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Graft-Tunnel Healing
Introduction
Graft healing within the bone tunnel and the intra-articular ligamentization process represent together the two main sites of biological incorporation after anterior cruciate ligament (ACL) reconstruction, since they contribute to the determination of the mechanical behavior of the femur-ACL graft-tibia complex.
Graft-tunnel healing is a complex process influenced by several surgical and postoperative variables, and most of our knowledge on the physiology of graft-tunnel healing has been largely provided by animal studies. Limited clinical investigations are available on this topic, mainly retrieved from patients with an ACL graft failure who underwent revision surgery or from imaging studies.
Although the experimental animal models on bone-tendon healing have a certain clinical relevance, their translational value is rather limited for several reasons. First, animal studies are based on two different models: an extra-articular and an intra-articular model. The extra-articular model consists of a tendon graft that is detached from one of its insertion sites and fixed within a drilled tunnel of an adjacent bone (i.e., the digital extensor tendon detached proximally and fixed within a bone tunnel in the proximal tibia). In the intra-articular model, an ACL reconstruction is performed using a free tendon graft, like the semitendinosus (ST) tendon, or the central third of the patellar tendon (PT). This model better reproduces the ACL reconstruction in humans. In fact, the extra-articular model does not consider the biological stimuli of the intra-articular environment that may influence graft healing within the tunnel. Second, experimental setting can vary according to the animal model selected, as knee kinematics of animals can be more or less similar to that of human beings. Third, animals are usually treated without a controlled postoperative regimen, and this can influence graft healing, depending on the forces applied to the graft during the early postoperative period. Finally, studies on bone-tendon healing differ for the outcome measurements considered: some authors reported histological results and some reported other mechanical properties of the bone-tendon graft complex; moreover, most of them only focused on the first 12 weeks after surgery because of the clinical relevance of this period for planning postoperative rehabilitation and return to physical activity.
Ligament-to-Bone Insertion Site
Tendon and ligament insertion sites to the bone can be physiologically distinguished into two different types: direct and indirect. Native ACL attaches to the bone through a direct-type insertion, which has a highly differentiated morphology. In fact, within 1 mm, four different layers are recognized: fibrous tissue, fibrocartilage, mineralized fibrocartilage, and bone ( Fig. 86.1 ). This region plays an important mechanical role, as it allows a progressive distribution of the tensile load from a soft tissue (ligament) to a hard tissue (bone). On the other hand, other ligaments, such as the medial collateral ligament, run parallel to the bone and insert through an indirect-type insertion. Indirect insertion sites do not gradually change from ligament to bone, but rather consist of specialized collagen fibers, the so-called Sharpey fibers, which are oriented obliquely to the long axis of the bone and ligament and provide anchorage between the two tissues ( Fig. 86.2 ).
Understanding the difference between this two types of insertion is fundamental, because although the anatomic ACL reconstruction enables one to restore the location of the native insertion sites on the femur and tibia, the structure and composition of the native insertion sites cannot be always completely reproduced and mostly require an indirect-type insertion.
Types of Graft
Autograft
Healing of a grafted tendon within a bone tunnel mainly depends on the type of graft. Several animal models have shown a slower incorporation rate into the bone tunnel with soft tissue grafts compared with bone-plug grafts such as bone–patellar tendon–bone (BPTB). Biomechanical testing comparing BPTB and soft tissue graft healing demonstrated that, for up to 3 weeks, both soft tissue and bone-plug tendon grafts fail for pullout from the bone tunnel; between 6 and 8 weeks after surgery, bone-to-bone interface appears mechanically stronger than tendon-to-bone interface, but this difference is no more significant by 12 weeks. Milano et al. actually showed in a sheep model that PT graft allows a faster incorporation into the femoral tunnel even at 1 month, whereas 6 months after ACL reconstruction both PT and soft tissue grafts are less strong (about 80%) but stiffer (about 110%) than a normal ACL.
The advantage of using the PT is that it consists of a direct-type insertion, similar to that of the native ACL, and it theoretically offers the strongest healing potential because it relies mainly on bone-to-bone healing. According to some animal models, bone plug incorporation at the tunnel wall occurs through a progression of partial graft necrosis, resorption, proliferation, and remodeling to becoming no longer distinguishable from the surrounding bone. However, it must be highlighted that even when using a BPTB graft some of the tendinous portion of the graft remains into the tunnel, leading to a tendon-to-bone healing at the intra-articular tunnel apertures.
Tendon-to-bone healing slightly differs from bone-to-bone healing. No signs of partial graft necrosis have been reported for soft tissue grafts, and the healing process usually progresses through different steps: inflammation, proliferation, matrix synthesis, and matrix remodeling. The inflammatory phase starts right after graft implantation and is characterized by the presence of distinct subpopulations of neutrophils and macrophages. These cells progressively repopulate the tendon graft, and produce cytokines and growth factors (GFs), such as transforming growth factor-beta (TGF-β), vascular endothelial growth factor, fibroblast growth factor, and bone morphogenetic proteins (BMPs), which contribute to the formation of lamellar bone in the outer part of the graft and a fibrovascular scar tissue into the interface. Three to four weeks after surgery, shear stresses between the tunnel and the graft lead to the formation of perpendicular fibers resembling the Sharpey fibers of an indirect insertion site. Moreover, their size and number are positively correlated with the graft pullout strength. Sharpey-like fibers continue to be present 1 year after surgery, while gradual osseointegration occurs from the bone tunnel wall, eventually incorporating the outer portion of the graft and improving graft attachment strength.
Clinical Studies
Limited evidence of graft healing has been provided by clinical studies. Although animal findings were confirmed by clinical studies, biopsies in the central region of the BPTB autograft did not actually show any sign of necrosis, but the presence of vascularity of the grafts as early as 3 weeks after reconstruction.
Imaging studies also increased knowledge on early vascularization and bone remodeling at the bone-tendon junction in humans. Sörensen et al. used positron emission tomography scanning to evaluate the bone-tendon healing after ACL reconstruction with either autologous BPTB or doubled ST and gracilis tendon. The highest level of bone turnover was found 3 weeks after surgery, but the activity at the femoral fixation points was markedly increased, even after 7 months. The bone turnover was almost normalized 22 months after surgery. Conversely, a recent study on 20 patients who were evaluated by computer tomography at 4 and 8 weeks after ACL reconstruction through BPTB autograft showed that bone plug integration was almost complete 8 weeks after surgery. Terauchi et al. performed a magnetic resonance arthrogram in 30 patients 2, 3, and 6 months after single-bundle ACL reconstruction with an autologous hamstring tendon (HT) graft. The authors showed that revascularization around the femoral and tibial tunnels occurred at 2 months postoperatively, with blood flow subsequently decreasing over time until 6 months.
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