Lumbar Total Disc Arthroplasty

Summary of Key Points

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Lumbar total disc replacement (TDR) has been studied extensively since its initial use in the 1980s in Europe. Results of long-term follow-up studies in Europe as well as multiple prospective, randomized trials in the United States have found TDR to produce good outcomes that are similar or superior to fusion.
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There are several concepts important to understanding the biomechanical function of TDRs, including degrees of freedom, constraint, rotation, and translation. TDRs can be classified in terms of the characteristics they possess and how these items affect the motion pattern of each implant design.
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One of the potential benefits of TDR is maintaining motion and thereby reducing the stress on adjacent segments. The literature supports lumbar TDR as having a protective effect in reducing the occurrence of adjacent segment degeneration compared with fusion. One study found that adjacent segment degeneration was reduced if there was at least 5 degrees of motion at the TDR level.
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Clinical outcomes and risks of complications related to lumbar TDR can be optimized by rigorously employing appropriate patient selection criteria. Of particular note, bone quality and the condition of the facet joints must be evaluated.
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Several studies, each using very different methodologies, have investigated the costs of TDR to fusion. Results have consistently found TDR to be less expensive or similar in cost.

Lumbar degenerative disc disease (DDD) is a common cause of low back pain—the most frequently encountered complaint in a primary care physician’s office. In most cases, DDD can be successfully treated nonoperatively with physical therapy, medications, and lifestyle modifications. Surgical treatment for refractory cases classically involves fusion of the affected spinal segment to eliminate motion and the pain the motion generates. An alternative to fusion is replacement of the painful disc with an artificial disc. Currently, one lumbar disc replacement device has been approved by the U.S. Food and Drug Administration (FDA) and marketed, ProDisc-L (DePuy Synthes, Raynham, MA). Multiple other designs are in trial and development stages or being used in other countries. Extensive research has been published on outcomes of lumbar disc replacement, with studies with 5- to 10-year follow-up periods and studies of more than 10 years available in Europe.

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 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. 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.

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 gold standard for operative treatment for DDD in patients who fail conservative therapy with persistent functionally incapacitating pain is arthrodesis of the affected segment and decompression of stenosis, if needed. This may be accomplished through a dorsal approach, ventral approach, lateral approach, or a combination. Fusion 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%. Evidence of adjacent-segment degeneration is observed in 30% to 40% of fusions. There is definitely an alteration in the balance of the spine after fusion. The optimal position of fusing the spine is not known but one potential advantage of arthroplasty is that it allows each patient to find his or her own balance and maintain motion.

The lumbar disc’s structure and biomechanical characteristics are complex due to the segmental motion and the correlation of different centers of rotation throughout the spine. A successful total disc replacement (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 ingrowth into vertebral end plates without significant subsidence. Most current TDRs do no attempt to replace the normal viscoelastic disc structure but instead replace it with a sliding rotational joint without inherent stiffness. The implant materials must be durable without accruing significant wear. However, there are newer discs that incorporate a viscoelastic component.

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 184-1 . 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 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 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 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 6 months. TDR candidates should also have a history and physical findings indicative of activity-related back pain that worsens with loading and flexion. They must have significant pain and disability to justify the potential risks and recovery period associated with operative intervention and having failed exercise-based therapy, medication therapy, injections, and activity modification. Because the majority of current TDR implants are designed for a ventral surgical approach, patients must be able to tolerate such an approach from the standpoint of prior retroperitoneal surgery and medical comorbidities including morbid obesity.

Box 184-1

Common Indications and Contraindications for Lumbar Total Disc Arthroplasty

Indications

Age 18 to 60 years
Symptomatic DDD L3-4 to L5-S1 with or without confirmatory discography
Failure of nonoperative therapy for at least 6 months
Bone density dual energy x-ray absorptiometry (DEXA) T-score > −1.0
Previous lumbar surgery if facet joint is not compromised
No previous retroperitoneal approach
No significant arterial calcification
Recurrent disc herniation contiguous with disc space with significant low back pain in a patient with no other contraindication who would be considered to be a fusion candidate

Absolute Contraindications

Poor bone quality (e.g., osteoporosis, osteopenia, metabolic bone disease, tumor)
Severe facet degeneration
Spondylolisthesis and spondylolysis
Circumferential stenosis
Scoliotic deformity > 11 degrees
Current or past trauma to involved vertebrae
Morbid obesity
Infection
Autoimmune disorder

Relative Contraindications

Age older than 60 years
Psychosocial disorder
Multiple degenerative disc disease levels
Obesity

TDR implants rely on fixation to vertebral end plates and bony ingrowth. Therefore, the patient’s bone quality must be adequate as quantified with a preoperative BMD evaluation with a T-score of at least −1.0. 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, particularly the facet joints. Radiographic studies and facet blocks may be used to diagnose facet arthropathy. Even though ventral insertion of a TDR 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 TDR increases rotational instability of the spine. The currently available unconstrained disc replacements do not fully restore this stability. Therefore, TDR in a spine with preexisting rotational instability (Cobb angle > 11 degrees) might be expected to result in higher failure rates.

Current Designs

Although to date only two TDR implants have been approved by the FDA, there are multiple designs in various stages of testing and development and in use in other countries. The Charité is the oldest and one of the most extensively studied current-generation TDR designs. The first version, SB Charité I, was first implanted in 1984. The device underwent two modifications and has been used in its current design, SB Charité III, since 1994. It was first implanted in the United States in March of 2000. It consisted of two cobalt-chromium end plates with a biconvex sliding central core of ultra-high molecular weight polyethylene (UHMWPE). This resulted in a floating center of rotation, allowing angular motion and translation, making it unconstrained. To encourage bony ingrowth, the end plates were 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. The implant was inserted using a standard ventral retroperitoneal approach and was approved for single-level use in 2004. The Charité was voluntarily taken off the market in the United States in September 2011 (as part of a business decision, not due to any problems with the device). That leaves only the ProDisc L on the market in the United States ( Fig. 184-1 ). It was developed in 1990 in France and has undergone one design revision. It was first implanted in the United States in October 2001. It also contains two cobalt-chromium end plates, but the UHMWPE insert is monoconvex and locks into the distal metallic end plate. This results in a ball-and-socket joint that limits translation and allows rotation, making it a constrained device in terms of translation. A central keel on the end plates and plasma-sprayed titanium coating allow for bony fixation and ingrowth.

Other TDRs are FlexiCore (Stryker Spine, Allendale, NJ), withdrawn by Stryker, Maverick (Medtronic Sofamor Danek; Memphis, TN) and not marketed in the United States due to patent issues, and the Kineflex-L (SpinalMotion, Inc., Mountain View, CA), which was withdrawn voluntarily by the sponsor. These products have end plates with metal-on-metal, ball-and-socket articulations that limit translation and allow rotation. Another product is the Activ-L (Aesculap, Center Valley, PA; Fig. 184-2 ) consisting of two metal end plates with a polyethylene insert using a ball-and-socket articulation, which is currently in review by the FDA. The Activ-L is inserted as a single unit and therefore does not require distraction as the ProDisc-L implant for fixation of the UHMWPE insert. This difference in insertion method is intended to reduce the risk of overdistraction. Also there are the M6-L (Spinal Kinetics, Sunnyvale, CA), the Mobidisc (LDR Medical, Troyes, France), and Freedom (Axiomed, Garfield Heights, OH) implants that are being used in Europe, and some of these are being evaluated in FDA trials in the United States. Key features of current TDR designs are provided in Table 184-1 .

TABLE 184-1

Current Lumbar Total Disc Replacement Designs

Implant	Manufacturer	Key Features	Approved Use
SB Charité	DePuy Synthes	Two metal end plates with biconvex mobile with unconstrained motion and UHMWPE insert	Single-level lumbar disc replacement (discontinued in September 2011)
ProDisc-L	DePuy Synthes	Two metal end plates with fixed uniconvex UHMWPE insert, ball-and-socket articulation with fixed center of rotation and constrained motion	Single-level lumbar disc replacement
Maverick	Medtronic Sofamor Danek	Two metal end plates with metal-on-metal articulation ball-and-socket type with constrained motion	Withdrawn due to patent issues
FlexiCore	Stryker Spine	Two metal end plates with metal-on-metal articulation ball-and-socket type with constrained motion	Withdrawn by Stryker
Kineflex	SpinalMotion	Two metal end plates with metal-on-metal articulation and semi-constrained motion	Withdrawn by sponsor
ActivL	Aesculap	Two metal end plates with UHMWPE insert with constrained motion	Single-level lumbar disc replacement
Freedom	Axiomed	Two titanium bead-coated end plates with rails for short-term fixation, notched end cap for device positioning with viscoelastic polymer core	FDA IDE study data collection is ongoing
M6-L	Spinal Kinetics	Two metal plates with UHMWPE artificial annulus and artificial polymer nucleus and viscoelastic motion	Not yet studied in the US but used in Europe
Mobidisc	LDR	Two metal end plates with self-centered UHMWPE creating an unconstrained motion	Being implanted in Europe