A Morphological Analysis of Metal Transfer on Retrieved Ceramic and Cobalt Chrome Femoral Heads

Abstract

Total hip arthroplasty is one of the most common procedures to treat end-stage osteoarthritis, with many variations on the components used. Although cobalt chromium-on-polyethylene is the gold standard, other materials combinations have recently gained popularity such as ceramic-on-polyethylene and ceramic-on-ceramic bearing couples due to their increased hardness and reduced wear. Metal transfer has been observed on the bearing surface of retrieved femoral heads in total hip replacements; described as dark metallic smears of titanium or cobalt chromium alloy. In vitro wear testing has shown increased wear of the polyethylene acetabular liner with the presence of metal transfer on femoral heads. Little is known though about the effects bearing surface materials may have on the morphology of metal transfer. This retrieval study sought to investigate the extent of metal transfer surface damage on the typical bearing surface of Cobalt Chrome (CoCr) and ceramic femoral heads, and to identify prevalent patterns and morphologies for later recreation. Femoral heads from three types of bearing couples (CoCr-on-polyethylene (n = 50), ceramic-on-polyethylene (n = 35), and ceramic-on-ceramic (n=15)) were assessed for the presence and extent of metal transfer. Visual evidence of metal transfer was observed on 75% (n = 75/100) of the femoral heads with a median metal transfer surface area coverage of 1.22% across all cohorts (n = 89/100). Metal transfer was found to cover more surface area for ceramic-on-ceramic bearing couples than CoCr-on-polyethylene bearing couples (Kruskal Wallis Test: p = 0.027), suggesting the influence of material factors. The most prevalent metal transfer morphologies observed were random stripes (n = 21/75), longitudinal stripes (n = 17/75), and random patches (n = 13/75). CoCr-on-polyethylene femoral heads showed significantly taller metal transfer than ceramic-on ceramic femoral heads for the longitudinal stripe pattern, suggesting that material factors could be linked to metal transfer severity (Kruskal Wallis Test: p = 0.025). Understanding the morphology of metal transfer may be useful for more accurate polyethylene wear studies through more realistic re-creation of metal transfer in in vitro pin-on-disk and joint simulators studies. This would allow for metal transfer and its correlation to polyethylene wear to be more accurately studied under normal and adverse activities, and for predictive wear studies of metal transfer producing failure mechanisms to be performed on various bearing couple combinations.%%%%M.S., Biomedical Science – Drexel University, 2017