Micro-Implants Biomechanics: A Comparative Study to Assess Variations in Anterior Tooth Movement using Different Height of Retraction Hooks

This clinical study was undertaken to determine the optimum height of the retraction hook when bringing two basic types of anterior tooth movement i.e. bodily movement and tipping movement when retracting from a buccal implant and to take advantage of these variations and apply them in various clinical situations of retraction. A total of 10 cases were selected and divided into two groups of 5 cases each. Group A had long hooks (8mm) soldered on 0.019 x 0.025 inch SS wire between the upper lateral incisor and canine. Group B had short hooks (2mm) soldered on 0.019 x 0.25 inch SS wire between the upper lateral incisor and canine. En-masse retraction was done for both the groups using a microimplant (Dentos no.1312-08) placed at the standard height of 10mm from the arch wire between the roots of the upper first molar and second premolar. Initially a force of 150gm measured with dontrix gauge was given using a NiTi- coil spring (Dentos-13mm length) for both the groups and after one month the force levels were increased to 250gm and 300gm in subsequent appointments using a shorter NiTi- coil spring (Dentos-8mm length). Different angular and linear cephalometric values were compared at the start (Pre- stage, T1) and after the correction of overjet (Post-stage, T2) and the results were statistically analysed. Results of this study showed that maximum lingual tipping of anterior teeth is seen when the height of the retraction hook is as low possible (Group B). In cases where torque control and bodily movement is the criterion, the hook length should be at the maximum height possible (Group A). The molar position in both the groups however remained unchanged .The results found in our study correlate to the results of the FEM study of H.M. Kyung [et al.] From this study it can be concluded that that implant biomechanics has a substantial variation from the biomechanics used in standard retraction and different types of tooth movement can be achieved by varying the height of the retraction hook. Thus microimplant assisted anchorages have not only stretched the boundaries of camouflage treatment but also have forced the orthodontist to inspect the variation in biomechanics which have been proved to be substantially different from conventional orthodontic biomechanics. Further long-term in vivo studies with more number of subjects will be forthcoming to further evaluate this approach of treatment