Like you I am not allergic to titanium, but my ortho did say just what you did, that there are trace metals in titanium. I'm sorry about you are getting hives and although I have eczema I don't have hives. Can they do testing for hives like they do for eczema?
I also believe constant contact of a foreign object in or outside of our bodies can lead to allergies. What's in titanium probably is raising your histamine army trying to get rid of the titanium. At least you can have the plates removed if you have healed. I didn't get Allergic Contact Dermatitis until middle age. Our body, in time, believes these foreign substances are dangerous invaders and raise up our histamine armies to purge the rejected substance causing eczema and I am assuming hives also. I do believe I would have been a good prairie wife from a century ago- making my own soap from lye and lard without preservatives, wearing 100% cotton clothing, cleaning with natural products like vinegar and so on. I am free of all 3 types of my eczema today because of diligent research and blogging on several sites. We learn from each other.