what injury does ahab sustain as a result of his fall

From the text:

"...he had been found one night lying prone upon the ground, and insensible; by some unknown, and seemingly inexplicable, unimaginable casualty, his ivory limb

having been so violently displaced, that it had stake-wise smitten, and all but pierced his groin; nor was it without extreme difficulty that the agonizing wound was entirely cured."