Response to activator treatment in Class II malocclusions.

A clinical study was designed to disclose the effects of activator treatment in the correction of Class II malocclusions. The rationale for the use of the activator appliance was based on the premise that correction of distocclusion can be achieved by inhibition of forward growth of the maxilla, inhibition of mesial migration of maxillary teeth, inhibition of maxillary alveolar height increase and extrusion of mandibular molars, increased growth of the mandible, anterior relocation of the glenoid fossa, mesial movement of mandibular teeth, and combinations of these effects. The appliance, as designed for this study, could potentially have an effect on all of these factors. Measurements were obtained from cephalometric head films obtained at 6-month intervals. Matched-pairs analyses of control versus treatment change after 6 months and after 1 year of treatment were done on 36 and 29 pairs, respectively. Pretreatment versus treatment changes were analyzed on 33 subjects by means of the spline regression analysis; posttreatment versus treatment changes were analyzed on 18 subjects by means of the Student Newman-Keuls multiple comparison test. The matched-pairs analyses of mean values demonstrated significant reduction in forward growth of the maxilla, uprighting of the maxillary incisors, reduced overjet, leveling of the mandibular occlusal plane, improved molar relationship, downward and forward relocation of the glenoid fossae, increased advancement of all mandibular structures, increased face profile angle, and increased lower face height. The two longitudinal analyses yielded similar findings, but some differences were noted. Because rather severe dental malocclusions were corrected, the slight average inhibition of maxillary growth and the anterior relocation of glenoid fossae alone could not account for the correction of the Class II dental arch relationship. It was therefore concluded that, in addition to the statistically significant changes, smaller changes occurred in several areas without being consistent enough or of a large enough magnitude to become statistically significant in the analyses of mean values. Comparison of group averages may mask treatment effects that significantly contribute to the correction of malocclusions in individual cases.

Morphologic response to changes in neuromuscular patterns experimentally induced by altered modes of respiration.

The present experiment was designed to test whether specific recordable changes in the neuromuscular system could be associated with specific alterations in soft- and hard-tissue morphology in the craniofacial region. The effect of experimentally induced neuromuscular changes on the craniofacial skeleton and dentition of eight rhesus monkeys was studied. The neuromuscular changes were triggered by complete nasal airway obstruction and the need for an oral airway. Alterations were also triggered 2 years later by removal of the obstruction and the return to nasal breathing. Changes in neuromuscular recruitment patterns resulted in changed function and posture of the mandible, tongue, and upper lip. There was considerable variation among the animals. Statistically significant morphologic effects of the induced changes were documented in several of the measured variables after the 2-year experimental period. The anterior face height increased more in the experimental animals than in the control animals; the occlusal and mandibular plane angles measured to the sella-nasion line increased; and anterior crossbites and malposition of teeth occurred. During the postexperimental period some of these changes were reversed. Alterations in soft-tissue morphology were also observed during both experimental periods. There was considerable variation in morphologic response among the animals. It was concluded that the marked individual variations in skeletal morphology and dentition resulting from the procedures were due to the variation in nature and degree of neuromuscular and soft-tissue adaptations in response to the altered function. The recorded neuromuscular recruitment patterns could not be directly related to specific changes in morphology.

Primate experiments in malocclusion and bone induction.

It has been recognized for many years that muscle function influences bone formation and morphology. It is hypothesized here that the movement of bone into new positions within a muscle system results in rearrangement of the stress distribution and reorganization of shape and internal structure. To test the above hypothesis, clinical and animal experiments involving the use of posterior occlusal bite blocks, Herbst appliances, and temporal and masseter muscle stimulation were undertaken. Chronic or continuous alteration in mandibular position within the neuromuscular environment with the posterior occlusal bite block and the Herbst appliance in a sample of monkeys produced extensive condylar remodeling and change in mandibular size. Short periods of induced hyperactivity in the masseter and temporal muscles were associated with the production of malocclusion symptoms. Excess induced temporal muscle activity of specific frequency and intensity may interfere with the remodeling of bone grafts placed under the temporal muscle, while a lesser level of activity was associated with bone remodeling.

Primate experiments on mandibular growth direction.

Induced oral respiration in eight growing monkeys led to a lowering of the chin, a steeper mandibular plane angle, and an increase in the gonial angle as compared with eight control animals. The posterior border of the mandible maintained its normal inclination relative to the skull. The mandible underwent a posterior rotation in the experimental animals and an anterior rotation in the controls. The findings suggest that the position of the chin and the inclination of the mandibular plane are controlled by the balance between the suprahyoid and the orofacial muscles. Since the posterior border of the ramus undergoes gradual remodeling and maintains its normal inclination, it is assumed that the masticatory muscles are not significantly affected by the altered respiratory function. It is therefore concluded that the ramus with the masticatory muscles and the chin with its suprahyoid and orofacial muscles represent two relatively independent systems.

Primate experiments on oral respiration.

Oral respiration associated with obstruction of the nasal airway is a common finding among patients seeking orthodontic treatment. The primate experiments reported here are part of a series designed to test some of the current hypotheses regarding the relationship between mouth breathing and dental malocclusions, that is, between deviations in orofacial muscle recruitment and jaw morphogenesis. Mouth-breathing was developed in the animals of this experiment by obstruction of the nasal passages with silicon nose plugs. The experiments showed that the monkeys adapted to nasal obstruction in different ways. In general, the experimental animals maintained an open mouth. Some increased the oral airway rhythmically, while others maintained the mandible in a lower position with or without protruding the tongue. All experimental animals gradually acquired a facial appearance and dental occlusion different from those of the control animals. From these and the previously reported primate experiments in this laboratory, it can be deduced that orthodontic appliances in general affect the morphology of the orofacial structure in two ways: by direct force and by sensory stimulation. (1) The appliance exerts a direct physical force which alters the strain distribution in the bone and elicits bone remodeling and tooth movement. (2) The presence of the appliance initiates the sensory input which triggers a neuromuscular response. This change in neuromuscular activity, in turn, affects both muscle development and bone remodeling. The fixed orthodontic appliance may work mainly on the first principle. Certain removable appliances may have a significant effect based on the second principle.

Research on control of craniofacial morphogenesis: an NIDR State-of-the-Art Workshop.

To assess the wide clinical ramifications of control of craniofacial morphogenesis, a State-of-the-Art Workshop was conducted by the National Institute of Dental Research at the initiative of Richard L. Christiansen, Chief of the Craniofacial Anomalies Program. In conjunction with the authors listed above, the format for the workshop was developed and participants were selected. The workshop was designed to provide an in-depth review of present knowledge and to identify future goals and directions for research on guiding, altering, and thus controlling growth and development of the cranofacial skeleton. The agenda for discussion ranged from molecular biology to clinical arts such as orthopedics and surgery. It was evident during the workshop that the mechanisms and procedures for controlling craniofacial morphogenesis must be derived from many biologic, physical, and clinical fields of knowledge. It is hoped that there will evolve an interdisciplinary clinical art which is aimed at preventing and correcting craniofacial deformities. Substantial biologic information has already been accumulated on the craniofacial skeleton. The clinical art of correcting malocclusion through mechanical forces is now applicable to the entire skull. The outstanding technical accomplishments of radical surgery in the correction of congenital craniofacial anomalies show that the needed surgical skills are now available. When these resources are combined, an area of knowledge and a clinical discipline which might be called "orthocephalics" is already identifiable. The workshop was held at the National Institutes of Health in Bethesda, Maryland, on Feb. 12 and 13, 1974. The ideas exchanged were integrated and summarized by the planning committee to produce this report.