There are many reasons for having an amputation of a limb:
- The death of tissue, aka gangrene in the limb, caused by a loss of it's blood supply, usually at the tips and spreading. Also, it can be resulting from the disease atherosclerosis (hardening of the arteries), prolonged exposure to cold, frostbite, or as a complication of diabetes, where a blood clot clogs a major artery.
- Severe infection, either life-threatening (flesh-eating virus?) or has seeped into the bone tissues (osteomyelitis as to be uncontrollable with antibiotics
- Cancer, especially sarcoma of bone. Amputation may offer the only reasonable chance of saving a patient's life.
- Severe Deformity and malfunction. It may be present at birth (congenital) or due to artery or nerve damage, longterm infection, large bedsores (pressure sores), or extreme swelling through the obstruction of lymph nodes (lymphedema).
- Severe injury in which the tissues, arteries, and nerves and nerves are so damaged that there is no chance of saving the limb.
There are 2 forms: Guillotine Amputation, done in an emergency, which requires additional surgery later to make a usable stump. Most of the time it's a normal amputation, which creates a stump to aid in a future prosthesis. Of course they use anesthetic.
Ereneta says to check out http://pdm.medicine.wisc.edu/kampen.htm for a study on field amputations.