If a good nerve is reconnected to a sheath, it will grow back through the hollow tube at the rate of about an inch per month. The closer to the trunk an amputation is, the less the likelihood of restoring function to the hand. We actually measure from the zone of injury to the extremity to estimate regeneration time. Bobby is at the outer limit. This is why there aren't many successful arm replants.
Success rates are much higher for amputated hands and fingers.
Fingers, especially, because they don't have muscles. Researchers are looking for biologic agents that could accelerate nerve regeneration, but they appear years away from use in humans. In the meantime, scientists have experimented with electrical stimulators designed to preserve muscles during nerve regeneration. External stimulators currently available do not work very well, however. Looking for a better alternative for Bobby, Daley referred him to a Canadian colleague who implanted an experimental electrical muscle stimulator in his forearm.