Tissue reaction to orthodontic tooth movement--a new paradigm

Direct or indirect resorption are both perceived as a reaction to an applied force. This is in contrast to orthopaedic surgeons who describe apposition as 'the reaction to loading of bone'. The article reviews the literature on intrusion of teeth with periodontal breakdown, and on the basis of clinical and experimental studies. The conclusion is reached that intrusion can lead to an improved attachment level, and that forces have to be to low and continuous. The tissue reaction to a force system generating translation of premolars and molars in the five Macaca fascicularis monkeys is described. Three force levels, 100, 200, 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 so that areas anticipated to be subject to differing stress/strain distributions were isolated. A posteriori tests were utilized in order to separate areas that differed with regard to parameters reflecting bone turnover. Based on these results and a finite element model simulating the loading, a new hypothesis regarding tissue reaction to change in the stress strain distribution generated by orthodontic forces is suggested. The direct resorption could be perceived as a result of lowering of the normal strain from the functioning periodontal ligament (PDL) and as such as a start of remodelling, in the bone biological sense of the word. Indirect remodelling could be perceived as sterile inflammation attempting to remove ischaemic bone under the hyalinized tissue. At a distance from the alveolus, dense woven bone was observed as a sign of a regional acceleratory phenomena (RAP). The results of the intrusion could, according to the new hypothesis, be perceived as bending of the alveolar wall produced by the pull from Sharpey's fibres.