The majority of musculoskeletal injuries involve the soft connective tissues including cartilage, meniscus, tendon, and ligament. In contrast to bone tissue, which regenerates after injury, these soft tissues tend to heal poorly through formation of weak, disorganized scar tissue. In fact certain soft-tissue injuries cannot heal on their own so surgical repair or complete reconstruction is required to restore function. Biological grafts (autografts and allografts) and “permanent” synthetic prostheses are currently used to reconstruct large soft-tissue defects. Autografts (a patient's own tissue transferred to another site) can be successful, but as you “rob Peter to pay Paul,” complications at the donor site can be significant. Allografts (transplants from another human) are in short supply and carry the risk of rejection or disease transmission. In general, permanent synthetic prostheses fail to withstand long-term mechanical loading associated with the musculoskeletal system. New strategies are clearly needed to manage musculoskeletal soft-tissue reconstruction.
Several tissue-engineering strategies are potentially useful to enhance repair of these troublesome injuries or to induce regeneration. One interesting approach (developed for skin and cartilage regeneration) combines scaffolds, cells, and cell signals to grow living tissue analogues in the laboratory. Scaffolds are natural or synthetic biomaterials upon which the cells attach and grow. The cells may be fibroblasts, chondrocytes, etc., depending on the specific application. Influential cell signals can include attachment factors, growth factors, and mechanical loads.
Our laboratory is developing a similar approach for ligament reconstruction. Ligaments are the strong, flexible bands of collagenous tissue that connect bone-to-bone to provide a delicate balance of stability and flexibility to the joints of the body. The anterior cruciate ligament (ACL) is the primary stabilizer of the knee and is frequently injured in sporting activities and accidents. Serious ACL injuries can lead to disability and progressive degeneration of cartilage and meniscus that normally serve as shock absorbers in the joint. Anterior-cruciate-ligament ruptures do not heal without surgical intervention, due to a poor intrinsic healing response and poor vascularity. To restore normal joint function, ruptured ACLs can be surgically reconstructed using a patellar-tendon autograft or allograft. While significant progress has been made in understanding ACL anatomy, structure, biomechanics, and healing, there is still no biological graft or biomaterial ideally suited for ACL reconstruction.