January 7, 2005

Tracing the life cycle of a manmade disease

A remarkable story of how a new disease was inadvertently caused by successful medical treatment, ultimately understood and eventually defeated by scientific innovation has been told by a major player in the process. In the December issue of Clinical Orthopedics and Related Research, William Harris, MD, DSc, of MGH Orthoaedics described how the development of total hip replacement led to an unexpected problem, erosion of bone adjacent to the implant, and how his team and others identified the process underlying that breakdown and helped to develop new materials that avoid the problem.

"The history of the unraveling and prevention of this worldwide, unique, severe disease is a fascinating story of the integration of surgical innovation, molecular biology and material science," wrote Harris, who is a pioneer in the field of joint replacement since the late 1960s. He and other surgeons gradually observed that hip implants could loosen starting about five years after surgery and eventually fail completely.

In 1976, Harris reported that implant failures appeared to be caused by a biological response at the site of the implant, which resulted in erosion of the bone. The MGH team later found that the metal head of the implant rubbing against the polyethylene joint socket caused small particles of polyethylene to break off over time. This led to an entirely new manmade disease called periprosthetic osteolysis, in which the immune reaction against these foreign particles eventually destroyed the bone tissue, loosening the implant to the point of failure.

In the early 1990s, Harris and his team focused their attention on finding a way to decrease the wear and tear of the polyethylene cushion in the joint and ultimately eliminate osteolysis. Working with a team of polymer chemists from the Massachusetts Institute of Technology, they developed a polyethylene that would hold up through years of constant motion and weight. The material proved strong and reliable in several studies, showing virtually no wear even after being subjected to excessive use and intense abuse. It was approved by the FDA for use in hip implants in 1999.

Because implants using this new material last for many years, the procedure now is considered appropriate for many more patients. "The key beneficiaries of this work are patients throughout the world who, thanks to Dr. Harris and his team, now have the chance to experience a better quality of life for a much longer time," says
Peter L. Slavin, MD, president of the MGH.