My Spotsie suffered an injury to one of her stifles.  Physical
examination showed damage to ligaments and x-rays showed one of the
tiny bones that anchor the ligament was displaced.  The vet and I
talked about orthopedic surgery - large dogs sometimes get the same
sort of injury and need surgery to prevent lameness - and decided to
wait and see how cage rest worked before doing any operation.  She
recovered function fairly quickly.  The outcome was that the cat still
can jump to the kitchen windowsill.  The explanation is that there is
less strain on the limb in an 8 pound cat than there is in an 80 pound
dog.

Obviously there is a huge difference between a cat and a human, but when I
tore (clean through) my ACL 10 years ago, the doctor actually gave me the
option of not having surgery.  He said if I wasn't going to be a very active
person, then I could actually live a normal life without going through it as
the main reason the ACL is there is to keep the shin bone in the correct
place next to the knee cap.  If you aren't going to be running hard and
stopping, then you would be alright.  I have no idea how the difference
would be for a cat because of the difference in leg structures, and there is
no way to tell a cat not to run and stop hard, but its still another 2 cents
to throw in : )