Healing process of torn human anterior cruciate ligament (ACL) was studied histologically. Torn anterior cruciate ligaments were obtained from 21 patients at primary ACL reconstruction and stained with hematoxylin and eosin. Patients age was from 16 to 40 years. The interval between initial injury and operation ranged from 2 to 30 days. At 2 days after ACL injury, fibroblasts were not seen in the torn ligament and lymphocytes appeared in the injured ligament. Although vascular endothelial capillary buds and fibroblasts appeared around the torn ligament at 11 days, few fibroblasts were seen in it. At 30 days, torn ligament shrank macroscopically and resembled scar tissue with irregular alignment of fibroblast and collagen fiber. In conclusion, torn ligament is not healed probably due to few fibroblasts proliferation.
