Acetabular strains produced by oversized press fit cups.

The purpose of this study was to determine the amount of acetabular deformation that occurs when acetabular components of different sizes are press fit. The change in acetabular volume was calculated for 1- to 3-mm oversized cup diameters with acetabuli ranging from 40 to 80 mm. An axisymmetric finite element model was used to determine the maximum acetabular strain that occurs when oversized cups are inserted in different acetabuli. The highest acetabular strains occur at the cup periphery, indicating that the resultant compressive force between the lateral pelvis and cup enhances press fit stability. In addition, higher maximum strains occur when the same millimeter amount of oversizing is used in a small compared with a large acetabulum. When a constant millimeter increment in cup oversizing is used, there may be a greater risk of pelvic fracture in a small acetabulum and a greater risk of inadequate press fit stability in a large acetabulum. To achieve the same amount of acetabular deformation and magnitude of strain, more oversizing is needed in a large acetabulum and less oversizing is needed in a small acetabulum. These results suggest that 1 mm of oversizing should be used for cups that are less than 52 mm in diameter, 2 mm of oversizing should be used for cups from 52 to 76 mm in diameter, and 3 mm of oversizing should be used for cups that are more than 76 mm in diameter.