What is a strong painkiller with a relatively low or moderate risk of dependance for back/spine injury?

No such animal: Strong; low risk; spine pain ? I am not sure that such an animal exists, but strong and risk are both words that are so dependent upon the individual. I do not think we could find an answer without knowing you and your actual problems.

Tramadol : Tramadol is fairly strong pain killer, specifically when combined with NSAID and have very low potential for addiction.