TYPES OF JOINT INJURY

A dislocation is a complete and persistent displacement of the articular surfaces of the bones that make up a joint relative to one another, with disruption of at least part of the supporting capsule and/or ligaments (see Plate 7-20). After a dislocation, a muscle spasm locks the two displaced bone ends in an abnormal position, usually creating an obvious and significant deformity.

A subluxation is a partial dislocation of a joint; that is, the articular surfaces are partially separated from each other and are no longer congruent. Although not as severe as dislocations, subluxations usually also damage part of the joint capsule and some of the supporting ligaments. After subluxation, the patient may still be able to move the joint to some degree. Failure to recognize and treat a subluxation may result in permanent ligament laxity and joint incongruity. In both dislocations and subluxations the incongruent alignment of articular surfaces may cause abnormally high pressure to be placed on the articular cartilage, which can damage the cartilage. Because articular cartilage has limited capacity to regenerate, this predisposes to longer-term problems such as arthritis.

Joint injuries are often a combination of a fracture and a dislocation. In a fracture-dislocation or fracture-subluxation, there is a fracture with incongruency of the adjacent joint surfaces. A bimalleolar fracture of the ankle is a good example of a fracture subluxation. In this condition, fractures of the medial and lateral malleoli create instability of the ankle joint, resulting in subluxation of the tibiotalar articulation.

A sprain is a stretch and/or tear of a ligament resulting from force causing angulation, subluxation, or dislocation that may be seen even if the articular surfaces subsequently return to their normal alignment. Even though the displacement is transient, significant damage may occur to the joint capsule and ligaments. Except for swelling, sprains cause no significant deformity. Joint movement is limited only by pain and not by joint incongruity. A sprain should not be confused with a strain, which is the stretching of a musculotendinous unit (muscle and/or tendon).

Sprains are graded into three categories according to the severity of damage to the joint capsule and some of the supporting ligaments. A grade I (mild) sprain is characterized by a slight stretching and damage to the fibers of a ligament with a firm end point still noted on physical examination. Grade I sprains usually heal in 3 to 4 weeks without significant loss of function. A grade II (moderate) sprain describes a partial disruption of the supporting ligaments and capsule that may demonstrate some laxity on physical examination. Most grade II sprains also heal in 3 to 4 weeks if the injured structures are protected from excessive loads or stretching and if appropriate rehabilitation and activity modification are provided. A grade III (severe) sprain refers to a complete rupture of the capsule and supporting ligaments. A grade III sprain is as severe an injury as a complete dislocation. The only difference between a grade III sprain and a dislocation is that in the sprain the articular surfaces spontaneously return to their normal positions. In severe sprains, surgery may be required to repair the completely torn ligaments and capsule.