A radiographic evaluation of birotational head motion in the bipolar shoulder hemiarthroplasty

The concept of bipolar hemiarthroplasty has been described in the hip for more than 20 years, its role being to decrease acetabular wear. Shoulder bipolar hemiarthroplasty was described shortly afterward. The purpose of this study was to determine if the prosthesis acted as a bipolar device moving primarily at the inner metal on polyethylene bearing as intended, or as a unipolar hemiarthroplasty moving at the outer metal on glenoid surface articulation. Eleven bipolar shoulder hemiarthroplasties, with a minimum follow-up of 22 months, were examined fluoroscopically. The proportion of arm abduction occurring at the scapulothoracic articulation and between the two components of the bipolar hemiarthroplasty was assessed and compared with that of normal patients and those with total shoulder replacements previously reported in the literature. The results of this study show that most of the movement in active arm abduction occurred at the scapulothoracic articulation and that the bipolar hemiarthroplasty acted predominantly as a unipolar device.