LAUSR

Purpose To investigate whether anatomic and straight stems could reproduce the anteroposterior distance (AD) of the native femoral head and evaluate the effect of AD of the femoral head on range of motion (ROM) and bony impingement. Methods This retrospective simulation study included 64 patients who had undergone primary total hip arthroplasty between 2012 and 2014. Using computed tomography (CT)–based templating software, anatomic and straight stems were inserted with same alignment. AD of the head centre was compared between the two stems and native anatomy. Furthermore, post-operative ROM was calculated, and correlation between AD and ROM was assessed. Results There was a strong positive correlation between native anteversion (mean 21.9°) and anatomic stem anteversion (mean 22.5°) ( R  = 0.975, P  < 0.001). There was no significant difference in AD between the native and anatomic stems (mean 37.7 and 38.8 mm, respectively), but AD of the straight stem was significantly lower than that of the native and anatomic stems. The straight stem showed a significantly lower ROM in flexion and internal rotation angles with 90° flexion (IR) than the anatomic stem ( P  < 0.05 and P  < 0.001, respectively). AD showed a stronger correlation with ROM of IR than with stem anteversion. Conclusions The anatomic stem could reproduce AD of the native femoral head centre, but the head centre of the straight stem in the same anteversion with anatomic stem translated significantly posterior, significantly decreasing the ROM of flexion and IR and increasing bony impingement of IR. To avoid bony impingement and acquire sufficient ROM, reproducing AD was important.