How Teeth Move
Teeth are held in the jawbones by a sling-like attachment called the periodontal ligament (PDL), which forms a very thin lining between the roots and the bone. The fibrous attachments comprising the PDL become squeezed in one area and stretched in another when a force is placed on a tooth. At the microscopic level, PDL compression stimulates cells called osteoclasts to remove bone while opposite PDL tension causes osteoblast cells to lay down bone.
This remodeling of bone is what allows teeth to move through bone. Any force above a certain threshold and duration can affect the position of teeth. These forces can be intentional ones produced by your orthodontist’s adjustments to appliances worn on your teeth or they can be natural ones produced by structures such as your tongue and lips as they function during such activities as swallowing and resting against your teeth.
The fact that teeth move due to applied and natural forces is what makes orthodontics possible, but it is also what makes retention appliances necessary. While your orthodontic forces will cease one day, natural forces will be constant yet variable throughout your lifetime. Because Mother Nature does not consistently balance forces to keep your teeth in the healthy and aesthetic relationships which orthodontic forces achieve, appliances worn periodically to offset compromising natural forces are necessary. These appliances are your retainers. (See retainer types below.)
Sequence of Steps
On a mechanical level, there are several steps, which we take successively to straighten teeth and correct the bite:
- Align – A first step is to resolve all crowding between teeth by pulling the center of each tooth towards a gradually stronger archwire to remove all rotations and overlaps between teeth.
- Level – After or shortly into aligning, the teeth in each arch need to be pushed up or down in the jawbone so that the tops of all the teeth are touching on one flat plane. A level arch can best be illustrated by imagining laying a stiff piece of cardboard on top of all the teeth in one jawbone. If all the teeth cusps and or top edges of the teeth touch this cardboard without it curving or bending, then the teeth positions are producing a level arch.
- Coordinate the bite – This step refers to the need to position the upper teeth correctly over the lower teeth and vice versa. All lower teeth should fit within the confines of the upper arch of teeth. Also, all lower side and back teeth should be ½ a tooth more forward towards the midline of the face than their corresponding upper teeth in order to produce the proper mesh or intercuspation which is so important for optimal health, proper function, and protection of the teeth. For this correct positioning to occur, all spaces need to be closed in the correct direction and many times rubber bands called elastics will need to be worn to pull upper arches of teeth forward or backward in relation to lower arches of teeth, or to pull upper teeth vertically closer to the lower teeth to enhance the proper contacts. This stage of treatment is one that can be heavily dependent on good cooperation from the patient in terms of wearing elastics or other appliances affecting this coordination.
- Detail – Perfecting individual teeth positions. Certain teeth will need slight improvements in any one or in several of the three dimensions that determine their position. One efficient and effective way to achieve these changes is to place bends in the archwire near the teeth that are to be moved. The wire will lay differently in the brace attached to that tooth and produce the movement. Sometimes reaffixing some braces in slightly different orientations on the teeth is performed to achieve desired movements. Another way to hasten this detailing is to move to a positioner appliance when treatment is this close to the end. By removing the archwire that connects the teeth, prior to using the positioner, each tooth can move more freely and independently towards its perfected position that is captured in the positioner. Precise lab work on the patient’s teeth models that were generated to make the positioner places the sectioned stone replicas of each tooth in their exact positions for the ideal result. The stretchy, clear, mouthguard-like positioner is then made over that perfected lab model of the teeth and bite. As a patient bites into his/her customized positioner, the teeth are gently pushed into their final positions. (See positioner under “removable appliances” for more information.)