Effects of Acetabular Cup Orientation and Implant Design on Psoas Impingement in Total Hip Arthroplasty

Abstract

Background:  Durable fixation has been demonstrated with use of large (jumbo) cementless cups in revision total hip arthroplasty (THA).    However, anterior protrusion of the cup rim may impinge on the iliopsoas tendon and cause groin pain.  The purpose of this study was to assess the effect of cup position and implant design on iliopsoas impingement.Methods:  THA was performed on six cadaver hips using oversized (jumbo) acetabular components, 60 to 66mm. A stainless steel cable was inserted into the psoas tendon sheath to identify the location of the psoas muscle.  CT scans were performed on each cadaver and imported in an imaging software. The acetabular shells, cables, and pelvi were segmented to create separate solid models of each. The shortest distance between each shell and cable was measured. To determine the influence of cup inclination and anteversion, the inclination (30°/40°/50°) and anteversion (10°/20°/30°) angles were varied in the virtual model for both a hemispheric and offset head center shell design. Results:  The shell to wire distance increased linearly with greater cup anteversion (R2>0.99) while inclination had less effect.  The distance was greater for the offset head center cup in comparison to the hemispheric cup.  Our results indicate that psoas impingement is related to both cup position and implant geometry. Conclusions:  For an oversized jumbo cup, psoas impingement is reduced by greater anteversion while cup inclination has little effect.   An offset head center cup with an anterior recess was helpful in reducing psoas impingement in comparison to a conventional hemispherical geometry.