Abstract
Fluid pressure generated in the hip during activity has been implicated in component loosening. Animal studies show both the adverse effect of direct pressure on osteocytes and the resorption of bone subjected to cyclic loads. Pressure fluctuation measured in contained pelvic osteolytic lesions during manipulation of the hip at revision surgery suggests cyclic pressure may have a direct effect on bone resorption leading to pelvic osteolysis.
To determine the cause of pressure fluctuation in pelvic structures supporting a hip implant, we conducted an experimental and numerical analysis of relative motion at modular interfaces of acetabular cups as load was applied and removed.
We showed that for polyethylene bearing inserts supported primarily at the rim, the application of cyclic load caused cyclic motion between the insert and the inside surface of the acetabular shell. In a fluid environment, this motion can generate cyclic pressure pulses that may be applied to bone directly through the holes in the shell provided for screw fixation.
We conclude that motion at modular interfaces of acetabular components may contribute to pelvic osteolysis. Our hypothesis is that the motion of a bearing liner under cyclic load can produce fluctuating pressure pulses that are applied to bone directly through screw holes. In addition, the pulses may aid the transport of polyethylene wear debris particles into fixation interfaces. It is possible that lytic lesions previously associated with backside wear of the liner may be related to pumping of joint fluid by the liner.
The abstracts were prepared by Nico Verdoschot. Correspondence should be addressed to him at Orthopaedic Research Laboratory, Universitair Medisch Centrum, Orthopaedie / CSS1, Huispost 800, Postbus 9101, 6500 HB Nijmegen, Th. Craanenlaan 7, 6525 GH Nijmegen, The Netherlands.
