This is from the link in my signature:
Contrary to most people's understanding, declawing consists of amputating not just the claws, but the whole phalanx (up to the joint), including bones, ligaments, and tendons! To remove the claw, the bone, nerve, joint capsule, collateral ligaments, and the extensor and flexor tendons must all be amputated. Thus declawing is not a “simple”, single surgery but 10 separate, painful amputations of the third phalanx up to the last joint of each toe.
From me (but my knowledge comes from sites like this one): Cats walk on their toes. When the toes are amputated their entire balance and gait and ability to stretch are affected for the remainder of their lives. Since cats instinctively hide pain, it is my opinion that most cats who have been declawed suffer pain, back pain and probably toe pain, their entire lives.
They no longer can walk properly which puts stress on muscles and joints and bone that were not designed to have that kind of stress. They can no longer stretch properly, so they can't ever EVER get relief.
Every time I hear of another cat who has had this evil done to him or her, I break down in tears.
And don't be lulled into thinking the laser procedure is any better. The method is different, the results are the same: life time of crippling.
Statistics on post amputation problems, both health and behavior, are skewed, because most people who have had it done, and feel justified, and intend to do it again, will never admit there were any problems with their cat. People who say their cats are "fine" after a declaw just don't want to see. People who are honest with themselves and regret doing it, are quick to admit it, and to detail issues they may have had to deal with.
Often vets are greatly to blame. Vets, eager for the fee, offer spay/neuter/declaw packages to new kitten owners. And the vets who willingly do it to adult cats are just.....criminal in my opinion. This type of vet does not give full disclosure about what it entails. They call it lightly "removal of the claws" making it sound so easy and pretty.
I have more to say on this but am out of time at the moment.