Around three years ago, I mashed my left thumb at work. This caused the fingernail to bruise and eventually come off. However, it has never grown back right. It has yellowing underneath the nail, like a fungus which the dermatologist tested for and was negative, and whiteness on top where the nail looks dead, and you can see the new nail underneath. The doctor didn't think I had damaged the nail bed because it was growing out straight, but is funny looking. He prescribed a urea nail stick to aid with the fungus like matter under the fingernail. I keep it trimmed back to allow for new nail growth, but it gets to a certain point and develops this gross looking end to it. Any idea why it's taking so long for the nail to regenerate, or is this an indication of a possible underlying cause?