Effect of orthodontic force on periodontal tissue metabolism. A histologic and biochemical study in normal and hypocalcemic young rats.

The relationship between force and degradation activity after application of orthodontic force is still obscure. Of particular clinical interest are the etiologic factors behind excessive root resorptions appearing in connection with orthodontic movement of teeth that has been proposed to be influenced by systemic factors regulating the tissue-degrading activity in periodontal tissues. Thus, the aim was, by histologic and new biochemical methods, to investigate the effect of orthodontic forces on the periodontal tissues in the normal and the hypocalcemic situation with secondary hyperparathyroidism. Root resorptions were induced in upper incisors of normal and hypocalcemic rats by subjecting the teeth to a moderate orthodontic force. In both groups the resorption of the roots occurred consistently in the vicinity of reorganizing areas of the periodontal ligament (PDL) with ongoing degradative activities and alveolar bone resorption. Furthermore, specific cell metabolic changes in alveolar bone and PDL in tension and pressure zones were detected and quantified by biochemical determination of alkaline phosphatase activity. This biochemical quantification of the metabolic changes together with the morphologic observations gave the clinically valuable information that the observed increase in occurrence and severity of root resorptions in moderate hypocalcemia was related to an increase in alveolar bone turnover. This study has shown that root resorptions were clearly related to the degradation process occurring in the vicinity of the hyaline zone and that in the hypocalcemic situation, the increase in root resorptions was related to an enhanced alveolar bone resorption. Thus, factors that minimize the time for resorptive/degradative activity should be discussed in this context rather than force per se.