A Study On The Stability Of Upper First Molar Position During Postretention In Three Dimensions

Purpose:The aim of the present study was to examine changes of the upper first molars in three dimensions during the postretention period and to evaluate the stability.Analysis the correlation between some risk factors and the relapse tendency.The measurements of sagittal and vertical variables were derived from cephalometric radiographs,meanwhile the horizontal ones from plaster models.Methods:1.32 patients of Class I and Class II malocclusion who had finished orthodontic treatment were examined,and all of them were well-retained for at least one and half a year.Cephalometric radiographs of pretreatment(T1),posttreatment(T2),and postretention(T3)were taken.The measurements of sagittal and vertical variables of upper first molars were derived frorm cephalometric radiographs.Sagittal linear distance contains U6MD-PTV(the average distance of the most distal and mesial aspect of the upper first molar to Pterygoid Vertical),U6Occ-PTV(linear distance from the mid-occlusal point of the upper first molar to Pterygoid Vertica).The sagittal position changes of the ANS were used as the maxillary growth extent.Sagittal angular variables include ? U6MD-SN,?U6-SN(posterior-inferior angle formed between the SN plane and the long axis of upper first molar),?U6MD-PP,?U6-PP(posterior-inferior angle formed between the PP plane and the long axis of upper first molar).The vertical variables contain U6MD-PP(the average distance of the most distal and mesial aspect of the upper first molar to PP plane),U6Occ-PTV(linear distance from the mid-occlusal point of the upper first molar to PP plane).Descriptive and inferential statistics were calculated to see whether the posttreatment change were statistically significantly different from zero.The specimen was divided into several groups based on the initial FMA,ANB and whether impacted third molar exists during retention,extraction or not during treatment.ANOVA or t test was used to compare the posttreatment changes between groups.2.42 patients who had finished orthodontic treatment and well-retained for at least one and half a year were examined.Plaster models of pretreatment(T1),posttreatment(T2),and postretention(T3)were collected.The intermolar widths and distances between the bilateral mesiobuccal,mesiolingual,distolingual,distobuccal cusps of the maxillary first molars were measured.Statistics were calculated to see whether the posttreatment change were statistically significantly different from zero,and to analysis the rotation tendency.Divide the specimen into two groups based on tooth extraction or not during treatment,and t test was used to compare the posttreatment changes between the two groups.Results:(1)During retention period,upper first molars exhibited significant mesially drift and tipping(P<0.05),the increments of U6MD-PTV and U6Occ-PTV are(1.77±2.10)mm,(1.72±2.15)mm.The Class II group emerges more mesially drift and the low angle group shows less sagittal tipping.(2)Significant vertical extrusion occurs(P<0.05),the increments of U6MD-PP and U6Occ-PP are(0.82±0.96)mm,(0.85±1.02)mm.The high angle group shows significantly more extrusion.(3)The intermolar width exhibited significant increment of(0.49±0.88)mm(P<0.05),and seemingly rotate mesiolingually during retention.(4)No significant correlation was found between whether the impacted third molar exist during retention and the stability of upper first molar.(5)No significant correlation was found between extraction or not during treatment and the stability of upper first molar.Conclusions:(1)The upper first molars exhibited significant mesially drift and tipping.Class II group emerges more mesially drift and the low angle group shows less sagittal tipping.(2)Continued extrusion occurs for vertically during postretention.The high angle group shows significantly more extrusion.(3)The intermolar width shows significant increase and seemingly rotate mesiolingually.No significant correlation was found between extraction or not and the stability of upper first molar.