What Causes Constant Headaches After Cervical Fusion Surgery?

After Cervical fusion on c4-6 I have had constant headaches now since May 2004. I was first on antidepressents and now I am on topiramate and nadolol which has helped to give me my lif back. I have also gone through 4 phases of physical thearphy. I am still under the care of a neuroligist at Henry Ford. My surgery was done by Dr, Soo. My question is what could have happen during surgery that created the problem with my constant headaches?