[Orthodontic movement through compact bone and spongious bone. The difference in tissue reaction with 2 different forces].
The objective of this study, was to study the contribution of the osseous density factor at the orthodontic movement. For this purpose we compared the histophysiological responses frontally to the roots of the mandibular first molar (at the experimental Wistar rat) according to the direction of displacement (mesio-lingual region). In fact, frontally to the mesial and lingual roots (at the mesio-lingual region) there is compact bony tissue in contrary at the vestibular and distal roots the density is less and there is spongiose osseous tissue. We found the following: the region of pressure is subjected to compression and hyalinisation periodontally from the first day, even with the "ideal" force; the extent of compression and hyalinisation periodontally, reaches the maximum approximately after two days; the extent of compression and hyalinisation periodontally is greater frontally to spongiose osseous tissue and even more greater with a heavy force; the immediate displacement of a tooth is greater frontally to spongiose bone and even more greater with a heavy force; there is bony resorption of the internal alveolarwall adjacent to the hyaline zone, resorption (direct laterally or peripherically) frontally to the compact and to the spongiose osseous tissue; the direct lateral resorption: is slower for the compact bone; frontally to the spongiose bone the lateral direct resorption is associated to an indirect resorption at the endoostic side; the real displacement of a tooth is less rapid in compact bone because the lateral direct resorption is slower and also because of the absence of indirect endoostic resorption; at the time of the real dental displacement, the periodontal space has reestablished; on regard of the spongiose bone, the periodontal space not only reestablishes, but also increases during the real dental displacement; at the level of the periosteum, we can observe a compensatory osseous apposition to the direct lateral bony resorption, only with light forces, on the contrary with heavy forces there is periostic resorption and the tooth movement occurs within the bone (especially frontal to a compact bone); the threshold for stimulating the osteoclasts is lower than the ideal force frontal to the compact bone. On the contrary, frontal to the spongiose bone, the threshold for stimulating the osteoclasts, is situated higher to the ideal force.