Describe tom robinson’s physical disability and how he got it. why is this so important?

Tom Robinson was a black man and a character from "to kill a mocking bird". Tom has physical disability, his left hand was 12 inch shorter than his right hand, and it is useless and he got it caught in a cotton gin when he was 12 years old.
It is important, because tom was accused for of a girl. The condition of girl shows that he used his left hand to beat and bruise her face. But tom was disable to use his left hand.