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Objective:The aim of this study was to determine how sensitive dental specialists and lay people are to maxillary incisor crowding when viewed from the front of the individual.Methods:Computer technology was used to create a series of photographs of the incisors of as miling woman as viewed from the front.The photographs showed varying degrees of maxillary incisor crowding as classified according to Little’s irregularity index(LⅡ).The incisors illustrated in the photos were ranked on a scale that ranged from perfect alignment to being severely crowded.The rating was done by the following groups of people:orthodontists(ORT),general dentists(GD),lay people with experience of orthodontic treatment(LWO),or lay people with no history of orthodontic treatment(LNO).Results:The ORT and the GD noted misalignment of one central inc is or when the LⅡ reached 1.5mm,whereas the LWO and the LNO were sensitive to2.0mm of crowding.When the LⅡ reached 2.0mm for one lateral incisor,it triggered ORT to consider providing orthodontic treatment,whereas this degree of irregularity was ignored by GD,LWO and LN O.When both central incisors were misaligned,the ORT were sensitive to the fact at 2.0mm of LⅡ,whereas the GD and the LWO became sensitive at 3.0mm of LⅡ,and the LNO were sensitive at 4.0mm of LⅡ.When both lateral incisors were misaligned,the ORT noted the crowding at a LⅡ of 3.0mm,the GD became sensitive at a LⅡ of 4.0mm,whereas both the LWO and the LNO ignored.When the crowding of all of the maxillary incisors reached a CⅡ of 4 mm both the ORT and the GD were alerted to the fact while both the LWO and the L NO were only sensitive to a total incisor crowding equal to a LⅡ of 6.0mm.Conclusions:Orthodontists are more critical than other groups when evaluating the misalignment of the upper incisors.It appears that the central incisors play a more important role than do the lateral incisors when dental crowding impacts on smile esthetics.For all of the observer groups,it also appears that people are more sensitive to the misalignment of a single tooth than they are to the same level of crowding distributed over multiple teeth.