Histomorphometric study of alveolar bone turnover in orthodontic tooth movement

Selected histomorphometric parameters were measured in alveolar bone adjacent to rat molars treated with a 40gram tipping force designed to tip the molar mesially and with a sham procedure. Undecalcified parasagittal sections of teeth and surrounding tissues were prepared for static and dynamic histomorphometry at 1, 3, 5, 7, 10, and 14 days. Initial tooth displacement was assessed by measuring differences between groups in the widths of the day-1 periodontal ligaments (PDL) at various vertical locations and correlating these using linear regression analysis. All parameters were measured in the alveolar bone adjacent to four quadrants around the teeth (mesial-distal; occlusalapical). Means and standard errors for each parameter in each group were calculated and compared for time- and group-specific differences using ANOVA and pairwise comparisons with Scheffe's multiple comparison tests. In shamtreated rats, bone resorption predominated on the distal alveolar surface, but significant surface-related differences between mesial and distal surfaces in bone formation were not demonstrated. Time-specific effects in bone resorption were not evident on either surface in the shams. These findings suggested that molar distal drift in the sham rat is facilitated by resorption in the distal alveolar bone. PDL width changes in orthodontically-treated rats were greatest in the mesial occlusal half of the root and decayed linearly toward the apex indicating that the greater initial tooth displacement occurred in the occlusal half of the root surface. Histomorphometric parameters of alveolar bone turnover were also seen to be greater in these locations, suggesting that these processes were sensitive to the increased tooth displacement. In orthodontically-treated rats, the total paradental alveolar bone and both pressure (mesial) and tension (distal) surfaces experienced a wave of resorption that lasted for four days, was preceded by an activation period of two days, and was followed by a wave of formation that persisted until the end of the experiment (14 days). The relative balance between bone formation and resorption waves was location-specific, with formation predominant on the tension side and resorption predominant on the pressure. These findings are consistent with the conclusion that pressure and tension initiate similar bone turnover events, and that bone deposition or removal is controlled by the degree of balance between the relative amounts of bone formation and resorption that occur at these sites.