attempt for relief of symptoms. Arthrodesis has long been the gold standard for the treatment of chronic, disabling low back pain of discogenic origin that has failed to respond to conservative treatment, with the thought being that the pain generated by a degenerative joint is linked to its mobility. Theoretically, the elimination of this motion will result in pain relief and clinical success, but the clinical success rate of arthrodesis varies depending on the diagnosis, number of previous surgeries, prior fusion attempts, and number of levels fused. The clinical role of fusion in DDD has been controversial because of short follow-ups and nonrandomized study designs in the literature. A 20-year critical review of the literature suggests a fusion rate of 87% and a clinical successfully result of 76% for DDD. In general, arthrodesis can provide pain relief and restoration of spinal stability, but several disadvantages are inherent to this procedure. Although fusion is reasonably effective in relieving low back pain, it can significantly change the normal loading of the adjacent disc, which is thought to lead to degenerative acceleration of the adjacent level. Researchers and physicians have long realized the anatomic and biomechanical shortcomings associated with discectomy and fusion, and it was therefore critical that new, effective treatment mechanisms be developed. An effective treatment option can be nucleus replacement.

HISTORY

The clinical history of nucleus replacement can be traced back to the early 1950s with the injection of polymethylmethacrylate (PMMA) into the disc space by Cleveland and Hamby and Glaser. Although the initial data on 14 patients appeared to be encouraging, a subsequent small, randomized study using discectomy as a control found that the PMMA nucleus device provided no obvious clinical advantages over the control. The status of pain in the back, leg, or both essentially remained the same, and no differences were found in preoperative and postoperative films. It was therefore determined that, in routine cases of disc protrusions, this provided no obvious clinical benefits. Even though improved efficacy was not shown over discectomy, important observations were made in regard to this first attempt at nucleus replacement. The authors realized that the polymer would conform to the disc space and provide a broad surface for bearing weight, criteria that are included in the design of some of the present-day nucleus replacement systems.

At around the same time, Paul Harmon between 1959 and 1961 performed 13 cases in which he implanted through an anterior retroperitoneal approach Vitallium balls marketed by Austenol, which later became Howmedica. Harmon himself never published the clinical results; however, one is known to have extruded and compressed the nerve root, but on extraction, the patient became asymptomatic. Paul Harmon later went on to teach Ulf Fernström his technique, and in 1962 to 1964, Fernström used SBF (Swedish Ball Bearing Factory) stainless steel balls, now famously known as the Fernström Ball, and implanted them in 125 lumbar patients in 191 levels. In 1966, his published results conclude that, with this form of disc arthroplasty, the outcomes were better than with discectomy alone, and similar to the results for discectomy with fusion. In 1963 to 1964, Hjalmar Reitz in South Africa implanted the same prosthesis in 19 lumbar discs in 12 patients for discogenic back pain and sciatica; he reports similar results and concludes that the main problem is avoiding overdistraction. He notes that some degree of subsidence occurred but was exceedingly minimal, even at 2 years. Although this procedure of using metallic spherical balls produced acceptable clinical results, it was ultimately abandoned because of subsidence of the steel balls into the vertebral end plates. However, it should be pointed out that regardless of the amount of subsidence, the clinical outcome for the Fernström Ball, in both the short and long term, was fairly good, with more than 75% of patients reporting good-to-excellent outcomes, and range of motion being observed after the implantation. The issue of subsidence did not discourage Alvin McKenzie, and in 1969 to 1971, he implanted the Fernström Ball in another 103 patients. In 1995, he published the 17-year follow-up on 67 of the original 103 patients. He reports an 83% success rate in patients with one or more disc protrusions with sciatica, and a 75% success rate in patients with DDD or postdiscectomy states that were candidates for fusion. More importantly, 95% of the patients thought the procedure was worthwhile and returned to work. In the end, he concludes that if the patient population had been subject to present-day (1995) diagnostic techniques and instrumentation, some of the patients destined for poor and fair outcomes would have been excluded. From these early clinical studies, it is apparent that nucleus replacement could be a clinically viable alternative to discectomy and fusion.

The issue of subsidence of the Fernström Ball remains controversial, even though Fernström himself advocated subsidence as a way to avoid extrusion of the implant. The driving force for an alternative technology for nucleus replacement was probably the issue of using a material much harder than the native nucleus leading to subsidence. Naturally, it would make sense to replicate the biomechanical and biologic functions of the disc as a shock absorber and the ability of the nucleus to imbibe and release water. Therefore, the use of elastomers or materials with viscoelastic properties has been proposed or studied. The most common of these materials are silicone and polyurethane, two of the most widely used medical elastomers, but other materials have now come to the forefront for use in nucleus replacement.

Nachemson was probably the first to experiment with an elastomer for nucleus replacement. In the early 1960s, he injected silicone rubber into the degenerative discs of cadavers and noted that their biomechanical response was similar to that of younger, nondegenerative discs. This was followed by Schulman, who in 1977 reported on the posterior placement of polyurethane in 83 patients, with 62 followed for 5 years, and noted improved results in those with radiculopathy. In 1978, Fassio and Ginestié used a silicone prosthesis. After a biomechanical assessment and animal study in monkeys, Hou implanted a preformed silicone nucleus of horseshoe shape into more than 30 patients with a maximum follow up of 4 years. Unfortunately, the details of the study were not published. Throughout the 1980s, the use of hydrogels for soft contact lenses was gaining in popularity. Therefore, in 1990, the concept of using a hydrogel as a material for nucleus replacement was developed. The material, polyvinyl alcohol (PVA), demonstrated an ability to imbibe and release water similar to that of the native nucleus, and restore the biomechanical function of the index disc; also, a large animal study showed no evidence of local or systemic toxicity. Unfortunately, implant extrusion in an early clinical trial is undeniably a major reason why its further development was halted. Whether this early clinical failure is part of the learning curve because of nonoptimized instrument and/or surgical technique or a fatal fundamental design deficiency remains debatable, and unfortunately, it may never be known until a hydrogel nucleus device proves to be viable clinically. However, the proven preclinical benefits of this hydrogel acted as a springboard for other nucleus replacement technologies.

The prosthetic disc nucleus (PDN) is the most well-known nucleus replacement device, with the first clinical use in 1996. It also consists of a hydrogel (Hypan [Kingston Technologies, Inc. Dayton, New Jersey]), which is enclosed in a polyethylene terephthalate jacket to restrict expansion on hydration. Although it has the most clinical use worldwide, it has yet to see ultimate clinical success as evidenced by multiple design changes, the development of the ALPA (AnteroLateral transPsoatic Approach) approach, and several failures in U.S. Food and Drug Administration (FDA) pilot studies, mainly because of implant extrusions. It has seen several design evolutions from a two-piece design with and without sutures, a single pillow with gross end-plate matching geometry, to now a single pillow with a softer hydrogel (HydraFlex [Raymedica Inc. Minneapolis, MN]). The lack of clinical success leading to these design changes has been because of the reported high extrusion rate of up to 36% and, to a lesser extent, incidences of subsidence as a consequence of end plate remodeling leading to poor clinical outcomes. HydraFlex apparently has more distinct design features that provide a more anatomic contoured device for greater fit and fill, a softer core with a larger footprint to potentially reduce the risk for subsidence, and faster hydration to allow faster stabilization in comparison with the older PND-SOLO design. It is also implanted via an ARPA (AnteroLateral Retroperitoneal Approach) approach. An Investigational Device Exemption (IDE) pilot study was initiated in June 2006.

INDICATIONS AND CONTRAINDICATIONS

Regardless of the success of preclinical testing and animal model evaluations, the efficacy of nucleus replacement devices can be determined only in human clinical trials. The indications and contraindications are largely determined by the intended objectives and the unique design of nucleus replacement devices. Although the indications and contraindications for the initial clinical study are determined a priori, it is not until at least limited clinical follow-ups are performed that a better idea of these criteria can be established. They will then be based on the outcomes of the clinical study and the risk/benefit and cost/benefit ratios. If the efficacy and safety are proved in the IDE study, the procedure may be indicated for a large portion of the discectomy population because it is designed to address the pitfalls of the discectomy procedure while not sacrificing the benefits.

The primary indication for nucleus replacement is single-level discogenic pain caused by DDD with or without leg pain. The technology is indicated for mild-to-moderate DDD—that is, a disc height of 5 mm or greater, or at least 50% of the adjacent level. A significant loss in disc height is an indicator of late-stage DDD. The annulus may have already adapted structurally and morphologically to the significant disc height loss, making distraction difficult, and on distraction, leaving it biomechanically incompetent. An incompetent annulus alone is a contraindication, because this may lead to an unacceptable risk for extrusion. Nucleus replacement devices by design are unconstrained, relying on the natural tissues to be competent enough for biomechanical restraint, and rely on load sharing with the annulus. Therefore, it may be difficult for a nucleus replacement device to correct spinal deformities such as spondylolisthesis and moderate-to-severe scoliosis or kyphosis without additional instrumentation. For instance, a kyphotic segment may be caused by a biomechanically unstable annulus that may have concentric or radial tears, disturbing the load sharing and increasing the extrusion risk. Severe end plate degeneration and abnormalities such as Schmorl’s nodes would most likely predispose the implant to subsidence, because the end plates are likely incapable of supporting the contact stress between the implant/end plate interface. End plate geometry may play a crucial role also, because a small percentage of the population has double recessed, convex, and hyperconcave end plates. In the case of double recessed end plates, this may increase the risk for scoliosis, and for convex end plates, this may increase the risk for extrusion. In the end, only large, randomized, controlled studies will determine to a large extent what the indications and contraindications will ultimately be. They may also be dictated by the material and device design. Table 49-1 summarizes the general indications and contraindications.

TABLE 49-1

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