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.

6_B_Material_e_Methods: 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 (SXL). Density was measured according to ASTM D1505. Crystallinity and thermal properties were determined according to ASTM D3417. Crystallite size/lamellar structure was determined by small angle x-ray scattering. Accelerated aging was carried out in an oxygen bomb under 5 atmospheres of oxygen at 70 degrees C for 14 days.

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.

Introduction Contemporary highly crosslinked polyethylenes fall into two classes (annealed or remelted). Annealed polyethylenes contain free radicals. Remelted polyethylenes have reduced mechanical properties but no free radicals. SXL provides the advantages of both classes.

Materials and Methods GUR 1020 polyethylene was sequentially crosslinked using three separate gamma radiation doses of 3 Mrad with an annealing step at 130 degrees C after each irradiation (SXL).

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)

Results Free radical concentrations were 14 x 1014 and 1550 x 1014 spins/g for SXL and GUR 1020 irradiated to 3 Mrad in nitrogen (gamma-N2) respectively. Maximum oxidation index was 0.09 for SXL, 0.09 for unirradiated UHMWPE, and 1.27 for gamma-N2.

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.

Discussion and Conclusions Sequential irradiation and annealing provides more complete crosslinking with 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. SXL is the basis for next generation highly crosslinked UHMWPE.

Introduction: Highly crosslinked UHMWPEs have been widely used in total hip replacements but have seen limited use at the knee due to concerns over strength characteristics. A new process, sequential irradiation and annealing, overcomes these limitations.

Materials and Methods: GUR 1020 polyethylene was sequentially crosslinked using three separate gamma radiation doses of 3 Mrad with an annealing step at 130 degrees C after each irradiation (SXL).

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.

Results: Scorpio gamma-N2 CR knees under normal walking conditions had a weight loss of 32.6 +/− 1.9 mg/million cycles compared to 6.5 +/− 1.6 mg/million cycles for SXL (p of 0.024). With Scorpio PS knees, the wear was 33.5 +/− 1.6 for gamma-N2 versus 7.7 +/− 0.7 mg/million cycles for SXL (p of 0.000009) subject to stair climbing simulation. Wear particle size was similar for SXL and gamma-N2. SXL knees showed no effect of oxidative challenge in a 10 million-cycle knee study.

Discussion and Conclusions: Wear is reduced by 80 percent and 77 percent respectively for CR and PS knees with SXL compared to gamma-N2. SXL has high resistance to oxidative challenge as shown by the lack of effect on knee wear results.