Mandibular Arch Length Research Articles

Serial extraction in Class I mixed-dentition cases

I n a case of Class I malocclusion in the mixed dentition, the question often is whether the correction can be achieved with a full complement of permanent teeth or by extraction of four premolars. Naneel believed that the problem could be solved on the basis of the difference between the mesiodistal widths of deciduous canines and molars and the mesiodistal widths of unerupted permanent canines and premolars in the mandibular arch. According to whether this difference, called “leeway,” is found to be favorable, slightly unfavorable, or decidedly unfavorable, Nance and Carey2 suggested a few therapeutic rules which are followed by most American orthodontists. Although of basic importance, leeway is not the only element on which correction of crowding in the mixed dentition is dependent. Before considering and evaluating these various other elements, I would like to stress that it is rarely possible to increase mandibular arch length. Nance demonstrated that arch length mesial to the first permanent molars actually decreases in the mandible between the mixed dentition and the permanent dentition. This final reduction in length also takes place when the original length has been increased by orthodontic treatment. He further observed that the maximum permanent increase in lower intercanine distance which can be achieved by enlargement of the arch in the mixed-dentition period never exceeds 2.6 mm. This limit was substantially confirmed in a recent infestigation by Lucchese and Alleva.7, 8 Extensive espansion of the mandibular arch is thus quite ineffective. Treatment of a crowded Class I malocclusion depends on many elements. Among them are space reduction for the mandibular incisors and the values of the following elements: (1) changes in lower intercanine distance, either nat-

Alveolar ridge resorption following tooth extraction

F ollowing tooth extraction, the fundic part of the bony socket fills in with connective tissue and bone, while the alveolar crest resorbs. After the wound heals and is covered with epithelium, the edentulous ridge remains.l-lo An important question is whether the center of the resulting residual ridge lies over the center of the original dentulous ridge or has shifted towards the original lingual or labial plate. This question is important to the prosthodontist who asks whether the artificial tooth should be placed over the center of the edentulous ridge or whether the artificial tooth should be placed more to the buccal or the lingual side of it in order to occupy more accurately the position of the natural tooth. On the basis of observations of dry skull specimens, Cryer’l stated that the edentulous lower jaw lies external to and is wider than the upper jaw after all teeth have been extracted. Rogers and Applebaum12 concluded from measurements made in cadavers with dentulous and edentulous jaws that in the maxillae the vertical height of the ridges had decreased, and the crest of the edentulous ridge had shifted palatally after tooth extraction. They felt that in the mandible the most extensive resorption of alveolar bone occurred on the superior surface of the ridge and on the lingual surface of at least the posterior part of the ridge. Thus they suggested that resorption of the labial plate in the maxillae results in a loss of edentulous arch length and width, while, in the mandible, resorption of the lingual plate increases the mandibular arch length and width so that the edentulous maxillary arch comes to lie within the edentulous mandibular arch. Tylman and Tylman13 wrote that, in the maxillae, the labial and buccal alveolar plates resorb much faster than the palatal plates, while, in the mandible, the amounts of bone resorbed at the lingual and labial plates are approximately the same. Swenson14 stated that after tooth extraction the alveolar process of the