THE EFFECT OF NEUTRON BEAM IRRADIATION ON CONVENTIONAL AND HIGHLY CROSS LINKED POLYETHYLENE WEAR

Abstract

Neutron beam irradiation is currently being explored as an alternative modality to improve local control of sarcomas. The purpose of this study was to investigate the effects of a sarcoma-dose fast neutron therapy on the wear properties of standard and highly cross-linked polyethylene total hip arthroplasty liners

Two groups of 28 mm I.D. polyethylene liners were used in this study – conventional polyethylene liners (N2vac: 3Mrads innitrogen, Howmedica Osteonic, Allendale, NJ), and highly cross-linked liners (Crossfire: 10.5MRads total radiation dose, Howmedica Osteonics, Allendale, NJ). All liners were sterilized in a oxygen free environment and stored in inert nitrogen packages. The plastic cups were sandwiched between two tissue-equivalent blocks to simulate the human hip region and brought to the fast neutron therapy unit. The neutron beam is produced in a super conducting cyclotron by bombarding an internal beryllium target with 48.5 MeV deuterons [d(48.5)+BE]. The cups were exposed to a dose of 15 Gy represented a typical neutron-dose given to a sarcoma patient. Wear testing was then performed utilizing a hip simulator (MTS, EdenPrairie, MN) with matched 28 mm diameter CoCr femoral heads. Physiologic loading was simulated with biaxial cross-path motion and peak loads of 2450 N. All tests were performed in 50 percent diluted alpha-calf serum(Hyclone Laboratories, Logan, UT) to simulate human serum exposure. Every 250,000 cycles the serum was changed and samples were removed from the machine, cleaned and weighed. The volume loss measurement shown below used the weight loss to calculate the wear rate. The wear rate was converted to volume loss by dividing by the density. The value is given as millimeter scubed per million cycles (mm3/mc). Phase one cups were tested within one month of radiation. Phase two cups served as soaked controls, and spent 7months in calf serum prior to wear testing. A total of five million wear cycles were performed for each cup to simulate five years worth of use.

The Averaged volumetric wear loss data demonstrated significantly less wear in CrossfireÒ compared to N2vac in both neutron irradiated and non-irradiated samples. This suggests that in sarcoma cases of the hip involving adjuvant fast neutron therapy, highly crosslinked poly-ethylene should be utilized. Averaging all data there was no statistically significant difference between the neutron radiation and non-treated components for both material conditions (N2VacÒ and CrossfireÒ). A trend towards decreasing wear in phase two samples was noted which may represent a material change in the liners exposed to serum over time. In addition, larger than normal variability in wear rates was seen within each group. Further testing of these liners is planned to elucidate these phenomenon. Table 1: – Volumetric wear loss per group. UHMWPE Material Volumetric wear Loss (mm3/mc) Standard Deviation N2VacÒ Phase 1 29.6 1.6 N2VacÒ Phase 2 14.0 N/a Neutron treated N2VacÒ Phase 1 52.2 18.1 Neutron treated N2VacÒ Phase 2 20.9 0.8 CrossfireÒ Phase 1 3.0 0.9 CrossfireÒ Phase 2 2.0 0.6 Neutron treated CrossfireÒ Phase 1 2.5 0.5 Neutron treated CrossfireÒ Phase 2 1.9 0.03 Graph 1– Averaged volumetric wear loss values

Sarcoma-dose (15Gy) fast neutron therapy adversely affects the wear of standard polyethylene acetabular cups. The wear rate of the conventional gamma-inert sterilized polyethylene increased by more than 50% following a 15Gy fast neutron treatment. The highly cross-linked polyethylene (Crossfire), on the other hand, was immune to the effect of neutron treatment at the same dose. The mechanisms responsible forth is difference are unclear and warrant further investigation. The clinical implication of this study is that for sarcoma patient receiving total hip replacement, highly crosslinked poly-ethylene rather than standard polyethylene should be used for the acetabular cup.