Your doctor is correct - once you have damaged the Matrix where the nail plate cells are 'born' from repeated and prolonged trauma, the Matrix becomes permanently damaged. Once this happens, the nail will never grow the same again, which is why it is concave. The ridging can be helped somewhat by twice daily applications of a good quality nailand cuticle oil that contains Jojoba and Vitamin E oils. The ridging is actually the rails and grooves upon which the nail plate rides forward on the nail bed, much like a train rides forward on its tracks. When the nail plate becomes dried out, the rails and grooves become apparent on the nail plate surface as ridging. This can also occur due to aging.