Wow! With that shoulder surgery, I recommend that you implement *all* of the suggestions for reducing stress on that shoulder. Yet another thing you can do, which I highly recommend, is quilt standing up. My dh made a large surround for my sewing machine out of a styrofoam sheet purchased at the hardware store. By placing my sewing machine on my cutting table and positioning the styrofoam around it, I was able to quilt for much longer periods of time because my shoulders and back were much less stressed.
Here are links to the Youtube videos that showed me how to make a styrofoam surround. She quilts sitting down, but I found standing up to be much better.
https://www.youtube.com/watch?v=g14govA4pIM
https://www.youtube.com/watch?v=yAS25v3ZTk0
https://www.youtube.com/watch?v=A0lk7UBQgZY
https://www.youtube.com/watch?v=hwrA...F28BF9&index=6
And here is a link to her video about sewing ergonomics:
https://www.youtube.com/watch?v=hbmQ...F28BF9&index=4
In my opinion, it's very important for quilters (especially those of us who have gotten up there in years) to pay attention to our bodies. I am careful not to do any one thing for too long a period of time, as I don't want to be hampered by repetitive stress injuries. In your case, with a joint replacement, it's even more important that you pay attention to the stresses in your upper body.