Effects of masticatory muscle function and bite-raising on mandibular morphology in the growing rat.

The aim of this series of investigations was to study the effect of masticatory muscle function on the growth pattern and on the internal structure of the mandible during growth. The muscular and dentoskeletal growth adaptation to prolonged bite-raising and the role of the functional state of the masticatory muscles in this adaptation were also to be elucidated. Differences in masticatory muscle function were induced in young rats by altering the consistency of the diet. Bite-raising was produced by the insertion of posterior bite blocks. Morphometric analysis of the internal bone structures of the mandible was performed on microradiographs, and videodensitometric analysis was performed on lateral radiographs and microradiographs. The effect of muscle function and bite-raising on mandibular growth was studied on a series of lateral cephalograms, superimposed on bone markers. Muscle belly, sarcomere and aponeurosis length adaptation to bite-raising was studied in situ with a digital caliper and under a microscope after fluorescent vital staining of the deep masseter muscle. A soft diet altered the pattern of growth of the mandible and reduced bone growth in the angular region. Transversal dimensions and cross-sectional area of the dentoalveolar process were smaller. Bone mass in areas possibly subjected to direct loads or bending forces was smaller. This was due to either less trabecular bone or thinner cortical bone. Only a few sites showed lower bone density. Posterior bite-blocks affected the size of the mandible as well as its growth pattern, and intruded lower molars. The soft diet influenced the effect of bite-blocks and caused less intrusion of upper molars and less inhibition of bone growth at the angular process. The deep masseter muscle adapted to bite-raising by elongation of the aponeurosis, but less in rats on a soft diet. Changes in masticatory muscle function affected the growth of the mandible in both the sagittal and transversal plane. Reduced loads on molars and condyle and smaller bending forces in other regions of the mandible possibly reduced the levels of stimulation of the osteocyte network and osteoblasts, thus inducing less trabecular bone and cortical bone formation in specific areas. In rats fed the soft diet, smaller increase in bone density represented an adaptation process in areas characterised by a lower bone apposition rate. The forces produced by the passive stretching of the masseter muscle affected the skeletal growth pattern and dental eruption. Weaker forces possibly produced by passive stretching of hypofunctional muscles resulted in more eruption of the upper molars and less inhibition of periosteal bone apposition in the angular region. Length adaptation in the masseter muscle through lengthening of the aponeurosis and dentofacial growth adaptation possibly decreased passive forces applied to teeth and skeletal structures, particularly in rats with higher functional demands. This may have caused a gradually decreasing effect of the appliance.