Anterior cruciate ligament injury is one of the most frequent sports injuries in humans and one of the most important causes of lameness in dogs. Anterior cruciate ligament injury results in pain, swelling and knee instability and without timely therapeutic intervention, subsequently causes osteoarthritis. According to the limitations of anterior cruciate ligament reconstruction, application of biological treatment such as mesenchymal stem cells is increasing. There are various types of stem cells such as embryonic, induced pluripotent and mesenchymal stem cell. Among these, mesenchymal stem cells are most widely investigated. Estrogen has variety of roles in integrity of anterior cruciate ligament. The purpose of this study was to evaluation of the estradiol effect on mesenchymal stem cells in healing of experimentally induced cranial cruciate ligament injury. Twenty-four mature male rabbits were neutered, then the anterior cruciate ligaments were transected partially. After 1 week the rabbits were randomly divided into 4: the control group, stem cell group, 17-β estradiol group and stem cell and 17-β estradiol group. The rabbits received 500 μl intracapsular injection of one to the following: (1) PBS only, (2) 106 MSCs in PBS, (3) 10-4 molar 17-β estradiol in PBS or (4) 10-4 molar 17-β estradiol and 106 MSCs in PBS. Histological evaluation of cellularity, cell morphology, collagen arrangement, vascularity and ground substance of the ligaments has been done at 7 weeks after transection. The anterior cruciate ligaments were evaluated using modified Bonar’s scale. The best result has been observed in the mesenchymal stem cells group compared with other groups. Because of interaction with collagen synthesis 17-β estradiol is not recommended.

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