Restoring Global Offset Enhances Gluteus Medius Muscle Activity Following Total Hip Arthroplasty

Background: There is limited evidence investigating how the summed measure of femoral offset (FO) and acetabular offset (AO), known as global offset (GO), correlates with clinical outcomes, especially for postoperative abductor muscle strength. The aim was to assess (1) how changes in GO impact hip joint abductor muscle activity of the treated hip following total hip arthroplasty (THA) using surface electromyography (sEMG) and (2) to determine if such variations affect clinical and functional outcomes. Methods: 123 consecutive hips undergoing primary unilateral THA between October 2015 and October 2016 were prospectively assessed. The anterolateral approach in a supine position was used for each patient. Patients were divided into three groups according to postoperative GO measurements on the operated side and the contralateral side as follows: Group 1, GO < 4.5 mm (reduced); Group 2, GO = 5 ± 0.5 mm (restored); and Group 3, GO > 5.5 mm (increased). Based on their inclusion as the initial members of that group, 20 patients were selected consecutively for each cohort. The Kruskal–Wallis test was used to compare differences of mean sEMG improvements across the cohorts and to examine the influence of GO measurements on improvement in patient-reported outcome measures. Results: Significant improvement in postoperative muscle activity was observed across all groups (p < 0.05), with the restored GO group showing the greatest enhancements in gluteus medius and tensor fascia latae EMG activity (p > 0.05). The Kruskal–Wallis test revealed statistically significant differences in improvement in gluteus medius activity, particularly between the reduced and restored GO groups (p < 0.05), indicating that restoring the GO is associated with higher postoperative muscle activation. Significant improvement in the Harris Hip Score and EuroQol 5 Dimension scoring was observed within each cohort. Conclusion: Surgical restoration of GO for optimizing postoperative muscle activity should be respected. However, the direct impact of these biomechanical adjustments on patient-reported outcomes in the short term appears minimal.