Correlation of stress and strain profiles and the distribution of osteoclastic cells induced by orthodontic loading in rat.

The aim of this study was to investigate whether stress or strain within the periodontal ligament (PDL), or rather in the bone, plays a role in initiating biological responses in orthodontic tooth movement. The upper first molars of 11 Wistar rats were moved mesially with a closed nickel titanium alloy coil spring for 3-12 d and three rats served as controls. After preparation of the maxillae for paraffin histology, tartrate-resistant acid phosphatase (TRAP) activity was evaluated to detect osteoclasts and their mononuclear precursors in four regions around the mesial root. Based on histological sections, finite element models were developed. The stress/strain distributions associated with the orthodontic loading were determined in the alveolar bone and in the PDL and compared with the osteoclast distributions. In the PDL, the normal stresses and strains were compressive (negative) in regions where the number of osteoclasts was highest, whereas the positive normal tensile stresses and strains were coincident with regions with small numbers of osteoclasts. The results indicate a direct correlation of the calculated stress/strain values in the PDL with the distributions of osteoclasts in the alveolar bone and PDL. Thus, these seem to be the dominant mechanical factor of tissue reaction to orthodontic force application, whereas the mechanical loading of the bone does not seem to play a role in orthodontic bone remodeling.