Three-dimensional Change Of Facial Soft And Hard Tissues In Adults After Third Molars Extraction

Whether the removal of wisdom teeth will cause the volume change of facial soft tissue,is a problem that many patients are concerned about.There are two parts in this study:Experiment 1 and Experiment 2.Experiment 1Objective:In this study,a standardised procedure was proposed to control involuntary motion and other factors during the capture of structural light scanning which may influence the morphology of three-dimensional facial models,and the inter-operator reproducibility was evaluated.Methods:20 volunteers each was taken nine facial scans in the same day by three researchers(each takes three times),via FaceScan structural light scanning system following the proposed procedure.Each capture had a 5-minute interval.Three facial scans acquired by the same researcher were compared by reverse engineering software(Geomagic).Six facial regions,including forehead,nose,paranasal,upper lip,lower lip and chin,cheek,were divided.With the first scan as a reference,the other two scans were registered,and surface-to-surface distance maps were acquired to calculate mean(μ),standard deviation(SD)and root mean squares(RMS)between two surfaces.The reproducibility between three researchers then was evaluated by one-way ANOVA.Results:The μ of 6 facial regions were close to 0.The RMS of lip regions were largest(0.48-0.53mm),the forehead was smallest(0.21mm),and the others ranged 0.37mm to 0.42mm.SD is slightly smallerthan RMS and has the same trend of change.There was no significant difference in RMS among the three researchers(P>0.05).Conclusions:With the constraint of the standardised procedure,the morphological reproducibility of facial models in six regions is satisfying.Experiment 2Objective:To explore the 3D changes of facial soft and hard tissues after wisdom tooth extraction.Methods:A total of 23 volunteers with ipsilateral third molars extracted were recruited.The body weight was controlled within 2kg during the study.Structured light scanning(SLS)was taken pre-extraction(T0),three months post-extraction(T1)and six months post-extraction(T2);cone beam CT(CBCT)was taken at T0 and T2.SLS was acquired under the standardised procedure proposed in Experiment 1.In Geomagic software,the mean distance(μ),standard deviation(SD)and root mean square error(RMS)between the buccal regions were measured by distance-map.The volume from the buccal region to midsagittal plane was measured by projection method,and the buccal volume change was calculated.CBCT data was processed in Dragonfly software.Manual alignment,segmentation,Boolean subtraction and pixels counting were done for hard tissue volume change(V-max,Vman,V-total)measurement.Two-way repeated measures ANOVA,one-way repeated measures ANOVA,and paired t test(Wilcoxon rank test was instead for non-normal distribution)were used to explore the influence of tooth-extraction and time on buccal volume.Intraclass correlation coefficient(ICC)was used to verify the repeatability of buccal volume measurement.Pearson correlation coefficient was used to assess the relationship of buccal volume change and hard tissue change in extraction side(T2-T0).Results:There was no significant interaction between extraction and time(P=0.051).Paired t test(and Wilcoxon rank test)showed that there was no significant difference in μ.SD,RMS and buccal volume change between the extraction side and the non-extraction side(P<0.05).In extraction side,compared with T0,the buccal volume of T1 decreased by 1.396(95%CI:0.323-2.470)cm3(P<0.05),T2 increased by 1.753(95%CI:-0.01-3.507)cm3(P=0.05),there was no significant difference of buccal volume between T2 and T0(P>0.05).There was no significant difference of the buccal volume of T0,T1 and T2 in non-extraction side(P>0.05).The inter and intra-raters ICC of projection method was 0.959 and 0.974 respectively.The hard tissue volume decreased by 2.33±0.46cm3 in extraction side(T2-T0).There was no correlation between the hard tissue change and buccal volume change(P>0.05).Conclusion:The volume of hard tissue on the extraction side decreased after the extraction of wisdom teeth.The removal of third molars may reduce the soft tissue volume in a short time,but in the long run,it will not cause buccal volume changes.