Initial stability of porous coated acetabular implants. The effect of screw placement, screw tightness, defect type, and oversize implants.

The effects of screw location, screw tightness, acetabular bony defects, and implant size on the initial stability of porous coated acetabular implants was studied in vitro using 12 fresh frozen cadaver pelves. The fixation of porous coated acetabular implants was compared following fixation with either rim or center screws. Implants were tested first under axial and torsional stress. Then, either a peripheral or central bony defect was created in the acetabulum and testing was repeated in a similar fashion. Rim screws were noted to significantly improve the initial fixation of implants in intact acetabula and in acetabula with central bony defects. It was also shown that the tightness of the screws is an important factor affecting the strength of fixation. Although bony defects compromise the fixation of implants, specimens with central defects were very stable, particularly when tested with axial load. Acetabula with peripheral defects lost significant stability. The use of larger implants in acetabula with bony defects was shown to increase bony coverage and the overall implant stability.