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Direct Compression Molded Polyethylene for Total Hip and Knee Replacements

Ritter, Merrill, A.

Clinical Orthopaedics and Related Research: December 2001 - Volume 393 - Issue - p 94-100
SECTION I SYMPOSIUM: Papers Presented at the Twenty-Ninth Open Meeting of the Hip Society and the American Association of Hip and Knee Surgeons
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Direct compression molded polyethylene is a process of creating a net-shaped component with finished articular surfaces from ultrahigh molecular weight polyethylene by applying heat and pressure to the raw resin (usually Hi-fax 1900) with the use of fixed geometry metallic tools. The final product has no machining or finishing. Sterilization was by gamma radiation in air. Between 1974 and 1978, there were 378 cemented T-28 stems (321 patients) and 171 cemented TR-28 stems (158 patients) articulating with a cemented direct compression molded acetabular component. Radiographically, linear wear was 0.06 mm per year for the T-28 stem and 0.05 mm for the TR-28 stems. The revision rates for the acetabulum were 9.5% and 7.9%, respectively with only two acetabula having osteolysis. On the femoral side, the revision rates were 11.1% and 12.8%, with 11 cases and one case of osteolysis, respectively. Between 1983 and 1996, 4583 AGC cemented total knee replacements were done with direct compression molded nonmodular tibial components. There was no osteolysis seen in these nonmodular compression molded total knee replacements even with 0.1% failure of the femoral components and 0.4% failure of the tibial components. Failure of all of the hip and knee implants was because of poor cement technique, instability, or both. Wear and osteolysis were minimal. Direct compression molded polyethylene has stood the test of time and is a good choice among the currently available polyethylenes.

From the Center for Hip and Knee Surgery, and St Francis Hospital-Mooresville, Mooresville, IN.

Dr. Ritter is a consultant for and has stock ownership in Biomet.

Reprint requests to Merrill A. Ritter, MD, 1199 Hadley Road, Mooresville, IN 46158.

Osteolysis, a biologic destruction of bone, frequently leads to revision of the total joint replacement even if it is not loose. Since approximately 1992, polyethylene debris has been implicated as the major source of osteolysis. 6 In an era of rapidly changing designs and materials, polyethylene has gone from machined molded sheets in the 1970s to machining ram extruded polyethylene in the 1980s, only to have problems with wear and osteolysis in the 1990s. Because of osteolysis in hip and knee replacements, attempts are being made to reduce the production of particulate wear debris by altering the polymers used for total joint bearings. Unfortunately the clinical experience with these modified polymers is limited in numbers and years.

Direct compression molding is a process of creating net-shaped components with finished articular surfaces from ultrahigh molecular polyethylene by applying heat and pressure to the raw resin with the use of fixed geometry metallic tools. The final product requires no machining or finishing (polishing). One of the original products fabricated by direct compression molding was the cemented all-polyethylene acetabular component of the T-28 total hip replacement system (Zimmer, Warsaw, IN) (Fig 1). Another direct compression molded component is the tibia component of the AGC total knee replacement system (Biomet, Warsaw, IN) (Fig 2).

Fig 1.

Fig 1.

Fig 2A–B.

Fig 2A–B.

The current study is a clinical review of direct compression molded polyethylene that has been used for 25 years as it relates to wear, osteolysis, and loosening.

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MATERIALS AND METHODS

The T-28 total hip replacement (Zimmer) was used between 1974 and 1978 and the TR-28 total hip replacement (Zimmer) was used between 1978 and 1980 (Fig 1). Both prostheses used a cemented direct compression molded all-polyethylene acetabular component that was fabricated from Hi-fax 1900 resin (Montell, Wilmington, DE). The T-28 monobloc femoral stem was fabricated from stainless steel and the TR-28 was fabricated from a CoCr alloy. The demographics of the T-28 study revealed 321 patients (378 hips) with an average age of 63.8 years (range, 22–85 years), 180 were women and 216 had osteoarthritis. The average followup was 21 years (range, 18–25 years). There were 171 TR-28 total hip replacements in 158 patients with an average age of 64.7 years (range, 34–88 years), 90 were women and 94 had osteoarthritis. The average followup was 17 years (range, 15–22 years). 1,16 There were 169 patients (193 hips) with the T-28 system who died, 64 patients (72 hips) with the TR-28 system who died, 30 patients (35 hips) with the T-28 system who were lost to followup before the system failed, and 19 patients (19 hips) with the TR-28 system who were lost to followup before the system failed.

The AGC total knee replacement (Biomet) has been used exclusively since 1983 with no changes except for the strength of the metal tibial base plate casting in 1992 and 1996. Similarly, the polyethylene bearing surfaces were nonmodular direct compression molded from Hi-fax 1900 resin (Fig 2).

There were 4583 AGC total knee replacements in 3054 patients who were followed up 4 to 15 years with an average followup of 8 years. The average age of the patients was 70 years (range, 18–93 years); 60% were women, and 87% had osteoarthritis. There were 407 deaths (8.9% of patients).

Followups were every 1 to 2 years after the implantation. Radiographs of the hip were evaluated for radiolucency at the bone-cement interface, wear as described by Livermore et al, 13 lysis within the acetabulum and the femur, and loosening. Radiographs of the knees were evaluated for alignment, changes in alignment, osteolysis, and loosening. Revision of any hip or knee component and the diagnosis for revision were recorded. Revision for dislocation or infection was excluded. Two patients with acetabular components had revision surgery because of acetabular loosening at 20 and 25 years after surgery, respectively, and their components were evaluated only for cross-linking and oxidation. The tibial components of the AGC system were revised because of patellar complications (disassociation and metallosis). None of the total knee components were removed because of complications associated with the polyethylene components.

There were seven tibial components of the AGC total knee replacements that never were implanted but remained on the shelf for 5 to 10 years, and six tibial components that remained on the shelf for 7 to 10 years (three Miller-Galante, two Insall-Burstein, and two AGC) were evaluated for percent oxidation, percent cross-linking, density, percent crystallinity, and white banding. 10

All of the clinical followups were done in the office and the patients were consecutive. The radiographs were reviewed and coded by the author, which may be a limitation of the study.

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RESULTS

Acetabular polyethylene linear wear was 0.06 mm per year for the stainless steel T-28 and 0.05 mm per year for the CoCr alloy TR-28. 1

Acetabular loosening was 14% in the T-28 group and 11% in the TR-28 group with only two cases of osteolysis in the T-28 group and none in the TR-28 group. Eleven percent of the femoral components of the T-28 group and 16% of the TR-28 group were loose, and there were 11 cases of osteolysis in the T-28 group, all of which were distal (Table 1).

TABLE 1

TABLE 1

The likelihood of osteolysis was minimal in the acetabulum. Osteolysis on the femoral side was related to inadequate cementation. The evaluation of the first radiograph obtained postoperatively showed Zone 1 radiolucency in 100% of the loose acetabular cups (Fig 3).

Fig 3A–B.

Fig 3A–B.

Two cups removed 21 and 25 years after implantation, respectively revealed high cross-linking (average, 81.98% and 77.4%) and low oxidation levels (average, 0.164 and 0.300) throughout the thickness of the entire specimen.

Loosening in the total knee replacements was present in 0.1% of the femurs, and 0.4% of the tibias. There were no cases of osteolysis in the 4583 AGC total knee replacements. One hundred eighty-three patellas were loose but this was statistically correlated with severance of the superior lateral genicular artery. There were no cases with any linear or global osteolysis (Table 2).

TABLE 2

TABLE 2

Table 3 shows that direct compression molded polyethylene (on the shelf for more than 5 years) had a maximum density less than 0.9430, crystallinity less than 72%, and no white bands.

TABLE 3

TABLE 3

Six specimens implanted for more than 7 years had similar amounts of density, crystallinity, and white bands. The two specimens with occasional white bands were not beneath the articular weightbearing surfaces (Table 4).

TABLE 4

TABLE 4

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DISCUSSION

Historically, many types of ultrahigh molecular weight polyethylene have been used for total joint bearings. The differences in the polyethylene include changes in resin, consolidation methods, machining practices, and sterilization procedures. Initially components were fabricated from sheets of molded Chirulen resin (Hoechst, Oberhausen, Germany) and marketed as RCH 1000. In the United States, this material was replaced with ram extruded materials with modifications such as carbon reinforcement (Poly II), 25 heat pressing (PCA), 17–26 and pressure crystallization (Hyalmer). 4,5,14,21 These efforts were designed to reduce the debris production of the bearing surface but with varied levels of success.

Implants that were not direct compression molded were machined from extruded bar stock and therefore subject to the problems of machining, including fiber orientation at the surface, plowing leading to surface defects, and alteration of the wear secondary to the cutting speed and tool feeding. 22 There is an increase in polyethylene density, percent crystallinity, and oxidation and a decrease in the cross-linking with time if the polyethylene is gamma irradiated in air and left on the shelf for more than 4 years. 3,20 The resins used were GUR 4150 or 415 HC and there were multiple clinical failures with gross wear secondary to delamination. 3,20

In the current study, failure of the T-28 and TR-28 total hip replacement systems and the AGC total knee replacement system has not been related to wear but to technique such as poor cementing or instability or both. The osteolysis in the acetabulum was low; in the femur distally the osteolysis was associated with loosening of the femoral prosthesis, areas of an inadequate cement mantle around the prostheses, or both. No osteolysis was seen around the AGC total knee replacements.

The direct compression molded polyethylene components show limited alteration of the material with time, especially little to no oxidative damage to the material such as white banding. In addition, in the components analyzed there were no fusion defects, the density was less than 0.950, and the crystallinity was less than 72%, all of which have been shown to reduce the risk factors for increased wear. 27 Also, the retrieved direct compression molded tibial bearings had only mild evidence of burnishing on the bearing surface but no evidence of delamination as seen in other knee replacement systems that failed. 7,24

The reason for such excellent clinical and laboratory results is not known; however, the resin and consolidation are factors. 7,10,24 Wear, however, is a minor problem.

There is a need to improve bearing surface technology, such as the new highly cross-linked polyethylene acetabular components, but new technologies must be evaluated to avoid the problems such as those that occurred with the Poly II, 25 Hylamer, 4,5,14,21 and heat-pressed polyethylene components. 17,26 Even though the wear performances show little wear 18 under simulator testing, there is a reduction in the fracture toughness, elongation to breakage, yield strength, and ultimate tensile strength 8,12 that accompanies exposure to higher doses of irradiation. Associated with these are wide variations in implant loading as described by Lombardi et al 15 and Nevelos et al, 19 which are not matched with the current laboratory simulators and which may lead to unexpected complications.

Current efforts, using high-dose gamma radiation and subsequent annealing have been to increase the cross-linking and decrease the oxidation. In the direct compression molding with Hi-fax 1900, increased cross-linking and decreased oxidation have been obtained but maybe not to the same extent as the newer polyethylenes. However, greater than 25-years of clinical followup show that direct compression molded polyethylene is effective.

In the current study, the mode of failure typically associated with a total hip replacement was aseptic loosening, most likely a problem secondary to poor technique. Conversely, the wear associated with total knee replacement usually has been delamination, producing large wear particles; however, osteolysis was not a major concern in the past. With the advent of modularity, backside wear mechanisms have produced significant quantities of submicron debris 9,11,23 and are probably design specific. Modularity has been implicated as another source of wear-induced osteolysis 22 in total knee replacements. Nonmodular knee systems can avoid this. 9

Therefore, bearing surfaces have been a problem associated with wear and osteolysis and alternative bearing materials should be evaluated. Direct compression molding has been used for 25 years with excellent clinical results. 2 The observations in this study of minimal osteolysis and minimal wear showed minimal oxidation and increased cross-linking in hip and knee prostheses, and may provide insight into the requirements for new and improved materials and help avoid past pitfalls. Until controlled, clinical randomized studies are done direct compression molded nonmodular implants should be used.

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References

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Section Description

Richard A. Brand, MD; and John R. Moreland, MD, Guest Editors

© 2001 Lippincott Williams & Wilkins, Inc.