A Computer-aided Research About The Change Of Arch Form After Extraction Treatment On Patients With Class Ⅱ Division1Malocclusion

In China，Class Ⅱ division1malocclusion accounts for about23.8%in thepermanent dentition of adolescents. The main clinical manifestations are maxillaryprotrusion, mandibular retrusion，deep overbite and overjet of the anterior teeth. For thismalocclusion，extraction orthodontic treatment is a common treatment method, whichaims to relieve dental crowding and alleviate deep overjet by retracting maxillary anteriorteeth with the extracted maxillary tooth space，and to level Spee’s curve、correct deepoverbite and improve the relationship between molars by moving mandibular posteriorteeth forward with the extracted mandibular tooth space. Therefore extraction treatmentusually brings about changes of the dental arch form，which mainly reflect in dental archwidth, length and curve.After extraction treatment of Class Ⅱ division1malocclusion，the extent of changesin dental arch form is closely related to the treatment mechanism and the stability of thetreatment effect. In this study，we chose some Class Ⅱ division1malocclusion patientswith four first-premolars extraction and analyzed the orthodontic correction mechanism bycomparing changes in arch width、length and dental arch curve after the treatment. Furthermore，our study will provides a reference for determining the treatment program ofthis malocclusion in clinical orthodontic. It also provides a reference for the stability ofextraction treatment effect about this malocclusion in our further research.This thesis consists of two parts:Part I: The Establishment of Computer-assisted Method for Dental Arch FormAnalysisThe purpose of this part is to establish a computer-aided method capable ofmeasurement and analysis of arch width、length and dental arch curve, aimed at theresearch of arch form changes after the extraction treatment of Class Ⅱ division1malocclusion. Specific study contents and effective methods are as follows:1. Acquisition of three-dimensional digital dental models：Before and after treatment，dental cast of patient was scanned by Zenotec Scan S100laser scanner.Then we obtaineda point cloud files of the three-dimensional surface. After that we tried to get detailedthree-dimensional digital dental models after denoising、smoothing、point subtractionprocessing through the software of Geomagic Studio.2. Fitting of dental arch curve: Using Reverse Engineering principle, based on theplatform of Visual Studio2005and Visualization Toolkit (VTK)， we establish acomputer-assisted system for dental arch curve analysis. First of all，we determined theprojection plane for measurement and analysis，then we determined the landmark pointsfor fitting arch curve on three-dimensional surface of digital dental model and generatedthe points for fitting dental arch curve in the projection plane，finally we realized thefunction of fitting arch curve and exporting the point’s coordinates.3. Establishment of computer-assisted method for dental arch form analysis: Basedon the step2，we inputted the point’s coordinates into MATLAB and programmed somemathematical operation to calculate the arch width and arch length between different partsof the dental arch，to analysis the characteristic of dental arch curve.Part II: The Study of Arch Form Changes after Extraction Treatment for Patients withClass Ⅱ division1malocclusionObjective: To compare and analyze the changes in dental arch form after extraction treatment for patients with Class Ⅱ division1malocclusion and to provide a reference forclinical treatment.Methods:30cases of Class Ⅱ division1malocclusion were selected to acquiredigital dental models with a3D laser scanner before and after the extraction treatment.Using those digital dental models，we fitted dental arch curve and exported the relatedpoint’s coordinates with our computer-assisted system for dental arch curve analysis.We calculated the arch width and arch length between different parts of the dentalarch by programming some mathematical operation in MATLAB. Statistical analysis wasperformed by paired t-test method for the changes of arch width、length before and aftertreatment. Through comparing the change of average dental arch curve in the projectionplane before and after extraction treatment，a comparative analysis was shown to explainthe changes in dental arch form after extraction orthodontic treatment.Results:(1) Changes in dental arch width: for Class Ⅱ division1malocclusionpatients, after orthodontic treatment their dental arch width of anterior maxillary lateralincisor and canine region increased, but slightly in amount, with the arch width increasingby1.74±1.48mm (P＜0.01) in the lateral incisor region and by0.90±1.20mm (P＜0.05) inthe canine region. Posterior maxillary dental arch width in each measuring positiondecreased significantly. The width of the second premolar and the first molar changedmore largely than that of anterior teeth, decreasing by4.08±1.32mm（P＜0.001) and3.19±1.35mm（P＜0.001) separately, while the width of the second molar decreasedslightly only by1.89±0.96mm（P＜0.001).The changes in mandibular dental arch width are basically the same as those inmaxillary arch. Anterior dental arch width had only a small increase in the incisor region,increasing by0.26±0.30mm（P＜0.05) after the treatment, with no obvious change in thelateral incisor and canine regions. Posterior mandibular dental arch width showed adecreasing change adaptive to the change of the posterior maxillary dental arch, with theaverage width of dental arch decreasing by4.47±3.06mm（P＜0.001) in the secondpremolar region and2.48±1.80mm（P＜0.001) in the first molar region. Yet the dental archwidth in the second molar region showed no obvious change. (2) Changes in dental arch length: after the orthodontic treatment on the Class Ⅱdivision1malocclusion patients, the average length of dental arch in maxillary canineregion increased slightly by1.18±1.19mm（P＜0.05) while the average posterior maxillarydental arch length in the second premolar, the first molar and the second molar regionsdecreased by5.96±3.14mm（P＜0.001)，6.53±2.04mm（P＜0.001) and6.35±1.79mm（P＜0.001) separately, which is greater in change than the anterior dental arch. Changes ofthe mandibular arch length are consistent with those of the maxillary arch length.(3) Changes in dental arch form: based on30patients, the average points on thedistribution map were worked out by dental arch curve fitting method. Seeing from thedistribution map, the maxillary and mandibular arch forms are basically the same beforeand after the treatment, with no obvious change.Conclusion: The results of this study suggest that there is no obvious change in thedental arch form after Class Ⅱ division1malocclusion extraction treatment. For thoseClass Ⅱ division1malocclusion patients with an obvious displacement of tooth positionin sagittal plane before and after tooth extraction treatment, the changes between the sameteeth on both sides can not accurately reflect the change of dental arch width. Based on theanalysis of the result that there is a decrease in arch length both in the anterior andposterior, an important mechanism of changes in the above mentioned dental arch widthbetween the same teeth on both sides is the displacement of tooth position in sagittal plane,which causes the changes of arch width and length in the horizontal plane. The results ofthis study also shows that fitting and extracting dental arch form plays an important role inanalyzing the change. The computer-aided method established in this study for theanalysis of arch form can be used as a tool to fit and extract dental arch form and thereforehas certain application value.