Broken wrist - why would a wrist fusion surgery be needed, and which bones get fused together? Any advice?

Depends on bone. The most common wrist fracture is of the distal radius (a forearm bone). Unless it is completely shattered a fusion of the wrist is rarely needed and than a partial fusion can maintain some motion. The second most common fracture is a scaphoid fracture. If it does not heal or the bone dies then a partial mid-carpal fusion is often performed to preserve motion. See an orthopaedic hand surgeon.