I'd recommend a Track Ball. There are two basic types, one with the
trackball on the side under the thumb, so you click the buttons w/ fingers
(just like a mouse), and another with the ball under the fingertips, so
you use the thumb for the primary click and wheel (you get used to it,
really! and fingers-on-the-ball does give finer control). Some units have
a scroll wheel while others omit it - if you are already used to having a
scroll wheel then by all means get one that has it, you'll really miss it
otherwise.
The main advantage to the trackball is that your arm and wrist stays still
and straight, just resting - all movement is in the digits. This can help
w/ Carpal Tunnel, and also w/ elbow and shoulder issues. Use it w/ a
wrist rest to get comfortable alignment of the arm.
MS had two trackballs, both now discontinued but you may find 'em if you
search around (they are getting scarcer by the day). Logitec has several,
some discontinued models and some new ones; most are still available. You
should try before you buy if you can, and understand that it will take a
day or so to get used to, but I'd predict once you do you'll love it and
will never go back to a regular mouse again.
FWIW I love my MS Trackball Explorer 5-button model. It's a shame they
don't make it anymore.