A: A sprain is caused by trauma- a fall, a twist, or a blow to the body, for example- that applies stress to a joint and overstretches or even ruptured supporting ligaments.
Ina mild sprain, a ligament is stretched, but the joint remains stable and is not loosened. A moderate sprain partially tears the ligament, causing the joint to be unstable. With a severe sprain, ligaments tear completely or separate from the bone. This loosening interferes with how the joint functions. You may feel a tear or pop in the joint. Although the intensity varies, all sprains commonly cause pain, swelling, bruising, and inflammation.
The ankle is the most commonly sprained joint, and a sprained ankle is more likely if you've had a previous sprain there. Repeated sprains can lead to arthritis, a loose ankle or tendon injury.