How did Tom Robinson injure his left arm in To Kill a Mockingbird?

When Tom Robinson stands up, Scout observes "he looked oddly off balance, but it was not from the way he was standing" (Chapter 18). Jem whispers that Tom Robinson is crippled. His left arm hangs limply beside his body. It also appears shorter than the other arm. His hand is misshapen, and it is clear he could not throw a punch with it. Atticus points out that the witness testimonies accuse Tom of punching Mayella with his left fist. Given his crippled state, this is impossible.

Reverend Sykes tells Scout and Jem how Tom's arm and hand were injured. Many years before, when he was a boy, Tom worked for Mr. Dolphus Raymond. While working, young Tom's hand got caught in the cotton gin. It was severely injured, and he never regained full use of it.