To determine the motion distribution of bipolar hip prosthesis, we performed experimental and clinical analyses.

In the experimental studies, component motion was examined according to surface lubrication, cup angle, and acetabular reaming in fresh-frozen cadaveric bone. In the clinical examination, thirty patients with a bipolar hip prosthesis were examined by fluoroscopy.

In the experimental condition when both joints were well lubricated, motion occurred mainly at the inner joint. However, slight motion of the outer joint appeared in terminal abduction if the acetabular cup angle was low and joint pressure was applied. When the only the outer joint was lubricated, motion occurred mostly at the outer joint. When both joints were not lubricated, motion occurred at outer joint unless the acetabular reaming was performed. In every situation having acetabular reaming, motion occurred only at the inner joint. Clinically, motion distribution varied between patients. However, we found a predominant contribution was made by the outer joint, in contrast to theoretical prediction, and the persistence of the combined motion of two joints in the majority of the patients studied (67-70 percent).

We found that the motion was combined in both outer and inner joint in two-thirds of patients, and that outer motion was more predominated.