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Orthopaedic Proceedings
Vol. 99-B, Issue SUPP_5 | Pages 9 - 9
1 Mar 2017
Wannomae K Micheli B Konsin Z Muratoglu O
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Introduction

Oxidation of ultrahigh molecular weight polyethylene (UHMWPE) can lead to failure of implants used in total joints. Cyclic loading is postulated to be one mechanism of in vivo oxidation in UHMWPE components as one previous study has shown [1]. We developed an accelerated aging test that incorporated compressive cyclic loading that the UHMWPE components would be exposed to in vivo. Surgeons are moving towards larger femoral heads in hip arthroplasty and removing less bone in knee arthroplasty necessitating thinner UHMWPE components. We hypothesized that, in this accelerated aging test, thinner UHMWPE components would be more susceptible to oxidation caused by the cyclic loading due to higher stresses in the material.

Materials and Methods

All samples tested in this study were Conventional PE: GUR1050 was machined into test specimens, vacuum packaged and gamma sterilized. Test samples were blocks 100 mm × 89 mm in cross-section with 3 different thicknesses: 1 mm, 3 mm, and 10 mm (n=3 each). Three cylinders were cored out of each test sample to serve as controls (Fig 1a) that were physically separated and thereby isolating the oxidation attributable to an applied compressive cyclic load. The controls were placed back into the holes from where they were cored during testing. Compressive loading was administered by a 12.5 mm diameter applicator affixed to a hydraulic test frame (Fig 1b), and all testing was done at 80°C in air. A sinusoidal compressive cyclic stress between 1 and 10 MPa was applied at 5 Hz for 7 days.

Microtomed thin films from all samples were analyzed via Fourier Transform Infrared Spectroscopy (FTIR) to quantify oxidation [2] after testing. Oxidation was measured through the thickness of the sample at targeted points along the length from directly underneath the center of the load applicator to 10mm away (Fig 1a). Oxidation was also measured through the thickness of the cylindrical controls.


Orthopaedic Proceedings
Vol. 99-B, Issue SUPP_5 | Pages 10 - 10
1 Mar 2017
Micheli B Wannomae K Muratoglu O
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Introduction

The fatigue strength of ultrahigh molecular weight polyethylene (UHMWPE) in total joint implants is crucial to its long term success in high demand applications, such as in the knee, and is typically determined by measuring the crack propagation resistance in razor-notched specimens under cyclic load [1]. This only tells part of the story: that is, how well the material resists crack propagation once a crack is present. A second, equally important component of fatigue strength is how well the material resists crack formation. Previous studies cyclically loaded a cantilevered post until failure [2], postulating that the post would break very quickly after crack initiation. Parran et. al. proposed a novel method to measure the crack initiation time by holding a sample in constant tension until a crack was visually observed [3]. We hypothesize that the crack initiation times of various UHMWPEs will follow similar trends as the more omnipresent crack propagation resistance tests.

Materials and Methods

The following UHMWPE formulations were tested: (i) virgin, (ii) gamma sterilized in vacuum, (iii) 91 kGy gamma irradiated, and (iv) 91 kGy gamma irradiated and subsequently melted. GUR1020 and GUR1050 bar stock of varying irradiation doses were machined into compact tension specimens [4] with a notch depth of 17 mm and a blunt notch root radius of 0.25 mm, mimicking a geometry of a joint replacement component.

Specimens were held in constant tension until failure; 3 to 5 different loads between 1 kN and 2.25 kN (n=3 samples per load per material) were tested. A video camera was focused on the face of the notch and took a picture every 10 seconds. The photos were reviewed to manually determine the crack initiation time (Fig 1). The time it took for the sample to completely fail – that is, shear into two separate pieces – was also recorded.


Orthopaedic Proceedings
Vol. 99-B, Issue SUPP_6 | Pages 80 - 80
1 Mar 2017
Wannomae K Lozynsky A Konsin Z Muratoglu O
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Introduction

Corrosion of the femoral head-trunnion junction in modular hip components has become a concern as the corrosion products may lead to adverse local tissue reactions. A simple way to avoid trunnion corrosion is to manufacture the femoral head with a non-metallic material, such as ceramics that are widely. An alternative solution may lie in advanced polymers like polyaryletherketones (PAEKs). These thermoplastics have high mechanical strength necessary for use as femoral heads in hip arthroplasty, but they must be tested to ensure that they do not adversely affect the wear of the ultrahigh molecular weight polyethylene (UHMWPE) liner counterface. Pin-on-disc (POD) wear testing has been extensively used to evaluate the wear properties of UHMWPE prior to more extensive and costly analysis with joint simulators. We hypothesized that the wear of crosslinked UHMWPE would not be adversely affected in POD tests when articulated against an advanced thermoplastic counterface.

Methods

0.1 wt.% VitE blended UHMWPE stock was e-beam irradiated to 100, 125, 140, 160, and 175 kGy and machined into cylindrical pins for testing. An additional group of 100 kGy e-beam irradiated and melted UHMWPE (with no vitamin E) was also machined and tested.

Three different counterface materials were tested: (1) Cobalt-chrome (CoCr) with a surface roughness (Ra) of <0.5 μm, (2) Biolox™ ceramic (CeramTec), and (3) Polyetheretherketone (PEEK), a member of the PAEK family (Fig 1).

A bidirectional POD wear tester [1] was used to measure the wear rate of UHMWPE specimens, where the specimens moved in a 10 mm × 5 mm rectangular pattern under a Paul-type load curve [2] synchronized with the motion. The peak load of the loading curve corresponded to a peak contact pressure of 5.1 MPa between each UHMWPE pin specimen and the counterface disc. Each test was conducted at 2 Hz in undiluted bovine serum stabilized with ethylenediamine tetraacetate (EDTA) and penicillin. The pins were cleaned and weighed daily, and a wear rate was calculated at the end of each test by linear regression.


Orthopaedic Proceedings
Vol. 98-B, Issue SUPP_3 | Pages 64 - 64
1 Jan 2016
Muratoglu O Rowell S Wannomae K
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Introduction

Radiation cross-linked UHMWPEs were developed to address osteolysis-induced joint arthroplasty failure by improving wear resistance and reducing associated particulate debris. Introduced clinically fifteen years ago, they are the primary bearing surface in use with excellent clinical outcomes and wear resistance. First generation materials sought to maintain oxidative stability by reducing or eliminating free radicals through thermal treatments, while second generation aimed to further balance oxidation resistance and improve mechanical properties through sequential irradiation and annealing or the incorporation of an antioxidant. Recent reports have identified lipid absorption and cyclic loading as potential in vivo oxidation-inducing mechanisms. In this on-going retrieval study, we report on the current status of oxidative stability in these two generations of UHMWPE bearings.

Materials & Methods

Six types of highly cross-linked UHMWPE hip and knee bearings (Table 1) were surgically-retrieved and collected under IRB approval. Standard material analysis was performed on cross-sections of loaded and unloaded bearing surfaces of the components. Thin sections (150 µm thickness) were extracted in boiling hexanes under reflux for 16 hours followed by vacuum drying for 24 hours. FTIR was used to evaluate oxidation and calculated from post-hexane absorbance spectra by normalizing the area under 1740 cm−1 (1680–1780 cm−1) to the area under 1370 cm−1 (1330–1390 cm−1), per ASTM F2102-13. Gravimetric swelling of regional cross-sectional blocks (1–2 mm3) for 2 hours in 130°C boiling xylenes was used to assess cross-link density, per ASTM 2214.


Orthopaedic Proceedings
Vol. 94-B, Issue SUPP_XXXVII | Pages 130 - 130
1 Sep 2012
Wannomae K Oral E Neils A Rowell S Muratoglu O
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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.


Orthopaedic Proceedings
Vol. 94-B, Issue SUPP_XL | Pages 132 - 132
1 Sep 2012
Oral E Neils A Ghali B Wannomae K Spiegelberg S Muratoglu O
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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.


Orthopaedic Proceedings
Vol. 93-B, Issue SUPP_II | Pages 223 - 223
1 May 2011
Wannomae K Rowell S Micheli B Malchau H Muratoglu O
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Radiation crosslinking decreases the wear of ultra-high molecular weight polyethylene (UHMWPE) and subsequent heating increases its oxidative stability. Clinical trials are showing lower femoral head penetration rate with highly crosslinked vs. conventional UHMWPE liners. Recently, a follow-up report showed a surprising increase in the femoral head penetration rate with a highly crosslinked UHMWPE, prompting us to closely analyze surgically explanted highly crosslinked UHMWPEs.

Thirty-four highly crosslinked components, all irradiated (100kGy) and melted, were included in the study. The components were surgically removed from patients for non-polyethylene related reasons. Oxidation was determined at the rim immediately after explantation. After shelf storage in air for 5–77 months, oxidation and crosslink density were measured at the rim and articular surfaces. An additional retrieval (92 mos. in vivo) was tested on the hip simulator; oxidation and crosslink density were determined after simulator testing.

All components showed no detectable oxidation immediately after explantation; however, surprisingly oxidation levels increased during shelf storage. Areas with increased oxidation showed a decrease in crosslink density. These changes did not correlate with in vivo duration; however, they correlated strongly with ex vivo duration. The component subjected to hip simulator testing showed no measurable wear and showed no detectable oxidation or marked decrease in crosslink density.

Two mechanisms may have reduced the oxidation resistance of highly crosslinked UHMWPE upon exposure to in vivo elements and subsequent exposure to air. One mechanism is based on free radical formation during cyclic loading; the other is based on an oxidation cascade initiated by absorbed lipids. Further studies are necessary to determine the impact of these mechanisms, if any, on the stability of components during in vivo service.


Orthopaedic Proceedings
Vol. 93-B, Issue SUPP_II | Pages 183 - 183
1 May 2011
Wannomae K Rowell S Oral E Muratoglu O
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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.


Orthopaedic Proceedings
Vol. 92-B, Issue SUPP_II | Pages 280 - 280
1 May 2010
Wannomae K Nabar S Muratoglu O
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Introduction: Two second generation highly crosslinked UHMWPEs have been cleared by the FDA for clinical use in the United States: sequentially crosslinked UHMWPE (X3™ UHMWPE, Stryker Inc., Mahwah, NJ, USA) and α-tocopherol stabilized UHMWPE (E-Poly™ UHMWPE, Biomet, Inc., Warsaw, IN, USA). Both have been shown to be oxidatively stable under standardized aging methods (ASTM F2003); however, these conventional aging methods did not consider the effect of mechanical loading on the oxidative behavior of the materials. By coupling the adverse effects of thermal aging and mechanical stress, we sought to investigate if either material was prone to environmental stress cracking (ESC). We hypothesize that the residual free radicals remaining in sequentially crosslinked PE will lead to oxidative degradation in this adverse test; furthermore, we hypothesized that the α-tocopherol infused in E-Poly™ will continue to protect the highly crosslinked PE even under such unfavorable conditions.

Materials and Methods: Three materials were tested:

Conventional: UHMWPE gamma sterilized in inert,

SXL: sequentially irradiated and annealed UHMWPE irradiated to a cumulative dose of 100kGy (33 kGy irradiation + 8 hour annealing in air, repeated 3 times) and gas plasma sterilized, and

E-Poly: UHMWPE irradiated to 100kGy, stabilized with α-tocopherol, and gamma sterilized in inert.

Four specimens from each group were subjected to a reciprocating mechanical stress of 10 MPa at a frequency of 0.5 Hz in an environmental chamber maintained at 80°C. Control samples were placed in the chamber but not subjected to cyclic mechanical stress. When a visible crack was observed on a sample’s surface or when a sample fractured, it and its corresponding control sample were analyzed by FTIR to quantify oxidation.

Results: All conventional specimens, half (2 of 4) of the SXL specimens, and none of the E-Poly specimens failed prior to the completion of 1,530,000 cycles (5 weeks of testing at 0.5 Hz). Cyclic loading had an adverse effect on the oxidation of the conventional and the SXL groups; the peak oxidation levels were higher in the cyclically loaded samples as compared to the control samples removed at the same time which were not loaded, likely due to an increase in chain scission induced by the mechanical load. The E-Poly specimens did not fail during the 5 weeks of testing, and FTIR did not reveal detectable oxidation in either control or loaded samples.

Discusssion and conclusion: Though the sequential processing of SXL creates a material with a lower free radical content compared to once-annealed material, it still yields a material prone to oxidation under extreme conditions, raising questions about its long-term oxidative stability. E-Poly™, protected by α-tocopherol, continues to exhibit high oxidation resistance even under adverse conditions.