The ability of various acetabular components to resist protrusio migration.

A foam model of a protrusio acetabulum with cylindrical voids of varying diameters was used to test acetabular components for their ability to resist migration within the defect. A bipolar component, a hemispherical cup, and a hemispherical cup with peripheral rim augmentation were tested in defects with diameters ranging from 1 to 3 mm less than the major diameter of the component. The cup with peripheral rim augmentation demonstrated 20% to 40% greater resistance to migration than the hemispherical cup, depending on the extent of under-reaming, with the bipolar component consistently demonstrating the lowest load to failure. The results of this study confirm the less-than-satisfactory clinical results seen with bipolar reconstruction of protrusio acetabuli, and support the use of "fixed" cups in these settings.