I have about a quarter inch missing from the ring finger of my left hand, and the tendon missing from the middle finger, the result of a car accident. The middle finger won't straighten on it's own, but will when pushed against the fretboard, so no problem there. The ring finger doesn't callus the same as the others, presumably because the skin is different lower down on the finger than at the tip, though I can't say for sure. If I just live with the pain for a few weeks and play frequently for short periods, it does develop some callus, though usually in just a small strip rather than the whole tip. After the accident, my dad mad the mistake of changing the 2-inch neck on my guitar for a narrower one, thinking it would be easier to play with the short finger. Actually, the short finger is wider at the tip and brushes against other strings, so it didn't help. The wider neck is easier. I just keep trying slightly different hand positions to compensate.