It is well-known that the bone shape and its internal structure will be affected by the functional activity of associated muscles. As to the masticatory organ, many researchers have suggested that masticatory muscles, especially the masseter muscle and lateral and medial pterygoid muscles affects the formation of the dentofacial skeleton. This study was performed to verify the hypothesis by means of morphometric analyses using CT scanned images,ultrasonography and roentgenograph cephalometry.

Sublects and method: The subjects of this study were 61 adult volunteers. Their mean age was 24years for male, and 23 years for female. All the subjects participated after giving their informed consent. CT image in each subject was scanned from the region of mandibular condyle to mandibular border according at the interval of 2 to 5 mm. The masseter superficial muscle thickness was observed at the middle region by ultrasonograph, using a 7.5MHz scanning probe.24 evaluation items were measured for analysis of dentofacial pattern using lateral roentgenographic cephalogram.

Results: Mean values of all measurements for evaluation items showed light sexual differences except for the case of masseter muscle thickness.

According to correlation coefficients analysis, volume of the medial pterygoid muscle was observed to be correlate with mandibular sizes, such as the mandibular length, mandibular body length and mandibular ramus height. This muscle also showed to have a high level correlation coefficient with the distance between mandibular backside and coronoid process or zygomatic bone that represent the facial depth.

Otherwise, the masseter muscle thickness showed a correlation with the mandibular ramus height as well as molar height, and also relate to both the inclination and the length of masseter muscle that represent the axis and distance between zygomatic processes and point gonion, respectively.

Thus, it seems true that masticatory muscles affect the development and formation of the mandibular posterior area, where is the attachment region of these muscles, via the stress of muscle activity.