Types of ligament damage that are most often reported are strains, when a ligament is stretched beyond its normal limits, and ruptures, which occur when a ligament is partially or completely torn. Ligaments are dense bands of fibrous connective tissue that connect two bones to one another at a joint, holding them together and protecting the joint capsule. Because they are situated at movable parts of the body, they are constantly absorbing and transferring various forces and therefore are vulnerable to injury. Ligament damage is typically caused by either an abrupt blow to the joint or by repetitive, excessive forces placed on the joint over time, which can cause ligaments to break down and leave them susceptible to strains and ruptures. It is also more likely to occur in individuals who have weak and unstable joints, particularly at the knee and ankle joints.
Especially among athletes and weekend warriors, the knee and ankle are the two joints in the body that most commonly sustain ligament damage. In the knee joint, the ligaments that are most often injured are the anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), and the medial collateral ligament (MCL). Of these, the ACL sees the most damage. An intra-capsular ligament, meaning that it is found inside of the knee joint capsule, the ACL runs diagonally between the femur bone in the thigh and the tibia in the shin. It is typically strained or torn during abrupt lateral or external rotation of the knee, as in changing direction when playing soccer or football, can require arthroscopic surgery in the case of tears, and can take months or even a year to heal.
Another intra-capsular ligament is the PCL, which crosses behind the ACL in the opposite direction; this ligament tends to see damage from hyperextension of the knee joint caused by a blow to the front of the knee or a sudden extension of the legs, as in explosive jumping movements. The MCL, on the other hand, is an extra-capsular ligament, running vertically outside of the joint capsule along the medial side of the knee joint between the femur and tibia. Ligament damage to the MCL most often occurs as the result of a sideways blow to the knee that knocks it inward, overstretching the ligament to the point of straining or tearing.
In the ankle, the most commonly injured ligaments are those on the lateral or outer side of the ankle: the anterior talofibular ligament, the posterior talofibular ligament, and the calcaneofibular ligament. These see the most ligament damage because, on rolling the ankle, it is more likely to invert, or roll outward so that the sole of the foot turns inward, than evert, or roll inward. Inversion of the ankle overstretches the ligaments on the outside of the joint, particularly the anterior talofibular ligament. This ligament connects the fibular or lateral malleolus, the large bony prominence felt on the outside of the ankle, with the lateral surface of the talus bone just beneath it; as such it runs the most parallel to the direction of the ankle roll when the foot is flexed and is most likely to sustain a strain or tear.
Another ligament near the ankle that is vulnerable to strains is the anterior inferior tibiofibular ligament, or AITFL. This is the ligament that links the bottoms of the tibia and fibula together in the lower leg just above the ankle joint, and injury to this ligament is known as a high ankle sprain. As this joint is a syndesmosis and therefore not very movable, the AITFL can also sustain ligament damage from inversion of the ankle.