Caribou to the best of my knowledge are thriving, and not included in bans. The only issue I can see, would be in the sale, purchase of these antlers, internationally. Even within ones home country, there are legal ramifications in the sale/purchase of "animal parts" If you harvested these antlers, or bought them, as long as you have the tags/permit for the animal, your fine.
As a knife handle, be aware, unstabilized antler, will move more than say buffalo horn.