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Sixty 10-mm bone-patellar tendon-bone allografts from young human donors were placed into four test groups, a control fresh-frozen group and three fresh-frozen irradiated groups. The irradiated groups were exposed to 2.0, 3.0, or 4.0 Mrad of gamma irradiation. The specimens were tested to tensile failure. The initial biomechanical strength of fresh-frozen allografts was reduced up to 15% when compared with fresh-frozen controls after 2.0 Mrad of irradiation. Maximum force, strain energy, modulus, and maximum stress demonstrated a statistically significant reduction after 2.0 Mrad of irradiation (P < 0.01). Stiffness, elongation, and strain were reduced but not with statistical significance. A 10% to 24% and 19% to 46% reduction in all biomechanical properties were found after 3.0 (P < 0.005) and 4.0 (P < 0.0005) Mrad of irradiation, respectively. After irradiation with a 4.0 Mrad dose, the ultimate load was below that of reported values for the human anterior cruciate ligament. It is clinically important to observe and document changes in human ligaments that result from currently used doses of gamma irradiation. The results from this study provide important information regarding the initial biomechanical properties of fresh-frozen human bone-patellar tendon-bone allografts after bacterial sterilization with gamma irradiation. The current accepted dose for sterilization is between 1.5 and 2.5 Mrad. There appeared to be a dose-dependent effect of irradiation on all the biomechanical parameters studied. Four of seven parameters were found to be reduced after 2.0 Mrad of irradiation. Reductions were found in all parameters after 3.0 and 4.0 Mrad of irradiation.

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The biomechanical properties of the rabbit medial collateral ligament (MCL) as a function of maturation and age were investigated. Femur-MCL-tibia (F-M-T) preparations were obtained from rabbits of different age groups (open or closed epiphysis). Parallel increases in the animal body weight and ligament cross-sectional area were recorded with age. Cyclic and tensile failure tests were performed to obtain the structural properties of the F-M-T complex and the mechanical properties of the MCL substance. There were significant increases in the load at failure, energy-absorbing capability of the bone-ligament junction, and in the tensile strength of the ligament substance as a result of maturation and subsequent aging. Increases in the area of hysteresis obtained during cyclic loading-unloading were also documented. At the closing of the epiphysis, the mode of failure of the F-M-T structure progressed from tibial avulsion to failure in the midsubstance of the ligament. An asynchronous rate of maturation was observed between the structural properties of the bone-ligament complex and the mechanical properties of the ligament substance.