On the side where tension is applied, meanwhile, new bone builds. This alternation of resorption and deposit is the basic orthodontic process. Frequently, the necessary anchorage is found in the opposing jaw. If the upper teeth protrude, elastic bands are attached to link the front part of the appliance in the upper jaw with the back part of a similar appliance in the lower jaw. The upper teeth are pulled back and the lower one tends to come forward.
Serial extraction is a preventive measure used in cases of severe overcrowding of the teeth.
Selected primary teeth are extracted at intervals to allow the permanent teeth to emerge in their proper places in the dental arch. This procedure is usually supplemented with mechanical orthodontic treatment. After even the most successful orthodontic treatment, relapse may occur as the teeth return to their original position. Consequently, the retention phase of orthodontic treatment is as important as the movement phase. Various devices forestall relapse.