What happened to Tom Robinson as a boy in To Kill a Mockingbird?

Atticus makes a big deal at the trial about how Tom Robinson’s left arm was crippled.

The trial of Tom Robinson is a big deal for Maycomb.  Tom Robinson is a black man accused of raping a white woman.  In the eyes of Maycomb, being accused makes him guilty.  Atticus Finch is appointed to defend the man, and during the trial he makes a big deal out of physical evidence about where Mayella was hurt.

Mayella had injuries to her face.  This is part of the evidence presented at the trial.  Atticus makes sure that everyone knows what side of her face the injuries are on.

Mr. Tate said, “Oh yes, that’d make it her right. It was her right eye, Mr. Finch. I remember now, she was bunged up on that side of her face…” (Ch. 17)

Atticus has her father, Bob Ewell, demonstrate that he writes with his left hand.  Atticus wants to use this to prove that he is the one who hit Mayella, not Tom Robinson.  As soon as Tom Robinson takes the stand, everyone understands what he was trying to do.  It is clear that his left arm is useless.

His left arm was fully twelve inches shorter than his right, and hung dead at his side. It ended in a small shriveled hand, and from as far away as the balcony I could see that it was no use to him. (Ch. 18)

Reverend Sykes explains to them that Tom Robinson got his arm caught in a cotton gin with he was a boy, and almost bled to death.  The accident “tore all the muscles loose from his bones” (Ch. 18).  Because of this, he has lost the use of his left hand for his entire adult life.  Even so, he is still strong enough to bust up chiffarobes, apparently.

Atticus's case mostly rests upon the evidence of where Mayella was hurt and the premise that Tom Robinson could not have physically caused her injuries.  Mr. Gilmer's case is based on proving that Robinson was still strong enough to do it.  Even though the jury convicts, everyone in town seems to think it was Bob Ewell and not Tom Robinson who hurt Mayella.