The effects of processing techniques on the mechanical properties of bone-anterior cruciate ligament-bone allografts

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Abstract
There has been a growing interest in the use of allo grafts as ACL substitutes. Allografts are often freeze dried to increase shelf storage time and sterilized with ethylene oxide. This study was conducted to determine the effect of a specific ethylene oxide sterilization pro cedure and freeze drying process on the initial mechan ical properties of femur-ACL-tibia preparations. Twelve knees (stifle joints) from six mature goats were divided into two groups (one knee of pair to each group). Knees were cleaned of all soft tissue except for the anterior cruciate, posterior cruciate, and collateral ligaments. Group 1 was sterilized with ethylene oxide (simulating clean procurement) then freeze dried. Group 2 was freeze dried only (simulating sterile procurement). The knees were rehydrated and then tested in tension to failure to determine their structural mechanical proper ties. The ethylene oxide-freeze dried specimens (Group 1) had a mean maximum load before failure of 2059 ± 273 newtons (N) (± SE) which was not statistically different than the maximum load of the freeze dried specimens (Group 2) of 2023 ± 214 N. The average strength of Group 1 and 2 combined was not signifi cantly different than a third group of 12 normal femur- ACL-tibia controls which had an average maximum force of 2403 ± 133 N. No significant differences be tween the groups were observed for stiffness, energy to maximum strength, or elongation to maximum force. It appears the freeze dry processing, with or without ethylene oxide sterilization, using the procedure we used, has, at most, a small effect on the initial mechan ical properties of the preparations. The low mechanical properties of implanted and revascularized allografts, as compared to normal femur-ACL-tibia complex ap pears to be related more to fixation strength and alter ations which occur in the ligament substance after implantation.

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