Plates and screws are used sometimes to temporarily restructure the body while bone tissue re-grows to support the body without them.  They can be made of many various materials including polymers.  Plates sometimes are left in the body when significant damage is done to the body's rigid structure and/or the patient is older and bone-tissue growth is minimal. 

It is convenient to think of screws and plates as a temporary fix for a larger problem.  Gene Therapy (see Hip Replacement for more info) can also contribute to this aspect of biomaterial applications greatly.