Orthopedic surgery frequently requires replacing a damaged ligament, such as the anterior cruciate ligament (ACL) in the knee, to restore joint stability and function. Since a ligament cannot effectively heal itself after a complete rupture, surgeons must use a tissue graft to reconstruct the structure. The most successful and commonly used option is an autograft, tissue harvested from the patient’s own body. The central question is why a tendon, a structure with a seemingly different function, is the ideal biological choice for replacing a ligament. The answer lies in the deep similarities of their microscopic composition and their shared capacity to withstand significant mechanical forces.
Defining the Roles of Tendons and Ligaments
The body contains two primary types of dense, fibrous connective tissue: tendons and ligaments. A tendon’s primary role is to connect muscle to bone, acting as an intermediary to transmit the force generated by muscle contraction, which allows for skeletal movement. Tendons are built to endure high, unidirectional tensile loads.
In contrast, a ligament connects one bone to another bone across a joint. Its function is to stabilize the joint by limiting excessive or abnormal ranges of motion. Ligaments must primarily resist forces that attempt to pull the bones apart or twist them beyond their functional limits. Although their functions are distinct, their underlying composition allows for a functional interchangeability in a surgical setting.
The Biomechanical Rationale for Substitution
The fundamental reason a tendon can successfully replace a ligament is that both tissues are constructed from nearly identical biological raw materials. Both tendons and ligaments are classified as dense, regular connective tissue, and their dry weight is largely composed of Type I collagen fibers. This specific type of collagen provides tremendous resistance to stretching and tearing, giving both structures their characteristic tensile strength.
While their material composition is similar, their internal organization reflects their function. Tendon collagen fibers are packed into a highly parallel arrangement, optimized to resist the high, straight-line tension from a muscle. Ligament fibers are also generally parallel but feature slightly more interwoven bundles, which allows them to resist multidirectional stresses necessary for joint stability. This subtle difference does not diminish the tendon’s inherent strength, which is often initially stronger than the native ligament it is intended to replace.
The Process of Ligamentization
After a tendon autograft is surgically placed to replace a ligament, it must undergo a profound biological transformation known as “ligamentization.” This process is the key to the long-term success of the reconstruction, as the tendon gradually adapts to its new mechanical and biological environment. The transformation is divided into three main, overlapping phases that can span over a year or more.
Early Necrosis and Healing
The first phase, known as early necrosis, begins immediately after implantation when the central cells of the tendon graft die due to loss of blood supply. Over the next few weeks, the graft’s mechanical strength temporarily decreases significantly as it is biologically weakened by this cell death and the initial remodeling process.
Revascularization and Cellular Repopulation
The second phase involves revascularization and cellular repopulation, which includes the ingrowth of new blood vessels and the migration of new cells into the graft.
Maturation and Remodeling
During the final phase of maturation and remodeling, the graft tissue begins to slowly adopt the microscopic characteristics of a native ligament. The collagen fibers start to reorganize and become less tightly parallel, mimicking the slightly more interwoven structure of a ligament. Full maturation of the graft may take up to two years or longer in human patients.
Common Tendon Sources for Ligament Reconstruction
The selection of a tendon autograft is a balance between obtaining a graft strong enough for reconstruction and minimizing the functional impact on the donor site.
The most common tendons used for ligament replacement are the semitendinosus and gracilis tendons, harvested from the hamstring muscle group on the inside of the thigh. These tendons are often folded multiple times to create a strong, multi-strand graft.
Another frequent source is the central third of the patellar tendon, often harvested with small bone blocks from the kneecap and shinbone, creating a bone-patellar tendon-bone graft. This graft is favored because the bone-to-bone healing at the attachment points can be faster than soft tissue-to-bone integration.
The quadriceps tendon, which connects the large thigh muscle to the kneecap, has also gained popularity as it provides a strong, thick graft with potentially fewer issues at the front of the knee compared to the patellar tendon graft.