Microcracks in the alveolar bone following orthodontic tooth movement: a morphological and morphometric study.

Microcracks and microdamage have been associated with bone remodelling. The aim of this study was to investigate the role of microcracks as a trigger for alveolar bone remodelling after the application of an orthodontic load. In 25 3-month-old male Danish land-race pigs, the lower right first molar was moved buccally with a force of 130 cN. The contralateral molar was not treated and was used as an internal control. After 1, 2, 4, 7 and 15 days of treatment, the regions containing the right and left molars were excised and en bloc stained in basic fuchsin. The presence of microcracks on the buccal and lingual sides of both treated and untreated teeth was detected and expressed as crack density (number/mm2). The buccal treated side showed significantly more cracks than the buccal untreated side at day 1. This difference was significantly larger than that observed at days 2, 7 and 15. The same side showed significantly more microcracks than the lingual treated side at day 1, and this difference was larger compared with that observed at days 4 and 15. The presence of more microcracks on the treated side than on the untreated side suggests a role for microcracks in the initiation of bone remodelling after orthodontic loading. The increased presence of microcracks on the side towards which the tooth was moved, and where bone resorption is usually observed, suggests that microcracks could represent the first damage induced by orthodontic force that has to be repaired by bone remodelling. Finally, the strain levels in the alveolar bone during the orthodontic load transfer in the experiment were examined by finite element (FE) analysis. Although this showed that the strains were very low (10-100 microstrain), it should be noted that occlusal loading was not taken into account. In addition, high-resolution microtomography of the alveolar bone/periodontal ligament (PDL) interface revealed that the actual surface of the alveolar bone was very rough, predisposing it to high local stress/strain peaks.