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Friday, March 12, 2004

Simulators now mimic in-vivo wear of ceramic hip cups

By Mary Ann Porucznik

Although ceramic acetabular cups have been used for total hip replacement (THR) for more than 30 years in Europe, they only received FDA-approval for use in the United States in 2003. With the availability of newer designs, researchers for Scientific Exhibit SE007 wanted to evaluate the outcome of ceramic wear reported in retrieval cases with the predictions from hip simulator laboratories.

European studies of wear in THR with ceramic cups generally showed two types of wear on femoral balls: a central region of mild wear and a peripheral region of stripe wear. Wear varied from less than 5 microns per year up to an inexplicable 3 mm per year in one rare report.

Recently, Australian retrieval studies of third-generation ceramic THR have shown volumetric wear of 1 mm3 to 3 mm3 per year for periods of up to two years in patients. Stripe wear was also demonstrated and corresponded to cup-rim contact in the flexed position.

Investigators noted that past simulator studies did not generally examine the microseparation mode of the hip joint as demonstrated in fluoroscopic gait studies. Thus, alumina-bearings produced ultra low wear, of the order of less than 0.05 mm3 per million cycles of the simulator machine.

Additionally, the simulators did not duplicate the stripe wear found in-vivo. This resulted in 1,000-fold less wear than with ultrahigh molecular weight polyethylene (UHMWPE). With the latest micro-separation kinematics, the simulators can now duplicate the stripe wear seen in-vivo. This has raised the run-in wear rates by fourfold and the steady state wear by up to 30-fold, which now gives reasonable agreement with the Australian retrieval data.

Despite the increase in wear rates, such short-term 'worst-case' performance was still 100-fold less than with UHMWPE cups. It is also likely that the microseparation test mode is relevant for metal-metal and metal-UHMWPE bearings.

Researchers include Ian C. Clarke, PhD; Toshiyuki Tateiwa, MD, and G. Allen Gustafson, MD, all of Los Angeles, and Takaakishi Shishido, MD, of Tokyo, Japan. Research funding was provided by Howmedica-Osteonics, Biocream Inc., Smith and Nephew.

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Last modified 20/February/2004