Pathophysiology of Ligament Degeneration, Targets for Future Therapy

Ligaments and tendons are vital musculoskeletal soft tissues, which are commonly injured due to overuse and trauma. Their distinct functions are well described; tendons transmit force from muscle to bone and ligaments connect bone to bone. Their unique structure and biochemical composition and how they change with disease are however poorly described. The composition of ligaments is very different to that of tendons and changes with their location and function within the skeleton. In general, they have a fibrillar and fascicular structure but are more loosely organised than tendon. In common with tendon they are comprised of over 70% water, cells and extracellular matrix. Their extracellular matrix (ECM) consists of mostly collagen type I, proteoglycans (PGs), elastin and other minor glycoproteins. Ligament ECM has been reported to have higher PG content than tendons and this has previously been considered to be a pathological change in tissues such as the cranial cruciate ligament (CCL). It is however most likely to be normal with PG type and content changing with exercise and ligament function. Different cell types have also been described in ligament with both mixed populations of round and spindle cells predominantly compared to the more consistent spindle type in tendons.  [...]