The Position of the Bipolar Cup Reflects the Direction of the Hip Contact Force Acting on It

We radiographically measured the bipolar cup position to analyze the direction of joint force acting on the bipolar cup. The abduction angle of the bipolar cup was measured in each radiograph taken immediately and at six 12 weeks and yearly after the operation. Radiographs in patients with weight bearing were also investigated. The results indicated that the abduction angle of the bipolar cup was 24.1° ± 11.2° immediate postoperatively and was 16.2° ± 5.1° at 6 weeks, 16.1° ± 5.1° at 3 months, and 16.2° ± 5.1° at 1 year. The cup abduction angles with weight bearing were not different from those without weight bearing and were 15.9° ± 4.9°, 16.2° ± 4.4°, and 16.1° ± 4.7° on the supine, double-legged stance, and single-legged stance radiographs, respectively. Because the position of the bipolar cup reflects the direction of loads pivoting on it, the direction of the joint force in the frontal plane acting on the bipolar prosthesis is about 16° to vertical.