I can't give a precise medical reason as I'm not a medic, but can describe my experience.

I would regularly get pain in my wrist due to misaligned bones/joints. The problem is that a regular mouse requires the wrist to bend 10-20 degrees in towards the computer, and that would cause the small wrist bones to misalign. It takes a $50-$70 physio visit to get these bones reset, so I needed a permanent solution. I tried a $70 medical wrist brace to keep my wrist perfectly straight, and that definitely helped but can be frustrating to use and would place additional strain on my shoulder/elbow joints.

When used correctly, the Evoluent mouse keeps my wrist almost perfectly straight and doesn't place any additional strain on other joints. The pain disappeared within 2-3 weeks.

That said, the Evoluent mouse is not perfect by any means. The scroll-wheel is starting to die on mine. I need to have my chair at just the right height, else I get the same problems as in a regular mouse. And my fellow developers struggle to use the mouse because of the sideways grip. There is also precisely *one* way to use the mouse (ie. you need to be right in front of the computer, my mouse cannot be used by left-handers, etc).

Anyway, I'd swear by the mouse. If only because it keeps my wrist straight without needing a brace.

As a software developer, the biggest risk you'll have is just wearing your hands/arms/wrists out. The "best" device of any kind should be the one which minimises the amount of strain.

For the mouse, the first thing is to avoid it as much as possible. Learn your keyboard shortcuts. Reaching for the mouse on a regular basis will only strain things. Placing 20 buttons on the mice will only encourage you to use it, when in reality your hands should be staying put on the keyboard.

The second thing is to get a mouse that places the minimum strain on your arms. I use an Evoluent Vertical Mouse, but I imagine you can find similar (and less extreme?) mice around. I spent a few hundred dollars on physio due to wrist pain, and it didn't go away. I spent $80 on the mouse and the pain evaporated in two weeks. That was the "best" hardware investment I ever made.