If you have a "knot" type pain in your shoulder blade area on the same side of your body as your mouse hand, I cannot strongly recommend a track ball mouse like this enough:

http://www.tigerdirect.com/applications/searchtools/item-det...;

I had a continual knot and painful spot in my shoulder blade from age 16 (when I started using a computer regularly) until age 35 when I tried the trackball mouse. Within a month or so of switching, the achey spot was entirely gone. I started a new job and used their mouse, and the pain came back. I brought in my mouse and it went away. Also, trackballs are faster once you get used to them.