Association between periodontal changes and biomechanical factors from tooth movement

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Histological changes in the periodontal ligement (PDL) from experimental tooth movements were investigated and integrated with analytical findings of stresses. A distally directed force of 250 gf was applied to the lower second premolars of adult dogs by use of a calibrated sectional arch wire with a closed loop. Mesiodistal sections of the tooth and supporting tissues were preparated at 2 hours and at 4 and 7 days after the beginning of experimental tooth movements. Further, stress levels in the PDL were analyzed by means of finite element analysis. A three-dimensional model for the analysis was constructed. The model consists of 3651 nodes and 2938 solid elements. A distally directed force of 250 gf was analytically applied to the tooth. Three principal stresses were determined in the PDL, and volumetric principal stress and shear stress component were evaluated. The nature of principal stress distributions in the PDL was highly coincident with histological findings in terms of site specific tension and compression of the PDL and alveolar bone remodeling. These results strongly emphasize the relationship between principal stresses and biological changes in the PDL, bone remodeling in particular, and hence its subsequent tooth movements.The distribution of calcitonin gene-related peptide (CGRP)- and substance P (SP)-immunoreactive (IR) nerve fibers and their correlation in the PDL of mouse incisors were examined by indirect immunofluorescence. Both CGRP-IR and SP-IR thin nerve fibers were abundant in the apical and middle third of the PDL. In the lingual protion of the incisal PDL, these nerve fibers were localized in the alveolar half of the PDL and were observed as free nerve endings. No CGRP-IR and SP-IR specialized nerve endings, such ans Ruffini-like corpuscles, were observed. In the labial PDL, CGRP-IR and SP-IR nerve fibers ran along the incisal axis. The distribution of CGRP-IR nerve fibers was very similar to that of SP-IR nerve fibers.