The primary purpose of the current study was to evaluate and compare the wear properties of vitamin E-doped, highly-crosslinked PE (VEPE) and one formulation of moderately cross-linked and mechanically-annealed ultra-high molecular weight PE (ModXLPE) in patients five years after primary THA. We also sought to understand whether polyethylene wear is associated with radiographic evidence of bone resorption or with deterioration in patient-reported outcome measures (PROMs).

A total of 221 patients from four international centers were recruited into a prospective RSA and clinical outcomes study. Seventy percent (76%) of patients received VEPE (vs. ModXLPE) liners, and 36% received ceramic (vs. metal) femoral heads. PROMs and radiographs were collected preoperatively and at one, two, and five years postoperatively. In addition, RSA radiographs were collected to measure PE wear.

We observed similar bedding in through the one-year interval and wear through the two-year interval between the two liner types. However, there was significantly more femoral head penetration in the ModXLPE cohort compared to the VEPE cohort at the five-year follow-up (p<0.001). The only variables independently predictive of increased wear were ModXLPE (vs VEPE) liner type (β=0.22, p=0.010) and metal (vs. ceramic) femoral head type (β=0.21, p=0.013). There was no association between increased wear and radiolucency development (p=0.866) or PROMs. No patients were found to have evidence of osteolysis.

At five-years postoperatively, patients treated with VEPE (vs. ModXLPE) and ceramic (vs. metal) femoral heads demonstrated decreased wear. At the longest follow-up (five years postoperatively), the wear rates for both liner groups were very low and have not led to any osteolysis or implant failures via aseptic loosening.

Osteolysis secondary to ultra-high molecular weight polyethylene (UHMWPE) wear is a leading cause of late-term implant failure via aseptic loosening in patients treated with total hip arthroplasty (THA). Radiation crosslinking of UHMWPE has been shown to decrease wear. However, the resulting polymer (crosslinked-PE) has a high free radical content. Two different methods that have been used to reduce the remaining free radicals are mechanical annealing and chemical stabilization using Vitamin E, a free radical scavenger.

The primary purpose of the current study was to evaluate and compare the wear properties of vitamin E-doped crosslinked-PE (VEPE) and one formulation of mechanically annealed crosslinked-PE using radiostereometric analysis (RSA) in patients five years after primary THA. We also sought to understand the association between polyethylene wear and patient-reported outcome measures (PROMs).

Three-hundred and five patients from six international centers were enrolled. Seventy-six percent were treated with highly-crosslinked (95 kGy) VEPE liners, and the rest received moderately-crosslinked (50 kGy) (ModXL), mechanically annealed liners. Data was collected prospectively at one-, two-, and five-year intervals.

At the 5-year follow-up, proximal femoral head penetration into the VEPE liners (median = 0.05mm (range, −0.03–1.20)) was significantly lower than the penetration into the ModXL liners (median = 0.15mm (range, −0.22–1.04)) (p<0.001). In the VEPE cohort the median proximal penetration did not increase from one- to five-year follow-up (p=0.209). In contrast, there was a significant increase in femoral head penetration for the ModXL group (p<0.001) during that same time. Multivariable regression showed that the only variable predictive of increased wear was ModXL liner type (B=0.12, p<0.001). There were no differences in PROMs between the liner groups, and there was no correlation between polyethylene wear and PROMs for the cohort as a whole.

The current study is the largest analysis of polyethylene wear at five-year follow-up using the RSA technique. We observed similar bedding in through the two-year interval between the two liner types, however, there was significantly more wear in the ModXL cohort at five-years. Currently, the wear rates for both liner groups are below the osteolysis threshold and have not led to any implant failures via aseptic loosening. Continued follow-up will provide a better understanding of the association between wear rate and clinical outcomes.

Aims. The material and design of knee components can have a considerable effect on the contact characteristics of the tibial post. This study aimed to analyze the stress distribution on the tibial post when using different grades of polyethylene for the tibial inserts. In addition, the contact properties of fixed-bearing and mobile-bearing inserts were evaluated. Methods. Three different grades of polyethylene were compared in this study; conventional ultra high molecular weight polyethylene (UHMWPE), highly cross-linked polyethylene (HXLPE), and vitamin E-stabilized polyethylene (VEPE). In addition, tibial baseplates with a fixed-bearing and a mobile-bearing insert were evaluated to understand differences in the contact properties. The inserts were implanted in neutral alignment and with a 10° internal malrotation. The contact stress, von Mises stress, and equivalent plastic strain (PEEQ) on the tibial posts were extracted for comparison. Results. The stress and strain on the tibial post for the three polyethylenes greatly increased when the insert was placed in malrotation, showing a 38% to 56% increase in von Mises stress and a 335% to 434% increase in PEEQ. The VEPE insert had the lowest PEEQ among the three materials. The mobile-bearing design exhibited a lower increase in stress and strain around the tibial posts than the fixed-bearing design. Conclusion. Using VEPE for the tibial component potentially eliminates the risk of material permanent deformation. The mobile-bearing insert can help to avoid a dramatic increase in plastic strain around the tibial post in cases of malrotation. The mobility allows the pressure to be distributed on the tibial post and demonstrated lower stresses with all three polyethylenes simulated. Cite this article: Bone Joint Res 2020;9(11):768–777

Aims

Radiostereometric analysis (RSA) studies of vitamin E-doped, highly crosslinked polyethylene (VEPE) liners show low head penetration rates in cementless acetabular components. There is, however, currently no data on cemented VEPE acetabular components in total hip arthroplasty (THA). The aim of this study was to evaluate the safety of a new cemented VEPE component, compared with a conventional polyethylene (PE) component regarding migration, head penetration, and clinical results.

Radiation cross-linked ultrahigh molecular weight polyethylene (UHMWPE) is the bearing of choice in joint arthroplasty. The demands on the longevity of this polymer are likely to increase with the recently advancing deterioration of the performance of alternative metal-on-metal implants. Vitamin E-stabilized, cross-linked UHMWPEs are considered the next generation of improved UHMWPE bearing surfaces for improving the oxidation resistance of the polymer. It was recently discovered that in the absence of radiation-induced free radicals, lipids absorbed into UHMWPE from the synovial fluid can initiate oxidation and result in new free radical-mediated oxidation mechanisms. In the presence of radiation-induced free radicals, it is possible for the polymer to oxidize through both existing free radicals at the time of implantation and through newly formed free radicals in vivo. Thus, we showed that reducing the radiation-induced free radicals in vitamin E-stabilized UHMWPE would increase its oxidative stability and presumably lead to improved longevity. We describe mechanical annealing, low pressure annealing, and warm irradiation of irradiated vitamin E blends as novel methods to eliminate 99% of radiation-induced free radicals without sacrificing crystallinity. These are significant improvements in the processing of highly cross-linked UHMWPE for joint implants with improved longevity

Introduction. Vitamin E stabilization of radiation crosslinked UHMWPE is done by (1) blending into the resin powder, consolidating and irradiating or (2) diffusing into already consolidated and irradiated UHMWPE and terminally gamma sterilizing. With blending, a higher radiation dose is required for crosslinking to the same level as virgin UHMWPE. With diffusion, the vitamin E amount used is not limited by the crosslink density, but, vitamin E is exposed to terminal sterilization dose of 25–40 kGy, less than the 100–150 kGy used with blending, which may decrease the grafting of the antioxidant onto the polymer. We investigated the efficiency of grafted vitamin E against squlene-initiated accelerated aging. Methods. Medical grade GUR1050 UHMWPE with vitamin E (0.1 wt%) was irradiated to 150 kGy. Tibial knee insert preforms were irradiated to 100 kGy, diffused with vitamin E using a doping and homogenization procedure. This UHMWPE was used either before or after gamma sterilization. One set of machined blocks (10 × 10 × 6 mm; n = 6) were extracted in boiling hexane for 4 days, then dried. The extracted blocks were doped with squalene at 120°C for 2 hours. One block each was analyzed after doping. The rest were accelerated aged at 70°C and 5 atm. of oxygen for 6 (n = 2) and 14 days (n = 3). Thin sections (150 micron thick) were microtomed and analyzed by Fourier Transform Infrared Spectroscopy to determine a vitamin E index (1245–1275 cm. −1. normalized to 1850–1985 cm. −1. ) and an oxidation index (1700 cm. −1. normalized to 1370 cm. −1. ) after extraction with boiling hexane for 16 hours and drying. Results. After extraction, 92% of the original vitamin E was removed from diffused and sterilized UHMWPE and 99% of the vitamin E was removed from the diffused and unsterilized UHMWPE. Vitamin E content of the blended, irradiated UHMWPEs could not be detected. As a result of accelerated aging in the presence of squalene, all extracted vitamin E-stabilized UHMWPEs showed increased oxidation except diffused, sterilized UHMWPE. The small amount grafted vitamin E in these samples (8%, ∼0.02 wt%) protected irradiated UHMWPE under these conditions. All vitamin E-stabilized, extracted UHMWPEs showed higher oxidative stability than irradiated and melted virgin UHMWPE in the presence of squalene. In the blended, irradiated UHMWPE, there was less effective vitamin E compared to the diffused, sterilized UHMWPE due to the high dose irradiation. Conclusions. Radiation grafting of vitamin E onto UHMWPE was effective against squalene initiated oxidation in accelerated aging. Vitamin E-diffused, sterilized UHMWPE showed no oxidation and diffused, unsterilized UHMWPE and blended, irradiated UHMWPE showed higher oxidative resistance than irradiated/melted UHMWPE

Revision of fractured ceramic-on-ceramic total hip replacements with a cobalt-chromium (CoCr) alloy-on-polyethylene articulation can facilitate metallosis and require further expensive revision surgery [1–3]. In the present study, a fifty-two year old male patient suffered from fatal cardiomyopathy after undergoing revision total hip arthroplasty. The patient had received a polyethylene-ceramic acetabular liner and a ceramic femoral head as his primary total hip replacement. The polyethylene-ceramic sandwich acetabular liner fractured in vivo after 58 months and the patient underwent his first revision surgery where he received a Vitamin E stabilized acetabular Polyethylene (PE) liner and a CoCr alloy femoral head with documented synovectomy at that time. After 15 months, the patient was admitted to hospital in cardiogenic shock, with retrieval of the bearing components. Before the second revision surgery, peak serum cobalt levels measured 6,521 μg/L, 78-times greater than serum cobalt levels of 83μg/L associated with cobalt poisoning [4]. Serum titanium levels found in the patient measured 17.5 μg/L) normal, healthy range 0–1.4 μg/L). The retrieved CoCr alloy femoral head had lost a total of 28.3g (24% or an estimated amount of 102 × 10. −9. wear particles (∼2 μm diameter) [1]) within 16 months of in vivo service. Despite initiating a cobalt chelating therapy, the patients' cardiac left ventricular ejection fraction remained reduced at 6%. This was followed by multi-organ failure, and ultimately the patient passed away shortly after being taken off life support. Embedded ceramic particles were found on the backside and articular surfaces of the Vitamin E-stabilized PE acetabular liner. Evidence of fretting wear on the titanium (Ti) alloy acetabular shell was present, possibly explaining the increased serum Ti levels. Scanning electron microscopy and energy dispersive X-ray analyses confirmed Ti alloy transfer on the embedded ceramic particles on the backside PE liner surface and CoCr alloy transfer on the embedded ceramic particles on the articular PE liner surface. A fractured ceramic-on-ceramic total hip replacement should not be revised to a CoCr alloy-on-polyethylene articulation irrespective of concurrent synovectomy [5] as it can cause severe, third-body wear to the CoCr alloy femoral head that can lead to metallosis with fatal, systemic consequences

Vitamin E (alpha-tocopherol) is a free-radical stabilizing agent used to maintain oxidative stability in radiation crosslinked UHMWPE for total joint replacements. Diffusion of vitamin E into UHMWPE after irradiation is one method of incorporation, while an alternative is blending vitamin E with UHMWPE resin powder and subsequently irradiating the consolidated mixture. With the latter method, it is possible for the antioxidant properties of Vitamin E to be exhausted in blends during irradiation, leading to oxidation. We report on the relative oxidative resistance of both irradiated (100kGy, 150kGy, 200kGy) vitamin E blends (0.02 wt%, 0.05 wt% and 0.1wt%) and post-irradiation vitamin E-diffused UHMWPE after three years of real-time aging in an aqueous environment at 40°C. Blocks of each type, including irradiated virgin UHMWPE, were also accelerated aged per ASTM F2003. Oxidation was measured with FTIR per ASTM F2102. Oxidation potential was determined through nitric oxide staining of hexane extracted thin sections, FTIR analysis and calculated using the height of the nitrate peak (1630 cm^-1). Our results showed that unstabilized samples exhibited substantial oxidation and oxidation potential throughout the surface and bulk with both types of aging. Post-irradiation diffused UHMWPE showed no detectable oxidation and decreasing oxidation potential with both aging methods. The vitamin E concentration at the surface of the diffused blocks decreased and the initial non-uniform profile with high surface concentration (3.4 wt%) shifted towards a uniform profile, equilibrating at an index of 0.1 or 0.7 wt% vitamin E. Samples showed a reduction in their initial vitamin E content by 47%– 61% over 36 months, but oxidative stability was not compromised. The non-uniform profile presumably created a driving force for elution into the aqueous environment, while the difference in solubility of vitamin E at 40°C, compared to the initial diffusion temperature at 120°C, may have also contributed. After six months of real-time aging, all irradiated blends showed surface oxidation, while 0.02 wt% blends additionally showed subsurface oxidation potential. However, oxidation was not induced by accelerated aging Methods: in any blended, irradiated samples. In conclusion, real-time aging resulted in greater differentiation in the relative oxidative stability of vitamin E-stabilized, radiation crosslinked UHMWPEs than accelerated aging. Irradiated blends with vitamin E concentrations as high as 0.1 wt% showed surface oxidation after 3 years; higher vitamin E concentrations cannot address this shelf oxidation as that will also reduce the crosslinking efficiency and increase wear. Post-irradiation diffused UHMWPE, which was not limited by the amount of incorporated vitamin E, showed oxidative resistance up to 3 years with a reduction in oxidative potential

Aims

The main aims were to identify risk factors predictive of a radiolucent line (RLL) around the acetabular component with an interface bioactive bone cement (IBBC) technique in the first year after THA, and evaluate whether these risk factors influence the development of RLLs at five and ten years after THA.