December 2001 Bulletin

Clinicians, scientists probe wear problems

New AAOS book summarizes causes future directions for research

Total joint replacement is a highly successful and cost effective procedure that has brought relief from pain and increased mobility to millions of people throughout the world, but it has its problems, too.

Despite its success, failures continue to occur, most commonly because of wear of the implant components, say editors of a new book, "Implant Wear in Total Joint Replacement: Clinical and Biological Issues, Material and Design Considerations." The book is edited by Timothy M. Wright, PhD, senior scientist, biomedical mechanics and materials, the Hospital for Special Surgery, New York, and Stuart B. Goodman, MD, PhD, professor and chief of orthopaedic surgery, Stanford University, Calif.

Based on an October 2000 symposium sponsored by the National Institute of Health and the AAOS, the book summarizes recent efforts of clinicians, bioengineers, material scientists and biologists to understand and address the wear problem and their suggestions for future directions for research. The symposium was an update of a 1995 NIH-AAOS workshop.

There is a much clearer picture emerging of the clinical, biologic and material design issues, says Wright. Clinicians know more about "how to treat implants in cases of excessive wear and have developed paradigms of how to operate and when," he explains. "They have a clearer idea of the techniques to follow patients to determine when there is wear."

Wear of total joint replacements "is generally insidious, with few clinical signs and symptoms until the late stages of failure," say Wright and Dr. Goodman in a summary of the clinical issues. "Furthermore, wear of implant components is affected by how the patient uses the joint and not merely how long the implant is in service. Thus, risk factors for increased wear and the need for revision surgery include younger, heavier, more active individuals."

On the engineering side, the "buzz" is new forms of polyethylene developed at higher levels of radiation to produce crosslinks that increase the resistance of the material to abrasion. This allows the use of thinner components and larger head sizes in hip implants which, in turn, produces more stability, Wright says.

Significant advances have been made in understanding the cellular and molecular mechanisms of loosening which opens new avenues of research on osteolysis and may lead to interventions in the biologic cascade.

Orthopaedic surgeons are more aware of wear-related failure of joint replacements today, says Wright. "If you go back eight or nine years ago, and look at 100 charts, you’ll see diagnoses such as ‘loosening’ or ‘mechanical failure.’ We don’t know when the joints were put in and why came out for joint failure."

Orthopaedic surgeons and researchers "don’t have the tools to know how big a problem this is," says Wright. "Looking forward, there is increased use of crosslinked polyethylene, but how will we find out if we have advanced the ball?," asks Wright.

The two implant symposia and the NIH Technology Assessment Conference in January 2000 help assemble the known information and an AAOS task force is exploring the potential of establishing a total joint registry. Registries in other countries help surgeons learn how many implants and revisions occur and which prostheses and procedures are successful and which are not.

The implant workshop was supported by the National Institute of Arthritis and Musculoskeletal and Skin Diseases, with additional contributions from the American Association of Hip and Knee Surgeons; Biomet, Inc.; The Knee Society; National Institute of Standards and Technology; Orthopaedic Research and Education Foundation; Orthopaedic Research Society; and Stryker Howmedica Osteonics.

The 200-page book is priced at $35 for AAOS members, $45 for nonmembers. To order, contact customer service at (800) 636-6726.

Facts

Total knee replacements

revisions

1990

129,000

12,000

1995

216,000

18,000

1999

267,000

25,000

Total hip replacements

revisions

1990

119,000

24,000

1995

134,000

29,000

1999

168,000

30,000


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