Shannon L. Rowell

Ex Vivo Stability Loss of Irradiated and Melted Ultra-High Molecular Weight Polyethylene

BACKGROUND Radiation crosslinking reduces wear of ultra-high molecular weight polyethylene (UHMWPE), and subsequent annealing or melting increases oxidative stability. Little is known about the oxidative stability of polyethylene total joint components after in vivo service and subsequent shelf storage in air. METHODS We analyzed thirty-four surgically retrieved, radiation crosslinked acetabular liners to determine their oxidative stability after in vivo service (range, 0.5 to 84.0 months). Oxidation was determined at the time of explantation. After shelf storage in air (range, 7.0 to 72.0 months), oxidation, crosslink density, and thermal properties were determined. Oxidation of one control liner that was shelf-aged in air (for eighty-four months) was also determined. RESULTS At the time of explantation, all components showed minimal oxidation; however, oxidation levels increased during shelf storage, with a concomitant decrease in crosslink density and increase in crystallinity. Increasing oxidation, increasing crystallinity, and decreasing crosslink density correlated with the duration of ex vivo storage. The shelf-aged control liner showed no detectable oxidation. CONCLUSIONS The oxidation and loss of crosslink density of the irradiated and melted UHMWPE was surprising. Two potential mechanisms that might alter the oxidative stability of UHMWPE in vivo are cyclic loading and absorption of lipids. Both of these mechanisms can generate new free radicals in UHMWPE and can initiate and propagate its oxidation.

A surface crosslinked UHMWPE stabilized by vitamin E with low wear and high fatigue strength

Wear particle-induced periprosthetic osteolysis has been a clinical problem driving the development of wear resistant ultrahigh molecular weight polyethylene (UHMWPE) for total joint replacement. Radiation crosslinking has been used to decrease wear through decreased plastic deformation; but crosslinking also reduces mechanical properties including fatigue resistance, a major factor limiting the longevity of joint implants. Reducing UHMWPE wear with minimal detriment to mechanical properties is an unaddressed need for articular bearing surface development. Here we report a novel approach to achieve this by limiting crosslinking to the articular surface. The antioxidant vitamin E reduces crosslinking efficiency in UHMWPE during irradiation with increasing concentration, thus we propose to spatially control the crosslink density distribution by controlling the vitamin E concentration profile. Surface crosslinking UHMWPE prepared using this approach had high wear resistance and decreased crosslinking in the bulk resulting in high fatigue crack propagation resistance. The interface region did not represent a weakness in the material due to the gradual change in the crosslink density. Such an implant has the potential of decreasing risk of fatigue fracture of total joint implants as well as expanding the use of UHMWPE to younger and more active patients.

Fracture of a Cross-Linked Polyethylene Liner Due to Impingement

We report a case of fracture at 2 years after implantation of a 50-kGy moderately cross-linked ultrahigh molecular weight polyethylene liner with an extended lip (Marathon, DePuy, Warsaw, IN). The extended lip section had fractured. The liner showed no oxidation. The articular surface was grossly deformed, likely due to wear, creep, and/or plastic deformation, and the liner showed no recovery of machining marks upon melting, indicating that some wear had occurred. Electron microscopy revealed fatigue striations on the fracture surface. The likely cause of failure was femoral neck impingement-induced wear and fatigue on the liner.

Delamination and Adhesive Wear Behavior of α-Tocopherol-Stabilized Irradiated Ultrahigh-Molecular-Weight Polyethylene

Wear and delamination of conventional ultrahigh-molecular-weight polyethylene (UHMWPE) components used in total knee arthroplasty can compromise long-term performance. Radiation cross-linking and melt-annealing reduced wear and increased delamination resistance of UHMWPE. An alternative material is the alpha-tocopherol-stabilized irradiated UHMWPE (alphaTPE), with improved mechanical and fatigue properties vs irradiated and melted UHMWPE. We studied the wear and delamination resistance of alphaTPE and conventional UHMWPE (direct compression molded GUR 1050 and Himont 1900) under reciprocating unidirectional motion. Wear resistance was improved, and no delamination was observed in alphaTPE. Accelerated aging did not alter the wear and delamination behavior of alphaTPE. The GUR 1050 UHMWPE showed delamination and pitting when subjected to unidirectional reciprocating motion after accelerated aging. Himont 1900 UHMWPE showed no delamination when subjected to unidirectional reciprocating motion after accelerated aging. alpha-Tocopherol-stabilized irradiated UHMWPE is advanced for use in total knee arthroplasty due to its high resistance to wear, delamination, and oxidation.

Increasing irradiation temperature maximizes vitamin E grafting and wear resistance of ultrahigh molecular weight polyethylene

Vitamin E stabilization of radiation crosslinked ultrahigh molecular weight polyethylene (UHMWPE) for total joint implants can be done by blending of UHMWPE resin powder with vitamin E, followed by consolidation and irradiation of the blend. It is well known that vitamin E prevents crosslinking in UHMWPE during ionizing radiation. We hypothesized that there would also be a significant amount of grafting of vitamin E onto UHMWPE during irradiation. Spectroscopic analysis of radiation crosslinked vitamin E-blended UHMWPE before and after extraction with boiling hexane showed vitamin E grafting in up to 30% of the blended vitamin E. Grafting increased with irradiation temperature. We also discovered that increasing irradiation temperature resulted in better preservation of active vitamin E in the polymer and increased crosslinking efficiency of UHMWPE. As a result, warm-irradiated vitamin E-blended UHMWPEs had significantly less wear than those irradiated at ambient temperature. It may be desirable to graft vitamin E on UHMWPE to decrease the possibility of elution and increase long-term stability. Warm irradiation of vitamin E blends may present an advantage in increasing vitamin E potency, as well as decreasing the wear of UHMWPE, which is crucial in decreasing the incidence of periprosthetic osteolysis in total joint replacement patients.

Comparative oxidative stability of α-tocopherol blended and diffused UHMWPEs at 3 years of real-time aging.

Vitamin E (α‐tocopherol) is a free‐radical stabilizing antioxidant used to maintain oxidative stability in radiation cross‐linked ultra‐high molecular weight polyethylene (UHMWPE) used in total joint replacements. We investigated the oxidative resistance of vitamin E‐stabilized UHMWPE in (i) postirradiation vitamin E‐diffused UHMWPE, (ii) vitamin E blended and irradiated UHMWPE, and (iii) unstabilized, irradiated UHMWPE after accelerated aging and real‐time aging in an aqueous environment at 40°C for 36 months. Unstabilized samples exhibited substantial oxidation throughout the surface and bulk with both types of aging. While vitamin E‐stabilized, radiation cross‐linked UHMWPEs were all superior to unstabilized samples, irradiated blends showed surface oxidation and subsurface oxidation potential beginning at ten months in real‐time aging. In contrast, postirradiation vitamin E‐diffused UHMWPEs showed no detectable oxidation and no increase in oxidation potential despite elution of some vitamin E. We also showed that current thermal accelerated aging methods were unable to differentiate among the oxidative stability of vitamin E‐stabilized, radiation cross‐linked UHMWPEs prepared by different processes.

In Vivo Oxidative Stability Changes of Highly Cross-Linked Polyethylene Bearings: An Ex Vivo Investigation

The development of highly cross-linked UHMWPEs focused on stabilizing radiation-induced free radicals as the sole precursor to oxidative degradation. However, secondary in vivo oxidation mechanisms have been discovered. After a preliminary post-operative analysis, we subjected highly cross-linked retrievals with 1-4 years in vivo durations and never-implanted controls to accelerated aging to predict the extent to which their oxidative stability was compromised in vivo. Lipid absorption, oxidation, and hydroperoxides were measured using infrared spectroscopy. Gravimetric swelling was used to measure cross-link density. After aging, all retrievals, except vitamin E-stabilized components, regardless of initial lipid levels or oxidation, showed significant oxidative degradation, demonstrated by subsurface oxidative peaks, increased hydroperoxides and decreased cross-link density, compared to their post-operative material properties and never-implanted counterparts, confirming oxidative stability changes.

Investigation of surgically retrieved, vitamin E-stabilized, crosslinked UHMWPE implants after short-term in vivo service

BACKGROUND Antioxidant stabilized highly crosslinked ultra-high molecular weight polyethylene (UHMWPE) components have been in clinical use since 2008. In vitro testing has shown excellent oxidation resistance, wear resistance, mechanical properties, and fatigue strength. In this study, we analyzed surgically retrieved components to investigate in vivo behavior and changes in the material. METHODS Fifteen surgically retrieved, vitamin E-stabilized, and radiation crosslinked UHMWPE components were analyzed to determine their oxidative stability, extent of lipid absorption in vivo, free radical content, hydroperoxide index, and extent of visible wear damage after in vivo service (0.1-36.6 months). RESULTS Retrievals showed no significant carbonyls at the time of surgical removal, while free radical content was observed to decay with increasing in vivo duration. There was no increase in hydroperoxide index. Lipid penetration increased with time. Ex vivo oxidation was not observed after 18 months of aging in air at room temperature. CONCLUSIONS The free-radical scavenging activity of the vitamin E appears to successfully prevent both in vivo and ex vivo oxidation for short durations, while reducing free radical content overall. Without an increase in hydroperoxides, the oxidation cascade initiated by radiation-induced and lipid-derived free radicals appears to have been inhibited. Further investigation is required with longer duration implants.

Material Properties of a Highly Anteverted Vitamin E-Stabilized Polyethylene Liner After Sixteen Months In Vivo

Highly cross-linked ultrahigh molecular weight polyethylene (UHMWPE) implants have produced lower wear rates compared with conventional UHMWPE implants in laboratory testing and in clinical use1. Highly cross-linked UHMWPE has resulted in reduced osteolysis in clinical follow-up studies2. The various formulations of highly cross-linked UHWMPE used today are cross-linked with ionizing radiation and then annealed or melted to reduce or eliminate the resultant free radicals that could lead to oxidation. An alternative approach to stabilizing radiation-induced free radicals while maintaining the mechanical properties of the highly cross-linked UHMWPE is to incorporate vitamin E, an antioxidant. Laboratory studies have shown that after irradiation, vitamin E-stabilized UHMWPE has improved oxidation resistance, equivalent wear, and improved mechanical strength relative to non-vitamin E-stabilized highly cross-linked UHMWPE3-7. Nevertheless, as we describe in this case report, vitamin E-stabilized UHMWPE remains susceptible to rim fracture in a highly anteverted shell, similar to first-generation highly cross-linked UHMWPE8,9 without in vivo material property changes. The patient was informed that data concerning the case would be submitted for publication, and she provided consent. Institutional review board approval was obtained. Twelve years after a fifty-five-year-old woman with rheumatoid arthritis had undergone primary left total hip arthroplasty, she was referred to us for evaluation of episodic, activity-related left groin pain. Radiographs showed a well-fixed cementless hip arthroplasty with an eccentric femoral head, indicating polyethylene wear and osteolysis (Fig. 1-A and Fig. 2-A). The acetabular component was in approximately 45° of abduction and 44° of anteversion. The infection workup was negative. Fig. 1 Figs. 1-A, 1-B, and 1-C Anteroposterior radiographs of the left hip. Fig. 1-A Preoperative radiograph before the first revision. Fig. 1-B Six-week postoperative radiograph after the first revision. Fig. 1-C Radiograph one year after the first revision. Fig. 2 Figs. 2-A, 2-B, and 2-C …

In-vitro oxidation model for UHMWPE incorporating synovial fluid lipids: IN-VITRO OXIDATION MODEL FOR UHMWPE

Post‐irradiation melting of ultra‐high molecular weight polyethylene (UHMWPE) reduced the oxidation potential of UHMWPE in vivo. After mid‐term (5–10 years) use in vivo, there is detectable oxidation in irradiated and melted joint implant retrievals. The absorption of the synovial fluid lipid squalene was identified as a possible factor initiating oxidation. We investigated the role of lipids in UHMWPE oxidation by asking: (1) Do other synovial fluid lipids initiate oxidation in irradiated and melted UHMWPE?; (2) What is the effect of the absorption of multiple lipids on UHMWPE oxidation?; (3) How does lipid‐initiated oxidation in vitro compare to what is observed in long‐term retrievals? We diffused emulsified single and mixed lipids into irradiated and melted UHMWPE and accelerated aged them. We analyzed the oxidation in these samples and in four long‐term highly crosslinked, irradiated, and melted Longevity™ UHMWPE liner retrievals (in vivo for up to 190 months) using Fourier Transform Infrared Spectroscopy (FTIR). We showed that lipids other than squalene could initiate oxidation in UHMWPE and that the types of absorbed lipids determined the amount of resultant oxidation. Although mixed lipids doping and accelerated aging reproduced the average and maximum oxidation values and oxidation products observed in vivo, the oxidation depth profile and its effect on cross‐link density was different. One reason for this was the variability of oxidation in retrievals, suggesting additional factors contributing to oxidation. The understanding of oxidative processes in vivo and the development of clinically relevant in vitro protocols to evaluate implant materials is crucial for their long‐term performance.