Stability of press-fit acetabular cups

Mechanical tests were performed to characterize the initial stability of press-fit cups as a function of cup design, surface structure, and surgical preparation. Eight cups from six manufacturers were press-fit into acetabular cavities prepared in two densities of Sawbones polyethylene foam and in bovine knee trabecular bone. Cavity sizes and cup loading forces were varied. Acetabular defects were simulated in the Sawbones model. Preparations were tested to determine axial-rotatory and tangential ("levering-out") stability. Results suggested that cup geometry and proper surgical technique-in particular, proper sizing and depth of the acetabular cavity-are important in determining initial cup stability independent of adjuvant screw or spike fixation. Stability is a function of the area of interface contact between the cup rim and the substrate. If the cavity is too small or too shallow, and the substrate too dense, the cup will not seat to the rim and stability will be compromised. If there are defects in the rim, the area of interface contact will be diminished and stability compromised. Cups with a true hemispherical design have a greater area of rim interface contact than "low-profile" cups and are therefore more stable. 1 mm undersizing of the cavity (or 2 mm undersizing in less dense substrate) appears to provide optimal stability.