The Mechanical Performance of Cervical Total Disc Replacements In Vivo

Abstract

Prospective retrieval analysis of Prodisc-C cervical total disc replacements (CTDRs) from 24 explanting surgeons during a 6-year period. To determine the in vivo mechanical performance and fixation to bone of explanted Prodisc-C CTDRs. The nature and quantity of damage sustained by an implanted device has proven to be important in the prediction of clinical longevity. We hypothesized that retrieval analysis of the Prodisc-C will display characteristic modes of wear consistent with increased posterior angulation and translation of the functional spinal unit after resection of the discoligamentous anatomy. Thirty CTDRs from 29 patients (mean age, 45.1 ± 1.9; range, 31-57 yr) after a mean length of implantation of 1.0 ± 0.2 years (range, 2 d-3.5 yr) were studied. Operative level was C4-C5 in 20% (6 of 30), C5-C6 in 47% (14 of 30), C6-C7 in 20% (6 of 30), and unknown in 13% (4 of 30). Polyethylene and metallic (cobalt chrome molybdenum [CoCrMo]) components were examined using light stereo-microscopy (6X-31X), scanning electron microscopy, and energy dispersive x-ray analysis. CTDRs were explanted for indications of axial pain (n = 9), radicular symptoms (n = 6), atraumatic loosening (n = 6), trauma (n = 5), metal allergy (n = 1), myelopathy (n = 1), hypermobility (n = 1), and unknown (n = 1). Surface area of ongrowth (mean = 7.2 ± 1.4%) was not associated with operative level (P = 0.37), surgeon-reported axial pain (P = 0.56), or atraumatic loosening (P = 0.93). Burnishing consistent with metallic endplate impingement was present in 80% (24 of 30) of retrieved CTDRs, most commonly in the posterior quadrant (P < 0.001). There was no association between implant height (P = 0.19) or depth (P = 0.17) and posterior impingement. Backside wear was not observed on any of the disassembled implants (0 of 16). Third-body wear occurred in 23% (7 of 30) and the donor site was confirmed by scanning electron microscope/energy dispersive x-ray analysis to be the porous-coated surface of the CTDR. Early clinical failures of Prodisc-C CTDRs display surface damage evidence of metal endplate-endplate impingement, most commonly posteriorly. Backside wear was not evident; however, third-body wear was found. Future studies will determine the clinical impact of these predominant modes of wear on long-term metal-on-polyethylene semiconstrained CTDR performance.