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.)

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