The most common scenarios where molar protraction is performed3 is correction of Angle's class II malocclusion, open bite cases and replacing the extraction space of first molar by mesialization of second and third molars. Molar protraction is now considered as an alternative treatment plan instead of a prosthesis in missing first molar space. When this edentulous space of missing first molar is closed entirely by protraction of second molar the common side effects that occur while protracting the molar over a long edentulous space are mesial tipping and mesiolingual rotation of the second molar crown. Thus, it is necessary to incorporate tip back and toe in bends in the loop design used for protracting the molar. There is limited literature available for molar protraction using loops thus the present study was undertaken to find out the most efficient loop and appropriate tip back angle.
In this study as we increase the tip back Angle from 10°, to 15° to 20° the mesial tipping of molar reduces in all the loops. The mesial tipping of 5.9° was seen at 10° tip back angle, mesial tipping of about 3° was seen at 15° tip back angle and molar translated forward at a tip back angle of 20°. This occurs because as we increase the amount of vertical bend, there is an increase in the M/F ratio which leads to more amount of root movement.12 These results are similar to the study by Abhay Chaudhari13, which stated the effect of increasing the number of gable bends in a T loop. He found that greater gable bend produced higher moment to force ratio required for tooth translation. Similar results were seen in the study done by Paiboon et al14, they found that adding a vertical bend in T loop increased the M/F ratio enough to produce root movement.
At 10°, tip-back angle mesial crown and distal root movement with extrusion of distobuccal cusp and intrusion of mesiobuccal cusp was seen. If the molar tips mesially while protracting there are greater chances of relapse as the roots are tipped distally. Due to mesial tipping of molar during protraction molar extrusion may occur which affects the vertical dimension of the face as it causes clockwise rotation of the mandible. In low angle cases, molar extrusion is helpful as it helps in opening of the deep bite. However, in high angle cases or in cases of skeletal open bite, extrusion of molar has to be controlled by using appropriate treatment mechanics3.
At 15° tip back angle there was slight amount of mesial tipping of 3° but crown and root both showed movement in mesial direction. Changes in vertical plane were minimal at 15° tip back angle. Thus, it can be used in cases where the occlusal plane of the patient has to be maintained. The mesial movement achieved was more at 15° tip back angle than at 20°. Ryu et al8 performed a FEM study to find out the appropriate bending angles for mandibular molar protraction using a running loop. He found out that a tip back angle of 11.5° and toe in Angle of 9.9° helps in achieving bodily protraction of molar. Kim and Park15 also suggested that the optimal tip back angle should be 11° using a running loop for mesial translation of mandibular molar using indirect anchorage.
At 20° tip back angle along with bodily translation of molar a slight amount of intrusion is seen on the molar. This is helpful in cases of vertical growth pattern. As the tip back Angle increases, the intrusive component of force increases thus these results are obtained. This finding was analogous with findings of Burstone and Koeing16, they explained that when the loop is placed closer to the anterior segment there is an extrusive force at the segment where shorter arm of the loop is engaged whereas at the posterior segment a vertical intrusive force is seen. Proffit17 also explained this with the help of V bend principle, the loop position acts like an off centered V bend.
Wu et al18 also suggested that a tip back angle of 20 to 30° and a toe in Angle of 15° were given while protracted the mandibular molar using a L loop. Similar amount of tip back bend was suggested by Chae and Kim19 et al that is 20 to 30° while using a running loop for protraction of mandibular molar. Saga et al20 also suggested that a 20° tip back bend should be given while mesializing mandibular molar using a helical loop. Narmada et al21 in their case report applied a tip back angle on 20–30° but this caused the slight distal tipping of molar at the end of treatment.
Thus, the amount of tip back bend to be given during mandibular molar protraction should be between 15° to 20°. Tip back bends have the benefit of a wide activation range and constant moment application22.
There were no differences seen in movement of molar in the three loops thus the null hypothesis was rejected. The changes seen were mainly due to the change in tip back Angle. This finding can be explained as the length of wire incorporated in all three loops is equivalent, so the length of the wire incorporated in loop design has an effect on the moment to force ratio produced by the loop. Chen et al23 suggested that if we increase the vertical or horizontal dimension of the loop there is reduction in the load deflection rate. Reducing the dimension of the loop increased the load deflection rate and the stress in the loop and reduces the moment to force ratio dramatically.
Future Recommendations
The study can be further extended by performing clinical trials to evaluate the efficiency of T loop, Cherry loop and Open helical loop and the effect of tip back angles on molar protraction as well as the effect of patient related factors like age, periodontal health and occlusal forces that effect tooth movement.