The ergonomic trackball mice from multiple manufacturers are designed with a few things in mind. The shape is dictated by the natural shape of the hand; the bending points and most comfortable positions. The buttons and track ball are also designed with the shape of a hand as the determining placement factor.
There are many different sizes of ergonomic trackball mice and it is a good idea to let your hand rest on many examples before settling on the one that you like best. In a case like this, ordering online might net you a lower price for something that doesn’t fit your hand as well as one that you might have paid a slightly higher price for and tried before you purchased.
Ergonomically designed trackball mice are designed for comfort, in order to reduce the stress on not only your hand, but your wrist and arm all the way up to your shoulder. The repetitive movements required by a regular mouse can cause stress the full length of your arm, causing premature fatigue and in the long run possibly joint and tissue issues. Purchasing a trackball mouse now might be a little more expensive than a traditional mouse, but it can save you in the long run, making you more viable for longer hours at the computer and saving you the pain and inflammation of irritated joints and tissues caused by prolong use.