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Impingement, possible small labrum tear."(Impingement meaning pressure, encroachment, infringement.)
So after x-rays, an MRI arthrogram (large needle, dye), and several "does it hurt when I shove your shoulder this way?" sessions, that's the diagnosis.
As you'll see from the link above, impingement usually occurs at the top of the shoulder, directly underneath the clavicle at its highest point. Since high school I've had difficulty and some pain there but the sharp pain I felt 3 months ago and the subsequent weakness I've had has been in a much deeper and more central place (to find the location, put your left finger at the top of the armpit and then come straight up about 2 inches).
Dr. Eric McCarty has recommended physical therapy as a first course of action. If the pain doesn't subside or if the shoulder doesn't improve, then I can always elect to have surgery later.
For the most part, this is great news. Shoulder surgery is no small repair. Also, unlike knee injuries, shoulders often don't return fully. I'm a little disheartened because I don't know if there will be a real fix though. In my eyes, "use it less" is unacceptable.
One of the positives has been that I've begun strengthening and using my left arm for over the head motions. Although I hope to rehab my right arm, I still plan to continue throwing a football, softball, and swinging (volleyball) left handed.