Three-dimensional Analysis Of Mandibular Molar Distalization With Clear Aligners Assisted By Micro-implant Anchorage

Objective:To analyze the achieved and predicted movement of mandibular molars and central incisors(crown and root apex)through three-dimensional measurement during mandibular molar distalization by using clear aligners.Discussing the effects of different micro-implant anchorage design on molar distalization,type of movement and anchorage protection of the mandibular central incisors.In order to provide a reference for clinical diagnosis and treatment protocol of molar distalization.Materials and methods:Choose 17 adult patients who used clear aligners to move mandibular molars distally(females,average age: 25 years).Patients were divided into two groups(A and B)according to their clinical treatment plans.Group A(10 cases): no additional anchorage.Group B(7 cases): using micro-implant anchorage screw to strengthen anchorage.Used Clin Check software to design treatment plan and obtain digital models at initial and target position.Study models were made before treatment(T0)and after mandibular molars moved into target position(T1)respectively to conduct3D-scanning.CBCT was taken at the same stage to reconstruct the mandible and dentition through Mimics software.Digital models from above were integrated in Geomagic wrap software and measurement markers were added at crown-root region.Superimposed the integrated models of T0,T1 and target position(TX)in Geomagic control software based on the mandibular basal bone area.Calculate three-dimensional achieved and predicted movement of mandibular molars and central incisors.SPSS 20.0 software was used for statistical analysis.Analysed the difference of achieved and predicted movement of mandibular molars and central incisors between two groups,and compared their efficiency of crown-root distalization and difference of three-dimensional movement of mandibular molars and central incisors.Results:1.Group A: mandibular first molars distalization efficiency was 54.98±8.87% of the crown and 24.63±23.07% of the root apex on average.Mandibular second molars distalization efficiency was 56.59±9.42% of the crown and 16.17±21.04% of the root apex on average.Group B: Mandibular first molars distalization efficiency was68.66±11.16% of the crown and 28.87±20.51% of the root apex on average.Mandibular second molars distalization efficiency was 71.02±7.61% of the crown and33.45±17.53% of the root apex on average.Statistic difference was found in the efficiency of the crown distalization between two groups(P<0.05),but no statistic difference was found regarding the efficiency of root apex distalization(P>0.05).2.Group A: difference of mandibular first molars between achieved and predicted movement: the crown had distal tipping of 2.97±1.85°and buccal movement of 0.88±0.67 mm.Difference of mandibular second molars between achieved and predicted movement: the crown had distal tipping of 3.07±2.47°,buccal movement of1.58(1.00,1.87)mm(Quartile).Achieved movement of mandibular central incisors: the crown had labial movement of 0.61±0.67 mm and the root apex had elongation of0.35±0.21 mm.Differences above had statistic significant(P<0.05).3.Group B: difference of mandibular first molars between achieved and predicted movement: the crown had distal tipping of 4.89±2.03° and the root apex had buccal movement of 0.43(0.21,0.77)mm(Quartile).Difference of mandibular second molars between achieved and predicted movement: the crown had distal tipping of5.21±2.04° and intrusion of 1.56±0.90 mm,the root apex had intrusion of1.75±0.67 mm,differences had statistic significant(P<0.05).Three dimensional movement of mandibular central incisors showed no statistic significant(P>0.05).4.Compared the achieved movement of mandibular central incisors between two groups,group B showed less labial movement of the crown and the root apex moved mesially.Difference had statistic significant(P<0.05).Conclusions:1.Clear aligners can achieve the distal movement of mandibular molars.During distalization,molars appeared distal tipping and buccal movement of the crown,the mandibular central incisors appeared labial movement.2.Micro-implant anchorage screw combined with clear aligners for molar distalization can reduce the tendency of labial movement of the mandibular central incisors,better protect the anchorage of mandibular central incisors and improve the efficiency of molar crown distalization.During molar distalization,the crown of mandibular molars showed distal tipping and slight intrusion.