Influence of time in-situ and implant type on fixation strength of cemented tibial trays - a post mortem retrieval analysis

Abstract

Background: Loosening of the tibial tray is cited as the most common cause of failure in total knee arthroplasty but the mechanism remains unclear. Post mortem specimens provide a unique opportunity to investigate the clinical condition.

Methods: Twenty two cemented components were serially retrieved in situ at autopsy from a university clinic. They were investigated for mechanical stability by pull-out, which was related to cement morphology and bone quality from CT scans, and to polyethylene wear by score analysis. Implants were grouped into three types: a particular fixed bearing design (n=8), a particular rotating platform design (n=5) and other mixed designs (n=9).

Findings: Trends were observed for pull-out force to decrease with time in situ and increase with cement penetration but was unrelated to bone density or polyethylene wear. For the fixed bearing implants decreasing pull-out strength was related to an increasing proportion of failure at the bone-cement interface. For the mixed designs the opposite was observed. The rotating platform implants failed at the implant-cement interface.

Interpretation: The analysis demonstrated that interface failure is dependent on the implant design, but that both the stem and the bone interfaces weaken with time in situ. Published findings for laboratory implantations have demonstrated that greater cement penetration improves fixation and this was reflected for clinical samples in this study.