Braces have been around for a very long time. In fact, a simple form of orthodontic appliances has been documented as early as 1000 B.C. The design and components have changed and been improved over the years, but the basic idea has remained largely the same.
The basic idea behind braces is to produce a force on a tooth in a specific, controlled amount and direction to move a tooth (the biology of just how a tooth moves when a force is applied will be discussed later).
The way the body produces tooth movement is quite an amazing and complicated process. It involves many different tissues, cells, and cell signaling substances. When a force is applied to the roots of the teeth through the braces and the wire, cells in the bone and tissues surrounding the root are stimulated to act. Cells in the body called Osteoblasts and Osteoclasts add and remove bone, respectively.
Pressure at the bracket produces pressure and tension (pulling) at the root of the tooth, causing remodeling of bone and tooth movement. This type of force combination is called a couple, which is defined as two equal and opposite forces acting on a mass.
A couple is a necessary way to move teeth since the braces are attached at the crowns of the teeth, and not at their center of resistance. The center of resistance would be the most ideal position from which to apply forces and move teeth. However, the centre of resistance is located at the roots and would therefore be an impossible place to attach a bracket and apply a force.