One of my best friends and the person who originally taught me to play the guitar, had a svere hand disability. He had been shot in the arm a couple of years before and used the guitar for rehabilitation. The disability caused him not to be able to control the muscles to open the right hand. He could make a weak fist, but to open it, he had to relax and let it fall open. The way he played was by using a pick in a conventional manner and on occaision plucking the strings with the remaining fingers. Although it prevented him from playing certain styles, he was able to develop his own style and sound.