Ken, do a search here on the forum under the phrase or term: arthritis thumb
Lots of threads will come up. You will learn something.
My personal arthritic thumb problems are at the bottom (base) of my thumb. I believe the condition is called Basal Thumb Arthritis. If I understand it correctly, the cartilage between the base of my long thumb bone ('first metacarpal of the thumb') and the wrist bone (trapezium) is shot, so the bones grind on each other and become inflamed, etc.
I first noticed it about 15 years ago while driving on long trips. I would sometimes position my hand and thumb between the inside of the driver's side door and the steering wheel. At some point, I noticed that if I did that for a long time, something in my hand would start hurting. I finally figured out it was my thumb! [pretty dense, huh?]
A few years later, the bottom of my thumb would hurt when I played a couple of sets of songs on a steel string guitar. I began practicing with a nylon string, student-grade, short-scale classical guitar that I bought almost by accident about 8 years ago. The scale length was 24.75" instead of the more usual 25.5" scale, so there was less tension on the strings. As well the nut width was 1.75" instead of 2", so it was easier to play the Big Chords at the nut. I do almost all my practicing for gigs on the short scale classical. It has helped a lot.
One more thing. I don't practice a lot of repetitive motion licks and if my hand starts hurting, I stop playing. Might just be for a few minutes, but I stop.
I hope this helps, best of luck.