Tendon-to-bone healing: Basic science aspects and enhancement techniques

Many reconstructive procedures require healing of a tendon graft in a bone tunnel (for example, reconstruction of the anterior cruciate ligament [ACL]) or to the surface of bone (for example, rotator cuff tendon repair). Tendons and ligaments attach to bone by one of two methods: a fibrocartilaginous interface (direct insertion) or by Sharpey's fibers (indirect insertion). Basic science studies have demonstrated that tendon- to-bone healing can occur by formation of either a direct or an indirect insertion, most likely because of differences in species examined, local mechanical environment, and intra-articular versus extra-articular location. Healing occurs by ingrowth of bone and soft tissue into the interface zone that forms between the tendon and bone. Recent studies have demonstrated the potential to enhance tendon-to-bone healing using cytokines such as bone morphogenetic proteins. Augmentation techniques may be especially useful in clinical situations in which healing between tendon and bone may be impaired, such as may occur in the setting of excessive motion at the graft-tunnel interface; in bone tunnel enlargement such as may be encountered during revision cruciate ligament reconstruction; or during rotator cuff tendon healing in elderly patients with osteoporosis. This article also discusses some of the fundamental questions that remain unanswered about the basic biologic and biomechanical characteristics of devices currently used for fixation of tendon grafts into bone tunnels or to the surface of bone.