Comparative Evaluation of in Vivo Radiographic Outcomes between Different Implants in Single-Level Cervical Disc Arthroplasty at Greater than 2 Years

Abstract

Introduction Cervical fusion alters the biomechanics of the cervical spine. Adjacent levels compensate for motion lost at the fused levels. This may lead to or accelerate the development of symptomatic adjacent segment disease (ASD). Cervical disc arthroplasty (CDA) is a safe and effective alternative to fusion. It is postulated that by preserving motion at the operated level, the adjacent levels will be protected from ASD potentially related to fusion. ASD will develop over the long-term and therefore CDA implants must maintain motion and mimic the normal biomechanics of a native disc to have the protective effect. Implant design may influence a CDA implant's ability to do this. We aim to assess if the three implants with different designs do display different radiographic outcomes in vivo. Materials and Methods All one-level CDA patients with greater than 2-year radiographic follow-up were identified from hospital records. The C2 to C7 Cobb angle was measured on neutral lateral radiographs. The segmental Cobb angle was measured on lateral neutral, flexion, and extension radiographs with motion of the CDA calculated as the difference between the two later measurements. Any disc with < 2 degrees motion was identified as not exhibiting motion. Results A total of 55 patients were included. Average follow-up was 145 weeks. Three different implants were used-Discocerv (Scient'x-Alphatec Spine, France), NuNec (Pioneer Surgical Technology, United States) and PRESTIGE LP (Medtronic Ltd., United States). The most common operated level was C5/6 with 40 implants. Other levels consisted of one C3/4, four C4/5, and ten C6/7. The number of implants, cervical alignment, segment alignment, and segmental motion (in degrees) is shown in Table 1. There was no statistically significant difference in any outcome between the implants. One PRESTIGE LP required revision after 2 years. Bony fusion was observed in 12.5% of Discocerv, 14.3% of NuNec, and 12.1% of PRESTIGE LP. Twenty-five percent of Discocerv cases had some degree of subsidence, while this was seen in 7.1% of NuNec and 6.1% of PRESTIGE LP implants. Conclusion Our results indicate that CDA implant type influences the alignment of the replaced segment and preservation of motion. This may be due to the different biomechanical designs of the implants. While statistically significant differences were not observed, differences may be clinically significant. We acknowledge that limited conclusions can be drawn from our results due to differing group sizes, but our study serves as a basis and highlights the need for further in vivo and in vitro studies.