Did the Bearing Surface Influence Outcome

Study Design: Retrieval analysis. Objective: To analyze a cohort of all retrieved XL-total disc replacements (XL-TDR) to investigate failure modes. Background: The XL-TDR was a lumbar metal-on-metal disc replacement that was withdrawn before clinical approval. Clinical reports have not indicated if early failures were attributed to the bearing surfaces or other design features. Materials and methods: All XL-TDRs (N = 20) received by an independent academic retrieval laboratory were evaluated using nondestructive techniques. Demographic information and reported causes of removal were documented. Light microscopy, metrology, and radiographic evaluation were conducted. Maximum linear wear and sphericity deviations from a fit sphere were calculated. Soft tissue histology was performed for cases with available tissue samples. Results: Suboptimal surgical placement was identified in 75% of cases. Loss of fixation was identified in 65% of cases, typically evidenced by migration or loosening. Accordingly, minimal bony attachment was seen on the porous-coated endplates, with most surfaces covered by loosely adhered fibrous tissue. Finally, impingement and heterotopic ossification (HO) were each observed in 65% and 60% of implants, respectively. Articular surfaces showed minimal wear, with only three specimens exhibiting deviations >25 µm from sphericity. No CoCr debris was detected in any of the examined tissues. Conclusion: Reliance on bony ongrowth for fixation and geometric constraint likely contributed to high rates of observed impingement, HO, and loss of fixation. CAD-based modeling further supported that limited flexion range and early anterior contact predisposed the device to impingement. These findings suggest that fixation strategy and implant geometry, rather than the bearing material, were likely the primary contributors to XL-TDR failure. These observations, combined with articular surface measurements, that is, low spherical deviations and tissue analysis, indicated that metal-on-metal bearing wear did not play a prominent role in these failures.