Initial mechanical stability of acetabular prostheses

Secure initial stability is crucial for prostheses designed for biological ingrowth; without it, subsequent tissue ingrowth is impossible. Mechanical testing was performed in vitro to measure the interface shear stability of acetabular cup prostheses. Cemented, threaded, fined, and press-fit cups, both with and without supplemental screw fixation, were tested. A mechanically consistent polyethylene foam was used as a substrate to obviate the variations inherent in cadaver bones. Ramped, eccentric rim loads were applied using a servohydraulic materials testing system. Reaming, insertion, and loading variables were carefully controlled. Cups with external threads were significantly more stable than those fixed by fins or screws. Press-fit cups were significantly looser than all other tested designs. None of the uncemented prostheses approached the initial stability of cups secured with acrylic bone cement.