What was Tom's disability in 'To Kill a Mockingbird' and why was it important to his defense?

Tom's left arm been caught in a machine when he was a boy and was rendered useless. Therefore he could not have strangle the girl he was accused of raping because the marks where clearly made by both hands on either side of the neck.
Tom Robinson's handicap is that he has a partial right arm. Most of his right arm was cut off in an accident, where the machine that caused the accident stripped all of the muscle from his right arm. His right arm can no longer move, so Tom uses his left arm.