Bipolar hemiarthroplasty was introduced in the early 1970s. The theoretical advantages of bipolar compared with unipolar components include increased range of motion and stability as well as decreased wear of the acetabulum. Bipolar components have been used extensively for patients who have a fracture of the femoral neck. The indications for the use of a bipolar component have been broadened to include avascular necrosis of the femoral head, degenerative osteoarthrosis, and revision hip arthroplasty.
Polyethylene wear and subsequent osteolysis have been recognized as problems in association with fixed acetabular components. Bipolar implants are also susceptible to failure of the bearing surface and its sequelae. The polyethylene wear associated with a bipolar implant is difficult to measure radiographically because of the design of the implant. The purpose of the current report is to demonstrate that polyethylene wear and subsequent osteolysis also occur with this type of implant. We report the cases of five patients who had failure of the polyethylene after a bipolar hemiarthroplasty of the hip. Four of the five patients had marked osteolysis.
*No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. No funds were received in support of this study.
†3905 Harrison Boulevard, Suite W208, Ogden, Utah 84403.
‡Charlotte Orthopedic Specialists, 1915 Randolph Road, Charlotte, North Carolina 28207.
CASE 1. A male patient who had sustained a traumatic dislocation of the hip when he was five years old began to have notable pain in the hip in 1986, when he was thirty-eight years old. He subsequently had a bipolar hemiarthroplasty for the treatment of degenerative joint disease. The implant functioned well for seven years, at which time the patient again began to have pain in the hip. Radiographs revealed cephalad migration of the bipolar component and severe acetabular osteolysis (Fig. 1), and a revision was performed. During the operation, a fifty-seven-millimeter bipolar component and a thirty-two-millimeter fixed-head femoral component with a porous-coated stem were encountered. The head of the femoral component had protruded into but not completely through the polyethylene liner. According to the manufacturer, the initial polyethylene thickness had been 5.994 millimeters. There was massive circumferential acetabular osteolysis, which was treated with morseled allograft and insertion of a fixed acetabular component. The patient was pain-free three years after this procedure.
CASE 2. A fifty-seven-year-old woman who had a history of lupus erythematosus and corticosteroid use had a bipolar hemiarthroplasty because of avascular necrosis in 1983. The implant functioned well for nine years, at which time the patient began to have pain in the groin and thigh. Radiographs revealed marked femoral and acetabular osteolysis as well as protrusio of the acetabular component (Fig. 2), and a revision was performed. During the operation, osteolysis was noted in the acetabulum and in the proximal-lateral aspect of the femur, extending distally two centimeters. The polyethylene insert was damaged at the rim, and all but two millimeters had been worn away at the dome. According to the manufacturer, the initial polyethylene thickness had been 5.8 millimeters. Numerous polyethylene fragments of various sizes were found throughout the joint. A fixed acetabular component and morseled allograft were used to reconstruct the acetabulum. After three years, the bone graft was incorporated, the acetabular interface was stable radiographically, and the patient was able to walk with a pain-free gait.
CASE 3. A fifty-four-year-old man who had degenerative osteoarthrosis was managed with a bipolar hemiarthroplasty in 1983. The implant functioned well for ten years, at which time the patient began to have pain in the groin and buttock. Radiographs revealed that the porous-coated femoral stem was well fixed by bone ingrowth but that there was marked acetabular osteolysis. Operative exploration revealed that the thirty-two-millimeter-diameter fixed femoral head had completely worn through the polyethylene liner at the dome. According to the manufacturer, the initial polyethylene thickness had been 3.073 millimeters. The stem was stable, and there was no gross damage to the femoral head; therefore, the femoral component was retained. The acetabulum was reconstructed with use of morseled allograft and insertion of a fixed acetabular component without cement. After three years, the patient had returned to his previous occupation and was able to walk with a pain-free gait.
CASE 4. A fifty-three-year-old man who had avascular necrosis was managed with a bipolar hemiarthroplasty in 1983. The implant functioned well for twelve years, at which time the patient began to have pain in the groin, buttock, and thigh. Radiographs revealed that the porous-coated femoral stem was well fixed by bone ingrowth but that there was notable acetabular osteolysis. Operative exploration revealed that the thirty-two-millimeter-diameter fixed femoral head had worn through all but one millimeter of the polyethylene liner at the dome. According to the manufacturer, the initial polyethylene thickness had been 3.073 millimeters. The acetabulum was reconstructed with use of morseled allograft and a fixed acetabular component. Two years postoperatively, the patient had returned to his previous occupation. At the time of writing, he was being followed closely for potential osteolysis of the contralateral hip, which had been treated with insertion of a thirty-two-millimeter-diameter fixed femoral head and a similar bipolar implant.
CASE 5. A twenty-one-year-old man who had avascular necrosis was managed with a bipolar hemiarthroplasty in 1984. The implant functioned well for ten years, at which time pain in the groin suddenly developed. On the basis of our experience with three of the four patients already described, we were concerned that the fixed head of the femoral component was wearing through the polyethylene liner and that this would lead to synovitis. According to the manufacturer of the bipolar implant (which had an outer diameter of fifty-one millimeters and an inner diameter of thirty-two millimeters), the initial polyethylene thickness had been 3.073 millimeters. A revision arthroplasty was performed in order to prevent damage to the head of the femoral component. During the operation, it was noted that all but one millimeter of the polyethylene had been worn away at the dome. The extensively porous-coated femoral component was cemented in retroversion, but the femoral ball was not damaged. We therefore decided not to remove the femoral component. A bipolar implant with an eight-millimeter-thick polyethylene insert was used to reconstruct the acetabulum. The decision not to use a fixed cup was based on the retroversion of the femoral stem and concerns about stability in a marginally compliant patient. After thirty months, the patient was satisfied with the result but occasionally had pain in the groin.
Polyethylene wear debris and subsequent osteolysis have emerged as major concerns after hip arthroplasty. When selecting an implant, one must consider a number of variables in order to maximize the longevity of the implant and to avoid failure associated with polyethylene wear and osteolysis. The rate of polyethylene wear has been described by a number of authors3,5. Charnley and Halley initially reported an average rate of wear of 0.15 millimeter per year in forty-nine (68 per cent) of seventy-two patients who had had a hip arthroplasty with use of cement. The average rate in the remaining twenty-three patients (32 per cent) was more than 0.15 millimeter per year. Griffith et al., in a later study of 491 patients, reported an average rate of wear of only 0.07 millimeter per year; however, twenty patients (4 per cent) had an average rate of wear of 0.24 millimeter per year. Collier et al., in a study of eighty-five retrieved acetabular components, reported that one all-polyethylene acetabular component demonstrated nine millimeters of wear after fifteen years in situ (a rate of 0.6 millimeter per year). They also noted several instances in which the head of a femoral component had completely penetrated through the polyethylene liner of a metal-backed cup within five years after the arthroplasty.
Bipolar components frequently are inserted without reaming of the acetabulum; therefore, a relatively small prosthesis is used, resulting in minimum polyethylene thickness. Bartel et al. noted that a decrease in the thickness of the polyethylene to less than six to eight millimeters was associated with a rapid increase in stress within the polyethylene1,2. Wright et al. estimated that the minimum acceptable polyethylene thickness is six millimeters; if this value is used as the criterion, many implants have a polyethylene thickness that is below the minimum level. We prefer to use at least eight millimeters of polyethylene in our fixed as well as our bipolar acetabular components. Frequently, we must use a twenty-two-millimeter femoral head in order to ensure this thickness.
Six manufacturers (Biomet, DePuy, and Zimmer, all of Warsaw, Indiana; Howmedica, East Rutherford, New Jersey; Johnson and Johnson, Raynham, Massachusetts; and Osteonics, Allendale, New Jersey) agreed to participate in a survey regarding the polyethylene thickness of various sizes of bipolar implants. Only three of these manufacturers (DePuy, Zimmer, and Osteonics) offer implants with an inner diameter of twenty-two millimeters. All of the implants that have an inner diameter of this size and an outer diameter of more than forty millimeters have a polyethylene thickness of at least six millimeters, regardless of the manufacturer. All six manufacturers offer implants with an inner diameter of twenty-eight millimeters. All of the implants that have an inner diameter of this size and an outer diameter of fifty-two millimeters or more have a polyethylene thickness of at least six millimeters. When the outer diameter is reduced to forty-eight millimeters, the implants made by three of the six manufacturers have a polyethylene thickness of less than six millimeters, and when it is reduced to forty-four millimeters, the implants made by five of the six manufacturers have a polyethylene thickness that is less than this minimum. Only four of the manufacturers (DePuy, Howmedica, Osteonics, and Zimmer) produce implants with an inner diameter of thirty-two millimeters. Not until an outer diameter of at least fifty-nine millimeters is reached do the implants from all of the manufacturers have a polyethylene thickness of at least six millimeters. One manufacturer (DePuy) offers an implant with an inner diameter of thirty-two millimeters and a polyethylene thickness of only 2.04 millimeters.
Livermore et al., in a study of the results of 385 total hip arthroplasties, found that the highest rate of volumetric wear was associated with thirty-two-millimeter-diameter femoral heads. Maloney et al. recently reported on the characteristics of wear particles generated in patients who had had failure of a hip implant inserted without cement. An average of 1.7 billion particles per gram of tissue was isolated from membranes obtained during revision operations. On the average, the total number of particles associated with bipolar acetabular components was twice that associated with fixed acetabular components7. If bipolar components do in fact produce twice the number of polyethylene particles compared with fixed components, then the use of a thirty-two-millimeter-diameter femoral head in conjunction with a bipolar acetabular component may be cause for concern.
In each of our five patients, the combination of a femoral component with a large head and an acetabular component with a small outer diameter led to the use of a polyethylene liner that was thinner than six millimeters. Three liners had a thickness of only 3.073 millimeters. According to the findings of Rimnac et al., who reported a rate of polyethylene wear ranging from 0.12 to 0.52 millimeter per year, implants with a polyethylene thickness of three millimeters could predictably fail within six to twenty-five years.
Because our five patients came from an unknown pool of patients, it was not possible to determine the true prevalence of osteolysis or polyethylene wear associated with this type of implant. Additional factors, such as the quality of the polyethylene, the method of sterilization, and the design of the implant, also may have played a role in the failure of these implants.
In summary, bipolar hemiarthroplasty was developed to prevent some of the problems that have been observed after unipolar hemiarthroplasty, including wear and protrusio of the acetabular component and pain in the hip. Although many of their advantages remain theoretical, bipolar prostheses have been used successfully for many years. We have shown that polyethylene wear and osteolysis can occur in active patients who have a bipolar implant, especially when the inner diameter of the component is large (thirty-two millimeters) and the outer diameter is small. Failure of the bearing surface should be suspected if osteolysis is noted at the site of a bipolar hemiarthroplasty or if the patient has pain in the groin beginning several years after the implantation of a bipolar prosthesis.
We have three recommendations concerning the use of bipolar components. First, if such an implant is indicated, then one with a minimum polyethylene thickness of six or, preferably, eight millimeters should be selected. This frequently necessitates the use of a twenty-two-millimeter-diameter femoral head. Second, a patient who has been managed previously with a bipolar hemiarthroplasty, especially one who has received a thirty-two-millimeter-diameter fixed femoral head, should be followed closely for the development of polyethylene wear and osteolysis. Third, if a patient has pain several years after a bipolar hemiarthroplasty, the thickness of the polyethylene should be determined. Early intervention before complete failure of the polyethylene, damage to the fixed femoral head, and development of osteolysis may be preferable to observation.