Tendons attach muscle to bone and ligaments attach bone to bone. Although ligaments are strong and rigid by nature, strains and sudden forces can cause them to rupture and tear, something that is a common sports injury. Damage is caused when the fibres become torn and the severity depends on the extent to which they have torn and the pain experienced as a result. Because of the lack of blood supply to the tissue, sometimes tears become permanent which can end in their removal. Also, if a ligament is stretched past a certain point, it can result in the ligament never returning to its original state. Like with ligaments, tendons if over strained can become damaged and even snap. A partially torn tendon can cause swelling and discomfort but can be healed over time, whereas a clean break in a tendon can cause a complete loss of movement and may result in permanent damage. Stem cells have been shown to help both tendons and ligaments rebuild themselves.