Strength of initial mechanical fixation of screw ring acetabular components

This study was conducted to determine the effects of design on the initial fixation of several types of screw-ring acetabular components. The components were tested in polyurethane foam to assess relative screw fixation strengths with a consistent material. Embalmed pelves from anatomic specimens were used to conduct paired tests between designs that showed large differences in insertional torque to failure in foam. The quality of the initial fixation in foam was found to be dependent on the design features of the components. Components with widely spaced, deep threads, and minimal thread interruptions offered the strongest initial fixation in foam. Tests in bone revealed a wide range of fixation strengths reflecting the variability in bone quality. No differences in fixation strength attributable to component design were observed in bone. When the insertional failure torque was greater than 60 N.m, one-half of the pelves fractured, and these fractures occurred with all designs. At failure torques less than 60 N.m, failure was predominantly due to thread strippage of the screw, with only two of 20 specimens experiencing pelvic fracture.