Direct and indirect resorption are perceived as reactions to an applied force. This is in contrast to the view of orthopedic surgeons, who describe apposition as a reaction to loading of bone. A histomorphometric study of the circumalveolar bone reaction to a force system generating translation of premolars and molars of five maccaca fascicularis monkeys is described. Three force levels (100 cN, 200 cN, and 300 cN) were applied for a period of 11 weeks. Undecalcified serial sections were cut parallel to the occlusal plane, and a grid consisting of three concentric outlines of the root intersected by six radii was placed on each section. Areas anticipated to be submitted to different stress / strain distributions were isolated. A-posteriori tests were used in order to separate areas that differed with regard to parameters reflecting bone turnover. Based on these results, a new hypothesis regarding tissue reaction to orthodontic forces is suggested. Direct resorption could be perceived as a result of the lowering of the normal strain from the functioning PDL and as such, as a start of remodeling, in the bone biological sense of the word. Indirect remodeling could be perceived as a sterile inflammation attempting to remove ischemic bone under the hyalinized tissue. At a distance from the alveolus, dense woven bone was observed as a sign of a RAP (regional acceleratory phenomena). The apposition could, according to the new hypothesis, be perceived as a result of the bending of the alveolar wall produced by the pull from the Sharpey fibers. The above suggested interpretation of tissue reaction would be shared with bone biologists.

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