Total hip replacement by far is the most successful orthopedic operation for treating joint damage. About 700, 000 joint replacement surgeries are performed each year, with $ 2.5 billion spent on the material. However, the postoperative revision rate (e.g. ~50% in year 1996) is still
very high, largely due to the artificial replacement failure and creation of wear debris in the surrounding tissue.
A hip replacement consists of two parts: a formal ball and a socket. (See images on right from Zimmer Inc.).
The current generation of metal-on-metal prosthetic is made of Co-Cr-Mo alloys (ASTM 1547), with ~60% Co, 26% Cr and 5-7% Mo. The alloy can be as cast or wrought, the grain size ranging from a few microns to millimeters. The Co-Cr-Mo alloy adopts a metastable f.c.c. matrix at room temperature, with some h.c.p.-martensite formed through strain-induced transformation. In addition, a small amount of M23C6 (M: Cr, Co) carbides are present in the matrix. Metal-on-metal hip replacements have recently lost popularity, yet we are still interested in this alloy as it is a commonly used biomedical and engineering alloy.
The current research in our group is to understand the wear and corrosion mechanisms of the Co-Cr-Mo alloy. We study the hip replacement system from a materials science perspective, focusing on the intersections of wear, tribology, and corrosion. Previous work has explored the surface structure, which consists a thin nanocrystalline layer formed through recrystallization. Other work has looked into the composition, formation, and wear of the tribolayer that forms on the metal-on-metal hip replacements during implantation. Additonally, the corrosion properties of the alloy were examined, focusing on the heat treatment's effect on the localized grain boundary pitting corrosion and the relation to crystallographic orientation.
The structural and mechanical properties of the surface from nano to micro-scale can be identified using nanoindentation, XPS and other surface characterization techniques. The primary techniques for this project exploit state-of-the-art analytical electron microscopy (TEM/STEM/EDS/EELS). This research will illustrate the wear and corrosion processes of metal-on-metal hip implants, and provide us the insight that is essential for designing next generation of joint replacement materials.
For this project, we closely collaborate with Dr. Jacobs and Dr. Wimmer's group at Rush University Medical Center in Chicago, Dr. Fischer's group at Germany, and Dr. Shull's group at Northwestern University. This collaboration was previously funded by an NIH stimulus grant.