Cervical artificial disc replacement: still experimental? Introduction and perspectives on cervical artificial disc replacement.

Fernstrom spherical endoprostheses, utilized in the 1960s, may have been the first artificial discs implanted in humans. These were non-articulating, ball-bearinglike devices that replaced discs and preserved motion. In 1966, Fernstrom reported the use of these devices in the cervical, as well as the lumbar spine1 (Fig. 1). Fig. 1 Fernstrom cage: this implant is widely considered to be the first disc implant and was a simple ball bearing adapted from use in the cervical and lumbar intervertebral discs. Modern cervical artificial disc replacement (C-ADR) first made its debut in 1991, with the Bristol/Cummins disc, the first of numerous articulating C-ADR devices (Fig. 2). The original devices were implanted by Cummins in 20 patients, who later reported that some continued to function well, up to 12 years after implantation.2 Results from these early articulating mechanical devices were encouraging, enough that further interest in these devices appeared very reasonable. Fig. 2 Cummins device: this design introduces an articulated coupling attached to the vertebral bodies with plate and screw fixation. It was modified and reintroduced as the Prestige II disc. Over the past 20 years, the number of cervical arthroplasty devices has proliferated and now are becoming available in various shapes, materials and biomechanical concepts. Current design concepts include metal-on-metal, metal-on-plastic, non-articulating metal bonded to plastic and plastic encased in cloth. The initial cervical arthroplasty devices implanted in the United States are now a decade old. While there are a few isolated reports of osteolysis, infection and implant failure due to wear, for the most part, these devices seem to have withstood the test of time, at least in the intermediate term. Whether they will continue to fare well and have longevity comparable to the initial hip arthroplasty devices remains to be seen. Similar to large joint arthroplasties performed on hips and knees, the differences in outcomes and revision rates for spinal arthroplasty will likely not be measurable in increments of a few years but will manifest themselves after decade-long intervals. While there are interesting similarities to the concept of hip and knee replacements, there are important differences to consider: Unlike hips and knees, disc arthroplasties are not synovial joints. Unlike hips and knees, a reasonable surgical alternative does exist—fusion surgery. Unlike hip and knee surgery, continued motion in an intervertebral segment may lead to a recurrence of symptoms by overgrowth of bone and fibrous tissue into adjacent neural elements. Unlike hip and knee replacement, cervical discs are encased in a chain of somewhat interdependent adjacent motion segments, thus increasing the focus on individual disc mechanics. All of these considerations are important as the more long-term success of these devices will be considered. In the short term (arbitrarily defined as less than 2 years), C-ADR has certainly provided satisfactory results in all regards. As the perspective changes to longer-term follow-up factors such as reoperation rates, adjacent segment disease and patient outcomes will be increasingly scrutinized. Short- and intermediate-term results of C-ADR certainly have been encouraging enough compared to fusion procedures to warrant greater acceptance of arthroplasty as a surgical alternative for suitably selected patients. Nonetheless, and in the face of actual published evidence, there remain significant obstacles around the world to greater utilization of C-ADR in symptomatic cervical disc disease especially from several governmental institutions and third-party payor sides. The counterarguments to disc arthroplasty are usually based upon declarations stating that these procedures are ‘investigational’ or ‘experimental’ despite a growing body of evidence to suggest that C-ADR is indeed ‘safe and effective’ (Figs. 3,​,44,​,55). Fig. 3 Prestige II disc replacement. This is a stainless steel metal-on-metal bearing articulating device with screw fixation into the vertebral body through the anterior plate. Fig. 4 The Bryan disc is a composite device with articulating polyurethane ring within a saline-filled chamber. It relies on bone ingrowth along its porous-coated end plates for final fixation. Fig. 5 The Prodisc-C is a chrome cobalt metal device which is secured into the host vertebrae by a keel. It features a semi-constrained design with an ultra-high density polyurethane dome articulating in an oval-shaped metal cup.