The impact of increasing femoral offset and stem anteversion on postoperative dislocation in bipolar hemiarthroplasty

Background: Femoral offset and anteversion have been reported to affect the incidence of dislocation following bipolar hemiarthroplasty, although the magnitudes of contributions of the femoral offset and stem anteversion on dislocation, both singly and in combination is not fully understood.Methods: Using the CT data of 61 patients (61 hips), including 30 male and 31 female who underwent bipolar hemiarthroplasty, three-dimensional dynamic motion analysis was performed using a modular implant that enabled adjustment of femoral offset and stem anteversion independently. The pattern of impingement and relationship between femoral offset/stem anteversion and range of motion were evaluated using the software.Findings: We found that a higher femoral offset and stem anteversion correlate with a greater range of motion of flexion and internal rotation. Furthermore, an increased femoral offset has a great effect on increasing range of motion of flexion than stem anteversion, and increased both femoral offset and stem anteversion have fewer effect on the flexion, whereas increasing stem anteversion has a great impact on internal rotation than offset, especially in the case with lower femoral anteversion. However, a higher stem anteversion decrease the range of motion of external rotation, whereas a higher femoral offset leads to an increased range of motion of external rotation.Interpretation: We demonstrated that both a higher femoral offset and stem anteversion substantially affected the range of motion in flexion, internal rotation and external rotation. However, these are not independent, but rather mutually confounding, the surgeons should consider retaining femoral/anterior offset in bipolar hemiarthroplasty.