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cclusion would result. This statement, which 100 years of experience has proved to be correct except when there are aberrations in the size of teeth, brilliantly simplified normal occlusion.
     Angle then described three classes of malocclusion, based on the occlusal relationships of the first molars:
     Class I: Normal relationship of the molars, but line of occlusion incorrect because of malposed teeth, rotations, or other causes.
     Class Ⅱ: Lower molar distally positioned relative to upper, line of occlusion not specified.
     Class Ⅲ: Lower molar mesially positioned relative to upper, line of occlusion not specified.
 
Fig. 1-3  The line of occlusion is a smooth (catenary) curve passing through the central fossa of each upper molar and across the cingulum of the upper canine and incisor teeth. The same line runs along the buccal cups and incisal edges of the lower teeth, thus specifying the occlusal as well as interarch relationships once the molar position is established.

     Note that the Angle classification has four classes: normal occlusion, Class I malocclusion, Class Ⅱ malocclusion, and Class Ⅲ malocclusion (Fig. l-4). Normal occlusion and Class I malocclusion shares the same molar relationship, but differ in the arrangement of the teeth relative to the line of occlusion. The line of occlusion may not be correct in Class Ⅱ and Class Ⅲ.
 
Fig. 1-4 Normal occlusion and malocclusion classes as specified by Angle. This classification was quickly and widely adopted early in the twentieth century. It is incorporated within all contemporary descriptive and classification schemes.

     With the establishment of a concept of normal occlusion and a classification scheme that incorporated the line of occlusion, by the early 1900s orthodontics was no longer just the alignment of irregular teeth. Instead, it had evolved into the treatment of malocclusion, defined as any deviation from the ideal occlusal scheme described by Angle. Since precisely defined relationships required a full complement of teeth in both arches, maintaining an intact dentition became an important goal of orthodontic treatment. Angle and his followers strongly opposed extraction for orthodontic purposes. With the emphasis on dental occlusion that followed, however, less attention came to be paid to facial proportions and esthetics. Angle abandoned extraoral because be found that it was not necessary to achieve proper occlusal relatonships.
     As time passed, it became clear that even an excellent occlusion was unsatisfactory if it was achieved at the expense of proper facial proportions. Not only were there esthetic problems, it often proved impossible to maintain an occlusal relationship achieved by prolonged use of heavy elastics to pull the teeth together as Angle and his followers had suggested. Extraction of teeth was
reintroduced into orthodontics in the 1930s to enhance facial esthetics and achieve better stability of the occlusal relationships.
     Cephalometric radiography, which enabled orthodontists to measure the changes in tooth and jaw positions produced by growth and treatment, came into widespread use after world War Ⅱ.
     These radiographs made it clear that many Class Ⅱ and Class Ⅲ malocclus本论文由英语论文网提供整理，提供论文代写，英语论文代写，代写论文，代写英语论文，代写留学生论文，代写英文论文，留学生论文代写相关核心关键词搜索。