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Disassembly of Bipolar Cup with Self-Centering System: A Report of Seven Cases

Hasegawa, Masahiro; Sudo, Akihiro; Uchida, Atsumasa

Clinical Orthopaedics and Related Research: August 2004 - Volume 425 - Issue - p 163-167
SECTION II: ORIGINAL ARTICLES: Hip
Free
SDC

Disassembly of bipolar cups with a self-centering system occurred in six patients (seven hips; five women, one man) and the cause of the failure was evaluated. The mean age of the patients at the time of arthroplasty was 49.7 years (range, 27–85 years), and mean weight was 48.4 kg (range, 37–65 kg). The mean time to failure was 7.5 years (range, 4.8–9.2 years). Before failure, all implants functioned well and none of the patients had sustained trauma. Retrieval study showed that the cause of failure of the locking mechanism with a self-centering system was severe polyethylene abrasion at the rim attributable to impingement and deformity of the locking ring. After the locking ring detached, the inner head dislocated from the outer head resulting in a varus position of the outer head in the acetabulum. If the deformed locking ring did not detach, the inner head could dislocate, with the varus outer head remaining in the acetabulum. The incidence of this failure was 11%. This disassembly is not a rare occurrence numerous years after a well-functioning bipolar hemiarthroplasty, even with the self-centering system.

From the Department of Orthopaedic Surgery, Mie University Faculty of Medicine, Mie, Japan.

Received: September 9, 2003

Revised: December 1, 2003

Accepted: January 16, 2004

Each author certifies that he has no commercial associations (e.g., consultancies, stock ownership, equity interest, patent/licensing arrangements, etc) that might pose a conflict of interest in connection with the submitted article.

Correspondence to: Masahiro Hasegawa, MD, PhD, Department of Orthopaedic Surgery, Mie University Faculty of Medicine, 2-174 Edobashi, Tsu City, Mie 514-8507, Japan. Phone: 81 59 231 5022; Fax: 81 59 231 5211; E-mail: masahase@clin.medic.mie-u.ac.jp.

Disassembly of bipolar Bateman UPF-I cups has been reported previously.3,6,8 The mechanism of failure in Bateman prostheses is varus loading of the bearing insert with stress concentration directly over the deep circumferential groove of the superolateral leaflets.8 To prevent varus alignment of the outer head, a self-centering system was developed that involves eccentric offset of bearing centers, with the outer cup center located distal to the inner bearing center. With a self-centering system, normal force at the articulating surface between the acetabulum and outer head should rotate the outer head into a valgus position.13 Also, a locking ring mechanism was developed to replace use of leaflets. However, disassembly of acetabular components with a self-centering system has been reported.9,10,20

Disassembly of acetabular components with a self-centering system (Bi-Articular Cup; Zimmer, Warsaw, IN) after bipolar hemiarthroplasty occurred in six of our patients (seven hips). At our institution during the time represented by these cases (1987–1993), 61 devices were implanted. No other bipolar designs were used during that time. The hypothesis was that the cause of disassembly of the Bi-Articular Cup with a self-centering system would be different from that of the Bateman UPF-I without a self-centering system.

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MATERIALS AND METHODS

Six patients (five women, one man; seven hips), experienced prosthesis failure after primary hip hemiarthroplasty using a bipolar head. The mean age of the patients at the time of arthroplasty was 49.7 years (range, 27–85 years), and mean weight was 48.4 kg (range, 37–65 kg). The preoperative diagnoses were avascular necrosis of the femoral head in four patients (five hips) and osteoarthritis (OA) in two patients (two hips). The bipolar cup used in all hips was the Bi-Articular Cup, which is a factory-assembled nonmodular prosthesis in which the polyethylene insert is fixed to a Co-Cr alloy shell. The polyethylene was manufactured by ram extrusion, and was sterilized with gamma irradiation in air. All femoral components were implanted without cement. An Anatomic Stem (Zimmer) was used in five hips, and a Multilock Stem (Zimmer) was used in two hips; both were made of Ti alloy. The femoral head used in all hips was made of Co-Cr alloy with a diameter of 22 mm. The inner head was snapped into the shell (outer head) at the time of the operation. In all hips, postoperative radiographs showed good alignment and seating of the prosthetic components. Acetabular component dimensions and oscillation angle were recorded. The mode of failure of the locking mechanism was recorded.

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RESULTS

Sixty-one Bi-Articular Cups were implanted, resulting in an incidence of 11% for failure of the locking mechanism. When this failure was diagnosed, none of the patients had sustained trauma, but all had pain. The pain was severe in patients with inner head dislocation and mild in patients without inner head dislocation. All patients had mild pain for 1 week to 4 months before diagnosis of the failure. The mean time to failure was 7.5 years (range, 4.8–9.2 years). Before failure, all implants functioned well and patients had no problems. The original dimensions of the implants are shown in Table 1. Migration of the outer head was seen in six of the seven hips. The mean superior migration was 3.3 mm (range, 0–10 mm), and the mean medial migration was 1.4 mm (range, 0–3 mm). Acetabular osteolysis was present in four hips, and femoral osteolysis was present in one hip.

Table 1

Table 1

Three types of failures of the locking mechanism were identified. In three hips, the locking ring was detached and was seen around the neck of the stem (Type I; Fig 1A). In three hips, the outer head was in an extreme varus position in the acetabulum, with dislocation of the inner head from the outer head, and the detached locking ring was seen around the neck of the stem (Type II; Fig 1B). In one hip, the outer head was in an extreme varus position in the acetabulum, with dislocation of the inner head (Type III; Fig 1C).

Fig 1.

Fig 1.

In all cases, the polyethylene insert showed severe abrasion at the rim at the time of revision, attributable to impingement of the neck of the femoral stem against the rim of the polyethylene insert. Deformity of the locking ring was seen (Fig 2A) as compared with an unused implant (Fig 2B). The femoral component was well-fixed in six hips and loose in one hip. The loose stem was removed and replaced with a new implant. Stable stems were not removed, and new heads made of Co-Cr alloy were attached to the neck. In five hips, the acetabulum was reconstructed using a cementless porous-coated acetabular component (Trilogy; Zimmer); in two hips, a bipolar cup with an improved locking mechanism was used (Multipolar Cup; Zimmer). In all cases, histologic examination of the granulation excised around the cup revealed numerous macrophages and foreign-body giant cells with polyethylene particles (Fig 3).

Fig 2.

Fig 2.

Fig 3.

Fig 3.

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DISCUSSION

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.

Fig 4.

Fig 4.

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.

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Acknowledgments

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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