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 defect s, and implant size on the initial stability of porous coated acetabul ar implants was studied in vitro using 12 fresh frozen cadaver pelves. The fixation of porous coated acetabular implants was compared follow ing fixation with either rim or center screws. Implants were tested fi rst under axial and torsional stress. Then, either a peripheral or cen tral bony defect was created in the acetabulum and testing was repeate d 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 th e screws is an important factor affecting the strength of fixation. Al though bony defects compromise the fixation of implants, specimens wit h central defects were very stable, particularly when tested with axia l 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.