Femoral head to neck offset after hip resurfacing is critical for range of motion

Abstract Background Range of motion after hip arthroplasty may be limited by soft tissues around the hip, extra-articular contact and femoral stem-neck contact with the acetabular articular surface. Femoral head–neck diameter ratio is recognized as a major factor influencing hip range of motion. In hip resurfacing, range of motion is constrained by “cup component to femoral neck” contact. To avoid cup-to-bone contact or to increase the degree of flexion at which it occurs, anterior translation of the femoral component relative to the central femoral neck axis may improve anterior head–neck offset and hip flexion. We questioned whether low or high anterior femoral head to neck offset, cup inclination, stem anteversion, and component size influenced postoperative range of motion and hip flexion in patients who had undergone hip resurfacing. Methods We prospectively followed 66 patients (68 hips) who underwent hip resurfacing at a mean age at operation of 46.4 years (range, 19–60 years). Mean follow-up was 37.5 months (range, 33–41 months). No patient was lost to follow-up. All patients were evaluated clinically and range of motion was precised. Radiological measurement evaluated the anterior femoral head–neck offset. Findings Mean anterior neck–head offset was 7.5 mm (range, 5–12 mm). We found significant linear regression correlation between anterior offset and flexion (R = 0.66) and between anterior offset and global range of motion (R = 0.51). One millimeter of anterior offset increased hip range of motion by 5° in flexion. No significant correlations were found between global range of motion or flexion arc of motion and component size, stem anteversion, cup inclination, gender ratio, preoperative arc of flexion or global range of motion. Interpretation Restoring or improving deficient anterior femoral head–neck offset appears important for restoring postoperative range of motion and specifically hip flexion.