Bipolar hemiarthroplasty of the hip has been used to treat fractures of the femoral neck, avascular necrosis of the femoral head, and osteoarthritis.3,6,10,17,19,21,22 The theoretical advantages of bipolar components, compared with unipolar, include increased range of motion and prevention of acetabular erosion or protrusion; however, there have been numerous reports of acetabular erosion or protrusion in patients treated with bipolar components.3,6,10,17–19,21 There also have been reports of disassembly of the acetabular components and fracture of the polyethylene bearing insert after bipolar hemiarthroplasty using a Bateman prosthesis (UPF-I).3,6,8 This type of failure occurs at the junction between the leaflets and the main body of the bearing insert, where the insert has a deep circumferential groove. If the outer cup is in varus alignment, stress is concentrated on the thin circumferential groove, resulting in failure.
In this study, the cause of failure of the locking mechanism with a self-centering system was marked polyethylene wear at the rim because of impingement, followed by deformity of the locking ring. After the locking ring was detached (Type I), the inner head dislocated from the outer head, with the outer head remaining in the acetabulum in varus position (Type II). If the deformed locking ring was not detached, dislocation of the inner head could occur, with the varus outer head remaining in the acetabulum (Type III; Fig 4). The cause of failure of this prosthesis was different from that of the Bateman UPF-I.3,8 Ito et al10 reported the incidence of disassembly of bipolar cups with a self-centering system was 4% (one of 23 hips) for the Osteonics UHR and 17% (one of six hips) for the Bi-Articular Cup, although the denominator of the latter was very small. The incidence of failure (11%) in the current study is comparable with that of their study,10 and orthopaedic surgeons must be aware that problems with disassembly are not rare numerous years after a well-functioning bipolar hemiarthroplasty, even when a self–centering system was used.
Polyethylene wear debris from the rim of the polyethylene insert caused disassembly of the bipolar mechanism, osteolysis, and stem loosening.12,16,17 Osteolysis and stem loosening also have been recognized as problems caused by polyethylene wear from bipolar bearing surfaces.5 Maloney et al14 showed that the total number of polyethylene particles associated with failed bipolar hemiarthroplasties was higher than the number of particles present in failed total hip replacements. Kim and Rubash11 reported that the particles generated by impingement are large. Polyethylene with a thickness less than 6 mm is associated with rapid wear from bearing surfaces2,5; in the current study, the minimum polyethylene thickness in the bipolar cups was 7 mm. The activity level of patients with total hip arthroplasty is an important factor affecting polyethylene wear.7 After bipolar hip arthroplasty, patient activity level affects wear. Three of the current six patients were young (younger than 50 years) and their activity levels were relatively high, however, none did heavy manual labor. The problems of polyethylene wear and subsequent osteolysis in bipolar hemiarthroplasty have not been resolved. Messieh et al16 described a structural defect of the bipolar head in cases in which the self-centering mechanics were fully functional. Impingement of the femoral neck against the rim of the polyethylene insert is the main cause of failure. The oscillation angles, or prosthetic arc of motion, of the Bi–Articular Cup used in the current patients were 53° for the medium neck and 44° for the long neck with skirt. During normal gait, the arc of the hip is small and impingement is avoidable. However, when ascending or descending stairs or standing up from a chair, a wider arc of motion is required that exceeds the oscillation angle of the bipolar cup, resulting in impingement.18 Nakata et al17 reported progressive migration of the outer head and massive acetabular osteolysis 5 years or more after bipolar hip arthroplasty for OA. Cabanela4 reported the results of bipolar hip arthroplasty in avascular necrosis were inferior to those of total hip arthroplasty. Surgeons should not use bipolar prostheses in patients for the treatment of arthritis.
The Bi-Articular Cup used in our patients no longer is available. It has been replaced with the Multipolar Cup, which has an improved locking ring mechanism and increased oscillation angle. The ring is assembled from the side of the polyethylene insert. Even if the rim of the polyethylene insert is damaged by impingement, the ring does not pull out. According to the manufacturer, the torque required to dislodge a polyethylene insert from an outer shell in a lever-out test was 103 inch-pounds for the Bi-Articular Cup and 362 inch-pounds for the Multipolar Cup. The neck of the stem has been improved. It now is more slender to increase the oscillation angle and is polished to reduce wear at the rim of the polyethylene insert after impingement.21 However, the bipolar cup design still may generate polyethylene particles.
Gamma irradiation in air had been the standard sterilization technique since the early 1970s. During the 1990s, we became aware that sterilization with gamma radiation produces free radicals that subsequently can oxidize, reducing the strength and ductility of the polyethylene. These changes reduce the resistance to wear. To avoid the problems caused by oxidation resulting from gamma sterilization in air, orthopaedic implant manufacturers are using alternate sterilization techniques, including gamma sterilization in vacuum or inert atmosphere, ethylene oxidation, or gas plasma.1,15 With the proper sterilization technique, this failure would not occur.
This study has some limitations. First, we could not determine when the failure occurred. A diagnosis of the failure might have been delayed several months, because the patients experienced no characteristic symptoms except mild pain. In failure with inner head dislocation, previous mild pain might be a warning of the failure, however, dislocation occurred suddenly with acute severe pain. Second, we have not retrieved autopsy specimens with this implant and therefore, could not examine changes of the implant, such as wear, impingement, and locking ring deformity, in patients with well-functioning implants.
The cause of failure of the Bi-Articular Cup was marked polyethylene wear at the rim because of impingement, followed by deformity of the locking ring. The hypothesis that the cause of failure of the Bi-Articular Cup would be different from that of the Bateman UPF-I was proven correct. It seems likely that problems with the Bi-Articular Cup will be observed with greater frequency in the future, and patients with this prosthesis should be carefully followed up.
We thank Toshihiro Imahara, MD, PhD and Makoto Nishimura, MD of the Department of Orthopaedic Surgery, Matsusaka Saiseikai Hospital, for contributions to this study.
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© 2004 Lippincott Williams & Wilkins, Inc.
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