Biological response to corticotomy-assisted orthodontics.

Abstract

The aims of this study were to validate the local and regional biological response to pre-orthodontic corticotomy in an animal model; and to evaluate the local and regional bone response to a flap or corticotomy procedure with and without tooth movement. Thirty-six Sprague-Dawley rats, six to eight weeks old, were divided into six groups and treated over a seven-day period. All treatment procedures were performed on the right maxillary first molar. A custom appliance was placed at the start of the observation period and surgical procedures were performed at time of placement. In the surgery groups, a full-thickness flap was elevated along the buccal surface of the right maxillary first molar and a distal vertical relieving incision between the roots of the first and second maxillary molars. For the corticotomy groups, a slow-speed hand piece and a 0.5 mm round bur was utilised to create a trench through the cortical bone extending from the apices of the first molar horizontally and mesially beyond the mesial root in an L shape. A fixed appliance with 100 g NiTi spring was placed to produce buccal tooth movement over a total experimental time of 7 days. Following sacrifice, the specimens were prepared and resin-embedded. Micro-CT scans of the samples were performed utilising a Skyscan 1174. CTan (CT analysis program) was used to measure the bone mineral density (BMD) and bone fraction of a defined region of interest. The buccal and palatal plates of bone were assessed to evaluate local and regional bony changes, respectively. Three-dimensional images of the samples were reconstructed using the Paraview program and macroscopic differences within the buccal region of bone between treatment groups could be seen in the reconstructed images, with the greatest changes visible in the combined ‘Tooth Movement and Corticotomy’ group. Similar osseous changes were visible in the untreated region of palatal bone in the same group. After seven days of treatment, the BMD of the buccal region of bone on the treatment side tended to decrease relative to controls, although the differences were only statistically significant for the combined ‘Tooth Movement and Corticotomy’ group. Comparison of the buccal region on the treatment side of all groups showed a pattern of change in bone fraction consistent with the level of intervention involved. The bone fraction of the ‘Tooth Movement and Corticotomy’ group was statistically significantly reduced compared with all groups except for the ‘Tooth Movement and Flap’ group. In the assessment of the palatal regions, variable results were found when the control side was compared with the test side. In conclusion, following corticotomy and seven days of buccal orthodontic tooth movement in the rat model, there was a significant reduction in bone volume fraction in the buccal region of bone. This suggests that, in an animal model, corticotomy combined with orthodontics was able to accelerate the bone resorption and formation processes associated with tooth movement, which supports the clinical results observed in previous reports of corticotomy-assisted orthodontics. Furthermore, there were osseous changes in the palatal region of bone, suggesting that there is a regional and systemic regional acceleratory phenomenon effect.