The effect of dietary consistency on bone mass and turnover in the growing rat mandible.

Hard and soft diets were fed to weanling rats for up to 8 weeks. Some animals were switched after 4 weeks to the opposite diet. A histomorphometric study of bone formation activity at the mandibular ramus, body, and condyle was made after in vivo fluorochrome labelling. Mineral apposition rates at the lateral and inferior periosteal surfaces of the ramus were lower in the soft diet than in the hard diet animals. The rate of bone formation at the lateral periosteal surface of the ramus was significantly lower in soft than in hard diet animals. The medial periosteal surface of the ramus sometimes changed to bone formation in the soft diet groups. Condylar cartilage zones were somewhat thinner in soft diet groups. In the mandibular body, differences due to dietary consistency were less marked than near the gonial angle. Adaptation of periosteal bone and condylar cartilage to a new dietary consistency occurred within 4 weeks of switching. These results suggest that lateral and inferior periosteal bone growth of the ramus and condylar elongation were slowed in rats consuming soft diets. Decreased functional force during rapid mandibular bone growth causes changes in shape. The changes are due to regional decreases in osteoblast function, realignment of bone formation surfaces in the ramus area, and slowed growth in the condylar cartilage.