Long-term clinical studies of total hip replacement suggest a direct relationship between bearing wear and periprosthetic osteolysis, particularly if polyethylene wear is greater than a threshold value of 0.1 mm per year. The current clinical trend to cross-linked polyethylene and hard-to-hard bearings attempts to ensure that bearing wear remains below this threshold. Fluid pressure generated in the hip during patient activity also has been implicated in the formation of periprosthetic lesions. Pressure fluctuation measured during manipulation of the hip at revision, or the identification of modular components that pump fluid during loading, suggest cyclic pressure may be a causative factor in bone resorption. Animal studies show the adverse effect of direct pressure on osteocytes. At more than 10 years followup, the low incidence of osteolytic lesions in retrospective reviews of successful cemented and cementless implant designs suggest that osteolysis is not an inevitable consequence of particle or pressure generation in the hip. If the quality of implant fixation prohibits fluid access to the surrounding bone, the rate of osteolysis is minimal. It is evident that whether the active factor in osteolysis is pressure, wear particles, or both, adverse periprosthetic effects can be minimized if access to the fixation interfaces in the hip is denied.
From *Clinical Measurement Corp., Ridgewood, NJ; the **Greater Pittsburgh Orthopaedics Association, Pittsburgh, PA; the †Indiana University Medical School, Indianapolis; IN; and the ††School of Bioengineering, Drexel University, Philadelphia, PA.
The hydroxyapatite cup study is funded by Howmedica Osteonics Corporation.
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