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What's the difference between shoulder laxity and shoulder instability?

Laxity usually refers to looseness of the soft tissues around the joint. The affected structures can include ligaments, tendons, muscles, and/or the shoulder capsule. The capsule refers to the envelope of ligamentous tissue that surrounds the shoulder like an envelope. In the shoulder, the rotator cuff functions as a protective, stabilizing force to hold the head of the humerus (upper arm bone) firmly in the socket. Laxity can be a natural occurrence when a person is born with collagen (protein structure that makes up soft tissues) that is pliable (very flexible). There are fewer links holding the fibers together. Laxity can also occur as a result of overuse. For example, throwing athletes who cock the arm way back and externally rotate the shoulder before releasing the ball forward can overstretch the soft tissues, thus leading to laxity. Instability refers to the fact that the shoulder is not stable within the socket. It may partially dislocate (called subluxation) or fully dislocate. Repetitive trauma and microdamage of the soft tissues can lead to laxity, which in turn results in instability. These two terms represent a phenomenon that exists on a continuum from mild (laxity) to severe (instability). As we come to understand trauma, microtrauma, and instability of the shoulder soft tissues, the terminology is becoming more precise. For example, some experts refer to subtle instability to describe overhead throwing athletes who develop problems from overuse rather than from trauma. Repetitive shear forces during the cocking phase of the pitch and during the throw (acceleration phase) may start a series of events that eventually lead to microinstability. The athlete has pain when pitching but doesn't feel as if the shoulder is going to come out of the socket. Apprehension is the word used to describe a common feeling for athletes with an unstable shoulder. There may actually be a wide range of clinical presentations for both of these terms (laxity and instability). Some patients have pain; others do not. Some feel apprehensive and fearful that the shoulder is going to come out of the socket as it has in the past. Sometimes the pitcher experiences an inability to place the ball where he or she wants it to go. The speed, force, and direction of the ball are affected but the player doesn't have specific symptoms of pain or a sensation that the joint is dislocating. Efforts are being made to understand these two conditions more completely. It's likely that the terms may be modified as more information becomes available. Computerized motion analysis is giving a better three-dimensional picture of normal and abnormal shoulder movement in throwing athletes. This will help considerably in sorting out the various shades of gray between laxity and instability.

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