Why does John Thornton walk with a limp?

Thornton suffered "frozen feet", possibly what we would consider frostbite though not defined as such.

When John Thornton froze his feet in the previous December his partners had made him comfortable and left him to get well, going on themselves up the river to get out a raft of saw-logs for Dawson. He was still limping slightly at the time he rescued Buck, but with the continued warm weather even the slight limp left him.