The Evaluation Of Basal Bone And Dental Arch Forms In Skeletal Class Ⅱ Malocclusions Before And After Extraction Treatment

Skeletal Class II malocclusion is a common malocclusion.The main cause of this kind of malocclusion is maxillary protrusion or mandibular retraction,or both exist at the same time.The patient’s profile tends to be protruded and affects the appearance.Although the most obvious feature of the skeletal Class II is that the sagittal and vertical disorder,the lateral relationship is also important.For class II patients,tooth extraction is a common way to relieve crowding or to correct molar relationship.Tooth extraction treatment should also improves lateral relationship while correcting sagittal relationship.This change affects the stability and function as well as whether the smile is full.While the narrow maxillary dental arch on skeletal Class II malocclusion has been relatively widely accepted,the study on width of the mandibular dental arch is still controversial,especially after the extraction treatment,tooth movement in three-dimensional directions caused a lot of trouble to accurately describe the location and shape of arch.Previous studies on dental arch and bone morphology were mostly based on plaster model to measure the width and depth of dental arch.However,due to the lack of stable reference points and the restriction of two-dimensional measurement method,there may exist some errors in the results.As Andrews proposed the concept of WALA ridge,it provides a reliable reference for the study of the relationship between the arch and the base bone.And with the development of computer and three-dimensional scanning technology,the methods that can accurately describe the dental arch and the morphology of the base bone have been applied.All these provide the basis for further study of the form of the dental arch and the base bone.This study is divided into two parts:Experiment One: Study on the morphology of the mandibular basal bone and dental arch of skeletal class II malocclusionObjective: To establish a basic research method to study the relationship between the mandibular bone and the dental arch in the patients with skeletal Class II and compare it with the Class I.Methods: 25 patients with skeletal Class II malocclusion and 25 cases of Class I normal occlusion models were selected.The mandibular model was scanned and a three-dimensional digital model was set up.The reference plane and coordinate system are established after the determination of the WALA ridge and the FA point.The polynomial curves which represent the corresponding curves of the base bone and the dental arch are fitted with the coordinates of each point.The width difference between the base bone and the arch curve at the depth of 3mm,10 mm and 18 mm was measured respectively.To compare the width difference between the base bone and the arch of the skeletal Class II and Class I.Result: 1.The width difference between the basal bone of the mandibular anterior segmentand the dental arch was 2.08 mm in Class I and-1.58 mm in Class II(P<0.05).2.The width difference between the basal bone of the middle mandibular segment and the dental arch was 2.92 mm in Class I and 1.84 mm in Class II(P<0.05).3.The width difference between the base of the posterior segment of the mandible and the dental arch was 4.24 mm in Class I and 3.80 mm in Class II(P > 0.05).Conclusion: 1.The width of the mandibular basal bone is greater than the width of the dental arch in Class I,and the dental arch is located inside of basal bone.2.The width of the anterior segment of mandibular dental arch in skeletal Class II is greater than the width of the basal bone,that is,the arch is located outside the basal bone.3.The width difference between the basal bone and the dental arch in the middle part is less than that of Class I.Experiment Two: Study on the morphology of mandibular basal bone and dental arch after orthodontic treatment of with extraction treatmentObjective: To explore the morphological changes of dental arch and bas al bone before and after orthodontic treatment with extraction for patients with skeletal Class II,and to provide reference for the design and selection of clinical treatment.Methods: The complete model of 25 patients with skeletal Class II malocclusion w as scanned after orthodontic treatment.The relationship between the mandibular basal bone and the dental arch was detected with the method in Experiment One,and was compared with previous result.Result: 1.After orthodontic treatment,0.84 mm,1.70 mm and 2.17mm(P < 0.05)were increased in the anterior,middle and posterior segments of the bas al bone,respectively.2.After extraction treatment,the anterior segment of dental arch decreased 3.35mm(P < 0.05),the middle segment decreased 0.71mm(P > 0.05),and the posterior segment increased 1.11mm(P < 0.05).3.The width difference between the basal bone and dental arch of anterior,middle and posterior segment was 2.60 mm,3.80 mm,4.86 mm respectively.Conclusion: 1.The width of the bone was increased in each segment after extraction treatment.The width of the anterior dental arch decreased and the width of the posterior segment increased.2.The width of the basal bone in the anterior segment was greater than the width o f the dental arch,and the arch was located inside the basal bone;the difference between the bone and dental arch of each segment increased.3.After the treatment,there was no significant difference in the relationship between the basal bone and dental arch in Class II and that in Class I.