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 degrees +/- 11.2 degrees immediate postoperatively and was 16.2 degrees +/- 5.1 degrees at 6 weeks, 16.1 degrees +/- 5.1 degrees at 3 months, and 16.2 degrees +/- 5.1 degrees at 1 year. The cup abduction angles with weight bearing were not different from those without weight bearing and were 15.9 degrees +/- 4.9 degrees , 16.2 degrees +/- 4.4 degrees , and 16.1 degrees +/- 4.7 degrees 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 degrees to vertical.