Here's my experience with AC joint arthrosis:
I was diagnosed with it in my right shoulder in 2007 or so. I turned 24 that year. In 2008 I had surgery on it. Four weeks later I did push-ups. 7 weeks later I was doing metcons. The doc said he removed maybe 4-5mm from both the clavicle and the acromion. The movement that caused the worst pain, for me, was popping my trapezius up, as in trying to "show off your traps", for example. The MRI confirmed the arthrosis more than the conventional movement tests of crossing your arm over your body, etc. Also no pain at all with the doc pushing down the joint.
My fun, and proper workouts, lasted 4 months. In late November 2008 I injured my left shoulder. MRI in Feb. showed small signs of the same thing in this left side. Doc said it wasn't nearly as bad... tried to just recover, ya know. Even went through 3 weeks of NOTHING with that arm (this was hard!!). MRI in June.. it got worse. Open surgery again, had it the 7th day of this month, 2 weeks ago today. I think I am recovering faster than with my right shoulder one.
Funny thing is, once again, according to the movements tests, that joint was fine. It was the MRI that confirmed it, AND, again, trying to pop the trap up (this time the left one) was painful. I have yet to find any text on this, but I suspect in upcoming years someone will do a study on this.
What I personally think? Well, For many years, about 7-8, I played an old computer game at a very high level. It was the ONLY game I played since the year 2000 or so. I retired in 2007 though. I played this game in a HORRIBLE possition. I inclined my chair back, and had both arms almost straight, at shoulder level, parallel to the floor, sortof. Maybe some days I wouldn't play, but maybe some others I'd play 2 or 3 hours. So maybe it takes it's toll. This is what I think, just me, I could be wrong. The bad posture combined with exercising = bad joint.
The doctor mentioned a guy who played this instrument, I don't know the name in english, but you touch it with your hands, like drums but with your hands... and he had both AC joints in pain. When he saw how he played it, he sat on a chair and his arms were up, at shoulder level, out in front... he told him to use a higher chair so his arms would be lower, and that "cured" him. I think this might add some weight to my own theory?
I feel I've only properly
trained in 4 months out of the last 2+ years, it is a HORRIBLE feeling. Hopefully soon I'll be able to do everything nicely again
Need to move on, past this newbie strength level!!!