EFFECT OF LOCKING MECHANISM ON FLUID AND PARTICLE FLOW-THROUGH MODULAR ACETABULAR COMPONENTS

Six modular acetabular components were evaluated to determine whether screw holes in the metal shell offer a route for fluid and debris into the acetabular bone stock. A 56-mm acetabular shell for each trial wa s mounted to a sealed chamber and loaded at a 25 degrees angle under a xial loads of 270-2,700 N and +/- 2.5-N-m torsional load. Polystyrene microspheres (average diameter, 0.5 mu m) were placed in double-deioni zed water at 300 mmH(2)O pressure in a sealed chamber above the compon ent. The only channel between the fluid above and the collecting chamb er below was through the cup-liner interface and 1 screw hole. Fluid a nd debris in the collecting chamber were harvested after 1,000,000 cyc les. The collected sample was filtered through a 0.2-mu m-pore filter and analyzed under electron microscopy for evidence of microspheres. W ater and polystyrene microspheres were isolated in the collecting cham ber for all trials except the Reflection cup (Smith & Nephew Orthopaed ics, Memphis, TN) with a screw hole cover and the Micro-Seal cup (Whit eside Biomechanics, St. Louis, MO) with a peripheral seal. A screw pla ced in the screw hole of the Reflection cup failed to seal the interfa ce. The peripheral seal around the rim of the Micro-Seal polyethylene prevented fluid and particle flow between the metal shell and polyethy lene liner.