Gamma Irradiation In Air Research Articles

The influence of design, materials and kinematics on the in vitro wear of total knee replacements

Debris-induced osteolysis due to surface wear of ultra high molecular weight polyethylene (UHMWPE) bearings is a potential long-term failure mechanism of total knee replacements (TKR). This study investigated the effect of prosthesis design, kinematics and bearing material on the wear of UHMWPE bearings using a physiological knee simulator. The use of a curved fixed bearing design with stabilised polyethylene bearings reduced wear in comparison to more flat-on-flat components which were sterilised by gamma irradiation in air. Medium levels of crosslinking further improved the wear resistance of fixed bearing TKR due to resistance to strain softening when subjected to multidirectional motion at the femoral–insert articulating interface. Backside motion was shown to be a contributing factor to the overall rate of UHMWPE wear in fixed bearing components. Wear of fixed bearing prostheses was reduced significantly when anterior–posterior displacement and internal–external rotation kinematics were reduced due to decreased cross shear on the articulating surface and a reduction in AP displacement. Rotating platform mobile bearing prostheses exhibited reduced wear rates in comparison to fixed bearing components in these simulator studies due to redistribution of knee motion to two articulating interfaces with more linear motions at each interface. This was observed in two rotating platform designs with different UHMWPE bearing materials. In knee simulator studies, wear of TKR bearings was dependent on kinematics at the articulating surfaces and the prosthesis design, as well as the type of material.

Shelf age of the polyethylene tibial component and outcome of unicondylar knee arthroplasty.

A recent report linked shelf-aging of unicondylar polyethylene tibial components with accelerated fatigue wear and rapid failure. We retrospectively reviewed our experiences with another unicondylar knee system to investigate the relationship between the shelf age of the polyethylene components and clinical outcome. One hundred Single Compartment Replacement unicompartmental knee arthroplasties were performed with cement between 1990 and 1996. The median shelf age for the polyethylene inserts was 1.7 years. At the time of the review, four knees had been lost to follow-up, sixteen were in patients who had died, and nineteen had been revised. Sixty-one knee replacements remained in situ at a mean and standard deviation of 8 +/- 2 years postoperatively. With revision as the end point, Kaplan-Meier survivorship analysis demonstrated a six-year rate of survival of 96% when the shelf age of the insert was less than the median shelf age but only 71% when the median shelf age was exceeded. Fatigue wear was identified on all retrieved components except for two that had shelf ages under one year and had been revised within three years. At four to six years of clinical follow-up, the group of knees in which the median shelf age had been exceeded also had poorer Knee Society knee scores and function scores. Aging on the shelf accelerated fatigue failure of polyethylene inserts sterilized with gamma irradiation in air and compromised our intermediate-term clinical outcomes with this unicondylar knee replacement system. Attention to the relationship between the shelf age and the clinical performance of the polyethylene component continues to be warranted with unicondylar knee arthroplasty implants.

Analysis of a retrieved UHMWPE acetabular cup crosslinked in air with 1000 kGy of gamma radiation

The oxidation and degradation that accompany high dose gamma irradiation in air for crosslinking and sterilization are important because they could affect the clinical performance of polyethylene total joint implants. We report a clinical case of a 1000 kGy gamma-irradiated, highly crosslinked ultra-high molecular weight polyethylene acetabular cup retrieved 24 years postoperatively. Performance evaluations included absorbed radiation dose, total wear penetration, average wear rate, crystallinity, oxidation, and equibiaxial tensile properties. A retrieved acetabular cup of the same grade of polyethylene but gamma-sterilized using a conventional dose was used as control. The highly crosslinked and control cups took in about 1300 and 30 kGy of radiation, respectively, as measured using a trans-vinylene index. Direct dimensional measurements revealed average wear rates of the highly crosslinked and control cups were 0.04 and 0.06 mm/year, respectively. The oxidation index of the highly crosslinked cup was very high (0.679), but equivalent to that of a 1000 kGy irradiated reference cup. The retrieved highly crosslinked cup showed much higher equibiaxial ultimate tensile strength than the retrieved control cup. Based on these observations, the increased wear resistance and equibiaxial tensile properties that resulted from extensive crosslinking in the presence of air were partially offset by the adverse effects caused by immediate oxidation during the process.

Charge trapping and dielectric relaxations of gamma irradiated radiolytically oxidized highly oriented LDPE

The influence of gamma irradiation in air on the effects of charge trapping in highly oriented LDPE has been studied through dielectric loss (tan δ) analysis in the temperature range from 25 to 325 K and using thermally stimulated discharge current (TSDC) measurements. The radiation induced oxidation was observed using IR spectroscopy. The intensity and the position of the γ and β dielectric relaxation maxima were correlated with maxima of TSDC measurements. It was found that the positions of the relaxation peaks are strongly dependent upon the changes in the microstructure of the amorphous phase and on the surface of the crystallites induced by orientation and gamma irradiation.

Fractography evolution in accelerated aging of UHMWPE after gamma irradiation in air

We studied the fracture surface evolution of ultra high molecular weight polyethylene (UHMWPE) specimens, manufactured from GUR 1050 compression moulded sheets, after gamma sterilisation in air followed by different aging times after thermal treatment at 120°C. Degradation profiles were obtained by FTIR and DSC measurements after 0, 7, 14, 24 and 36 h aging. We observed by SEM the morphology patterns at these aging times, in surface fractographies after uniaxial tensile test of standardised samples. The results pointed out clear differences between short and long aging times. At shorter times, 7 h, the behaviour was similar to non-degraded UHMWPE, exhibiting ductile behaviour. At longer times, 24–36 h, this thermal protocol provided a highly degraded zone in the subsurface, similar to the white band found after gamma irradiation in air followed by natural aging, although closer to the surface, at 150–200 μm. The microstructure of this oxidation zone, similarly found in gamma irradiated samples shelf-aged for 6–7 years, although with different distribution of microvoids, was formed by fibrils, associated with embrittlement of the oxidised UHMWPE. In addition, the evolution of the oxidation index, the enthalpy content, the mechanical parameters, and the depth of the oxidation front deduced from the fractographies versus aging time showed that a changing behaviour in the degradation rate appeared at intermediate aging times.

Early head penetration into cemented Hylamer Ogee socket

The authors reviewed 18 consecutive cemented total hip arthroplasties with the Hylamer Ogee socket. The average follow-up period was 47 months. All Hylamer Ogee sockets were sterilized by gamma irradiation in air, and the average period from production to surgery was 12 months. Two-dimensional penetration of the femoral head into the Hylamer Ogee socket was determined from anteroposterior radiographs of the pelvis using a digitizer. The average head penetration rate of the present series was 0.36 mm/y. Osteolysis around both components was found in 6 cases (30%). Of 4 cases defined as aseptic loosening, 2 were revised during the follow-up period. To detect component loosening because of catastrophic high wear of the cemented Hylamer Ogee socket as early as possible, precise radiographic follow-up is essential.

Long-term in vivo wear performance of porous-coated acetabular components sterilized with gamma irradiation in air or ethylene oxide

This study evaluated the long-term in vivo wear performance of 2 groups of well-functioning cementless acetabular cups sterilized by different methods. The first group included 31 hips that were implanted with AML TriSpike cups (DePuy, Warsaw, IN) sterilized by γ-irradiation in air. The second group included 28 hips implanted with Arthropor cups (Joint Medical Products, Stamford, CT) that were sterilized with ethylene oxide. Time-dependent variations in the radiographic wear rates were compared within each group. Changes in the wear rates between 4- and 16-year follow-up times for the TriSpike cups were not significant ( P=.09), and there was no evidence to suggest a trend toward substantially increasing wear rates with longer follow-up times. Among the Arthropor cups, the wear rates remained relatively constant between 2 and 14 years of follow-up evaluation. Although clinically apparent late increases in radiographic head penetration rates were not evident, we will continue to monitor all patients for evidence of accelerated wear at late follow-up.

Responsibilities when confronting a defective surgical implant.

The senior author shares lessons from his experience with faulty knee implants that resulted in severe polyethylene wear and 51% failure of 73 unicondylar knee arthroplasties within 2 years after implantation. Ensuing research showed that the components had been sterilized by gamma irradiation in air and had shelf lives ranging from 4.5 years to 6.5 years. It was learned that gamma-irradiated components with more than 5-year shelf lives remain in inventories without information showing their sterilization dates. This experience reiterated the importance of staying abreast of the literature and reports concerning products and of being alert to unexpected patient outcomes relatively soon after implantation. If a surgeon suspects a faulty prosthetic device, he or she should contact the manufacturer and the United States Food and Drug Administration, so that an investigation can begin. In such situations, the physician's primary responsibility lies with the patient, and it is necessary to arrange extra time and staff resources to give the affected patients information, compassion, added clinical monitoring, and treatment. Because the potential exists for gamma-irradiated in air implants with lengthy shelf lives to remain undetected in inventories, the current authors recommend that surgeons select implants sterilized by a different process.

Fate of the unrevised all-polyethylene patellar component in revision total knee arthroplasty.

During revision total knee arthroplasty, the alternatives for addressing the well-positioned and secure all-polyethylene patellar component with minimal or no wear include retention, revision, and resection. The purpose of the present study was to determine the intermediate-term fate of all-polyethylene patellar components that were left in situ at the time of revision tibiofemoral arthroplasty. We retrospectively reviewed the results of 202 revision total knee arthroplasties, performed at four centers, in which a well-fixed, well-positioned all-polyethylene patellar component with minimal wear was retained. Clinical evaluation was performed to identify the presence of anterior knee symptoms or patellar failures, and the Knee Society clinical scores were recorded. Sixty-eight percent of the revision tibiofemoral components were from a different manufacturer than the retained patellar component. Twenty-one patients (twenty-one knees; 10%) reported anterior knee pain at a mean duration of follow-up of seven years (range, two to fourteen years). Eleven of the twenty-one knees with anterior knee pain had evidence of component loosening, excessive wear, or delamination either on follow-up radiographs or on direct inspection at the time of reoperation. In the other ten knees, the findings were attributable to osseous impingement on the femoral component or soft-tissue dysfunction (including soft-tissue impingement, subluxation, and scarring). Seventeen of the twenty-one patients with anterior knee pain underwent additional surgery to address the source of the pain. Failures due to loosening or wear occurred only in patients in whom the patellar component had been sterilized with gamma irradiation in air; these failures occurred at a mean 7.3 years after revision (eleven years after primary total knee arthroplasty). The likelihood of patellar component failure was significantly greater when the component had been gamma irradiated in air than when it had been sterilized with another method (p = 0.0008). The average knee score increased from 42 to 86 points, and the average function score increased from 46 to 70 points. Retaining a well-positioned, stable all-polyethylene patellar component at the time of revision tibiofemoral arthroplasty can be successful, provided that the polyethylene has not oxidized. Manufacturing mismatch is acceptable with most contemporary designs provided that the patellar component articulates appropriately with the femoral implant. Therapeutic study, Level IV (case series [no, or historical, control group]). See p. 2 for complete description of levels of evidence.

Howse II cemented titanium metal-backed acetabular cups: poor 10-year results in 107 hips.

We report a retrospective clinical and radiographic review of 107 consecutive primary total hip replacements, using the cemented Howse II prosthesis. The review concentrates on the long-term performance of the acetabular cup, which was designed with a cylindrical titanium metal backing and a polyethylene liner. In all the hips in the series, this cup was used with a modular titanium straight femoral stem and a 32-mm diameter titanium head. At a mean follow-up of 9.8 (7-11) years, 28 hips had already been revised and 10 others were recognized as clinical or radiographic failures. The indication for revision in 26 of the hips was aseptic loosening and the mean time to revision was 7.2 (3.7-10.9) years This metal-backed cup has only 42% survival at 10 years, which seems to be related to certain of its design features. These include an inadequate thickness of polyethylene sterilized by gamma irradiation in air, a lack of mechanical bonding between the liner and its metal shell, and the high wear rate from the use of a 32-mm titanium femoral head.

Oxidation-induced dynamic changes in morphology reflected on freeze-fractured surface of gamma-irradiated ultra-high molecular weight polyethylene components.

Oxidative degradation of ultra-high molecular weight polyethylene (UHMWPE) attributed to sterilization by gamma-radiation in the presence of air has been cited as one of the major causes of premature wear in total joint arthroplasty. For example, in retrieved UHMWPE tibial bearings, not only adhesive and abrasive wear, but also fatigue wear characterized by delamination and fracture is frequently observed. In this study, we examined the effects of gamma radiation on the microstructural morphology of UHMWPE tibial bearings, and propose a severe fatigue wear mechanism. Scanning electron microscopic observations were conducted on freeze-fractured surface of retrieved UHMWPE components that had been sterilized with gamma radiation in air before implantation, unused components that had been stored on the shelf for several years (5-11) after sterilization, and disc specimens given an accelerated aging protocol after gamma radiation. Scanning electron microscopic observations showed that the freeze-fractured surface of these components had a double layer, which was bordered below the surface. A closer observation of the subsurface layer below the border revealed an extremely rough and porous honeycomb-like structure. Fourier transform infrared analysis demonstrated that the honeycomb-like region in the subsurface had a high oxidation level. The internal morphology of oxidized UHMWPE was classified into four categories based on the level of the oxidation. According to these results, the morphological changes with oxidative degradation of gamma-irradiated UHMWPEs in the presence of air could consistently be explained as the result of major chemical and physical changes such as increased crystallinity, elevated density, and reduced mechanical strength. We relate the morphological changes caused by oxidative degradation in the subsurface to the location of the origin of fatigue wear in total knee joints.

Rapid polyethylene failure of unicondylar tibial components sterilized with gamma irradiation in air and implanted after a long shelf life.

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 < or = 4.4 years. 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. 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, r (2) = 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. The present study demonstrated early, severe wear of tibial polyethylene bearings that had been sterilized by gamma irradiation in air and stored for < or = 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.

Accelerated aging studies of UHMWPE. I. Effect of resin, processing, and radiation environment on resistance to mechanical degradation.

The resin and processing route have been identified as potential variables influencing the mechanical behavior, and hence the clinical performance, of ultra-high molecular weight polyethylene (UHMWPE) orthopedic components. Researchers have reported that components fabricated from 1900 resin may oxidize to a lesser extent than components fabricated from GUR resin during shelf aging after gamma sterilization in air. Conflicting reports on the oxidation resistance for 1900 raise the question of whether resin or manufacturing method, or an interaction between resin and manufacturing method, influences the mechanical behavior of UHMWPE. We conducted a series of accelerated aging studies (no aging, aging in oxygen or in nitrogen) to systematically examine the influence of resin (GUR or 1900), manufacturing method (bulk compression molding or extrusion), and sterilization method (none, in air, or in nitrogen) on the mechanical behavior of UHMWPE. The small punch testing technique was used to evaluate the mechanical behavior of the materials, and Fourier transform infrared spectroscopy was used to characterize the oxidation in selected samples. Our study showed that the sterilization environment, aging condition, and specimen location (surface or subsurface) significantly affected the mechanical behavior of UHMWPE. Each of the three polyethylenes evaluated seem to degrade according to a similar pathway after artificial aging in oxygen and gamma irradiation in air. The initial ability of the materials to exhibit post-yield strain hardening was significantly compromised by degradation. In general, there were only minor differences in the aging behavior of molded and extruded GUR 1050, whereas the molded 1900 material seemed to degrade slightly faster than either of the 1050 materials.

Observation on retrieved Hylamer glenoids in shoulder arthroplasty: Problems associated with sterilization by gamma irradiation in air

Hylamer is an ultra-high molecular weight polyethylene that was used in the glenoid prosthesis of the Global Shoulder. It was developed from a joint venture with DePuy Orthopaedics, Inc, and E.I. DuPont Company. In presterilization evaluations it was noted to have improved mechanical strength with superior creep and wear resistance. However, after sterilization was performed with gamma irradiation in an air environment from 1990 to 1993, the Hylamer glenoids were affected by oxidation, which altered the mechanical properties, and they became brittle and eroded. From 1993 to 1995 Hylamer glenoids were sterilized by gamma irradiation in a nitrogen environment. From 1995 to 1998 sterilization was by gas plasma processes, which do not create the oxidation problem. Since 1998 the glenoid prosthesis of the Global Shoulder has been made exclusively of Enduron and is gas sterilized. The primary purpose of this article is to provide orthopaedic surgeons the best information available about Hylamer glenoids. We make suggestions concerning the diagnosis and treatment of the patients who had a Hylamer glenoid inserted between 1990 and 1993. (J Shoulder Elbow Surg 2002;11:191-197)

Direct compression molded polyethylene for total hip and knee replacements.

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.

A comparison of the wear and physical properties of silane cross-linked polyethylene and ultra-high molecular weight polyethylene

Cross-linked polyethylenes are being introduced widely in acetabular cups in hip prostheses as a strategy to reduce the incidence of wear debris–induced osteolysis. It will be many years before substantial clinical data can be collected on the wear of these new materials. Silane cross-linked polyethylene (XLPE) was introduced into clinical practice in a limited series of acetabular cups in 1986 articulating against 22.225-mm alumina ceramic femoral heads and showed reduced wear rates compared with conventionally sterilized (gamma irradiation in air) ultra-high molecular weight polyethylene (UHMWPE). We compared the wear of XLPE manufactured in 1986 with the wear of UHMWPE manufactured in 1986 in nonirradiated and irradiated forms. In the nonirradiated forms, the wear of XLPE was 3 times less than UHWMPE when articulating against smooth counterfaces. The nonirradiated materials did not show signs of oxidation. In the irradiated forms, only UHMWPE showed high levels of oxidation, and this caused a substantial increase in wear. Antioxidants added to XLPE during processing gave resistance to oxidative degradation. When sliding against scratched counterfaces, the wear of UHMWPE increased by a factor of 2 to 3 times. Against the same scratched counterfaces, the wear of XLPE increased dramatically by 30 to 200 times. This difference may be attributed to the reduction in toughness of XLPE. Clinically, XLPE has been articulated against damage-resistant ceramic heads, and this probably has been an important factor in contributing to reduced wear. New cross-linked polyethylenes differ considerably from XLPE. This study indicates that it is prudent to examine the wear of new polyethylenes under a range of conditions that may occur in vivo.

Mechanical behavior, wear surface morphology, and clinical performance of UHMWPE acetabular components after 10 years of implantation

The relationships between the mechanical behavior of ultra-high molecular weight polyethylene (UHMWPE) and the long-term clinical performance of acetabular component bearings remain poorly understood. During previous hip simulator validation studies, researchers have correlated the mechanical behavior and surface morphology of polymeric biomaterials with the wear performance in a hip simulator. For the present study, we sought correlations between clinical performance and quantitative metrics of the mechanical behavior and surface morphology of retrieved cups. Nine all-UHMWPE acetabular cups of the same cemented design, 32mm head size, and manufacturer were retrieved after an average implantation time of 14.1 years (range: 11.5–16.4 years). The implants were manufactured from RCH-1000, sterilized by gamma radiation in air and implanted between 1980 and 1983. Mechanical behavior of the retrieved components was determined using the previously validated small punch test, which subjects miniature specimens to multi-axial loading conditions. Surface morphology of the retrievals was assessed using white light interferometry. No significant relationship was observed between the surface roughness measurements and patient related variables. However, statistically significant relationships were observed between the mechanical behavior determined by the small punch test and the implantation time and patient weight associated with the retrieved hip cups. These findings support the hypothesis that the mechanical behavior of UHMWPE is related, at least in part, to the clinical performance of acetabular component bearings for total hip replacement.

Fracture behavior of UHMWPE in non-implanted, shelf-aged knee prostheses after gamma irradiation in air

We studied non-implanted ultra-high molecular weight polyethylene (UHMWPE) knee components, that were gamma-sterilized in air and shelf-aged during 6 and 7 years, to explore fracture mechanisms in the degraded material. Prior to the fractography study, we assessed degradation through FTIR and optical density measurements in microtome sections. Then, we morphologically compared the surface fractography of degraded, oxidized UHMWPE with that of non-degraded material in specimens from the same implants, fractured after uniaxial tensile tests in standardized samples. Fractography surface analysis clearly differentiated the behavior of non-degraded and degraded UHMWPE. Failure in non-degraded samples occurred through craze formation, followed by fast crack propagation. Quantitative fracture analysis suggested a ductile behavior. This was supported by the identified local plasticity in the region close to the initial flaw. In the fractography surface analysis of degraded material, a fibrillar behavior was observed. A heterogeneous domain structure appeared, showing different coarseness with multiple void sizes. Our results, based on mechanical measurements and fracture morphology in the subsurface region, confirmed the embrittlement of oxidized UHMWPE.

Wear debris from total hip arthroplasty presenting as an intrapelvic mass

Abstract: A granulomatous reaction to the small particles worn off from an implanted prosthesis is a common complication in arthroplasty. The chronic inflammatory response to the wear debris from either the polyethylene or the metal components is thought to be responsible for osteolysis, implant loosening, and finally mechanical failure. Formation of an extra-articular granulomatous tissue mass is uncommon, however. We report a case of a huge granulomatous mass that presented as an intrapelvic tumor. The prosthesis was an Anatomical Medullary Locking (AML) hip arthroplasty with cementless fixation. The polyethylene insert was manufactured by ram extrusion and sterilized with gamma irradiation in air. Fragments of the insert were among the contents in the cystic mass together with black metallic debris. There was no loosening of the prosthesis, and the acetabulum floor was preserved.

Head penetration into Hylamer acetabular liner sterilized by gamma irradiation in air and in a nitrogen atmosphere

We reviewed 25 consecutive primary cementless total hip arthroplasties with Hylamer acetabular liners (Hylamer group) and 12 with conventional ultra-high molecular weight polyethylene (Enduron group). Two-dimensional penetration of the femoral head into the liner was determined from anteroposterior radiographs of the pelvis. Head penetration rate was 0.37 mm/y in the Hylamer group sterilized by gamma irradiation in air (n = 6; mean length of follow-up, 3 years), 0.21 mm/y in the Hylamer group sterilized by gamma irradiation in a nitrogen atmosphere (n = 19; mean length of follow-up, 2.7 years), and 0.11 mm/y in the Enduron group (n = 12; mean length of follow-up, 3.9 years). Osteolysis was identified in 6 of the 25 hips with Hylamer liners and 1 of the 12 hips with conventional liners. There was a positive linear correlation between period from production to operation and head penetration rate with Hylamer liner sterilized by gamma irradiation in air and no correlation in a nitrogen atmosphere. Rapid oxidation by irradiation in air might not be the main cause of high rate of wear in Hylamer liners.