Effects of screw fixation on the acetabular cup

The initial stability of a cementless acetabular cup would influence the bone ingrowth and long-term success rate. The objective of this study is to investigate the effect of screw fixation patterns on the stability of the acetabular cup by finite element simulation. A commercially available cup with five screw holes was used as the target model for analyses. The peak displacement on the cup under gait loading was used as the evaluation index. From the simulations, it showed that the screw provides a stable region around the purchased location and the peak displacement occurred at the cup rim, which indicated a lever behavior of loaded cup. For multiple-screw placement, the general principle is to place screws as separately as possible and, if feasible, place the screw near the cup rim so that it nears the peak displacement site and provide a larger stable region. Finally, a sound fixation with three screws is sufficient because the fourth and fifth screws provide minimal benefit.