Engineering considerations in the design of double-cup hip replacement prostheses.

The diameter of the femoral neck ought not to be reduced, and to provide sufficient ranges of motion without impingement requires the largest practicable articular surface radius and perhaps a less than hemispherical acetabular articular surface. Wear is less with plastic cups and metallic balls than vice versa, and wearing life requires some minimum acetabular wall thickness, which depends on the diameter of the articular surface. Fixation grooves etc. should not reduce the effective wall thickness in highly-stressed regions. In practice, a nearly hemispherical acetabular component will often protrude medially or (preferably) laterally; such a protrusion is unlikely to weaken the fixation but could cause impingement, and could be removed with no loss. The available volume is best used by having the thickest acetabular wall where the wear should be greatest. For the femoral component, the simplest internal surface, a flat-ended cylinder, is also the best. A part-spherical inner surface cannot easily be filled with cement, and a skirt surrounding part or all of the femoral neck offers no advantages and some disadvantages.