One of the major factors that determines pain post amputation (phantom pain) is pre operative pain severity and length of time. If someone has been dealing with an injury for months to years and then opts for amputation then they are almost definitely going to have phantom pain, likely severe. This makes sense considering the theory that chronic pain is due to a combination of neural reinforcement that occurs in the spinal cord and at higher cortical levels. Interestingly, I recently read that children who have limbs amputated during infancy or who are missing limbs at birth can have phantom sensations and pain. That means some of this neural circuitry is present at cortical levels and is much more complex then we had ever realized.