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© 1999 The Journal of Bone and Joint Surgery, Inc. Early Osteolysis Following Total Hip Arthroplasty with Use of a Hylamer Liner in Combination with a Modular Ceramic Femoral Head. A Case Report* , RALEIGH , CLEVELAND, OHIO , RALEIGH, NORTH CAROLINAInvestigation performed at the Raleigh Orthopaedic Clinic, P.A., Raleigh
In an effort to improve the surface and wear characteristics of ultra-high molecular weight polyethylene, an enhanced ultra-high molecular weight polyethylene (Hylamer; DePuy-DuPont Orthopaedics, Wilmington, Delaware)6 was introduced in 1991. It was anticipated that the wear characteristics of a Hylamer acetabular surface in combination with a ceramic modular femoral head would improve the long-term performance of prostheses in young, active patients4,25. Several recent reports, however, have described wear and early osteolysis in association with Hylamer acetabular liners coupled with cobalt-chromium modular femoral heads manufactured by various companies3,15. The purpose of the current report is to describe rapid wear and early osteolysis following use of a Hylamer acetabular liner in combination with a ceramic femoral head made by the same manufacturer.
In April 1994, a total hip arthroplasty was performed on a thirty-three-year-old woman (height, 167 centimeters; weight, fifty-two kilograms) who had arthritis due to congenital dysplasia of the left hip. The patient's preoperative work activities as an accountant had required frequent climbing and descending of multiple sets of stairs, which had greatly aggravated the problems with the hip. The work environment subsequently was modified to better accommodate the patient. The arthroplasty was performed with use of components from the same manufacturer. These included a femoral component (Stability; DePuy Orthopaedics, Warsaw, Indiana) measuring twelve by 150 millimeters, which was inserted without cement; an acetabular shell (Duraloc; DePuy Orthopaedics) with a diameter of fifty-two millimeters, also inserted without cement; a Ti-6Al-4V bone-screw (Low Profile; DePuy Orthopaedics) with a diameter of 6.5 millimeters and a length of thirty millimeters, for augmentation of the cup; and a neutral Hylamer liner measuring fifty-two by twenty-eight millimeters with a thickness of eight millimeters. Because of the patient's young age, activity level, and life expectancy, an aluminum-oxide-ceramic modular femoral head (Ceramic Hip Ball; DePuy Orthopaedics [manufactured by Feldmueler, Plochingen, Germany]) with a diameter of twenty-eight millimeters and a +5-millimeter offset was used. Postoperatively, the patient was allowed full weight-bearing as tolerated. The postoperative and rehabilitation course was normal and without incident. Early radiographs, made six weeks after the operation, showed the femoral component to be implanted in neutral alignment and the acetabular component to be implanted at a 32-degree inclination (Figs. 1-A and 1-B). The patient did well and returned for follow-up at ten months and at two years, at which times radiographs showed well positioned and fixed components. She had a full range of motion without pain in all planes, with equal limb lengths.
In November 1997, three years and seven months after the operation, the patient had no pain and still had a full range of motion, but radiographs revealed acetabular osteolysis in acetabular zones 1 and 2 of DeLee and Charnley5 and femoral osteolysis in zone 1 of Gruen et al.8 (Figs. 2-A and 2-B). There was no evidence of loosening of the components, but migration of the femoral head into the polyethylene was evident. The extent of migration of the femoral head into the acetabular liner, according to the technique of Livermore et al.14, was approximately 1.8 millimeters (0.5 millimeter per year). The extent and possible long-term implications of wear of the implant were discussed with the patient, and she elected to have an exchange of the femoral head and the acetabular liner, which was performed in February 1998. At the time of the operation, the femoral stem and the acetabular shell were stable and the modular Hylamer liner and the ceramic head were easily exchanged. Osteolytic lesions were identified, debrided, and packed with crushed cancellous bone allograft. Samples of tissue from the acetabular capsule, the proximal part of the femur, and the acetabular lesions were obtained for histological examination.
The extent of surface deformation of the retrieved Hylamer liner was measured directly with use of a modification of the shadowgraph technique, as described previously20,24. The implant also was studied with a dissecting microscope for evidence of impingement or abrasive wear over the dome. Two vectors of deformation were seen on the articular surface (Fig. 3). The first vector measured 1.7 millimeters in length and was located 52.4 degrees from the opening of the cup and 82.0 degrees from a plane defined by a line between the center of the cup and the center of the extended lip. The second vector measured 1.2 millimeters in length and was located -72.3 degrees from the opening of the cup (that is, lateral to the cup face) and 12.3 degrees from the apex of the extended lip (Fig. 3). The presence of two deformation vectors illustrates a major limitation of wear-measurement techniques that depend on migration of the head. The measured amount of migration of the head represents the sum of wear and polyethylene creep, which cannot be differentiated radiographically. Therefore, a gravimetric method was used to directly measure volumetric deformation of the Hylamer liner10. The results showed total volumetric wear of 554.7 cubic millimeters. On the basis of forty-six months in vivo, this represented a volumetric wear rate of 144.7 cubic millimeters per year. This is a greater extent of deformation than was seen in a previously reported series of acetabular components retrieved at autopsy20.
Evaluation of the dome of the liner showed indentations corresponding to screw-holes in the metal backing, which were probably the result of polyethylene creep. There was no evidence of notable burnishing or wear, suggesting minimum motion between the liner and the backing. There also was no evidence of impingement on the rim, and no third-body abrasive particles were identified in the polyethylene. The modular ceramic head was evaluated with a dissecting microscope and also showed no unusual patterns of abrasive wear. Ultra-high molecular weight polyethylene particles from the acetabular capsule, the proximal part of the femur, and biopsy specimens of the acetabular lesions were isolated and quantified with use of methods described previously11,16. The concentration of polyethylene particles ranged from 1.25 x 109 to 18.5 x 109 particles per gram of tissue (dry weight); this concentration was similar to that reported previously for tissue around failed total hip prostheses11,12,16,18, but considering the short duration that the implant was in vivo, the rate of accumulation of polyethylene particles in the tissues of this patient was high11,12. Most of the particles were smaller than two micrometers, and many were smaller than the detection limit of the electronic particle analyzer.
Osteolysis as a result of wear debris after total hip arthroplasty has become a major concern. Unfortunately, there are few alternatives to the use of ultra-high molecular weight polyethylene as the material1,2,9,18,19,22,23. In an effort to improve the wear properties of polyethylene, an enhanced polymer was developed by the DePuy and DuPont corporations6,13,15. To our knowledge, the case that is presented here represents the first occurrence of extensive, early osteolysis following total hip arthroplasty with use of a Hylamer polyethylene liner and a ceramic modular femoral head from the same manufacturer, a combination of materials that would be expected to have optimum wear properties4,25. Only three years and seven months after the total hip arthroplasty, there was radiographic evidence of extensive osteolysis, and the retrieved implant showed prominent polyethylene deformation. This polyethylene deformation actually represents the sum of wear and polyethylene creep. Although the relative magnitude of creep was unknown, the large amount of deformation and the high concentration of polyethylene particles in adjacent tissues suggest that most of the deformation in this implant was due to wear. The presence of multiple vectors of polyethylene deformation has been reported previously24, but the mechanism of their formation is unknown. Multiple vectors probably are not caused by loosening of the cup24. Although a localized area of roughness of the head might cause multiple vectors, no such area was visually identified on the retrieved head of the implant in our patient. Multiple vectors may be caused by so-called impact loading, recently described by Northcut et al.17, but additional studies are needed to clarify this phenomenon. The rate of wear (144.7 cubic millimeters per year) of the implant in our patient was greater than that in most previous reports on Hylamer liners3,15,21. The reason for the high rate of polyethylene wear in our patient is unclear. Although the composition and mechanical properties of the enhanced polyethylene represent one possible variable, it is clear that not all Hylamer implants have the rate of wear that was identified in our patient21. In order to obtain optimum conformity of the implant, manufacturers usually recommend using an acetabular component and modular femoral head made by the same manufacturer. Chmell et al. reported a relatively high rate of wear (0.48 millimeter per year) of Hylamer liners and a 4 percent rate of failure (six of 143) at a mean of 2.5 years3. A possible explanation for the accelerated rate of wear of the implants described by Chmell et al. is that the femoral heads and acetabular cups were made by different manufacturers. Similarly, Livingston et al. found evidence of high rates of wear of Hylamer liners that were used in combination with a variety of femoral heads, some of which were made by manufacturers other than DePuy15. Interestingly, those authors noted the greatest amount of wear (mean rate, 0.33 millimeter per year) in patients who had a combination of a Hylamer liner and a DePuy ceramic head. The method of sterilization and packaging has been described, to our knowledge, in only one previous report on wear of Hylamer liners21. Moreover, the possible role of shelf time has not been addressed in any report on Hylamer acetabular components of which we are aware3,15,21. Commercially available Hylamer polyethylene has been sterilized with use of several different methods. Implants manufactured in 1992 were sterilized with gamma irradiation in air. Gamma irradiation in a nitrogen atmosphere followed by vacuum-packaging was introduced in 1993, and implants that have been available since October 1995 have been sterilized with gas plasma without irradiation7. The acetabular polyethylene component that was used in our patient had been sterilized with gamma irradiation in air and had been packaged on July 6, 1992. It was stored at room temperature in air until it was implanted in April 1994, twenty-two months after sterilization and packaging. The combination of this sterilization method and storage in air may have resulted in a highly oxidized polyethylene with compromised mechanical properties. Several factors may contribute to wear of the implant, including the physiochemical properties of the articulating surfaces, the thickness of the polyethylene, the alignment of the implant, the surface roughness, the patient's weight and activity level, and the presence of third-body wear debris19. In our patient, the alignment of the femoral component and the position of the cup were satisfactory; the components did not loosen in vivo, and we found no evidence of unusual surface roughness or third-body debris. Although we have described only a single instance of rapid wear of a Hylamer acetabular liner articulating with a ceramic femoral head in a young patient, this finding is of concern as the extent of wear greatly exceeded our expectations. It is hoped that contemporary methods of sterilization will improve the wear properties of all types of polyethylene.
*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.
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