Rationale of Cervical Arthroplasty

The excellent clinical success rate and long-term experience with ACDF for the treatment of cervical degenerative disc disease raises the question as to the need for the development of alternate procedures. Although fusion is beneficial to the symptomatic level, it may be detrimental to the remaining motion segments as it induces increased motion and abnormal intradiscal pressure recordings in adjacent disc segments, which translate into increased stress on the adjacent nonoperated discs. Several studies have reported the radiographic development of adjacent segment degeneration (ASD) after ACDF procedures. Baba and associates assessed more than 100 patients who underwent anterior cervical fusion for cervical myelopathy and observed new spinal canal stenosis above the previously fused segments in 25% of patients at an average follow-up of 8.5 years. Similarly, Gore and Sepic reported a 25% risk of developing new radiographic adjacent segment spondylosis 5 years following ACDF. In a comparative radiographic study, Herkowitz and colleagues evaluated 44 patients with 4.5 years of follow-up randomized to either ACDF or posterior foraminotomy without fusion for the treatment of cervical radiculopathy and found that the chances of developing ASD were similar between the two groups (41% for ACDF versus 50% for foraminotomy group) with no correlation between the development of ASD and the onset of new clinical symptoms referable to those radiographic changes. In a series of more than 800 patients who underwent posterior foraminotomy without fusion by Henderson and coworkers, 9% of patients developed adjacent segment disease requiring additional surgery over an average follow-up of 2.8 years, demonstrating that the development of ASD may not be unique to anterior cervical fusion and could just reflect the natural history of the disease or failure to treat all of the symptomatic segments during the primary procedure.

Irrespective of the lack of clear correlation between radiographic changes following the anterior cervical fusion and clinical symptoms, a subgroup of patients with ACDF does develop symptomatic adjacent segment disease requiring treatment. There has been an attempt to distinguish clinical versus radiographic adjacent segment problems after cervical fusion. The term adjacent segment degeneration has been used to describe radiographic changes adjacent to a previous spinal fusion procedure that do not necessarily correlate with any clinical findings as compared to adjacent segment disease, which refers to the development of new clinical symptoms that correspond to segmental radiographic changes next to the level of a previous spinal fusion. Bohlman and colleagues, Gore and colleagues, and Williams and colleagues reported 9%, 14%, and 17% risks, respectively, of developing symptomatic adjacent segment disease following anterior fusion that required surgical treatment. Hilibrand and coworkers retrospectively reported the long-term follow-up of 409 anterior cervical decompression and stabilization procedures performed for radiculopathy or myelopathy. They found symptomatic ASD occurred at a relatively constant incidence of 2.9% per year during the 10 years after surgery. The results of this study have been interpreted by some as providing robust evidence for the development of ASD following ACDF. Patients with preexisting degenerative changes and who were more than 60 years old had a more rapid onset of symptomatic adjacent-segment disease. It is intriguing that patients who had multilevel arthrodeses were significantly less likely to have symptomatic adjacent-segment disease than were those who had had a single-level fusion (p < 0.001). Though still conjectural, with progression of natural history being an important factor in the development of ASD, the study by Goffin and colleagues showed that the rate of radiographic ASD following arthrodesis for traumatic cervical injuries was 60% over 5 years, indicating that the development of adjacent segment disease is at least partly related to the arthrodesis itself and may not be blamed totally on the natural history of the disease.

There is biomechanical evidence demonstrating that CDR may allow for a more normal restoration of load transfer and kinematics at adjacent levels when compared with fusion. Wigfield and associates recorded and compared intradiscal pressures in adjacent levels of normal/untreated cadaveric cervical spines, spines treated with a simulated fusion, and spines treated with a CDR and found that the motion and pressures of adjacent segments in specimens treated with a CDR did not significantly differ from those in the normal/untreated spines. Similarly, Puttlitz and colleagues demonstrated that spines treated with CDR approximated the intact motion in all three rotational planes at the affected level compared with normal cadaveric spines. In a clinical prospective study, Robertson and coworkers compared the incidence of radiologic documented changes and symptomatic adjacent-level cervical disc disease after single-level discectomy and cervical fusion or arthroplasty using the Bryan disc and showed the appearance of new radiographic changes in 34.6% of the fusion-treated patients as compared to 17.5% of the arthroplasty-treated patients at 24 months (p = 0.009). New symptomatic adjacent degenerative disc disease occurred in 7% of the fusion group and in none of the arthroplasty group (p = 0.018). This study strengthens the argument that maintaining motion with arthroplasty after single-level disc disease may delay or prevent to a significant degree the associated radiologic disc degeneration. Although the initial and midterm data failed to demonstrate any benefit of CDR on ASD as purported, long-term results from the Prestige ST trial as well as data from a two-level arthroplasty study using the Mobi-C implant have shown encouraging results with a decreased incidence of adjacent segment disease following CDR as compared to that with ACDF.

Another rationale for the use of CDR, as opposed to ACDF relates to the complications with pseudarthrosis and bone graft procurement for arthrodesis. It is generally believed that less soft tissue dissection, decreased esophageal retraction pressure, and the minimal profile of the current generation cervical disc replacements may result in a lower incidence of postoperative dysphagia following cervical arthroplasty as compared to instrumented ACDF.

Nomenclature, Biomaterials, and Biomechanics

Fundamental to the principle of arthroplasty are the corollary concepts of repetitive stress and the generation of wear-related debris. Various factors are involved in the design of a cervical artificial disc such as (1) implant kinematics, (2) implant materials, (3) device subsidence, and (4) fixation to bone. The choice of material used in creating the device must be governed by three principles: articular surface wear, generation of wear debris, and host inflammatory response. Many designs incorporate two different materials articulating with each other. In a metal-on-polymer pairing, the polymer wears preferentially, whereas in a metal-on-ceramic pair, the metal wears to a greater extent. Metal wear debris generates a lesser inflammatory response compared to polymer debris.

An ideal artificial disc should resist corrosion and wear, reproduce the movements of a normal cervical disc without overloading adjacent biomechanical structures, be easy to insert, and be x-ray and magnetic resonance imaging (MRI) compatible. Implant design characteristics are important for the function and longevity of CDR. The articulating surfaces must be able to tolerate anticipated load without fatigue or failure, while minimizing friction, and they should have superior wear characteristics with minimal debris generation and exceed patient life expectancy. The presence of wear debris and the associated inflammatory response can be detrimental to the stability of the device and varies depending on the design of the disc (metal on metal versus metal on polymer). In addition, the implants must remain permanently affixed to the adjacent vertebral bodies.

With the development of numerous artificial discs since the early 2000s, the Cervical Spine Study Group developed a nomenclature system for cervical arthroplasty. Artificial discs are categorized by material, articulation, fixation, design, and kinematics. They can be classified as nonarticulating, uni-articulating, or bi-articulating. Various designs include metal on metal (Prestige ST and LP; Medtronic Sofamor Danek, Memphis, TN), metal on polymer (Bryan, Medtronic Sofamor Danek; ProDisc-C, Synthes Spine, West Chester, PA; Mobi-C, LDR Medical, Troyes, France), ceramic on polymer, or ceramic on ceramic. Discs are either modular with replaceable parts or nonmodular. Some have points for vertebral body fixation and some have surfaces that promote bone ingrowth at the disc–end plate interface. With regard to motion, they may be constrained, semiconstrained, or unconstrained. Constrained devices restrict motion to less than that seen physiologically, semiconstrained devices allow physiologic motion, and unconstrained devices rely on soft tissue and the inherent compression across the disc space to limit motion. Each of these devices is available in a range of heights and depths to accommodate individual variances in anatomy. The currently approved cervical artificial discs are listed in Box 183-1 .

Indications and Contraindications

The indications for cervical arthroplasty are not nearly as robust as those for cervical fusion, and this technique should not be assumed to be appropriate for all patients who may benefit from an ACDF. Ideally, patients receiving an artificial disc should have normal cervical spinal alignment and mobility along with one of the following pathologic entities: radiculopathy caused by disc herniation or foraminal osteophytes or myelopathy caused by disc herniation producing spinal cord compression from C3 to C7 with or without axial neck pain.

It has been shown that CDR is as effective as ACDF for the management of cervical myelopathy due to single-level disc abnormality when used for a soft central disc herniation. Currently in the United States, CDR is FDA approved for intractable neck pain with radiculopathy or myelopathy at a single or two levels between C3 and C7 in a patient who has failed a minimum of 6 weeks of conservative treatment. At present there are six devices that are FDA approved for single-level implantation and one device approved for both single- and two-level implantation in the United States.

The importance of careful patient selection cannot be overemphasized for the success of CDR. Factors to consider when selecting suitable candidates for arthroplasty include the disc height at the level of degeneration, inherent range of motion, and cervical alignment. Cervical arthroplasty is contraindicated in the setting of significant segmental or global deformity. Similarly, patients without preexisting motion or spinal instability should not be treated by a CDR. Arthroplasty requires the dorsal elements to be intact and functional, so patients with suspected ligamentous or facet disease are not suitable candidates. Those with cervical kyphosis, cervical spondylolisthesis, previous laminectomy, osteoporosis, metabolic bone disease, or cervical trauma are not ideal either. A history of infection or osteomyelitis would preclude the use of a prosthetic disc device. Other relative contraindications include rheumatoid arthritis, renal failure, osteoporosis, cancer, preoperative corticosteroid medication, ankylosing spondylitis, ossification of the posterior longitudinal ligament, and diffuse idiopathic skeletal hyperostosis.

Cervical Arthroplasty: the Clinical Evidence

A number of randomized controlled trials have provided high-quality evidence demonstrating that the safety and efficacy of CDR are equivalent to that for ACDF. None of the studies showed that CDA resulted in inferior clinical outcomes relative to ACDF, and depending on the study there were superior outcomes in some parameters. The reader is referred to the specific papers to review the data in detail.

The 2- and 4-year results from the first prospective, randomized U.S. FDA investigational device exemption trial for two-level total disc replacement with the Mobi-C cervical artificial disc versus anterior discectomy and fusion were published demonstrating the safety and efficacy of cervical artificial discs even for two level implantation.