Degenerative disk disease (DDD) is one of the leading causes of pain and disability in adults in the United States.1 Approximately 80% of Americans have at least one episode of low back pain (LBP) at some point in their lives.1,2 This carries with it a large socioeconomic impact with respect to health care dollars and lost days of work. The development of new technology is driven, in part, by the opportunity to potentially and significantly alter the natural history of spinal disease, as well as to improve long-term patient outcomes and reduce health care costs. DDD is often successfully managed nonsurgically. However, a small percentage of patients fail to respond to conservative treatments such as physical therapy, anti-inflammatory or analgesic medications,
modification of activities, therapeutic spinal injections, and weight loss. Surgical intervention is recommended for those patients with LBP for 6 months or more and who have exhausted nonoperative treatments. Lumbar fusion continues to be the gold standard for surgical treatment of intractable axial back pain secondary to DDD.1,3,4
Fusion relieves pain by restricting motion. Fusion surgery does not always provide adequate pain relief, however, and other complications associated with the procedure include pseudarthrosis, development of adjacent-segment disease, and autograft donor site complications.5 It is unknown whether these postoperative problems are related to spinal fusion or simply represent the natural progression of the patient's pre-existing disease.6 Biomechanical studies have shown that rigid immobilization of one segment of the spine increases stress to the segments above and below the fused segment. This increased workload may factor into accelerated degeneration or instability of the adjacent segments.1,2 Additionally, radiographic evidence of fusion success does not always correlate well with clinical relief of symptoms and a return to normal functioning.1,2
HISTORY OF DISK REPLACEMENT
Although total disk replacement is currently a hot topic, the concept dates back to the 1950s.5,7,8 Early attempts at constructing artificial disks included an acrylic substance inserted into the disk space after diskectomy, a fluid-filled elastic chamber with metal endplates, and a Dacron mesh-covered silicon disk. From the late 1960s through the late 1980s, new prostheses included a nuclear replacement using a stainless steel ball, a polyolefin rubber between two titanium plates, and a posterior- hinged metal prosthesis with interposed titanium springs.1,7
The goal of modern artificial disk replacement (ADR) technology is to maintain spinal segment motion and prevent adjacent level degeneration.4,9 Artificial disks are designed to restore intervertebral disk-space height and maintain proper sagittal balance, thereby reproducing the biomechanical properties of a normal disk with a satisfactory range of motion (see Figure 1). Achieving these goals should eliminate pain and improve functional ability. Newer artificial disks possess long-term durability and stability.1 Four lumbar spine arthroplasty devices are available in the United States. The Charité artificial disk (DePuy Spine, Inc) and the ProDisc lumbar spine prosthesis (Synthes, Inc) are metal-on-plastic articulating surface devices. Both have received FDA approval. The Maverick lumbar prosthesis (Medtronic Sofamor Danek, Inc) and the Flexicore lumbar prosthesis (Stryker Spine) are metalon- metal articulating surface devices. These two devices are currently being studied in FDA investigational trials.1
DISK ANATOMY
A functional spinal unit consists of the two adjacent vertebrae and the intervertebral disk as well as the spinal ligaments that hold the unit together.10 The lumbar motion segment is a three-joint complex comprised of a disk and two facet joints.10-12 The interaction of these three articulations allows for normal motion of the lumbar segment including flexion/ extension, lateral bending, axial rotation, and axial compression. 13 Arthroplasty devices are classified by their ability to replicate the range of motion for each of these modes. A device that allows hypermobility beyond the normal physiologic range is classified as unconstrained for that mode. A device that allows unrestricted motion within the normal physiologic range is classified as semiconstrained for that
mode. If the device has mechanical restrictions within the normal physiologic range, it is classified as constrained for that mode. None of these four devices allow axial compression; therefore, they are classified as constrained for that mode of segment motion.1
The Charité artificial disk consists of two concave, cobaltchrome- molybdenum (CoCrMo) alloy endplates that articulate with a convex, high-molecular-weight polyethylene (PE) sliding core. A radiopaque wire surrounds the core, enabling the core to be seen on radiographs. Fixation teeth on each endplate secure the device to the vertebral body endplates. The Charité comes in seven endplate footprint sizes, four lordotic angles, and five core heights, allowing the surgeon to match the device to the patient's anatomy.7,14,15 This device is classified as unconstrained for flexion, extension, lateral bending, and axial rotation.5,11
In 1997, Lemaire and colleagues reported excellent results in 79% of 105 patients who received this device, with a mean follow-up of 51 months.7 The FDA investigational device exemption (IDE) trial, completed in December 2003, compared the Charité artificial disk with standalone Bagby and Kuslich (BAK) cages with iliac crest bone graft (ICBG) harvest. At 1-year follow-up, patient satisfaction with the Charité prosthesis was 93% compared to 81% for patients who underwent BAK spinal fusion.7 A retrospective study by David demonstrated the safety and efficacy of the Charité artificial disk at the L4-L5 or L5-S1 level. Long-term clinical outcomes (minimum follow-up of 10 years) were good to excellent in 87 of 106 (82.1%) patients.15
The ProDisc-II lumbar spine prosthesis consists of two CoCrMo endplates with an ultrahigh-molecular-weight PE core fixed to the lower endplate (see Figure 2). The prosthesis endplates are affixed to the vertebral endplates by a central keel, and their nonarticulating surfaces are coated with a titanium plasma spray.11 This prosthesis is available in two end plate sizes, three core heights, and two lordotic angles. ProDisc-II is the only device that is being evaluated for the treatment of multiple segment DDD.5 This prosthesis is classified as semiconstrained for flexion, extension, and lateral bending but unconstrained for axial rotation.11
The FDA IDE trial compared the ProDisc device with an anterior femoral ring allograft and posterior pedicle screw fixation with autologous ICBG.11,16 Delamarter and colleagues reported the results of the first 78 randomized patients (56 patients who received a ProDisc-II prosthesis, 22 patients who underwent the anterior-posterior fusion procedure).16 Based on Visual Analog Scale (VAS) and Oswestry Disability Index (ODI) scores, the patients who underwent disk replacement had statistically favorable results at 6 weeks and 3 months compared to the patients who underwent spinal fusion. At 6 months and from 6 months to 2 years, the difference was not statistically significant.16 In a retrospective study comparing the clinical and radiographic outcomes of the Charité and ProDisc devices, Shim and colleagues reported no clinically significant difference in either improvement of VAS and ODI scores or clinical success rates between the two devices.17
The Maverick total disk replacement, also made of CoCrMo, has a ball-and-socket design. Both endplates have a central keel and hydroxyapatite coating. The FDA IDE trial for the Maverick, which began in May 2003, is ongoing. The randomized trial compares the device with a single-level lumbar tapered fusion device combined with a bone morphogenetic protein allograft. The Maverick prosthesis is classified as semiconstrained for flexion, extension, and lateral bending but unconstrained for axial rotation.5,11,13
The FlexiCore artificial disk also has a ball-and-socket design. This device has teeth on the outer ring of the implant for fixation to the vertebral endplates, and both surfaces are covered with titanium plasma to enhance fixation to the bone.13 US clinical trials of the Flexicore began in August 2003.13 The device is being studied in patients with singlelevel disk degeneration. The control group consists of patients who underwent a 360-degree fusion with posterior instrumentation.