From this reference,

On July 20, 1993, while cutting down oaks in a Pennsylvania forest, Don Wyman got his leg pinned beneath a fallen tree. No one could hear his yells for help. After digging for more than an hour to try to free his bleeding, shattered leg, he hit stone. He would bleed to death unless he did something drastic.



Wyman made his decision. Using a wrench and the starter cord from his chain saw as a tourniquet, he cut off the flow of blood to his shin. Somehow he had the fortitude to amputate his own leg below the knee with his pocket knife. He crawled to his vehicle and drove to a farmer's home. The farmer got him the help that saved his life.



The story of the climber who amputated his arm was carried in my local paper a few days ago. My first thought was he's one tough dude. My second though was asking why he was climbing alone. Sure, he's a very experienced climber in excellent shape, but going into the back country alone isn't very smart. I've been to Moab, Utah. It's beautiful beyond words but very rugged. It isn't a good place to be by yourself.

Here's a morbid note, a Google search of +amputate +"pocket knife" returns 111 hits and they're not all about the same incident. There are several references to amputating toes and fingers, but arms and legs are much more rare.