First of all, it will help you to learn a bit about cat paw anatomy. This page provides excellent details, plus diagrams. The most important thing to understand from that article is: “Removal of the last digits of the toes drastically alters the conformation of their feet and causes the feet to meet the ground at an unnatural angle that can cause back pain similar to that in humans caused by wearing improper shoes,” and “A graphic comparison in human terms would be the cutting off of a person’s finger at the last joint of each finger.”
As far as the bit you are specifically asking about, this article states, “The rate of complication is relatively high compared with other so-called routine procedures. Complications of this amputation can be excruciating pain, damage to the radial nerve, hemorrhage, bone chips that prevent healing, painful regrowth of deformed claw inside of the paw which is not visible to the eye, and chronic back and joint pain as shoulder, leg and back muscles weaken.
Other complications include postoperative hemorrhage, either immediate or following bandage removal is a fairly frequent occurrence, paw ischemia, lameness due to wound infection or footpad laceration, exposure necrosis of the second phalanx, and abscess associated with retention of portions of the third phalanx. Abscess due to regrowth must be treated by surgical removal of the remnant of the third phalanx and wound debridement. During amputation of the distal phalanx, the bone may shatter and cause what is called a sequestrum, which serves as a focus for infection, causing continuous drainage from the toe. This necessitates a second anesthesia and surgery. Abnormal growth of severed nerve ends can also occur, causing long-term, painful sensations in the toes. Infection will occasionally occur when all precautions have been taken.”
From my own experience, my cat was permanently crippled. She was declawed at my parents’ behest; I was 10 at the time. She “chipmunked” for about a year after the procedure – preferred to sit on her haunches and not let her front feet touch the ground when sitting. She would whine to be picked up and carried places rather than jump or walk. Her personality changed, now she will always walk with a limp, and have pain when jumping. Look at this photo of her paw and compare it with this photo of my other, non-declawed cat’s paw.
So basically, it’s torture for these animals, and it’s done for no more than our convenience. Buy softpaws instead.