A larger sized cup accelerates cartilage erosion of acetabulum after bipolar hemiarthroplasty in elderly with femur neck fracture

Bipolar hemiarthroplasty (BHA) is one of the common procedures done for the treatment of femur neck fracture. One of the frequently encountered complication with this surgery is erosion of the acetabular cartilage. This study was conducted to investigate acetabular erosion after BHA according to the difference in diameter between femoral head and implanted cup at minimum 10-year follow-up.We retrospectively reviewed 117 patients (117 hips) undergoing BHA with fracture of neck of the femur. Their mean age was 77.8 years (range, 65-96 years) and male: female ratio was 32:85. Patients were divided into 3 groups; Group A - bipolar cup size > actual head size, Group B - cup size < head size, Group C - cup size = head size. The degree of both superior and medial acetabular cartilage erosion was identified and calculated on postoperative radiographs using line of acetabular margin and Kohler line.The mean superior and medial acetabular erosion were 1.62 ± 1.6 mm (range, 0-4.4) and 4.15 ± 2.7 mm (range, 0-8.2) in Group A, 1.30 ± 1.3 mm (range, 0-3.8) and 4.11 ± 2.7 mm (range, 0-7.8) in Group B, and 0.90 ± 1.1 mm (range, 0-2.6) and 3.16 ± 2.9 mm (range, 0-7.9) in Group C (P = .039 and P = .187, respectively). The superior acetabular erosion showed significant difference between the 3 groups. During mean follow-up period of 12.3 years, 5 patients (5/117, 4.3%) underwent conversion to total hip arthroplasty due to superior acetabular erosion. All of 3 patient underwent BHA with a larger bipolar cup than the actual femoral head.A lager sized cup accelerated superior cartilage erosion of acetabulum after BHA. An optimal cup size should be considered when undergoing BHA in elderly patients.