Paper 260: Electronic Beam Sterilization of Soft-tissue Grafts Maintains Significantly Improved Biomechanical Properties at High-dose Irradiation (34 kGy) than Standard Gamma Treatment

Abstract

Gamma irradiation is the most frequently used sterilization procedure for soft-tissue allografts as they are used in ACL surgery. However, biomechanical properties of gamma irradiated allografts at dosages > 20 kGy are significantly reduced. Electronic beam (Ebeam) irradiation dose and other parameters can be more accurately controlled than it is economically feasible than with gamma irradiation. Also, it has been shown that the addition of CO2 with E-beam sterilization at low temperatures allows for a significant reduction of free radicals build-up which is mainly responsible for the loss of tissue strength. It was the objective of this study to compare the impact of 34 kGy gamma vs. E-beam irradiation on the biomechanical properties of human bone-patellar tendon-bone grafts at the time of sterilization. Paired 10 mm-wide human bone-patellar tendon-bone (BPTB) grafts were harvested from 10 donors and split into two groups (each n=10): A) Ebeam, B) Gamma. All grafts were irradiated with 34 kGy. 10 non-irradiated BPTB grafts of identical dimensions were used as controls. All grafts underwent biomechanical testing: preconditioning (10 cycles, 0 – 20 N); cyclic loading (200 cycles, 20 - 200 N) and a load-to-failure (LTF) test. The strain rate was 150 mm min-1. Graft motion during cyclic loading was tracked with an optical measurement system. Failure load and displacement at failure were recorded and stiffness was derived from these values. The strain difference between the first and last cycles as well as creep were used as a determinant of viscoelasticity. Student t-test was used for statistical comparison of both study groups (paired) and controls (independent). Level of significance was set at p < 0.05. Stiffness of non-irradiated controls (199.6 ± 59.1 N/mm) and ebeam sterilized grafts (192.8 ± 58.0 N/mm) did not significantly differ, while gamma-irradiated grafts had significantly lower stiffness (170.6 ± 58.2 N/mm) than controls (p<0.05). Failure loads were significantly lower in both study groups (ebeam: 1139 ± 445 N, gamma: 1073 ± 617 N) than in the controls (1741 ± 304 N) (p<0.05). Creep was significantly larger in the gamma irradiated (1,06 ± 0,58 mm) than in the ebeam (0,26 ± 0,24 mm) and control (0,20 ± 0,17 mm) group that did not differ significantly. Strain difference was not different between either control or study groups. This study shows that the impairment of biomechanical properties of soft-tissue allografts at high irradiation doses of 34 kGy is substantially reduced with the ebeam procedure compared to standard gamma treatment. Considering the results of this study and the improved control of irradiation application with electronic beam, this technique might be a promising alternative in soft-tissue sterilization. However, a significant reduction of failure strength has to be conceded with electronic beam irradiation, too. Therefore, in future studies it is important to gain better understanding of the underlying processes that affect soft-tissue strength and to eventually eliminate these adverse effects.