Comparison of in vivo and simulator-retrieved metal-on-metal cervical disc replacements

BACKGROUND Cervical disc arthroplasty is regarded as a promising treatment for myelopathy and radiculopathy as an alternative to cervical spine fusion. On the basis of 2-year clinical data for the PRESTIGE(®) Cervical Disc (Medtronic, Memphis, Tennessee), the Food and Drug Administration recommended conditional approval in September 2006 and final approval in July 2007; however, relatively little is known about its wear and damage modes in vivo. The main objective was to analyze the tribological findings of the PRESTIGE(®) Cervical Disc. This study characterized the in vivo wear patterns of retrieved cervical discs and tested the hypothesis that the total disc replacements exhibited similar surface morphology and wear patterns in vitro as in vivo. METHODS Ten explanted total disc replacements (PRESTIGE(®), PRESTIGE(®) I, and PRESTIGE(®) II) from 10 patients retrieved after a mean of 1.8 years (range, 0.3-4.1 years) were analyzed. Wear testing included coupled lateral bending ( ±4.7°) and axial rotation ( ±3.8°) with a 49 N axial load for 5 million cycles followed by 10 million cycles of flexion-extension ( ±9.7°) with 148 N. Implant surfaces were characterized by the use of white-light interferometry, scanning electron microscopy, and energy dispersive spectroscopy. RESULTS The explants generally exhibited a slightly discolored, elliptic wear region of varying dimension centered in the bearing center, with the long axis oriented in the medial-lateral direction. Abrasive wear was the dominant in vivo wear mechanism, with microscopic scratches generally oriented in the medial-lateral direction. Wear testing resulted in severe abrasive wear in a curvilinear fashion oriented primarily in the medial-lateral direction. All retrievals showed evidence of an abrasive wear mechanism. CONCLUSIONS This study documented important similarity between the wear mechanisms of components tested in vitro and explanted PRESTIGE(®) Cervical Discs; however, the severity of wear was much greater during the in vitro test compared with the retrievals.