Although Prior has suffered through a variety of injuries over the last few years (tendinitis, fractured elbow after being hit by a pitch), his struggles of late have revolved around his throwing shoulder. Last year Cubs trainer Mark O'Neal was quoted on the Cubs' website as stating that Prior had "laxity," which he further defined as genetic looseness, in Prior's shoulder. This set off a bit of a media buzz about this "laxity" condition that Prior was battling. The fact is that many successful pitchers have laxity in their throwing shoulders. Laxity, which basically means extra motion or "play" in a joint, often develops in the thrower's shoulder as a result of the repeated extreme (and unnatural) ranges of motion induced by pitching.
Every joint in the body has two interfacing bony surfaces that move relative to one another to allow for normal motion. The two bony ends are encased by fibrous tissue, the joint capsule, which helps stabilize them. The capsule can be overstretched – either traumatically (which is what happens when a joint dislocates, often tearing the capsule) or gradually, over time, as a result of repeated stretching (which is usually the scenario for the thrower). So in essence, the presence of laxity in a pitcher, especially at the pro level, is not uncommon, and may even be beneficial in terms of allowing more wind-up to produce greater forces when pitching.
So why is it a problem for Prior? The less inherently stable a joint is, the more that joint relies on the support of the muscles around it. In the case of the shoulder, several muscle groups, most notably the rotator cuff, must provide that extra support. Additionally the scapular muscles (muscles that control the movement of the shoulder blade) must provide a strong base of support for the cuff. If these muscles are not all working synchronously, the joint can feel like it is slipping or sliding and can become painful. If this goes on for long enough, abnormal forces at the shoulder cause further tissue breakdown, and this is how a pitcher can find himself going under the knife. The take-away message here is that the key to a healthy shoulder is excellent muscular support. In the absence of a gross defect in the shoulder that would clearly respond to surgical repair, the first choice for this type of problem is rehab. This helps explain why the three orthopedic surgeons who evaluated Prior last year all recommended intensive rehab as the best course of action.
It remains to be seen how effective Prior can be in his return. The glass half-full observer can celebrate the fact that Prior is pitching without pain. If he can continue to do so while rediscovering his command and adding some speed, he could be a force. It is a big "if," but it's an "if" with upside, and so we, along with the Cubs, must wait and see.