Effect of masticatory function on the internal bone structure in the mandible of the growing rat

The influence of changes in the masticatory function on bone mass, bone den sity and cortical thickness was analyzed in different functional units of t he mandible of the growing rat. Young male albino rats were fed either a ha rd diet or a soft diet for 4 wk. Undecalcified coronal sections were select ed from five mandibular regions and microradiographic images of the specime ns alongside an aluminium stepwedge were obtained. Bone mass and density we re measured at selected sites with a computer-aided image analysis system. Cortical thickness was measured on diagrams. The reduced masticatory functi on in the soft diet group produced different adaptive responses in the test ed regions of the mandible. Some regions showed reduced cortical thickness (underneath the incisor and lateral to the first molar and some locations o f the ramus), a few showed reduced bone density (medial to the first molar and in the ramus), and only one showed both (in the ramus). Reduced bone ma ss was associated mostly with thinner cortical bone rather than lower corti cal bone density. Changes in cortical thickness and changes in bone density may be two different mechanisms for adjusting local mechanical properties in the mandible of the rat. Masticatory muscle function is a determinant fo r the amount and density of cortical and trabecular bone and may possibly i nfluence results of orthodontic tooth movement and its possible relapse.