Background

The intervertebral disc is a complex structure that plays a key role in range of motion and load transfer in the lumbar spine. The nucleus pulposus absorbs compressive loads, whereas the anulus fibrosus resists shear forces and contains the nucleus. A disc in the normal lumbar spine bears 80% of compressive loads. It is subjected to 1 to 2.5 times body weight on ambulation and up to 10 times body weight when lifting a heavy load. The lumbar disc also allows for rotation and translation in three orthogonal planes. The characteristics of motion vary according to the level, with more rotation occurring in the upper lumbar spine and more flexion and extension in the lower lumbar spine. The center of rotation in the sagittal plane is usually located dorsal and caudal to the center of the distal end plate, but varies slightly with flexion and extension.¹

With disc degeneration, the nucleus pulposus loses water content and becomes less compliant, leading to collagen degeneration and fissures in the anulus. Inflammatory cytokines are released from the nucleus and sensory nerve fibers proliferate deeper into the disc space, resulting in discogenic pain. The disc’s biomechanical characteristics also become altered. As the disc becomes more rigid and loses height, more stress is transferred to the facet joints. The natural history of this process results in eventual disc space collapse, foraminal narrowing, facet degeneration, soft tissue hypertrophy, and compression of neural elements.

The gold standard of operative treatment for DDD in patients who fail conservative therapy is arthrodesis of the affected segment and decompression of stenosis, if needed. This may be accomplished through a dorsal approach, ventral approach, or a combination of the two with or without iliac crest autograft. The procedure has been performed since 1911 and carries success rates of between 60% and 90%. This treatment, however, has several significant drawbacks. Pseudarthrosis rates are reported at 14%.² Evidence of adjacent-segment degeneration is observed in 30% to 40% of fusions.^3,4 Iliac crest autograft harvesting results in significant rates of postoperative complications and donor site pain.⁵

Given the limitations of the established operative options for patients with DDD and considering the very high and reproducible success rates in arthroplasty of hip and knee joints, the need for an effective total disc replacement (TDR) alternative in the lumbar spine becomes obvious. The lumbar disc’s structure and biomechanical characteristics are very complex. A successful TDR design must take into account multiple factors, including the multidirectional angular and translational range of motion, variable center of rotation, and need for ingrowth into vertebral end plates without significant subsidence. The implant materials must be durable enough to sustain approximately 8 million cycles per year without accruing significant wear.

Indications

As with many other spine operations, proper patient selection is one of the most important aspects of a successful TDR. The most common indications used in current disc replacement surgeries are listed in Box 161-1.⁶ Typical indications are illustrated in Figure 161-1. The vast majority of patients who qualify for TDR are younger than 60 years of age. This excludes most patients with degenerative processes in dorsal spinal structures (e.g., facet degeneration, ligamentum flavum hypertrophy, disc herniation) and with inadequate bone stock. Bertagnoli et al. have shown that in a carefully selected group of patients older than 60 years of age, a TDR procedure resulted in equivalent patient satisfaction rates.⁷ Even in this group of 22 patients, however, there were 2 cases of radiculopathy due to circumferential stenosis and 2 cases of implant subsidence. DDD must be shown to be the main, if not the only, source of the back pain. Positive radiographic findings and a concordant discogram are currently thought to be the best ways to confirm DDD. Because most cases of low back pain from DDD resolve with nonoperative treatment, surgical candidates must have failed those options. They must have significant pain and disability to justify the potential risks and recovery period associated with operative intervention. Because current TDR implants are designed for a ventral surgical approach, patients must be able to tolerate such an approach from the standpoint of prior surgical interventions in that area and medical comorbidities.

FIGURE 161-1 Indications for lumbar arthroplasty: typical case example. A, A 47-year-old male executive who has been disabled for 9 months due to lumbar spondylosis at L4-5, with mechanical back pain radiating down the posterolateral aspect of his right thigh and an Oswestry Disability Index (ODI) score of 55. B, Sagittal MRI shows single-level disc disease at L4-5, Modic type II changes. His discogram was also positive for concordant pain when his L4-5 disc was stimulated, but not when L3-4 or L5-S1 were injected. C, He underwent In Motion lumbar disc arthroplasty (the updated version of the Charité arthroplasty) with complete relief of his mechanical back pain (postoperative ODI = 5). He has returned to work full time. The lateral radiograph demonstrates good restoration of disc space height, which is uniformly successful with many types of arthroplasty. D, Anteroposterior radiograph of the In Motion Charité arthroplasty shows an ideal midline position of this nonkeeled type of lumbar disc replacement.

TDR implants rely on fixation to vertebral end plates and ingrowth. Therefore, the patient’s bone quality must be adequate. Osteoporosis, osteopenia due to a metabolic disorder, or tumor may cause implant failure by dislodgement or subsidence. Because lumbar TDR replaces only ventral elements of the spinal column, a good candidate for disc replacement should not have degeneration of dorsal structures, especially the facet joints. Radiographic studies and facet blocks may be used to diagnose facet arthropathy. Even though ventral insertion of a TDR device may offer some indirect decompression, circumferential stenosis is best treated by direct decompression of neural elements through a dorsal approach with fusion, if needed. Studies have shown that the disc space preparation necessary for insertion of a lumbar TDR increases the rotational instability of the spine. The currently available unconstrained disc replacement implants do not fully restore this stability.⁸ Therefore, TDR in a spine with preexisting rotational instability (Cobb angle >11 degrees) might be expected to result in higher rates of failure.

Current Designs

Although only two TDR implants are currently approved by the FDA, there are multiple designs in various stages of testing and development. Charité is the oldest and one of the most extensively studied of the current-generation TDR designs. The first version, SB CHARITÉ I, was first implanted in 1984. The implant underwent two modifications and has been used in its current version, SB CHARITÉ III, since 1994. It consists of two cobalt-chromium end plates with a biconvex sliding central core of ultra-high molecular weight polyethylene (UHMWPE). This results in a floating center of rotation, allowing angular motion and translation. To encourage bony ingrowth, the end plates are covered with plasma-sprayed titanium and electrochemically coated with calcium phosphate. This has been shown in animal studies to result in 48% osseointegration, compared with the 10% to 30% ingrowth seen in successful hip and knee replacement prostheses.^9,10 The implant is inserted using a standard ventral retroperitoneal approach and is approved for single-level use.

Another FDA-approved TDR implant, the ProDisc L, was originally developed in 1990 and has undergone one design revision since then. It also contains two cobalt-chromium end plates, but the UHMWPE insert is monoconvex and locked into the distal end plate. This results in a ball-and-socket joint that limits translation and allows rotation. A central keel on the end plates and plasma-sprayed titanium coating allow for bony fixation and ingrowth. Two other TDR implants designed for a ventral approach are currently in FDA Investigational Device Exemptions study stages: FlexiCore (Stryker Spine, Allendale, NJ) and Maverick (Medtronic Sofamor Danek, Memphis, TN). Both products have end plates with metal-on-metal, ball-and-socket articulations. Key features of current TDR designs are listed in Table 161-1.

Because of the known complications that have occurred during device implantation, including dislodgement and the need for revision surgery using the ventral approach, there is extensive work being done on TDR designs implanted by other means.¹¹