Lightning Podiums: Spinal Gumbo - 803A

Presented by: R. Havey

Author(s):

R. Havey(1), S. Khayatzadeh(1), L. Voronov(1), G. Carandang(1), F. Phillips(2), A. Patwardhan(1,3)

(1) Edward Hines Jr. VA Hospital, Musculoskeletal Biomechanics Laboratory, Hines, IL, United States
(2) Rush University Medical Center, Orthopaedic Surgery, Chicago, IL, United States
(3) Loyola University Medical Center, Orthopaedic Surgery & Rehabilitation, Maywood, IL, United States

Abstract

Introduction: The FDA recently approved a mobile-core cervical total disc arthroplasty (TDA) for 1- and 2-level implantations (Mobi-C, Zimmer-Biomet). The design includes a UHMWPE mobile core that articulates with the lower metal (Co-Cr) endplate in a plane bearing, allowing anterior-posterior and lateral translation. The core articulates with the superior metal endplate in a spherical bearing. This disc may be classified as a 5 degrees-of-freedom disc that is unrestrained since it does not resist angular or translational motions until reaching the built-in stops. Our previous experience with a mobile-core lumbar TDA with bi-convex articulations showed motion of the core was unpredictable; it tended to get trapped between metal endplates at extremes of extension or flexion range of motion (ROM) and would move in an uncontrolled stick-slip fashion resulting in poor quality of motion. This was consistent with the uneven wear patterns and core damage in explanted implants [Kurtz 2008]. However, the cervical spine differs from the lumbar spine in the physiologic loads and motions experienced by the TDA in vivo. The purpose of this study was to investigate the motion response of the mobile-core TDA when implanted in human cadaveric spines in 1- and 2-level reconstructions.

Methods: We tested eight cadaveric specimens (C3-T1) (age:42±6) under physiologic loads in flexion-extension (FE), lateral bending (LB), and axial rotation (AR); first intact, then after implantation of TDA at C5-C6 (1-level), and then at C6-C7 (2-level). The implant-height, footprint, and positioning in the disc space were based upon the judgment of an arthroplasty-trained surgeon. Motion data were analyzed to determine ROM and quality of motion before and after implantations.

Results: The ROM of C5-C6 and C6-C7 closely matched in their values of means, standard distributions, and medians; for the intact and post-disc arthroplasty states. Therefore, data from the two segments were combined. The TDA significantly increased ROM in FE from 13.7±4.3 to 16.7±4.2 degrees (P< 0.01), while decreasing it in LB from 10.0±1.8 to 7.7±3.3 degrees (P< 0.01). ROM in AR was unaffected (7.9±2.0 vs. 7.5±2.0, P=0.21).Postoperative FE-ROM was positively correlated with preoperative FE-ROM (Figure 1A). Six of the 16 implanted segments had postoperative FE-ROM greater than 20 degrees. In 8 of 16 implanted segments, the kinematic signature reflected acceptable quality of motion (Figure 1B), while the remaining 8/16 segments demonstrated instability, or large increases in motion for a small applied moment (Figure 1C).

Conclusions: In 6/16 implanted segments the postoperative FE motion was significantly larger than at the adjacent C4-C5 segment, far exceeding physiologic normative ROM in vivo. These segments had above normal preoperative ROM and the unrestrained nature of this prosthesis design likely allowed excessive motion to occur post-implantation. The mobile-core design does not lend itself to controlled motion of the core, which led to the poor quality of motion in some segments, a behavior noted previously for the lumbar mobile-core prosthesis.

Figure 1