Impact biomechanics and pelvic deformation during insertion of press-fit acetabular cups.

Five fresh cadaver pelves were cleaned of soft tissue and instrumented with strain gauges. The acetabula were reamed, and a cementless cup, oversized at the periphery, was inserted. The applied force and cup acceleration were measured during insertion and used to calculate an effective mass of the cup, insertion device, and pelvis during each impact. Periacetabular strains increased variably during cup seating. After the cups were seated, strains continued to increase with postseating impacts. The effective mass remained constant throughout the test, indicating that cup seating is not associated with a change in acceleration. This finding implies that an accurate assessment of cup seating cannot be inferred by surgeon proprioception during impaction, and use of an apical hole in the cup is necessary to determine when the cup has seated.