Masticatory muscle function and craniofacial morphology. An experimental study in the growing rat fed a soft diet.

The present series of investigations was performed in order to study the relationship between masticatory function and craniofacial morphology, and to elucidate the mechanism underlying possible functional changes in the craniofacial growth in rats fed a soft diet. The experimental model chosen was comprised of young rats with altered masticatory function, induced by changing the consistency of the diet. Possible influences on the craniofacial growth were studied longitudinally by x-ray cephalometry, and the bone graft reaction in the facial skeletal regions of rats was assessed in sutures and cortical bone by vital stain, micro-radiography and routine histology. The attrition and eruption rate of the rat incisors were measured "clinically", and the effect of an increased attrition was tested cephalometrically, to see whether it influences the craniofacial growth. In order to evaluate the forces developed by the masticatory muscles the active tetanic tension was measured after electrical stimulation. The muscle fibre composition and the fibre size of the masseter and digastric muscles were examined after enzyme histochemical analysis. The craniofacial morphology and the growth pattern of the rats fed a soft diet were found to be changed to a more orthocranial one, while a decreased bone apposition was found in the angle of the mandible and the upper viscerocranium, with an alteration in the cranial sutures. The morphological changes observed were independent of the attrition and eruption rate of the rat incisors but seem to be related to the low masticatory forces developed by the rats fed a soft diet. The alteration in the "biting" force level was possibly due to the masticatory muscles changes in the muscle fibre types and the smaller size of the fibres, caused by the decreased functional demand on the rats fed a soft diet. Thus, it seems that the alteration in the masticatory function caused structural changes in the masticatory muscles, as well as changes in their contractive capacity. These may be considered as the results of prolonged centrally decided functional alteration brought about by changing the consistency of the diet. The reduced functional forces and the changes in the masticatory muscle contraction caused a low functional strain on the bone and less tension on the periosteal membrane. This tension in the periosteal membrane has also been shown to regulate the displacement of bones in the craniofacial region. The change in the masticatory function may have induced changes in the interosseous movements, which can be underlying reason for the obliterative osteogenesis in the internasal suture.(ABSTRACT TRUNCATED AT 400 WORDS)