Taper-Trunnion Interface Stress Varies Significantly With Head Size and Activity

Total hip arthroplasty is performed with modular parts. Either a metal or ceramic ball is fastened to the trunnion of a femoral stem via a Morse taper. This implant scenario has been successful. However, recently larger (36 mm or greater) metal heads have become more popular as a means to reduce the incidence of hip joint dislocation. Today, a number of clinical failures have occurred due to mechanically assisted crevice corrosion at the head (taper) stem (trunnion) interface necessitating revision surgery. The objective of this research is to investigate how trunnion stress varies with head size, and how taper-trunnion geometric parameters including horizontal lever arm (HLA), taper engagement level, and a new parameter called trunnion load offset affect trunnion stresses. We hypothesized that trunnion stress may increase with increasing head size and HLA.

This hypothesis was tested by conducting finite element analysis of a titanium hip stem and 4 commercially available cobalt-chromium femoral heads subjected to 4 different moderate to severe physiological loading conditions.

Results showed that trunnion stress increases with increasing head size, increased HLA, and trunnion load offset. It was also found that under certain load cases the trunnion stresses get exceptionally high, especially for the larger head sizes.

This study suggests trying to avoid larger femoral head sizes that may result in higher implant stresses under certain loading conditions.