I have arthritis in my thumb root joints. It sometimes bothers me so much that I have been refraining from using my cameras. I finally decided that if I am going to continue with photography that I need to see how I can minimize the problems with my thumbs.
After trying various ways of holding my cameras I have decided that I just need to stop using my thumbs anymore than absolutely necessary when holding and carrying my cameras. These images show my best option so far.
The number one solution is to let the camera and lens rest on my hand and not use any thumb grip on the camera backs. Not having a rear thumb or front grip actually helps. In the first case I show my grip using the X100F camera. It has no grips and it has good balance. The camera weight actually helps me to hold it still without having a tight thumb grip with either hand. My thumbs just act as minimal guides while the camera weight rests on my left hand.
The second image shows how I hold the E-M5 III camera with the 14-150mm zoom lens. In order to balance the weight I hold the combined weight of the camera and lens by supporting them under the lens.
It is far better to use this grip with a pancake prime lenses since zooming a longer lens does require the use of the thumb when turning the zoom ring. A camera-lens combination with a prime pancake lens is also far easier to balance. In both cases I need to mostly use a neck strap just to make sure that I don’t drop the camera.
This particular way of carrying and holding a camera necessitates that the overall weight of the camera plus lens not be too heavy. That rules out larger heavier cameras with bigger sensors, heavier longer zoom lenses, etc. For example no full frame sensor cameras with long lenses.