What is the correlation of in vivo wear and damage patterns with in vitro TDR motion response?

This study combined the evaluation of retrieved total disc replacements (TDRs) with a biomechanical study using human lumbar spines. Thirty-eight CHARITE TDRs were retrieved from 32 patients after 7.3 years average implantation. All implants were removed because of intractable back pain and/or facet degeneration. In parallel, 20 new implants were evaluated at L4-L5 and L5-S1 in an in vitro lumbar spine model.

The purpose of this study was to correlate wear and damage patterns in retrieved TDRs with motion patterns observed in an in vitro lumbar spine model. We also sought to determine whether one-sided wear and motion patterns were associated with greater in vivo wear.

The comparison of polyethylene wear in TDRs after long-term implantation to those tested using an in vitro model had not yet been investigated.

The wear patterns of each retrieved PE core was analyzed at the rim and dome. Thirty-five cores were further analyzed using MicroCT to determine the penetration symmetry. For the in vitro study the implants were tested under physiologic loads using a validated cadaveric model. Motion patterns of the in vitro-tested implants were tracked using sequential video-fluoroscopy.

Fifteen of 35 retrieved cores (43%) displayed one-sided wear patterns. Significant correlations were observed between implantation time and penetration and penetration rate. In the in vitro study, there was evidence of motion at both articulations, motion at both articulation but predominantly at the top articulation, and solelyat the top articulation. Core entrapment and pinching was observed and associated with visual evidence of core bending or deformation.

This is the first study to directly compare the long-term PE wear and damage mechanisms in TDR retrievals with the motion patterns generated by a validated in vitro cadaveric testing model. The retrievals exhibited wear patterns consistent with the in vitro testing.