The UCL, highlighted in MS Paint.
The UCL is composed of two portions, the anterior and the posterior. The anterior portion connects the front of the medial epicondyle (part of the humerus) and the coronoid process, part of the ulna, one of the two bones in the forearm. The posterior portion connects the lower back portion of the medial epicondyle to the medial margin of the olecranon (also part of the ulna, and the bony part of the elbow visible when the arms are bent).
The UCL is at particular risk for pitchers. These ligaments can fray, loosen (ligamentous laxity), or tear as a result of repetitive motion strain, especially with pitchers. Pitchers tend to make at least 100 throws per game, plus many more during warmups and bullpen sessions outside game pitches. The basic problem is that the UCL has a moderately low ultimate tensile strength (pulling strength), and stress approaching that strength is often approached on every throw. Thus, more throws mean more stress and a greater chance of injury.
A lot of research has gone into the effects of breaking pitches on UCL injury, and the consensus seems to indicate that breaking pitches, particularly the slider, lead to increased incidence of elbow injury. Ervin Santana throws sliders, curveballs, and changeups. Why do sliders hurt the UCL? When throwing a slider, the pitcher supinates (palm turns upward) his throwing arm upon ball release. This causes the radius and ulna (the bones in the forearm) to twist while imparting valgus stress (lateral to medial, much like pushing on the side of a knee) on the elbow and placing much of that stress on the UCL.
So, looking at data on Santana's pitches, a couple factors are immediately in play. First, Santana threw 33.9% of his pitches as sliders last season, up from an average of 23% in the preceding seasons. Second, he threw 3428 pitches last season as compared to 2513 the year before. This is a substantial increase. Both the increased number of sliders and the increased number of overall pitches conspired to sprain Santana's UCL.
What, exactly, is a sprain? It's an intentionally ambiguous term, but without better medical information from the Angels' medical staff, I'm going to assume its a second-degree sprain, or a minor tear of a ligament. Anything more and I'd expect a longer potential layoff from baseball activities, and anything less would probably keep Santana in line to pitch at the beginning of the season. Second-degree sprain rehabilitation is nonsurgical and follows the R.I.C.E. method for rest, ice, compression, and elevation. This reduces swelling, inhibits further inflammation of the injury, encourages healing circulation, and provides stabilization to assist in healing. Santana will likely be put into a soft cast or sling and avoid baseball activities for 2-3 weeks before restarting weight training and throwing programs.
Interestingly, elbow injuries are almost always preferable to shoulder injuries. Tommy John surgery is used to repair a severely damaged UCL by replacing it with ligaments from other parts of the body. Recovery rates from TJ surgery are between 85 and 90 percent, much higher than rates for shoulder labrum tears. Although recovery time generally takes as much as a year, a severe UCL injury does not typically end careers, even for pitchers who rely on velocity.
Santana ranked #24 on the pitcher abuse rankings for the 2008 season and was predicted as one of the top 10 most likely pitchers to sustain an injury this season, although I can't find the damn SB Nation fanpost that shows that because their search system is awful.
Predicted to return some time in May, expect Santana to possibly reduce the number of sliders he throws (perhaps sacrificing some strikeouts and effectiveness) in an effort to keep his elbow healthy.
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