Neuromuscular and skeletal adaptations to altered function in the orofacial region

Abstract

The purpose of this study was to investigate the nature of intrinsic musculo-skeletal adaptations resulting from experimental alteration of the orofacial environment. A new occlusal configuration was devised which modified oral sensory stimuli, subsequently prompting anterior positioning of the mandible of rhesus monkeys ( Macaca mulatta) during functional jaw movements at four defined stages of maturation. Specific skeletal, dental, and neuromuscular adaptations were studied and interrelated by means of serial electromyography, serial cephalometric radiography with metallic implants, and microscopic analysis. The study was divided into two time periods. During the 13-week control period, normal growth data were gathered from the four age groups. During the 13-week experimental period, specific neuromuscular and skeletal alterations caused by the experimental conditions were identified. Postural activity in the muscles of mastication and presumably the postural position of the mandible were altered by the induced changes in the oral environment. The superior head of the lateral pterygoid gradually increased in activity, first during functional movements and then during the maintenance of mandibular postural position. The superior head appeared to function as a principal forward positioner of the mandible. This activity decreased or disappeared by the end of the experiment. At the end of the experimental period, ten of the twelve experimental monkeys demonstrated an anteroposterior alteration in molar relationship. No single adaptive process could be isolated and identified as the sole cause of any effective alteration in maxillomandibular relationship at any age level. Rather, each resulting relationship was the composite of specific complementary (and occasionally antagonistic) adaptations throughout the craniofacial complex. Mandibular skeletal adaptations occurred primarily in the infant and juvenile animals in which the extent and direction of growth at the condyle were altered. Dentitional adjustments in the mandible were most notable in the adolescent and adult animals. In the nasomaxillary area a decrease in the vertical displacement of the maxillary complex was noted in all but three of the experimental animals. Horizontal displacement of the maxillary complex and the drift pattern of the maxillary dentition were also affected, but the expression of this effect was variable. After 13 weeks, little histologic evidence of physiologic or pathologic responses to the induced protrusive function was evident in the sacrificed animals. These histologic findings were consistent with past studies which suggest that the adult temporomandibular joint was stable and resistant while the growing articulation was responsive to functional changes. The results of this study further indicated that a chronologic correlation existed between the occurrence and disappearance of altered neuromuscular function and the re-establishment of skeletal balance. As skeletal balance was restored through specific structural adaptations, the need for compensatory muscle function was reduced. Further, the nature and extent of the specific skeletal and dental adaptations depended upon the level of maturation of the animal.