Lumbar Total Disc Arthroplasty

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 annulus fibrosus resists shear forces and contains the nucleus. A normal disc in the lumbar spine bears 80% of compressive loads. It is subjected to one 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. 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 rostral end plate but varies slightly with flexion and extension.

With disc degeneration, the nucleus pulposus loses water content and becomes less compliant, leading to collagen degeneration and fissures in the annulus. 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. This process results in eventual disc space collapse, foraminal narrowing, facet degeneration, soft tissue hypertrophy, and compression of neural elements.

The traditional gold standard operative treatment for symptomatic DDD in patients who fail conservative therapy with persistent functionally incapacitating pain is arthrodesis of the affected segment and decompression of stenosis, if needed. In theory, fusion targets the pain associated with lumbar disc degeneration by removing the disc material and eliminating motion at the affected segements. This may be accomplished using a variety of approaches and fixation options. Arthrodesis has been performed since 1911, with fusion rates exceeding 90% in some reports of patients with DDD. However, fusion surgery in the setting of lumbar DDD remains controversial because of concerns regarding clinical benefit, pseudoarthrosis and ASD. Evidence of ASD is observed after 30% to 40% of fusions, ^, likely caused by altered spinal biomechanics. One potential advantage of arthroplasty is that it allows each patient to find his or her own balance and maintain motion. The rationale for lumbar TDR is to remove the painful disc while maintaining or restoring motion, thereby reducing the risk of adjacent segment degeneration.

A successful TDR design must take into account multiple factors, including the multidirectional angular and translational range of motion, the variable center of rotation, and the need for fixation without significant subsidence. Currently available TDRs in the United States do not attempt to replace the normal viscoelastic disc structure but instead replace it with a sliding rotational joint without inherent elasticity. The implant materials must be durable without accruing significant wear. However, newer designs not yet available in the United States incorporate a viscoelastic component. These single-piece implants contain deformable elastomeric cores that mimic the shock absorption of a normal nucleus pulposus.

Indications

As with many other spine operations, proper patient selection is one of the most important factors for successful TDR. The most common indications are listed in Box 125.1 . The 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) and those with inadequate bone stock as quantified by evaluating bone mineral density (BMD). Bertagnoli and colleagues found that in carefully selected patients older than 60 years of age, TDR resulted in significant clinical improvement and high patient satisfaction rates. Even in this group of 22 patients, however, there were two cases of radiculopathy attributed to circumferential stenosis and two cases of implant subsidence. DDD must be the main, if not the only, source of back pain. Because most cases of low back pain from DDD resolve with nonoperative treatment, surgical candidates must have failed those options for at least six months. TDR candidates should also have a history and physical findings indicative of activity-related back pain that worsens with loading and flexion. Significant pain and disability should be present to justify the potential risks and recovery period associated with operative intervention. Also, they must have failed exercise-based therapy, medication therapy, injections, and activity modification. Because current TDR implants are typically implanted via an anterior retroperitoneal surgical approach, potential contraindications include prior retroperitoneal surgery, retroperitoneal radiation, and morbid obesity with a markedly protuberant abdomen.