"As replacement joints are becoming increasingly common in aging populations, our results explain how such devices fail and suggest that improvements should be made in artificial joint design," said the study's senior author K.L. Paul Sung, Ph.D., UCSD professor of orthopedic surgery and adjunct professor of cellular bioengineering.
The team measured how titanium particles affected the bonding strength of pins implanted in rat thighs. The pins were shown to come out more easily when the titanium particles were present, with the smallest and largest particles causing the greatest weakening. The researchers demonstrated how different-sized titanium particles affected bone-building cells called osteoblasts and bone-destroying cells called osteoclasts. Microscopic studies revealed osteoblasts did not form proper adhesions, with small- and medium-sized titanium particles concentrated inside cells. Increased production of the protein RANKL by osteoblasts recruited and activated osteoclasts at the insertion sites, further weakening the bone. Larger titanium particles also activated metalloproteinases, which chop up the extracellular matrix that holds cells together.
Currently, Sung is leading a team in using nano-technology to improve implant material which has three to five times higher wear resistance and fatigue properties to reduce particle generation from implants.
In addition to Sung, additional authors were first author Moon G. Choi, M.D., UCSD Department of Orthopedic Surgery; and Hae S. Koh, M.D., UCSD Department of Orthopedic Surgery; Daniel Kluess, M.S. and Daniel O'Connor, M.A., UCSD Department of Bioengineering; Anshu Mathur, Ph.D., George Truskey, Ph.D., Department of Biomedical Engineering, Duke University; Janet Rubin, M.D., Department of Medicine, Emory University School of Medicine and Veterans Administration Medical Center, Atlanta; and David X.F. Zhou, Ph.D., UCSD Department of Bioengineering.
The study was supported by a Bristol Meyers/Zimmer Award for Excellence in Orthopaedic Research and the National Institutes of Health.