Press-fit acetabular cup fixation: principles and testing

Pre-clinical testing of the fixation of press-fit acetabular components of total hip prostheses relies on cadaver or synthetic bone, but the propertie s and geometry of bone models differ from those of physiological bone. Cup designs use varied mechanisms for initial stability in bone; therefore, usi ng different analogues and tests is appropriate. Press-fit cup stability was tested in the following: firstly, polyurethane (PU) foam modelling cancellous support; secondly, glass-fibre reinforced ep oxide (GFRE) tubes modelling acetabular cortical support; thirdly, cadaveri c acetabula. Three commercial cups [Harris-Galante II (H-G II), Zimmer; Opt ifix, Smith & Nephew, Richards; porous coated anatomic (PCA), Howmedica] an d an experimental cup with enhanced rim fixation were tested in three modes : direct pull-out, lever-out and axial torque. The fixation stabilities measured in the PU and the GFRE models showed tren ds consistent with those in cadaver bone, differing in the oversizing and c up geometry. The experimental cup was significantly more secure in most mod es than other cups; the H-G II and Optifix cups showed similar stabilities, lower than that of the experimental cup but greater than that of the PCA c up (analysis of variance and Tukey's highly significant difference test; p < 0.001). The stabilities measured in cadaver bone more closely approximate d those in GFRE. The use of several bone analogues enables separation of fi xation mechanisms, allowing more accurate prediction of in vivo performance .