A doctor examined my painful thumb and suggested I have surgery to fuse the middle joint of my thumb together. Won't this keep me from doing my normal activities?

Not necessarily. When the middle thumb joint is fused together with arthrodesis surgery, a person's ability to do day-to-day tasks isn't affected that much. This is because some people have a very small range of motion in this joint anyway. Fusing the joint keeps the joint from moving, but it also protects it from eventually becoming arthritic and painful.