A sprain is a tear of ligaments, the tough bands of fibrous tissue that connect bones to one another at a joint. Normally, ligaments stabilize a joint, keep the joint's bones aligned and limit the motion of a joint to the normal range. When a joint is sprained, its torn or stretched ligaments can lose part or all of their ability to reinforce the joint and to keep it moving normally. In severe cases, the sprained joint can become unstable and loose, bones can move out of alignment and the joint may extend beyond its normal range of motion.
Although ligaments can be sprained in a variety of ways, the actual ligament damage is usually caused by at least one of the following:
Flexing (bending), extending (stretching out) or twisting a joint beyond its normal range of motion — This injury is a common cause of wrist sprains in skiers. If a skier falls with a ski pole still strapped to the wrist, the attached pole can twist the wrist beyond its normal limits and cause a sprain.
Suddenly increasing the tension (strain or pull) on a ligament to the point that it snaps in two — This type of injury can happen in the knee joint if you suddenly stop short while running. The extreme tension of the braking force actually tears one of the knee ligaments in two, often causing a pop that can be felt or even heard when the ligament snaps.
Hitting the joint directly or hitting one of the bones near the joint — This type of sprain often happens in contact sports, especially when a shoulder or knee joint takes the full impact of a collision between two athletes.
Any unusual force across a joint can cause a sprain. Among athletes, more than any other group, sprains are common. Knee sprains and shoulder sprains are common among those who participate in football, basketball, soccer, rugby, wrestling, gymnastics and skiing. Foot sprains are a danger for ballet dancers, snowboarders, windsurfers, equestrians and competitive divers. Wrist sprains are common in skiing, football, basketball, baseball, roller hockey, boxing, basketball, volleyball and weightlifting. The specific joint that tends to be sprained during a particular sport is usually related to the kinds of joint movements that the sport requires or to the types of impact or collision that can happen. For example, many knee sprains in football players are caused by the extreme knee stress of cutting moves and sharp turns. Others are caused by the direct impact of tackles.
Off the playing field, sprains often occur because of high-impact accidents — for example, striking the knee on the dashboard during a car crash or slipping on a patch of ice and landing on a wrist or shoulder. Sprains are also common in the workplace.