(This is my personal idea, I'm not sure if it's correct, but it's just what I think)
Answer: In wild, tigers eat bones in order to get calcium. This is one of two main elements of bones, and it ensures the bone is hard enough to carry the whole weight of the animal. The diet they have in the zoo doesn't contain bones, so the bone problems are due the lack of calcium. To solve the problem, zoos need to provide the tigers some bones in their diet so as to ensure that they get enough calcium