Initial stability of a cementless acetabular cup design: experimental investigation on the effect of adding fins to the rim of the cup.

Different design solutions have been suggested for improvement of the initial stability of cementless acetabular cups, such as adding threads, spikes, or pegs to the hemispherical geometry, the pore structure of the surface; and screw fixation. This experimental study investigated the effect of fins on the initial stability of the acetabular cup. Three designs were studied, with none, 2, and 12 fins, respectively. The cups were press fit into cavities reamed in 2 different polyurethane foams, used to simulate 2 qualities of cancellous bone. Two millimeter press-fit and exact-fit conditions were investigated. The results show that the type of substrate and the interference value are important in determining the initial stability of the cup. The addition of fins on the cup rim enhances in vitro the initial stability, especially in cases of a poor press fit with a good substrate. This preclinical investigation suggests that the use of a cup design with fins may be beneficial in all cases in which press fit of the cup cannot be assured. However, further clinical studies are required to validate in vivo the efficacy of the fins as additional fixation devices.