It wouldn't surprise me if it was due to his torture at the "Hanoi Hilton." IIRC, both of his arms were broken, and they used that as part of the torture.
The North Vietnamese were absolutely brutal and evil. But it wasn't anywhere near as bad as waterboarding by the CIA, or the conditions at Gitmo!
Mark
You don't recall correctly. McCain was injured in the crash of his plane. He was treated at a NV hospital usually reserved for treating NV officers. In McCain's own words:
"Immediately the interrogations began and when I refused to answer their questions they would knock me into unconsciousness. Finally after four days of this I agreed to give information if they would take me to a hospital. The doctor, after taking my pulse said, "It's too late." They left the room. Sometime later, the chief interrogator again returned shouting "Your father a big admiral, now we take you to hospital."
As I recall they didn't directly torture him - more a case of serious neglect early on. He was injured from the plane crash, and beaten by villagers. After he was taken captive, his injuries were left to heal without treatment until the North Vietnamese figures out who he was...at which point they treated his injuries and fed him well enough for him to gain weight, hoping to use him as a bargaining chip. It was a big deal for them, and they even built a propaganda monument to those who captured him.