I had back surgery on August 9th. I had my L5-S1 fused and my L3-4,L4-5 replaced. Before surgery I used heat all the time but I vaguely remember hearing something a long time ago that after a fusion you can't use heat. I haven't been able to get a hold of my surgeon to find out. I am using ice but I know heat would help the muscle spasms I have.
By the way, since surgery I have nothing but good things to say. Other than the spasms my back pain is next to none. I've been injured since 2007 so this surgery was a saving grace. I would recommend disk replacements to anyone.