Establishment Of Computerized Analysis System For The Arch Length Measurement

Dental cast is an important medium to record and preserve the information about patient's teeth, dental arch, alveolar, basal bone, palate and occlusion. The result of dental cast measurement can help orthodontics diagnose and work out treatment plan. Dental crowding is one of important factors for the decision of extraction and non-extraction treatment. Dental crowding is defined as a discrepancy between tooth size and arch available length that results in a misalignment of the tooth row. Arch available length is an important part of dental crowding analysis. The accuracy of arch available length measurement directly affects analysis of dental crowding and treatment plan. Now the common measurement methods on clinic are wire and dividers. We find that the results measured by the two methods for a same cast are different, and the results measured by different people or at different time are different. Therefore, in this study, the differences of the repeatibility and reliability between results measured by the two ways were analyzed systematically. Then, we established digital models and computer program for arch length measurement to improve the reliability and repeatibility. And the established way was applied on the analysis of the Spee curve and the space requirement for levelling the Spee curve.The thesis consists of two parts:Part one is the comparative study of wire and dividers applied in orthodontic dental models of arch length measurement. This study shows that it's necessary to establish the computer analysis system for arch length measurement. Methods: 80 pairs of dental models were selected, including 40 normal occlusion models and 40 crowded models. The arch length of every model was measured with the methods of wire and dividers respectively, and the casual error and paired t test was subsequently employed to compare the differences of the measurements obtained. Results: The variance coefficients of the dental length results measured by the dividers was larger than that measured by the wire no matter in the models with crowded or normal occlusion(P〈0.05). The dental arch length measured by the wire method was significantly larger than that measured by the dividers method(P〈0.05). The differences between the measurement results in the two methods were 4.4±3.0mm and 5.2±1.7mm in the dental models with crowded and normal occlusion, respectively. It is suggested that the repeatibility and reliability of the two ways are not enough for the clinical diagnosis needs. Therefore, we decided to establish a computerized analysis system for arch length measurement.Part two is the establishment of the computerized system for the arch length analysis. Methods: Dental cast was scanned by a 3D (three dimensional) laser scanner (Minolta Vivid 910) to form a digital model. The mathematical method was used to evaluate the dental arch length. Results: Digital dental model measurements could be analyzed in plane and in space. The results were more reliable and repeatable than clinical methods of wire and dividers. The calculation of the space for levelling the curve of Spee in the digital model was more accurate than clinical estimation.The study began with the problem of clinical methods of wire and dividers in arch length measurement. Then we established computerized system applied in arch length analysis of dental cast. Based on the system, the reliable information could be provided for the orthodontic treatment plan decision. The established system can also be used for preservation of dental model as 3D digital model, manifestation of 3D digital dental model, assistance of teaching, etc.