Matrix remodeling in healing rabbit Achilles tendon

Biochemical, biomechanical and ultrastructural properties of the connective tissue matrix were investigated during the early remodeling phase of tissu e repair in experimentally tenotomized and repaired rabbit Achilles tendons . Sterile surgical tenotomy was performed on the right Achilles tendons of 14 rabbits and allowed to heal for 15 days. The animals were euthanized and the Achilles tendons excised from both limbs. The left contralateral Achil les tendon of each rabbit was used as a control in the experiments. Prior t o biochemical analysis, both intact and healing tendons were tested for the ir biomechanical integrity. The results revealed that the healing tendons h ad regained some of their physicochemical characteristics, but differed sig nificantly from the intact left tendons. The healing tendons regained 48% t ensile strength, 30% energy absorption, 20% tensile stress, and 14% Young's modulus of elasticity of intact tendons. In contrast, biochemical analysis showed that the healing tendons had 80% of the collagen and 60% of the col lagen crosslinks (hydroxypyridinium) of normal tendons. Sequential extracti on of collagen from the tissues yielded more soluble collagen in the healin g tendons than intact tendons, suggesting either an increase in collagen sy nthesis and/or enhanced resorption of mature collagen in healing tendons co mpared to intact tendons. Electron microscopic studies revealed remarkable differences in the ultrastructure between intact and healing tendons. These observations could explain, in part, the connective tissue response to hea ling during the early phases of tissue remodeling.