Evolution of Cervical Arthroplasty

Anterior cervical diskectomy with fusion (ACDF) has been the “gold standard” treatment of central and paracentral herniated cervical disks for decades. Significant advances such as plated fixation have contributed to the increasingly high fusion rates of ACDF since Cloward first described this technique in 1958.¹ Fusion rates, already greater than 90% with allograft and anterior cervical plate fixation, now approach rates of 100% with the use of synthetic interbody grafts such as polyetheretherketone (PEEK) and recombinant human bone morphogenetic protein-2 (rhBMP-2).² In the setting of such a successful operation, one must thoroughly investigate what if any role arthroplasty should play as an alternative to arthrodesis.

During the past decade, ACDF has been found to be associated with symptomatic adjacent-segment disease.^3–5 Hilibrand and colleagues reported that the adjacent-segment reoperation rate after ACDF was 2.9% annually. Furthermore, 10 years after ACDF, 25% of patients reported symptoms caused by adjacent-segment disease.^3–5 Likewise, Goffin and coauthors reported that 92% of postoperative ACDF patients had radiographic evidence of adjacent-segment degenerative disk disease at 5 years.⁶ This adjacent-segment degeneration may be related both to the natural history of cervical spondylosis and to the biomechanical effects of fusion on the adjacent level. Matsunaga and colleagues have discussed higher shear strain occurring at levels adjacent to anterior cervical fusions.⁷ It has been postulated that cervical disk arthroplasty might reduce the incidence of this shear strain by providing motion more similar to normal neck mobility.^8–10

The earliest attempts to replace the cervical intervertebral disk with a prosthetic device began in the 1960s.¹¹ In the last decade, there has been significant growth in the enthusiasm for cervical disk arthroplasty, with early designs of these implants modeled on artificial joints implanted in the knees, hips, and shoulders.

In arthroplasty of large joints (knees and hips), implants are subjected to repetitive stress that results in wear-related debris. Tribology (study of friction, lubrication, and wear) has been a primary focus for the developers of large-joint prostheses in recent years. Excessive wear debris in large-joint arthroplasty is an issue of great concern because it may stimulate a host response consisting of macrophages and multinucleated giant cells surrounding the wear debris, which can initiate a cascade that ultimately results in osteolysis and loosening of the implant. The cervical arthroplasty is under less load than a large weight-bearing joint prosthesis, and cervical disk prostheses have articulating surfaces that may generate a small amount of wear debris during the life of the implant.

To quantify the amount of wear debris generated by these devices, wear testing has recently been performed on two different cervical prostheses—a metal-on-metal disk (Prestige Artificial Disc; Medtronic Sofamor Danek, Memphis, TN) and a polyurethane-on-metal disk (Bryan Artificial Disc; Medtronic Sofamor Danek)—under normal physiologic loads and motions. Testing was conducted to a total of 20 million cycles in a cervical spine simulator that applied the loads and motions associated with activities of daily living. It was found that the mean wear rates were orders of magnitude less than the rates of currently available large-joint prostheses. Furthermore, evaluation of a cervical arthroplasty device explanted after 3.25 years has shown the wear equivalent to be approximately 311,000 cycles of in vitro wear testing.

Nomenclature of Cervical Arthroplasty

In the 1990s, numerous new-generation artificial disks were developed. To better understand the differences between evolving generations of artificial cervical disks, the Cervical Spine Study Group developed a new nomenclature system for cervical arthroplasty. They classified artificial disks into three types: nonarticulating, uniarticulating, and biarticulating. The implant concepts included a metal-on-metal design, a metal-on-polymer (e.g., ultrahigh-molecular-weight polyethylene), a ceramic-on-polymer design, and a ceramic-on-ceramic design. These prostheses can be either modular (having replaceable components) or nonmodular (lacking replaceable components). Some are fixated with supplemental vertebral body screw, whereas others are not. In addition, some disk arthroplasty devices promote biologic bone ingrowth at the disk–end plate interface. These implants may be constrained in terms of motion, or they may be semiconstrained or unconstrained. As such, classification is based on the following criteria: articulation, material, design, fixation, and kinematics (Fig. 293-1).

FIGURE 293-1 This algorithm allows classification of cervical arthroplasty devices. These artificial cervical disks are classified by the number of articulations, the design material, and their modularity (interchangeable parts).

(From Mummaneni PV, Haid RW. The future in the care of the cervical spine: interbody fusion and arthroplasty. Invited submission from the Joint Section Meeting on Disorders of the Spine and Peripheral Nerves, March 2004. J Neurosurg Spine. 2004;1:155-159.)

Clinical Trials

European, Australian, and American clinical trials have reported excellent results when using the metal-on-metal and metal-on-polymer disks as opposed to ACDF. These results were based on Neck Disability Index (NDI) and arm pain intensity visual analogue scale (VAS) scores, as well as neurological status.^10,12–16 Thus far, excessive metal debris, material fatigue/fracture, and catastrophic failure have not been problematic in these trials. These trials appear to suggest that the advantages of maintenance of motion and the potential improved clinical outcome of arthroplasty outweigh the putative disadvantages of wear debris, material fatigue, and joint failure. In the United Sates, several artificial disks are undergoing prospective, randomized clinical trials (Food and Drug Administration Investigational Device Exemption [FDA IDE] Trial) in which outcomes are being compared with ACDF. Several trials have been completed (the Prestige ST Artificial Disc, Bryan Artificial Disc, and ProDisc-C).

In 2007, we reported the 2-year clinical and radiographic outcomes of a prospective randomized trial comparing ACDF and arthroplasty with the Prestige ST Cervical Disc (Figs. 293-2 to 293-4). Five-hundred-forty-one patients with single-level cervical degenerative disk disease and radiculopathy were randomly assigned to one of two treatment groups: arthroplasty with the Prestige ST or a control group treated by ACDF. Eighty percent of the arthroplasty-treated patients and 75% of the control patients reached 2 years of follow-up at the time of publication, and further follow-up is still ongoing. The NDI score and the Short Form Health Survey (36 items) (SF-36) score improved in both groups. However, the arthroplasty group had a statistically significant higher rate of neurological success, as well as a lower rate of secondary revision surgeries and supplemental fixation than did the ACDF group. The cervical disk arthroplasties maintained segmental sagittal angular motion averaging greater than 7 degrees (see Fig. 293-3). It is important to note that there were no cases of implant failure or migration in the arthroplasty group.

FIGURE 293-2 Photograph of the Prestige LP device.

(From Mummaneni PV, Robinson JC, Haid RW Jr. Cervical arthroplasty with the PRESTIGE LP cervical disc. Neurosurgery. 2007;60:310-314; discussion 314-315.)

FIGURE 293-3 Illustration of the Prestige ST device.

(From Mummaneni PV, Burkus JK, Haid RW, et al. Clinical and radiographic analysis of cervical disc arthroplasty compared with allograft fusion: a randomized controlled clinical trial. J Neurosurg Spine. 2007;6:198-209.)

FIGURE 293-4 A and B, Flexion and extension radiographs demonstrating motion with the Prestige LP device.

(From Mummaneni PV, Burkus JK, Haid RW, et al. Clinical and radiographic analysis of cervical disc arthroplasty compared with allograft fusion: A randomized controlled clinical trial. J Neurosurg Spine. 2007;6:198-209.)

Sasso and coauthors reported the results of a 2-year trial comparing the Bryan artificial disk with ACDF in 2007. Similar to the Prestige trial, the functional outcome and radiographic results of this prospective, randomized trial for patients with one-level cervical disk disease were determined. They found that the Bryan disk compared favorably with ACDF in the group studied. At 2 years, they found statistically significant differences between the groups, with improvements in the NDI score, neck and arm pain, VAS scores, and SF-36 physical component score in the Bryan disk patients.¹⁷

At this point, the final results of the ProDisc C (Fig. 293-5) arthroplasty FDA IDE trial have yet to be published.

FIGURE 293-5 Illustration of the metal-on-polymer ProDisc-C.

(From Mummaneni PV, Haid RW. The future in the care of the cervical spine: interbody fusion and arthroplasty. Invited submission from the Joint Section Meeting on Disorders of the Spine and Peripheral Nerves, March 2004. J Neurosurg Spine. 2004;1:155-159.)

With further long-term follow-up of cervical arthroplasty, clinicians will be able to determine the ultimate benefits and drawbacks of this technology.

Conclusion

In the last half-century, the philosophy of spine surgeons was to perform cervical fusion to manage symptomatic cervical disk herniations. The focus of research has been to improve fusion rates to 100%. Spinal implants improved fusion rates significantly while minimizing morbidity, such as iliac crest donor site pain. In the next 50 years, spine surgeons may witness the emergence of a new philosophy centered around the maintenance of motion when treating spinal segmental disease. Cervical arthroplasty is a promising new option for the management of degenerative cervical disease.