The solvent extraction step applied in conventional oxidation measurement protocols for UHMWPE retrievals resulted in an elevated oxidation index (OI) in remelted highly cross-linked UHMWPE (RM-HXLPE). The present study seeks to confirm the effect of solvent extraction on OI measurement and to understand the relationships among soak-aging, fluid uptake, and resulting OI from various test protocols. Two materials were tested, representing legacy gamma-in-air sterilized (GammaAir-PE, GUR4150, 30 kGy) and remelted highly cross-linked (RM-HXLPE, GUR1050, 100 kGy, 147°C/5h) UHMWPE. Concave discs approximately 19 millimeters (mm) in diameter and 3 mm in dome thickness were machined from both materials prior to soak-aging. Soak-aging consisted of a combination of: (1) ASTM F2003 accelerated aging (5 atm O2, 70 °C for 14 days), and (2) either static soaking (SS, for 11.57 days) or dynamic load-soaking (LS, 2280 N at 1 Hz for 1 million cycles) in bovine synovial fluid at 37 °C to simulate the combination of shelf and in-vivo aging, respectively. Unsoaked samples were used as control (C) group. Thin films (150 μm) were harvested from cross-sections of all groups and were subjected to two solvent extraction protocols using Sohxlet (Heptane for 6 h (HEP6) or Hexane for 16 h (HEX16)) prior to be analyzed by two OI analyses using Fourier transform infrared spectroscopy (FTIR). FTIR analyses (128 scans/spectra, 4 cm−1 resolution) were carried out using both peak height at and peak area centering 1714 cm−1 for OI and 1734 for fluid uptake index (FI); carbon-carbon vibration at 1368 cm−1 was used for normalization. All GammaAir-PE data was further normalized using prewash control while RM-HXLPE data used computed results. The paired t-test was used with a significance level of p < 0.05.Introduction:
Materials and Methods:
Ultra-high molecular weight polyethylene (UHMWPE) has been successfully used as a bearing material in total joint arthroplasty. However, longevity of these implants has been compromised by wear and fatigue damage of the polyethylene. The addition of vitamin E to the polyethylene is a process recently introduced in the market to stabilize free radicals produced during radiation crosslinking. The objective of the present study is to investigate the effect of the addition of vitamin E on the wear characteristics of UHMWPE. Sequentially cross-linked and annealed UHMWPE material (X3™, Stryker Orthopaedics, Mahwah, NJ) was used as a control. Trident™ acetabular cups (Stryker Orthopaedics, Mahwah, NJ) with inner diameters of 36 mm and 44 mm and a wall thickness of 3.8 mm were tested on a 12 station MTS hip joint simulator. The simulator used a physiologic loading pattern with a maximum load of 2450N. The test was conducted under standard clean conditions with alpha calf fraction serum diluted to a protein concentration of 20 g/l for a total of three million cycles. All cups ran against CoCr femoral heads, and gravimetric measurements were taken every half-million cycles. Results show that sequentially crosslinked components, size 3 6mm, had an average volume loss of 9.4 ± 2.5 mm3, while vitamin E components of the same size had an average of 16.5 ± 3.1 mm3. This represents a 75% increase for vitamin E components that is statistically significant (p = 0.039). Size 44 mm sequentially crosslinked components had an average volume loss of 6.8 ± 3.7 mm3, while vitamin E components had an average of 19.7 ± 3.2 mm3. This denotes a statistically significant increase of 192% for material with vitamin E (p = 0.011). Linear regression analysis yielded wear rates of 4.1 ± 0.9 mm3/mc and 6.1 ± 1.3 mm3/mc for size 36 mm sequentially crosslinked and vitamin E components, respectively, which represents a non-significant increase of 49% for vitamin E components. Size 44 mm sequentially crosslinked components had a wear rate of 3.8 ± 1.3mm3/mc, while vitamin E components had a wear rate of 8.1 ± 0.7 mm3/mc. This represents a statistically significant increase of 117% in wear rate for vitamin E components (p = 0.013). The results of this testing indicate that the addition of vitamin E degrades wear performance relative to sequentially crosslinked material. Research shows that the introduction of Vitamin E affects the ability to create crosslinks during irradiation by reacting with some of the free radicals. Oral et al have shown that the crosslink density decreases when Vitamin E is blended into UHMWPE. Their research has also shown that a decrease in crosslink density causes an increase in wear rate. The results of the current testing show that the addition of vitamin E to polyethylene reduces the wear resistance of highly crosslinked polyethylene.
Remelted highly cross linked UHMWPEs have no detectable free radicals but the mechanical and fatigue properties are reduced because remelting changes the microstructure. Annealed highly cross linked UHMWPEs maintain the microstructure and mechanical properties but contain free radicals. A novel sequential irradiation and annealing process preserves the microstructure while providing enhanced oxidation resistance.
SXL density was 939.2 kg/cubic meter, identical to that for unirradiated UHMWPE and UHMWPE irradiated in nitrogen to 3 Mrad (gamma-N2). SXL crystallinity was 61.7%, compared to 61.3% and 59.2% for gamma-N2 and virgin UHMWPE, respectively. The long period spacing, crystal thickness and amorphous thickness were 38.2, 23.6 and 14.6 nm respectively for SXL and 38.9, 23.0 and 15.9 for gamma-N2. There was no statistical difference. Accelerated aging resulted in a white band for gamma-N2 with an oxidation index of 1.27. The response of SXL was the same as virgin UHMWPE e.g. crystallinity and density were unchanged with no white band formation and an oxidation index of 0.09. By avoiding remelting, sequential irradiation and annealing preserves polyethylene microstructure. The sequential process allows more efficient cross linking of free radicals resulting in an oxidation resistance equivalent to that of virgin UHMWPE.
Highly cross linked polyethylenes fall into two classes depending on whether annealing or remelting are used in processing. Annealed polyethylenes contain free radicals. Remelted polyethylenes have reduced mechanical properties but no free radicals. Research has now produced a highly cross linked polyethylene (SXL) that combines the advantages of each class. GUR 1020 polyethylene was sequentially cross linked using three separate gamma radiation doses of 3 Mrad with an annealing step at 130 degrees C after each irradiation (Mrad total). Free radical concentration was measured by electron spin resonance. Accelerated aging was carried out in an oxygen bomb under 5 atmospheres of oxygen at 70 degrees C for 14 days. Tensile properties were determined according to ASTM D638. Wear measurements to 5 million cycles were made on an MTS hip joint simulator at 1 Hz using the Paul load curve with maximum load of 2450 N with alpha fraction bovine calf serum. Free radical concentration was 14 x 10(14) spins/g for SXL compared to 1550 x 10(14)spins/g for GUR 1020 irradiated to 3 Mrad in nitrogen (gamma-N2). The maximum oxidation index was 0.09 for SXL, 0.09 for unirradiated UHMWPE, and 1.27 for gamma-N2 respectively. Mechanical properties exceeded the ASTM F648 specification and were unchanged by oxidative challenge. Wear rates were 1.35 cubic mm per million cycles for SXL and 46 cubic mm per million cycles for gamma-N2 respectively. Wear particle sizes were similar for the two materials Sequential irradiation and annealing provides more complete cross linking of free radicals with a consequent reduction in free radical level. SXL has the same resistance to oxidative challenge as unirradiated polyethylene. Mechanical properties exceed the ASTM F648 values. Wear is reduced by 97% compared to that of gamma-N2. Sequential irradiation and annealing preserves the microstructure by avoidance of melting yet minimizes free radicals.
The following were measured: free radical concentration (electron spin resonance), oxidation resistance (5 atmospheres of oxygen at 70 degrees C for 14 days), and tensile properties (ASTM D638). Hip simulator wear was determined (MTS machine, 5 million cycles, 1 Hz, Paul load curve with maximum load of 2450 N, alpha fraction bovine calf serum)
SXL tensile properties exceeded ASTM F648 and were unchanged by oxidative challenge. Wear rates were 1.35 and 46 mm3 per million cycles for SXL and gamma-N2 respectively; wear particle sizes were similar.
Wear was determined by weight loss under normal walking and stair climbing conditions (MTS knee simulator, 5 to 10 million cycles, 1 Hz, maximum load of 2600 N to 3800 N, alpha fraction bovine calf serum). Scorpio CR and PS knees were evaluated using SXL and UHMWPE gamma sterilized to 3 Mrad in nitrogen (gamma-N2). Oxidative challenge was in 5 atmospheres of oxygen at 70 degrees C for 14 days.
