BIPOLAR PROSTHETIC REPLACEMENT FOR THE TREATMENT OF AVASCULAR NECROSIS OF THE FEMORAL-HEAD

Eighty-three hips in 66 patients with nontraumatic avascular necrosis of the femoral head (ANFH) showing evidence of severe collapse or secondary osteoarthritic changes were treated with surgical implantation of bipolar hip prostheses either with or without cement fixation of femoral stems. The cases were observed for more than three years (range, three to ten years seven months; average, five years seven months) and assessed in terms of functional and roentgenographic results. These data were compared with the results of classic hemiarthroplasties using fixed head prostheses (Austin-Moore-type with curved or straight stem) that were performed at the authors' institutions before 1980. The results confirm that the clinical outcome is improved with use of the bipolar prosthesis. Satisfactory results (a score of 80 or greater on a hip function scoring system proposed by the Japanese Orthopaedic Association) were maintained throughout the follow-up period in most cases (71 of 83 hips) with the bipolar prosthetic replacements. In a group of patients with unsatisfactory results (12 hips), proximal migration of prosthetic head was seen in two cases. The incidence of proximal migration of bipolar heads, including those exhibiting minimal movement, was significantly lower when compared with that observed in an Austin-Moore-type head-fixed prosthesis group (7/83 versus 12/19) during a comparable postoperative follow-up period. It is noteworthy that the proximal migration of the bipolar head was not progressive, and, in most cases observed more than five years, the cartilaginous spaces of acetabulum were preserved, whereas in the Austin-Moore-replaced group, the migration was evident and progression occurred within three years of surgery. Radiologic loosening of cementless femoral stems of the bipolar prostheses were judged by their subsidence or the angular shift of femoral stems (or both), and the incidence of loosening was lower in the bipolar prosthesis group (14/74). Stem-loosening was seen predominantly in cases where the thinner Moore-type stems had been used, thus producing a lower medullary canal filling ratio. In cases involving the Moore-type stems either with or without bipolar bearing, the stem loosening rate was significantly higher (49%) than in cases where a thicker Omnifit or Harris-Galante type (6%) stem had been used, at three years postsurgery. In conclusion, the results indicate that the bipolar hip prostheses are preferred for the treatment of advanced ANFH, provided that stable initial fixation can be achieved. Whether this choice continues to be successful for longer postoperative periods remains to be seen.