I missed the original post so this may have been suggested so if I'm
repeating information I apologize.  My cat is 19 and began showing signs
of arthritis about 2 years ago.  I bought him a large heating pad bed,
its 2 feet by 4 feet.  I keep it on low in the evening when I am home.
It works really well, he loves it and he seems to have much less
stiffness and pain (I measure this by how he walks when he rises and how
active he is) and the creaking noise in his joints have completely
disappeared.  He's even jumping on things since about the last year or
so.  I think he likes the size of it because he can stretch out and feel
the heat all over his body.  It's a foam square covers with vinyl on the
side resting on the floor and thick fleece on the side he rests on.  To
wash it I simply remove the foam and toss it in the washer.

It might be something you can consider for your cat.

Trish