The Characteristic Of Tooth Movement With Class Ⅲ Elastics And Anatomic Border Of Mandibular Second Molar Distalization In Adults With A Skeletal Class Ⅲ:A CBCT Study

[Objective] The purpose of this investigation:(1)is to evaluate the effect of straight wire appliance in the camouflage treatment of skeletal Class Ⅲ malocclusion; and(2)determine the posterior anatomic limit by comparing the posterior available spaces measured at various levels of the mandibular second molar on axial slices of CBCT; and(3)predict the posterior available space and the presence of root contact with the inner lingual cortex of the mandibular body using lateral cephalograms.[Methods] Part 1:15 skeletal Class Ⅲ adults (8male,7female) were treated with straight wire appliance, and lateral cephalometric radiographs were obtained and analyzed before and after treatment. Part2:Cone beam computed tomography scans were obtained on 34 adults with skeletal Class Ⅲ normodivergent facial profile. Posterior available space was measured at the crown and root levels along the posterior occlusal line connecting the buccal cusps of the first and second molars on the axial slices. It was also measured at the occlusal level on the lateral cephalograms derived from the cone beam computed tomography scans. The measurements on the cephalograms were used to predict the actual posterior available space determined by computed tomography and to determine the presence of root contact with the inner lingual cortex by linear regression.[Results] 1. Satisfactory occlusion and face esthetic results were established for 12 patients after finished with straight wire appliance in skeletal Ⅲ malocclusion. However,3 patients’ tooth moved slowly and got poor occlusion. The upper incisors proclined while the upper first molar and second molar extruded (2.2±1.6)mm and (2.1±1.2)mm, tipped mesially 2.2°±4.4°nd 2.3°±8.6°, respectively. Mandibular first and second molar distally moved (-2.1±2.0)mm, and(-2.3±2.8)mm and tipped-9.1°±3.4°and-2.4°±2.6°ignificantly. The lingual inclination of the lower incisors increased-3.5°±2.5°and extruded (1.6±1.7)mm.The SN-MP and FMA increased 1.5°±1.2° and 1.2°±1.4°, respectively.2. The mandibular second molar is more similar to the outer lingual cortex, the space was smaller at the root apex. The distance to the outer cortex at the crown is larger than root levels on the axial slice(F=34.500, P=0.000) and the posterior available space was significantly smaller at the root level than at the crown level (P<0.01).The posterior available space measured on the lateral cephalograms resulted in a regression equation with a coefficient of determination of R2:0.263^ 0.226^ 0.226.[Conclusion] 1. (1)Upper molar extruded with Class III elastics is main reason that lead to the clockwise rotation of the mandible.(2)Mandibular molars and incisors distally moved and tipped is a way to improve the relationship and overjet.(3) When the mandibular second molar root makes contact with the inner cortex, tooth movement slows,which affected the result of treatment dramatically.So, more attention must be payed.2. (1)In patients with skeletal Class III normodivergentfacial profiles.The outer lingual cortex of the mandibular body, not the anterior border of the ramus, was the posterior anatomic limit when the molar was distalized along the POL.(2)Lateral cephalograms provided limited but valuable information for predicting the posterior available space.(3)The mandibular third molar is not a factor that affected the result of molar distalization.Howere, extraction the third molar is neccesarry before molar distalzation.(4) The posterior limit at the occlusal level was actually located approximately 4-6 mm distal to the anterior border of the ramus as observed on the lateral cephalograms. CBCT evaluation is recommended in patients requiring significant molar distalization in individual.