Ligaments and tendons are both connective tissues in the body, and they are both made of stacked bundles of collagen fibrils. The difference between these two are what they connect, which then affects their functions.
Ligaments attach bone to bone; tendons attach muscle to bone.
Tendons, also called sinew, merge with both the periosteum, which the thin membrane covering the bone, and the fascia, which is the thin membrane covering the muscle fibers. This merger happens at the insertion point, a movable attachment, and origin, a stationary attachment. The insertion and origin are the points at which a particular tendon binds the bone it acts on and the muscle that acts on the tendon. Each muscle usually has two tendons, which bind to two different bones. The muscle also usually crosses the joint of the two tendon-bound bones so that contraction causes the tendon to pull and act on the bone for movement. Thus, the tendons carry tensile forces and act like a pulley on the bone.
Ligaments attach to bones by forming a capsular sac around articulating joints. Articulating joints are those that move and are also surrounded by synovial fluid, cartilage, and other connective tissues that protect the bone ends upon movement. The ligaments hold the bones together when the bones are acted on by tendons and also limit the extension of the bones being acted on, preventing, for example, 360 degree rotation. Each joint can have several ligaments to direct the movement of the bone when pulled on by the tendons and muscles.
These two tissues also have many similarities. One similarity is that ligaments and tendons have similar structures and compositions, including a layering of type I collagen fibrils, fibroblasts, and proteoglycan matrix. However, tendons contain more organized collagen (fibrous) and ligaments contain more proteoglycan matrix. Also, tendons contain elastin and have been suggested to act as springs with elasticity and not just as pulleys. A second similarity is that both tissues lack a good blood supply and do not heal well, often requiring surgery and hampering mobility if torn or injured. However, tendons can receive blood supply from the surrounding tissues and ligaments from the microvascularity at the insertion point. A third similarity is that both tissues act on joints to facilitate movement by muscles – the tendons are the pulley cord and the ligaments are the wheel. The ability of the tissues to perform their functions alters with health, age, mobility, and previous injury.