Effects of initial stresses and time on orthodontic external root resorption.

Optimum stresses for a favorable response to orthodontics are still unknown. Here, we compared the effects of initial periodontal ligament (PDL) stresses over time in orthodontic external root resorption (OERR), necrosis, and the TRAP+ cell population. Forty-two rats (Fischer CDF) were treated with 10 cN of force for 5 different time periods. Finite element (FE) models of the first maxillary molars were constructed from μCT scans to calculate initial PDL stresses. The scans were also used for OERR measurements before histology. Time, stress, and their interaction were significant to result in an OERR increase only in the regions of medium and high stress. OERR was not significantly different between control and treated animals over time in the region of low stress. After 30 days, OERR was increased by 5- and 3-fold in the zone of high- and medium-stress regions, respectively. The TRAP+ cell population initially followed the stress gradient, but changed after bone and necrotic tissue resorption. In the 30-day modeling cycle, the correspondent 3rd principal stress range to promote direct bone resorption and insignificant OERR was between -9.92 and -7.75 KPa. These translate to approximate forces of 30 to 40 cN applied at the bracket level (tipping) of a human maxillary canine.