The wires are not to give any kind of permanent support, they were a temporary measure to keep the sternum aligned while natural healing took place, i.e. the sternum knitted back together. The way the bone fuses together again is a very strong bond, like when a bone is broken, it's actually stronger in the area than it was before. Wires can break years after surgery as the bone remodels itself, but should not be of any concern unless the it is sticking out, causing irritation or pain. An x-ray will give your Doctor a clear view if the wire is dangerous, you don't want your heart or a lung damaged. In most cases the breaks can be simply ignored. They can be surgically removed, but if there's no danger with the break, then there really is no need.
How was this discovered? Have you discussed this with your doctor?