An old injury, just revealed by the Mariners, Adrian Beltre has evidence of an injury to the radial collateral ligament at the base of the left thumb. Since Beltre throws right, I suspect he suffered this injury from one or more fielding attempts where the thumb of his glove struck the ground forcing his thumb toward the index finger.
If you are paying attention, Andy LaRoche just had surgery and will miss at least a couple months (and in all probability not be right for the balance of the 2008 season) for an injury to the ulnar collateral ligament of the base of the thumb - the opposite side of the joint where the base of the thumb is attached to the bone of the hand (first metacarpal) where the mechanism of injury is the opposite of Beltre's (the ligament in the webspace between the thumb and the index finger is the ulnar collateral ligament, while the ligament on the outside of the thumb is the radial collateral ligament).
LaRoche's injury is colloquially referred to in medical parlance as a "Gamekeeper's Thumb." No, not an injury to soccer's goaltender, but an injury to bird-handlers (in old England, as a matter of fact, where the malady was first described) who raise and harvest various birds for human consumption (chickens and game hens for example) and the "Gamekeeper" would sacrifice the bird by snapping its neck with their hands. When the maneuver is performed hundreds of times, the stress on the ligament at the base of the thumb became evident because the condition was so disabling to the gamekeeper and they complained about it to observant doctors.
The injury is also called a "Ski Pole" injury which is what happens to skiers when the ski pole is violently loosened from their grip in a fall when their hand is around the handgrip of the ski pole and the wrist is fixated in the pole's strap, ostensibly as a tether so as not to lose the pole in a fall. The thumb is taken violently beyond its normal range of motion, rupturing the ligament.
It's important to recognize that the thumb (along with the index finger) is responsible for about 90-percent of the function of the hand. The ulnar collateral ligament (LaRoche's injury), clinically, is far more important to the function of the thumb than the radial collateral ligament on the opposite side of the base of the thumb (Beltre's injury - which now chronic and likely healed).
If one examines Beltre's performance both before and after this ailment is believed to have originated, which has been done elsewhere (The Fantasy Hot Sheet presented by The Roto Times: Beltre, Granderson, Rolen and more, March 25, 2008, by Rick Wilton), you'll notice that his productivity is about the same, indicating that, while he has pain and perhaps some mild disability from that old injury he can still play and still produce.
All athletes, especially professional athletes, play with some degree of pain and disability. Just because something is found on an MRI and the athlete has some discomfort in that area does not necessararily mean that that finding is the cause of the problem and certainly not one that requires fixing. One third of the population taken at random off the street, for example, on an MRI has one or more bulging intervertebral discs in their back - and no symptoms, no disability.
When somebody has back pain or a back injury and an MRI is indiscriminately ordered, select unknowing (or unscrupulous) medical practitioners will ascribe their patient's problem to the disc and recommend surgery. The results of disc surgery (discectomy and/or laminectomy) are well known postoperatively: one third get better, one third get worse, and one-third are exactly the same as before the surgery. In well-controlled studies, randomized to surgery versus no surgery, two years after the diagnosis of disc disease producing back pain is made, the outcome of patients in both surgical and conservative management groups is precisely the same - the same number are better, the same number are worse, the same number are just the same.
Clearly, it's possible that the abnormal disc that is imaged by MRI may have nothing to do with the back pain. Along with lots of things MRI imaged in MLB players, so it could be true with the radial collateral ligament of Beltre's thumb. I would like to see a study where a series of asymptomatic baseball players undergo MRI of the thumb. I suspect there would be a significant number with evidence of old functional injuries not dissimilar from Beltre's, and this could all be much ado about nothing.