Bony incorporation of soft tissue anterior cruciate ligament grafts in an animal model: Autograft versus allograft with low-dose gamma irradiation

Background: The effect of low-dose gamma irradiation on healing of soft tissue allografts remains largely unknown. Hypothesis: The authors hypothesized that soft tissue allograft healing to bone would be delayed compared with that of autograft tissue and that low-dose (1.2 Mrad) gamma irradiation would not affect the healing response of allograft tissue after anterior cruciate ligament (ACL) reconstruction. Study Design: Controlled laboratory study. Methods: Forty-eight New Zealand White rabbits underwent bilateral ACL reconstructions with semitendinosus tendon graft. Sixteen rabbits were reconstructed with autografts and the remainder with allografts. The 32 allograft rabbits each received 1 irradiated allograft (1.2 Mrad), with the contralateral leg receiving a nonirradiated allograft. Animals were euthanized at 2 weeks or 8 weeks postoperatively. Tensile stiffness, maximum load, and displacement at maximum load were measured. Tibial and femoral segments were sectioned perpendicular to the tunnel axis allowing for histologic and histomorphometric analyses at the tendon-bone interface. Results: There were no significant differences between the maximum load or stiffness values among all groups at 8 weeks. At 2 weeks, autograft exhibited significantly (P<.01) lower maximum load than did the nonirradiated grafts. Regarding histology, at both 2- and 8-week time points, autograft tendon displayed more advanced degenerative and remodeling processes in comparison with irradiated allograft and nonirradiated allograft. Discussion: The maximum load and stiffness of a healing tendon allograft in ACL reconstruction appear to be unaltered by lowdose (1.2 Mrad) irradiation. At 8 weeks, there were no biomechanical differences in tendon-bone healing of allografts when compared with autograft controls. Histologic analyses suggested a faster remodeling response in autograft specimens in comparison with allografts at both time points. Clinical Relevance: The findings support the contention that low-dose gamma irradiation is safe for sterilization of ACL soft tissue allografts without compromise of graft properties at early time points.