[An experimental study of the orthodontic force effect on internal remodeling of mandibular bone]

The role of mechanical stress in determining bone structure by influencing its cellular processes during remodeling has been discussed. But previous studies have not provided an adequate background of knowledge to explain the effect of orthodontic force in internal remodeling of the bones. This study was performed to determine the relationship between stress distribution induced by orthodontic force and alteration of internal remodeling in mandibular bone. Seven young adult mongrel dogs of unknown age were used as experimental animals. The bilateral lower 4th premolars were extracted. After 106 days, the 3rd premolars on the right side were moved distally by coil springs for 106 days. Twice pulse labels of tetracycline and calcein were performed to obtain histomorphometric parameters of cortical bones prior and posterior to tooth movement. With the use of microscopic plotting tool and minicomputer, the direction of haversian systems was reconstructed three-dimensionally. To make a comparison with the alteration of histomorphometric parameters and direction of Haversian systems, the principal stress distribution of mandibular bone caused by orthodontic force was obtained by means of 3-D finite element method analysis. Experimental results indicate as follows. 1. The osteoid seams number per unit area (OsAf) and the labeled surface per unit area (TcAf) increased significantly at both alveolar and basal bone region compared with control side. 2. The resorption cavities number per unit area, (Ar) increased significantly at alveolar bone region. But no significant changes were noted at basal bone region. 3. No significant differentiations were noted in mineral appositional rate (Mo) between any bone regions nor label timings. 4. There is large variance between the direction of Haversian systems and the maximal principal stress vector. But a significant alteration of the direction of Haversian systems was found in highly stressed and younger bone areas. From these findings, it can be speculated that, as a primary reaction of bone against stress, the optimum microscopic structural change took place in areas of bone which bore tension or pressure due to excess stress, at the same time the internal remodeling of bone was accelerated by increase of activation frequency (mu), accompanied with no increase of remodeling periods (sigma).