Effect of interposed tissue and contamination on the initial stability of a highly porous press-fit acetabular cup.

For biologic fixation, press-fit acetabular cups should have initial stability with minimal micromotion and osteoconductive surfaces in contact with bone. Inadequate exposure potentially influences initial stability by increasing the possibility of soft tissue interposition and contamination at the implant-tissue interface. A sawbone model was used to examine how interposed tissue and contamination influence initial cup stability. Seven groups (n = 4) were tested with varying levels of interposed fatty and fibrous tissue placed around the rim of the cup. 54 millimeter in diameter highly porous hemispherical acetabular cups (Stryker, Mahwah NJ) and 54 mm reamed cavities in sawbone blocks were used. Shells were seated and maximum lever out force was recorded for each sample. Cups with fibrous tissue spaced evenly along the rim had a lever out force that was 150% of the control (107 ± 6 vs. 150 ± 12N, p = 0.005), and fatty tissue contamination had a lever out force that was 140% of the control (143 ± 18 vs. 107 ± 6N, p = 0.04). Cups with fibrous tissue placed eccentrically along the rim had a lever out force that was double the control 107 ± 6 N vs. 200 ± 15 N (p = 0.001). Surprisingly, fatty tissue contamination and fibrous tissue interposition at the rim increased initial stability. The eccentrically interposed tissue forced the opposite pole of the cup into the bone, resulting in a more secure press-fit. However, soft tissue interposition decreases implant/bone apposition, and the effect on long term fixation is unknown. Statement of Clinical Significance: Soft tissue interposition between the bone and cup may provide higher initial stability, but its long-term effects are unknown.