Abstract: BACKGROUND: The process of tendon-bone healing after anterior cruciate ligament reconstruction with allografts is complex, and tunnel enlargement may occur due to poor healing. The process of preoperative graft treatment may have some influence on tendon-bone healing, but the specific situation is not clear.
OBJECTIVE: To explore the effects of allograft disposal methods on bone remodeling during tendon-bone healing after anterior cruciate ligament reconstruction surgery. 
METHODS: Thirty-two male beagles were randomly divided into four groups (n=8 per group) for anterior cruciate ligament reconstruction by coin toss method after establishing bilateral posterior cruciate ligament rupture models. In group A, fresh autogenous Achilles tendon grafts were used. In group B, a fresh allograft Achilles tendon was used. In group C, an allograft Achilles tendon irradiated by gamma rays and refrigerated at low temperature was used. In group D, lyophilized allograft tendon after γ-ray irradiation was used. Micro CT scanning and osteoclast tartrate-resistant acid phosphatase staining were performed in the femor-graft-tibia complex at 3 and 6 months after anterior cruciate ligament reconstruction. Scanning images were selected to measure the bone microstructural parameters and a 3D bone tunnel model was established to measure the tunnel area.
RESULTS AND CONCLUSION: (1) Micro CT scan: 3 months after operation, the connection density, trabecular number, trabecular thickness, ratio of bone volume to total volume and structural model index in group D were lower than those in groups A and C (P < 0.05). The trabecular separation was higher in group D than that of group A (P < 0.05). The connection density, trabecular number and structural model index of group B were lower than those of group A (P < 0.05). The trabecular separation was higher in group B than that in group A (P < 0.05). The ratio of bone volume to total volume was lower in group B than that in groups A and C (P < 0.05). Six months after surgery, the connection density, trabecular number and the ratio of bone volume to total volume in group D were lower than those in group A (P < 0.05). Trabecular thickness and structural model index were lower in group D than those in groups A and C (P < 0.05). The trabecular separation was higher in group D than that in group A (P < 0.05). The connection density and structural model index of group B were lower than those of group A (P < 0.05). The trabecular thickness was lower in group B than that in groups A and C (P < 0.05). The trabecular separation was higher in group B than that in group A (P < 0.05). (2) Tartrate-resistant acid phosphatase staining: Compared with group A, osteoclasts in groups B and D were stained more deeply and more in number. The osteoclasts in group C were similar to those in group A in morphology and quantity, with small volume and light staining. (3) Tunnel area: At the same time point, the tunnel expansion of the femur and tibia in group D was the largest. The area of the inner opening of the tibial tunnel at 3 months after operation and the area of the inner opening and middle part of the femoral tunnel at 3 and 6 months after operation in group B were larger than that in group A (P < 0.05). The area of the inner opening of the tibial tunnel at 3 and 6 months after operation was larger than that of group A (P < 0.05). (4) Those findings suggest that bone tunnel enlargement is common during tendon-bone healing after anterior cruciate ligament reconstruction. The disposal methods may affect bone resorption and the changes in bone microstructure mediated by osteoclasts at the tendon-bone interface. Therefore, it affected bone remodeling during tendon-bone healing.

Key words: anterior cruciate ligament reconstruction, allograft, Achilles tendon, disposal method, tendon-bone healing, bone remodeling