A Study of Surface Modification on Current Orthopaedic Bioceramics after In Vivo Exposure to the Physiological Environment

Over 300,000 total hip replacement (THR) surgeries are performed annually in the United States with generally good clinical outcomes. Surgical registries around the world report that approximately 90% of implanted THR devices survive for ten years or longer after implantation. However, the trend of increasing incidence of THR, particularly among younger, more active patients has generated considerable interest in THR design improvements to increase the longevity of these devices. As a result, many new articular bearing materials have been introduced for THR in recognition of prosthesis wear as a leading failure reason for contemporary designs. Ceramic-on-ceramic articulations were first introduced for THR devices several decades ago, but the initial clinical effectiveness of such devices was mired by the low fracture toughness of first generation bioceramics. Improved ceramic manufacturing technologies and new composites of alumina and zirconia ceramics have led to a resurgence in the interest of bioceramics as an alternative bearing material for THR since the 1990’s. The broad objective of this thesis is to investigate the surface integrity of explanted alumina and zirconia toughened alumina (ZTA) femoral heads that have endured in vivo function. High hardness and good wettability are considered two essential characteristics contributing to fluid-film lubrication of hard-on-hard articular bearing surfaces. These properties were addressed with two primary studies on a set of alumina (n=21) and ZTA (n=7) prosthetic femoral heads, retrieved through an implant retrieval program which has been approved by an institutional review board. Never-implanted alumina and ZTA heads were used as controls. The first study was an analysis of surface roughness, performed on each head using optical interferometry. The second study was an analysis of surface wettability, performed on each head using a drop shape analyzer and controlled 1 microliter droplets of deionized water. To accurately quantify wettability on the spherical surfaces of the femoral heads, new methodologies were developed and verified. Results of these studies show that surface roughness and wettability could be highly variable from one point to the next on each explanted femoral head, suggesting that the effects of wear were localized on these advanced bioceramics. Undamaged surfaces on explants had lower average surface roughness than the average surface roughness on never-implanted control heads. The surfaces examined exhibited a large variance in wettability data, ranging from highly wettable on some surfaces to non-wettable on others. Using known wettabilities of alumina and ZTA materials, correlations were drawn between local surface topographies and their effect on wetting behaviors. Conclusions from this study provide important considerations about assessing wettability in the presence of roughness for future explant analyses