Graft-tunnel healing

Several experimental studies have shown that bone–tendon p0310 graft healing in ACL reconstruction occurs in a period varying from 3 to 12 weeks. The quality and rate of healing depend on many variables, predominantly the type of graft. Soft tissue grafts such as hamstring tendon grafts heal within a bone tunnel by formation of a fibrous transitional layer between the tendon and bone, which contains penetrating Sharpey-like fibers. This newly formed bone– tendon interface matures with time and resembles the indirect-type insertion observed in tendons and ligaments. Bone plug tendon grafts, such as patellar tendon, heal within bone tunnel by incorporation of the bone plug to the surrounding bone and formation of an indirect-type insertion at the interface between bone and the intraosseous fibrous portion of the graft. Bone–bone healing occurs more rapidly than tendon—bone healing. Resistance to pullout force appears to be similar between the two types of grafts by 8 to 12 weeks after surgery. Therefore soft tissue grafts need high primary fixation strength and stiffness because of the consistent risk of failure due to pullout from the tunnel during the first 2 months. Mechanical stresses can affect maturation and differentiation of the bone–tendon graft junction depending on many factors such as bone density, fixation, placement and tensioning of the graft, gap size, and postoperative immobilization. Compression of the graft within the tunnel can enhance healing for both the bone plug and soft tissue. However, during the first 3 months after an ACL reconstruction, regardless of the type of graft used, the strength of the bone–tendon graft junction does not influence the mechanical behavior of the femur–ACL graft–tibia complex because the weak link of the ligament replacement rapidly shifts from the fixation site to the midsubstance of the graft. Therefore the application of excessive loads during this period, such as a too-aggressive rehabilitation and early return to sport activities, may cause a permanent elongation of the graft, thus compromising the result of the reconstruction. Many efforts have been made to improve the quality and rate of bone–tendon healing. Tissue engineering and gene transfer techniques have been applied to obtain a direct-type fibrocartilaginous insertion of the ACL graft, similar to that of the native ligament, and to accelerate the healing process of tendon grafts within bone tunnel. However, more investigations will be necessary in the near future to evaluate the possible employment of these biological techniques in the clinical practice.