Shingo Mori

Threshold for Bone Resorption Induced by Continuous and Intermittent Pressure in the Rat Hard Palate

Osteoclastic bone resorption under the denture base has been considered to be influenced by mechanical force exerted through the denture base. However, no scientific evidence has been shown concerning whether osteoclastic bone resorption is a pressure-threshold-regulated phenomenon or merely a proportionally pressure-dependent phenomenon. In this study, a clear statistical criterion as an objective standard for the evaluation of histopathological bone resorption was established, and the relationship between the amount of continuous or intermittent compressive pressure exerted on the rat hard palate and induced osteoclastic bone resorption was studied based on this criterion. Eighty-five male rats of the Wistar strain, 15 weeks of age, were fitted with experimental dentures which were designed to exert a defined amount of continuous or intermittent compressive pressure on the intermolar region of the hard palate. No bone resorption was observed when continuous compressive pressure was << 1.96 kPa or when intermittent compressive pressure was << 9.8 kPa. In contrast, continuous compressive pressure ≧ 6.86 kPa or intermittent compressive pressure ≧ 19.6 kPa caused significant bone resorption in all rats studied. From these results, it was revealed that osteoclastic bone resorption under the denture base was a pressure-threshold-regulated phenomenon, and that the thresholds exist for the induction of bone resorption under the denture base by both continuous and intermittent compressive pressure, respectively. It was also clearly revealed from the results that the threshold for continuous compressive pressure is lower than that for intermittent compressive pressure.

Effects of ovariectomy and/or dietary calcium deficiency on bone dynamics in the rat hard palate, mandible and proximal tibia

The effects of ovariectomy and/or dietary calcium deficiency on bone dynamics were examined by comparing the histomorphometric changes in these bones. Five groups of rats were studied, (1) unoperated basal controls; (2) sham-operated, fed on a normal-calcium diet; (3) ovariectomized, fed on a normal-calcium diet; (4) sham-operated, fed on a calcium-deficient diet; (5) ovariectomized, fed on a calcium-deficient diet. The basal controls were killed at 6 weeks of age, and the remaining groups were killed at 18 weeks of age. The hard palate, mandible and proximal tibia were processed undemineralized for quantitative bone histomorphometry. Bone volume, eroded surface, osteoid surface and bone-formation ratio were calculated. A significant age-related increase in bone volume and a significant decrease in bone formation were observed in the hard palate and mandible, whereas no significant age-related increase in bone volume could be found in the tibia. In the hard palate, ovariectomy neither inhibited age-related increases in bone volume nor affected bone dynamics, while both the combined ovariectomy and dietary calcium deficiency and dietary calcium deficiency alone led to bone loss and increased bone turnover. In contrast, in the mandible and proximal tibia, ovariectomy alone as well as dietary calcium deficiency led to bone loss and increased bone turnover. Ovariectomy, therefore, produced no significant changes in the hard palate, but affected bone dynamics in the mandible and tibia. However, dietary calcium deficiency induced bone loss and increased bone turnover in the hard palate, mandible and proximal tibia, independently of ovariectomy.

A Qualitative Analysis on Bone Dynamics in the Tissue under Denture Base Lacking in Self-cleaning

The purpose of the present study was to estimate the qualitative dynamics of bone tissue under denture base caused by lack of self-cleaning as a result of coverage by denture base, utilizing double labeling technique with bone markers and contact microradiography.The experimental denture base was designed to make contact with molar region of the palate of rat and to transmit no occlusal forceto the tissues under the denture base. The denture base was not removed throughout the experimental period. This procedure caused lack of self-cleaning, and it also caused continuous pressure to be applied to palatal tissues under the denture base by the accumulation of exfoliative horny cells, which induced bone resorption and formation under the denture base.The results were as follows:1. Two to three weeks after the insertion of the denture base, number of osteoclasts and Howships lacunae were observed on the palatal surface of hard palate bone tissue under the denture base. In the same observation period, increased bone formation was observed on the nasal surface.2. Four to six weeks after the insertion of the denture base, increased bone formation was observed on the palatal surface of hard palate bone tissue under the denture base. Bone formationwas observed mainly in Howships lacunae, and only slight bone formation was observed in other region. 3. The first one week and from seven to ten weeks after the insertion of the denture base, bone formation was suppressed on the palatal surface of hard palate bone tissue under the denture base.