Bone Tunnel Healing
Because of insufficient healing potential in the bone tunnel or abrasion of the graft at the tunnel exit, all types of ACL reconstruction are associated with a proportion of grafts failure.[71] Tendon-bone incorporation of a tendon graft within the bone tunnel is a major concern when using a tendon graft for ligament reconstruction. Successful ACL reconstruction with a tendon graft requires solid healing of the tendon graft in the bone tunnels. Improvement of graft healing to bone is crucial to facilitate early and aggressive rehabilitation and a rapid return to full activity. Healing of a tendon graft in a bone tunnel requires bone ingrowth into the tendon. Indirect Sharpey fibers and direct fibrocartilage fixation of the tendon-bone interface provide different anchorage strength and interface properties.[10]
The insertion of the native ACL is characterized in four layers: tendon, fibrocartilage, mineralized fibrocartilage, and bone.[27] The collagen fibers of the tendon extend into both the fibrocartilage and the mineralized layer. This structure usually is destroyed when the ligament is removed and the bone tunnel is drilled. A replication of this direct type of insertion may be considered desirable when assessing bone tunnel healing for ACL grafts.[27] Based on normal ACL structure and function of the insertion site, the ideal tendon graft would attach broadly to the surface of the bone at the femoral and tibial attachment sites by an intermediate zone of fibrocartilage.
The bone-patellar tendon-bone graft undergoes a process of ligamentization. The graft undergoes initial processes of necrosis, revascularization, cellular proliferation, and then remodeling. The remodeling phase could be divided into consolidation and maturation phases. The biochemical and morphologic changes occur in the graft as it assumes a histologic pattern similar to native ACL, but it is not identical to either native ACL or tendon.[23]
The most rapid healing time at the insertion sites is in patellar tendon autograft or allograft with bone-to-bone healing times of 4–6 weeks. This graft has the additional benefit of having the natural insertion site of tendon preserved on the bone plug as previously described.
The mechanism by which graft-bone healing occurs depends on the type of the graft used. For bone-patellar tendon-bone grafts, healing in the tunnel resembles normal fracture healing but may be a more complex process. Incorporation of the bone block in the tunnel has been observed as early as 16 weeks after surgery.[23] Bone-patellar tendon-bone grafts have the advantage of allowing rigid fixation of the graft in the bone tunnel.
The tendon-bone healing process occurs through a different mechanism after implantation of a soft-tissue graft without bone plugs.[23,47] First, fibrovascular interface tissue forms between graft and bone, and progressive mineralization of the interface tissue occurs with subsequent bone ingrowth into the outer tendon and incorporation of the tendon graft into the surrounding bone.[10,75] Sharpey's fibers are made up of type I collagen and connect the periosteum to the bone. Progressive reestablishment of the continuity of collagen fibers between the tendon and the bone results in re-establishment of a tendoosseous junction.[16] Formation of the Sharpey-like fibers within the bone tunnel often are identified as a marker of indirect healing between the tendon and bone.[76,77] Formation of these collagenous fibers may start from 6 weeks after surgery.[16,76] However, complete bone tunnel healing of an ACL graft may occur as late as 6–12 months after surgery.[23,77,78] Some studies in the animal models suggest that tendon graft incorporation occurs more slowly than bone-patellar tendon-bone healing.[75,79] In addition to the choice of graft, surgical fixation, graft position, and interfacial motion within the bone tunnel may affect healing.[27] Graft motion within the bone tunnel has been shown to be inversely proportional to healing in animal models.16,75–78,80 Histology taken at revision surgery for mid-substance tears, shows that free hamstring tendon autograft has adequate osteointegration between 6–15 weeks.[81] If soft tissue-to-bone integration occurs at this time in the postoperative time period, a hamstring tendon graft may allow for earlier recovery and return to activity because of less donor site morbidity. A number of studies have shown no significant difference in clinical outcomes between bone-patellar tendon-bone graft and hamstring tendon graft for ACL reconstruction.6–8,82
The healing potential of a ruptured ACL as described before is considered to be extremely poor.[4,83] It is suggested that the intraarticular environment that inhibits ACL healing also may interfere with bony healing in the proximal parts of the osseous tunnels.[33]
Curr Orthop Pract. 2013;24(3):321-333. © 2013 Lippincott Williams & Wilkins
