Teeth move at a rate of about 1mm per month, when gentle forces of about 30 grams are placed on them. Braces are like a mini-engineering project on the teeth, guiding these forces to align the teeth.
Teeth are like icebergs: only part is visible. Unseen under the gum are the roots of the teeth. Like the iceberg, the root extends nearly twice as long as the visible part called 'the crown'. The root is attached to the jaw bone, but it is not part of it. It is connected with small fibres, and these allow a tooth to move a little in its socket in response to chewing. When a dentist removes a tooth, it is the breaking of these fibres that allows the tooth to come out.
The bone around a tooth gives it support and makes it feel solid. Bone is a living material that is constantly being remodelled. There are tiny cells in bone that constantly form new bone, and others that take it away. The state of our bones are held in balance by these cells: osteoblasts, the builders; and osteoclasts, the removers. Bones remodel as part of a constant ‘general maintenance’, but bones also respond to forces and to pressure. Orthodontics makes use of this property of bone. Placing a force on a tooth transfers this pressure to bone which, if the force is light enough, signals the osteoclast cells to take away bone at the point of pressure with the builder cells - the osteoblasts - laying down new bone on the opposite side. Teeth withstand heavy pressure because big forces temporarily stop the flow of blood to bone, and the cells do not get activated.
As an exercise, note that when you gently press the flat surface of your fingernail with the tip of a pen, it will go white. The light pressure required to do this is the force that blocks the flow of blood in your finger nail. Feeling this gentle force will give you an indication of the lightness of the force required to move teeth. With braces, the magnitude of the force is what starts a response in the bone and the constancy of the force that creates the effect of tooth movement.
Correct force amount in right direction + Adequate space = Tooth movement
The light pressures that get teeth to move are of the order of 20-30 grams per tooth. To move teeth, the orthodontist places a small handle on each, which are called ‘brackets’. These handles are what enable the light forces to be applied and give a grip to the slippery tooth surface, like the handle on the suitcase. The brackets are glued to the tooth surface with a material that causes no damage. Wires attach to the brackets which act as a guide to movement, and the teeth slowly slide on these wires like curtain hooks on a rail.
Braces are often called ‘train-tracks’ because they act as a rail on which the teeth travel. The orthodontist changes these wires at various times during treatment. Starting off with light and very flexible wires, stiffer and more rigid wires are placed as the teeth come close to their final positions. Forces are kept light and gentle throughout.
The forces or light pressures required to move the teeth are produced using stretchy elastic materials or by placing bends into the wires themselves. Orthodontists are an inventive bunch, and all sorts of systems have been invented to generate optimum forces and to speed up tooth movement.
Forces can be applied from any material that is stretchy and has a memory-like an elastic, a spring, or a wire. Even plastic can be stretched in a way that it will apply force to the teeth. This is the principle behind clear aligner systems such as Invisalign. The aligners are made slightly different to the teeth in such a way that when placed, they gently press them into a new position. Each individual aligner can put enough force to move a tooth 0.25mm. It is the elastic properties of the aligner material that produce the movement.
The orthodontist places braces on teeth other than just those which appear out of place. The reason for this needs to be explained, and it is all about controlling forces. Every action has an equal and opposite re-action. You will remember from studying science that forces always act in two directions; this is called Newton's First Law. If you push against a wall, the wall is essentially pushing against you. If we set up a force to move teeth backwards, we need to have a larger number of teeth providing the grip for that force. Essentially, the orthodontist uses a large number of strong back teeth to provide the anchor for the force to the front teeth. Moving teeth with braces is like setting up a mini tug-of-war, where the orthodontist fixes the competition so that one particular side will win.
The reason why it is good to know all this is that understanding how and why teeth move helps treatment to work better. Sometimes the orthodontist will ask a patients to wear small elastic bands from one part of the brace to the other. These are the force in the system, the petrol in the tank, so to speak. So, wearing these as instructed and changing them when they get weak is essential to making proper progress in treatment. Likewise, if a wire on your braces gets bent out of shape, don’t expect the teeth to continue moving in the right direction.
To re-cap..
Light forces get teeth to grow into a new position over time. Attaching brackets to teeth gives the orthodontist a handle to control each tooth; while elastics, springs and wires provide the force and direction to the braces.
