The mechanical performance of cervical total disc replacements in vivo: prospective retrieval analysis of prodisc-C devices.

STUDY
DESIGN: Prospective retrieval analysis of Prodisc-C cervical total disc replacements (CTDRs) from 24 explanting surgeons during a 6-year period.
OBJECTIVE: To determine the in vivo mechanical performance and fixation to bone of explanted Prodisc-C CTDRs. SUMMARY OF BACKGROUND DATA: 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.
METHODS: 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.
RESULTS: 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.
CONCLUSION: 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.