[An experimental study on internal structures in the Japanese dentulous mandible].

Measurement of the human substantia compacta and spongy bone. For the sake of a fuller understanding of the structure of the human mandible, a study was made to clarify objectively the thickness of the substantia compacta, the ratio of spongy bone to the total substantia spongiosa, and the width of the spongy bone. Subjects were 11 human mandibles from individuals ranging in age between 35 and 64. All teeth from the central incisor through the second molar remained in place. Measurement was performed with a personal-computer-based image-processing system. Results 1. Width of the substantia compacta Measurement of the width of the substantia compacta showed the thickest part (3.32-3.35 mm) to be roughly at the center of the lingual side of the incisors. The second thickest (2.85-2.90 mm) was located at the buccal-alveolar part of the molars. The thinnest (1.26-1.27 mm) part was located at the labial-alveolar part of the incisors. The next thinnest part (1.74-1.75 mm) was located at the central lingual portion of molars. In the mandible base, thicknesses at all locations fell in the range of 2.76-2.98 mm, with little location-related variation observed. In the labial(buccal)alveolar region, the substantia compacta grows thicker from the mental region toward the molars. In the central lingual region, it grows thinner from the mental region toward the molars. 2. Ratio of spongy bone to total substantia spongiosa Measurements showed the highest ratio (80.9-81.4%) of spongy bone to total substantia spongiosa to be in the lower lingual side of the incisors. This was followed by the central lingual portion of the mental region (80.36%) and the labial side and central regions of the incisors (69.71-78.36%). The ratio was lowest (29.79%) at the first and second premolars. In general, the ratio was high in the incisor region and low near the premolars. No difference was observed between the buccal and lingual sides of the incisor and premolar regions, although in the molar region, the ratio was relatively higher on the buccal than on the lingual side. 3. Width of the spongy bone within the substantia spongiosa.