Barbara H. Currier

Rim cracking of the cross-linked longevity polyethylene acetabular liner after total hip arthroplasty.

BACKGROUND Studies have suggested that cross-linked polyethylene bearings reduce wear rates from 40% to 100% compared with conventional polyethylene. However, the reduced mechanical properties of highly cross-linked polyethylene have the potential to be a limiting factor in device performance. We reviewed a series of retrieved acetabular liners with a fracture of the superior rim to assess the factors that played a role in their failure. METHODS Four Longevity acetabular bearings, which had been retrieved from two patients after seven to twenty-seven months in vivo, were visually examined for clinical damage, were assessed with use of Fourier transform infrared spectroscopy to determine the level of oxidation, and were analyzed for mechanical properties and fracture surface characterization. Control data were obtained from never-implanted devices and from global reference ultrahigh molecular weight polyethylene bar stock as an industry calibration material. RESULTS All four retrieved liners demonstrated articular surface wear modes, which in most cases were rated as moderate, and none were rated as severe. All showed cracking or rim failure of the liner at the superior aspect along the groove in the polyethylene that engages the locking ring of the shell. The retrieved liners had no measurable oxidation, and the mechanical properties were comparable with those of never-implanted material. CONCLUSIONS There was no notable in vivo degradation of the retrieved liners. Important factors related to failure appear to be thin polyethylene at the cup rim, relatively vertical cup alignment, and the material properties of the highly cross-linked polyethylene that are decreased relative to conventional polyethylene. The critical dimension with respect to rim failure in modular liners appears to be the minimum thickness at the equatorial region.

Comparison of cross-linked polyethylene materials for orthopaedic applications.

Cross-linked polyethylenes are being marketed by orthopaedic manufacturers to address the problem of osteolysis caused by polyethylene particulate wear debris. Wear testing of these cross-linked polyethylenes in hip simulators has shown dramatic reduction in wear rate compared with standard ultrahigh molecular weight polyethylene, either gamma irradiated in air or nitrogen - or ethylene oxide-sterilized. However, this reduction in wear rate is not without cost. The cross-linking processes can result in materials with lower mechanical properties than standard ultrahigh molecular weight polyethylene. To evaluate the effect of the various cross-linking processes on physical and mechanical properties of ultrahigh molecular weight polyethylene, commercially available cross-linked polyethylenes from six orthopaedic manufacturers were tested. This study was the culmination of collaboration with these manufacturers, who provided cross-linked polyethylene for this study, wear characteristics of the material they provided, and review of the physical and mechanical properties measure for their polyethylene. Cross-linked materials were evaluated as received and after an accelerated aging protocol. Free radical identity and concentration, oxidation, crystallinity, melt temperature, ultimate tensile strength, elongation at break, tensile stress at yield, and toughness are reported for each material. By comparing these physical and mechanical properties, surgeons can evaluate the trade-off that results from developing materials with substantially lower wear rates.

Overview of polyethylene as a bearing material: comparison of sterilization methods.

Polyethylene has been used for more than 30 years as an orthopaedic bearing material; however, there has been recent concern regarding the early failure of a small percentage of the polyethylene bearings. The damage seen in some retrieved polyethylene components has been linked to gamma radiation sterilization in air, which was widely used by the industry for years. Gamma radiation in air has been documented to cause an increase in oxidation and degradation of mechanical properties with time. The degradation of polyethylene initiated by gamma sterilization in air has led the orthopaedic industry toward alternative sterilization methods, including gamma radiation in an inert gas or vacuum environment, ethylene oxide gas sterilization, and gas plasma sterilization. For many of these alternative techniques, little clinical performance data exist. This study is a comparative evaluation of sterilization methods using the same analytic techniques that have been used to document the effects of gamma sterilization in air on polyethylene. Fourier transform infrared spectroscopy, electron spin resonance, and uniaxial tensile testing are used to compare, respectively, the oxidation levels, free radical concentration, and mechanical properties of material sterilized by each method. The polyethylene is evaluated before sterilization, poststerilization, and postartificial aging. All examined alternative sterilization methods, when compared with gamma sterilization in air, caused less material degradation during a components preimplantation shelf life.

Evaluation of Oxidation and Fatigue Damage of Retrieved Crossfire Polyethylene Acetabular Cups

BACKGROUND Crossfire cross-linked polyethylene is produced differently from other cross-linked polyethylene materials; a below-melt-temperature annealing process is used with the goal of avoiding compromised mechanical properties. The present study was performed to evaluate retrieved Crossfire acetabular cups to determine whether they had oxidized and to what extent oxidation might have influenced their clinical performance. METHODS Eleven acetabular cups were received at retrieval and a twelfth acetabular cup was received two years post-retrieval over a period of four years. None were retrieved because of polyethylene wear or fatigue. The cups had been in vivo from 0.1 to 5.3 years. Each was examined visually, clinical fatigue damage was rated, and oxidation was measured with use of Fourier transform infrared spectroscopy. RESULTS The cups exhibited oxidation that varied with its location on the cup: the oxidation value was generally low on the articular surface but more than an order of magnitude higher value on the rim. Maximum rim oxidation correlated significantly with the time in vivo (Spearman rho = 0.734, p = 0.010). Oxidation was identified visually by a white band in thin sections on the rim of seven of the cups and on the articular surface of one of these seven cups. Six of the seven cups also exhibited clinical fatigue damage. Eight of the twelve cups exhibited evidence of impingement or dislocation. CONCLUSIONS Acetabular cups made of Crossfire polyethylene oxidized to a measurable degree. The oxidation-related reduction of polyethylene mechanical properties was sufficient to allow the fatigue damage seen in these retrieved cups.

Shelf life and in vivo duration. Impacts on performance of tibial bearings.

Polyethylene has been used for more than 30 years as an orthopaedic bearing material; however, recently concern has been focused on the early failure of some polyethylene bearings. The damage seen in some bearings has been linked to gamma radiation sterilization performed in an air environment. Gamma sterilization in air has been documented to cause an increase in oxidation and degradation of mechanical properties that continue with time. However, not all retrieved bearings that are gamma sterilized in air exhibit the elevated oxidation and mechanical property degradation that lead to early component failure. Bearings that are gamma sterilized in air oxidize while sitting in inventory before implantation. Shelf oxidation rate was estimated based on analysis of a series of never implanted tibial bearings. This shelf oxidation rate allowed estimation of in vivo oxidation for retrieved tibial bearings of known sterilization date. Bearings with less than 1 year of shelf life after gamma sterilization in air had lower in vivo oxidation and better in vivo performance than did those with longer shelf life before implantation. Shelf time before implantation appears to be a significant factor in the success or failure of bearings that are gamma sterilized in air.

Current sterilization and packaging methods for polyethylene.

Gamma sterilization in an air environment can induce oxidation in polyethylene. Oxidation can lead to polyethylene embrittlement, compromising mechanical integrity and clinical performance of polyethylene bearings. For these reasons, orthopaedic manufacturers have modified their methods of sterilizing and packaging polyethylene. Two alternative approaches have emerged: sterilization by non-radiation methods and sterilization by gamma irradiation in inert environments. The current study presents a prognosis for clinical performance of polyethylene sterilized with new methods, based on material property analyses (oxidation levels, mechanical properties, crosslink density) of never implanted and retrieved bearings. Data from bearings that were never implanted which were sterilized with the new methods and shelf aged as many as 3 years, show negligible oxidation, ductility above 400%, and ultimate tensile strength near 50 MPa, all exceeding specifications of the American Society for Testing and Materials. There are significant differences in crosslink density (swell ratio) depending on the sterilization method. Retrievals indicate that bearings sterilized with these new methods are performing well clinically and that the majority are not changing with time. The current study suggests that the shelf oxidation problem has been addressed by these new sterilization techniques and that clinical performance at short followup is acceptable. However, long-term clinical performance must be evaluated in the future.

Rapid Polyethylene Failure of Unicondylar Tibial Components Sterilized with Gamma Irradiation in Air and Implanted After a Long Shelf Life

Background: The mechanical toughness of polyethylene that has been sterilized by gamma irradiation in air decreases after a long shelf life. The purpose of the present study is to report the high failure rate after unicondylar knee replacements performed with polyethylene bearings that had been sterilized with gamma irradiation in air and implanted after a shelf life of ⩽4.4 years. Methods: Between December 1997 and January 2000, seventy-five unicondylar knee replacements were performed in sixty-two patients. All patients were followed both clinically and radiographically. A revision operation was offered when the patient had pain, swelling, and radiographic evidence of rapid polyethylene wear. The effect of aging of the polyethylene during storage was evaluated by dividing the knees into three groups on the basis of shelf life and comparing them with regard to the rate of revision and the observed wear of the polyethylene. Four retrieved components were examined for the presence of oxidation. Results: At a mean of eighteen months after the arthroplasty, thirty knees had been revised and seven were scheduled for revision. The rate of polyethylene wear increased as the shelf life increased. There was a significant inverse linear correlation between the shelf life of the polyethylene and the time to revision (p < 0.01, r2 = 0.64). All retrieved components had greater-than-expected wear with pitting and delamination of the surface. Seven components had fractured, and ten had both fractured and fragmented. Analysis of four components confirmed severe oxidation of the polyethylene. Conclusion: The present study demonstrated early, severe wear of tibial polyethylene bearings that had been sterilized by gamma irradiation in air and stored for ⩽4.4 years. This risk can be minimized by ensuring that implants have not been sterilized with gamma irradiation in air and stored for several years.

In Vivo Oxidation in Remelted Highly Cross-Linked Retrievals

BACKGROUND Elimination of free radicals to prevent oxidation has played a major role in the development and product differentiation of the latest generation of highly cross-linked ultra-high molecular weight polyethylene bearing materials. In the current study, we (1) examined oxidation in a series of retrieved remelted highly cross-linked ultra-high molecular weight polyethylene bearings from a number of device manufacturers and (2) compared the retrieval results with findings for shelf-stored control specimens. The hypothesis was that radiation-cross-linked remelted ultra-high molecular weight polyethylene would maintain oxidative stability in vivo comparable with the stability during shelf storage and in published laboratory aging tests. METHODS Fifty remelted highly cross-linked ultra-high molecular weight polyethylene acetabular liners and nineteen remelted highly cross-linked ultra-high molecular weight polyethylene tibial inserts were received after retrieval from twenty-one surgeons from across the U.S. Thirty-two of the retrievals had been in vivo for two years or more. Each was measured for oxidation with use of Fourier transform infrared spectroscopy. A control series of remelted highly cross-linked ultra-high molecular weight polyethylene acetabular liners from three manufacturers was analyzed with electron paramagnetic resonance spectroscopy to measure free radical content and with Fourier transform infrared spectroscopy to measure oxidation initially and after eight to nine years of shelf storage in air. RESULTS The never-implanted, shelf-aged controls had no measurable free-radical content initially or after eight to nine years of shelf storage. The never-implanted controls showed no increase in oxidation during shelf storage. Oxidation measurements showed measurable oxidation in 22% of the retrieved remelted highly cross-linked liners and inserts after an average of two years in vivo. CONCLUSIONS Because never-implanted remelted highly cross-linked ultra-high molecular weight polyethylene materials had no measurable free-radical concentration and no increase in oxidation during shelf storage, these materials were expected to be oxidation-resistant in vivo. However, some remelted highly cross-linked ultra-high molecular weight polyethylene retrievals showed measurable oxidation after an average of more than two years in vivo. This apparent departure from widely expected behavior requires continued study of the process of in vivo oxidation of ultra-high molecular weight polyethylene materials.

Effect of fabrication method and resin type on performance of tibial bearings

Polyethylene has been used successfully for more than 30 years as an orthopedic bearing material. During this time, several polyethylene resins and fabrication methods have been used to produce bearings. Some bearings fail prematurely due to fatigue, which has been linked to oxidation and degradation of mechanical properties resulting from gamma sterilization in air. Fabrication method and/or resin have been hypothesized to govern whether oxidative degradation occurs in gamma-sterilized bearings. This study evaluates the effect of fabrication (machining/direct compression molding) and resin type on oxidation and the resulting mechanical properties for a large series of never-implanted bearings. While many molded bearings studied exhibit lower oxidation than machined bearings, fabrication method is not a significant predictor of oxidation. Resin type and shelf-age are found to be significant predictors of oxidation. Bearings fabricated from Himont 1900 exhibit lower oxidation than those from GUR 415/412 at comparable times after gamma in air. However, Himont 1900 bearings lose strength and elongation at lower oxidation levels than GUR 415/412 bearings. But since Himont 1900 oxidizes more slowly, Himont 1900 bearings retain mechanical properties for longer shelf times than comparable GUR 415/412 bearings. These effects are seen in retrievals as well.

An analysis of hylamer and polyethylene bearings from retrieved acetabular components.

Hylamer and conventional polyethylene acetabular liners of the same design, revised for a variety of reasons, were examined and compared to assess the performance of Hylamer as a bearing material. Clinical damage modes, linear wear rates, oxidation levels, and mechanical properties were measured. In both series, many liners were retrieved for dislocation. Wear/osteolysis was the most common reason for retrieval in the Hylamer series, while none of the conventional polyethylene liners were retrieved for this reason. Nearly all liners exhibited abrasion, burnishing, scratching, and creep. The Hylamer liners had more cracking, delamination, and pitting. The Hylamer liners had an average linear wear rate of 0.32 mm/year, while the conventional polyethylene liners had an average wear rate of 0.20 mm/year. Due to sample size, no statistical difference in wear rate was noted between the two groups. In general, both the Hylamer and conventional polyethylene showed oxidation peaks subsurface, resulting from their exposure to gamma radiation in air. Liners with elevated oxidation had decreased ultimate tensile strength, elongation, and toughness. For given oxidation levels, the corresponding mechanical properties of Hylamer appeared lower than those of conventional polyethylene. The ultimate tensile strength values ranged from 14 to 33 MPa for Hylamer and 19 to 32 MPa for conventional polyethylene. Elongation ranges were 19% to 350% (Hylamer) and 80% to 375% (conventional). The Hylamer retrievals in this study gave initial indications of performance; Hylamer appeared to behave similarly, but not superiorly, to conventional polyethylene, in the early functional period with respect to clinical wear and clinical performance. Both Hylamer and conventional polyethylene liners were degraded by gamma sterilization in air, with Hylamer liners demonstrating greater property changes.