Factors affecting the severity of backside wear of modular tibial inserts

Background: The use of modular tibial components in total knee arthroplasty introduces a possible source of polyethylene wear at the nonarticulating (backside) surface. However, it is not known whether this phenomenon is an incidental finding observed in unique specimens or is a feature common to all modular components. The purpose of this study was to determine the type and severity of backside wear in retrieved tibial inserts of several common total knee designs. Methods: One hundred and twenty-four polyethylene tibial inserts of twelve different designs were retrieved at revision total knee replacements after implantation periods ranging from zero to 180 months. Each insert was visually inspected with use of a stereomicroscope for seven different modes of surface damage in four quadrants defining the backside surface. Results: Pitting, burnishing, and measurable polyethylene protrusions were observed on the backside of polyethylene inserts of implant designs with a variety of different capture mechanisms. Across all implant designs, pitting was observed in 90% of the retrieved specimens; burnishing, in 77%; and protrusion, in 61%. Overall, implants of the IB-II (Insall-Burstein-II) design (Zimmer) exhibited the most severe burnishing, whereas those of the Duracon design (Howmedica) had the most severe pitting. Severe protrusions were noted with inserts of one design (AMK [Anatomic Modular Knee); DePuy). A longer time in situ was associated with larger polyethylene protrusions, but the severity of pitting and burnishing did not increase with increasing duration of implantation. Conclusions: Moderate-to-severe wear of the nonarticulating surface of the tibial insert was frequently observed in all designs of knee prostheses, independent of the capture mechanism. These results indicate that new designs of modular tibial components are needed to prevent the generation of polyethylene wear debris through backside wear of total knee replacements. Level of Evidence: Therapeutic study, Level III-2 (retrospective cohort study). See Instructions to Authors for a complete description of levels of evidence.