Surely, I realized that to stop RSI you need to tackle chair, posture, desk, keyboard, etc. So, keep that in mind when reading my review. So, lets talk about the mouse. I tried several vertical mouses, I tried ergonomic mouses and even some gaming mouses, the pain was the same and with some of them even worst.
https://www.amazon.com/Kensington-Expert-Wireless-Trackball-K72359WW/dp/B01936N73I

