I use an Microsoft Intellimouse Explorer. It is a fairly large size and I find that a relaxed hand fits over it very comfortably i.e. no gripping or distortion of the hand is necessary to move it or to operate any of its functions. There are also thumb buttons on it to double up some of the finger buttons if necessary. Note the thumb buttons are no use for lefthanded operation.
I've used one for ages now and it would take a whole new radical redesign to make me change.
My arthritis is not too severe but I can easily imagine that it would be highly suitable for quite advanced joint stiffening.