The limitations of orthodontic treatment

Abstract 1. 1. This report is based on a large number of cases, some of which have been treated actively, and some of which have been controlled only with a preventive lingual arch for holding arch length. Most have been followed a sufficient number of years after retainers were discontinued to determine definitely whether or not the final outcome was stable. Both the stable cases and those which relapsed have been carefully analyzed by means of measurements which are herein described. 2. 2. These measurements lead me to conclude that the length of the dental arch, from the mesial of one mandibular first permanent molar to the mesial of the corresponding tooth on the opposite side, is always shortened in the transition from the mixed to the permanent dentition. Furthermore, this arch length cannot be permanently increased in mixed dentition cases through orthodontic treatment, but, on the other hand, it will decrease. The only exceptions to this generalization are cases which show an abnormally marked lingual axial inclination of mandibular incisors, ∗ or cases which have sustained the premature loss of deciduous canines or deciduous second molars. These exceptions are included only in the interests of accuracy and do not involve cases in which any marked increase in this distance may be expected to be maintained. 3. 3. These measurements lead me to conclude that the distance from the mesiolingual of the mandibular first permanent molar to the mandibular midline likewise always shortens in the transition from the mixed to the permanent dentition, and that this distance cannot be permanently increased through orthodontic treatment in the mixed dentition, allowing again for the same exceptions. 4. 4. It is possible, using the measurements described, to estimate accurately the prognosis of most mixed dentition cases, and to determine whether or not extraction will be necessary in permanent dentition treatment. 5. 5. Active treatment in the mixed dentition period is desirable only in Class III cases, cross-bites, and Class II, Division 1 cases wherein facial appearance is markedly affected. 6. 6. The treatment of the above cases should follow the principles discussed in this paper; that is, it must be recognized that the amount of space between mandibular first permanent molars cannot be permanently increased. Since this distance is always shortened in the transition from the mixed to the permanent dentition, the accommodation of all the teeth in the mixed dentition period in nonrotated positions constitutes overtreatment through overexpansion. The treatment of the mandibular arch in Class II, Division 1 cases requires the extraction of the deciduous canines in order to position the mandibular incisors over basal bone, without resorting to overexpansion. 7. 7. The preventive lingual arch is useful only during a short interval of time; namely, from just prior to the exfoliation of the second deciduous molars until such time as the mandibular canines and premolars have fully erupted. Such use of the preventive lingual arch is indicated only in those cases which are given a favorable prognosis by the measurements herein described.