The effect of cup medialization and lateralization on hip range of motion in total hip arthroplasty

Background: There is little description of the effect of cup position on the hip range of motion in total hip arthroplasty. The purpose is to evaluate the effect of cup medialization/lateralization with a compensatory increase/decrease in femoral offset on the hip range of motion, and whether the bone morphology of the anterior inferior iliac spine affects hip range of motion in total hip arthroplasty. Methods: Using the CT data of 100 patients (male; 30, female; 70), 3D-dynamic motion analysis was performed in four scenarios with cup medialization/lateralization with the same/decreased global offset. We calculated the range of motion before component impingement and bony impingement in flexion, internal rotation and external rotation using the software. Furthermore, we measured bony morphological features of anterior inferior iliac spine, and we analyzed the correlations among them. Findings: We found that the cup medialization with the same stem offset had negative effects on hip range of motion in flexion and internal rotation due to bony impingement, whereas cup medialization caused external rotation to significantly decrease with the same global offset. On the other hand, cup lateralization with the same global offset had negative effects on flexion and internal rotation, whereas external rotation increased. Furthermore, there were negative correlations among flexion and laterally large and steep anterior inferior iliac spine. Interpretation: Our results demonstrated that the advantage of cup medialization can depend on the individual anatomy such as bony morphology of anterior inferior iliac spine in flexion.