Posttraumatic condylar hyperplasia.

Posttraumatic condylar hyperplasia can result in complex facial asymmetry composed of degrees of condylar and ramus overgrowth, malocclusion, and complementary maxillary deformity. Three patients with unilateral condylar hyperplasia are described to illustrate the scope of surgical and orthodontic treatment required to restore facial symmetry. Condylar hyperplasia with facial asymmetry should be recognized as a possible consequence of condylar injury, and further delineation of the onset and natural history of this deformity could lead to investigations or intervention before the establishment of all the components of the deformity.

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 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.