Joint degeneration is a common problem that can occur in a variety of joints throughout the human body. The condition typically is more prevalent as the skeletal system ages and is often treated with medications and/or physical therapy. These conservative treatments sometimes meet only limited success. If unsuccessful, the patient typically will continue to experience ongoing pain and limited mobility.
Often the treatment progression leads to a total joint replacement. These replacements have been performed for years in joints such as the hip and the knee. The replacement devices usually comprise some form of a metallic structural component or endplate with an intermediate polyethylene core. It is not unusual for replacements such as these to give 15-20 years of service before requiring some degree of revision.
In the spine, the surgical treatment of choice has been fusion for the treatment of intervertebral disc degeneration. The spinal intervertebral disc is situated between the vertebral bodies. The spinal disc is comprised of a tough outer ring called the annulus, and a jelly-like filling called the nucleus. The belief has been that removing the diseased spinal disc(s) and fusing between affected levels will not make a significant difference in the overall mobility of the spine. However, spinal fusion has proved to cause an increase in degeneration at other vertebral levels that must compensate for the loss of motion at the fused level commonly causing the patient to relapse into more pain and limited mobility.
Recently, there has been a focus on the use of “motion preservation” implants over implants that promote spinal fusion. These motion preserving implants, in the form of joint replacements in the spine, show promise to alleviate many of the problems associated with fusion devices in the spine. Intervertebral disc replacement devices are seen today typically comprising a pair of biocompatible metal plates with a polymer or elastomeric core, or a metal plate articulating on a metal plate. Some known implants incorporate springs to provide shock absorption or resistance to compression of the joint in a direction normal to the transverse axis of the spine. However, current motion preserving implants have certain disadvantages relating to range of motion, stability, and comfort.
Load-bearing implants, and in particular, intervertebral implants that preserve motion between the vertebral bones in the joint have been found to be preferable to fusion members in many circumstances. Specifically, to more closely mimic a healthy, natural intervertebral joint, such implants provide not only for polyaxial rotation between the vertebrae, but also for small amounts of translation therebetween. This translation is sometimes required when the vertebral joint is in extension, such as when the spine is bent backwards. Previously known implants developed to allow for both polyaxial rotation and translation have drawbacks, including high wear rates, unnatural or unsafe dynamics, and complex implantation and removal procedures. The following embodiments overcome these and other drawbacks of currently known implants.