Identification of Combined Anteversion Targets to Optimize Impingement-free Range of Motion Across Femoral Head Sizes: A Simulation Study.

Background: Dislocation remains a risk following total hip arthroplasty (THA). While prior studies have proposed optimal combined anteversion (CA) ranges to maximize impingement-free range of motion (IFROM), the influence of bony impingement and femoral head size remains underexplored. This study utilized computed tomography-based robotic planning software to identify a target CA that optimized IFROM and to evaluate the effect of head size. Methods: Five patients (mean age: 63.2 years) who underwent robotic-assisted THA were included. Using computed tomography-based planning software, femoral and acetabular version were systematically adjusted from 0° to 37°, simulating bony and implant impingement in flexion and internal rotation and extension and external rotation. Simulations were performed with 28-, 32-, 36-, and 40-mm heads, totaling 57,750 tests. Functional range of motion (ROM) was defined as ≥40° internal rotation at 90° flexion and ≥10° external rotation at 20° extension. For each head size, maximum IFROM and acceptable CA configurations meeting functional ROM criteria were recorded. A mathematical formula was developed to describe optimal CA per head size. Results: For all patients and head sizes, the optimal CA was described by (acetabular anteversion+(0.7×femoral anteversion) = 36 and was independent of native version. Larger femoral heads were associated with broader functional ROM. Acceptable CA angles increased by 158% from 28 to 32 mm, 86% from 32 to 36 mm, and 31% from 36 to 40 mm. Conclusions: Larger femoral head sizes increase IFROM and expand the range of CA positions achieving functional ROM. These findings highlight the importance of head size selection in optimizing IFROM and surgical planning for THA.