Date of Award


Document Type


Degree Name

Master of Dental Science (MDS)



Research Advisor

Edward F. Harris, Ph.D.


Russell A. Wicks, D.D.S., M.S. Richard A. Williams, D.D.S., M.S.


maxillary incisors, crown form, crowding


Dental crowding occurs when the mesiodistal tooth crown widths exceed the space available in the dental arch for proper alignment. Previous research dealing with this common orthodontic problem has measured mesiodistal (MD) tooth widths or clinical tooth height-to-width ratios from dental casts. The present study used full mouth periapical series of dental radiographs, which provided an opportunity to study anatomical crown form of the maxillary incisors, as measured from the cementoenamel junction (CEJ), in relation to crowding. The aim of this study was to evaluate the statistical association between maxillary incisor crown form and the extent of crowding in adolescent boys and girls who sought comprehensive orthodontic treatment. A recent suggestion is that incisor crown form (in contrast to size itself) affects the risk of anterior crowding. The sample consisted of 60 males and 91 females, with a mean age of 13.7 years and fully erupted maxillary central and lateral incisors. Periapical radiographs of the maxillary central (I1) and lateral (I2) incisors were scanned and digitized, and a computer program was used to measure linear dimensions and shape ratios for one maxillary central and one maxillary lateral incisor from each subject. Two complementary space analyses, Merrifield’s anterior space analysis (TSASD) and Little’s irregularity index, were performed on the dental casts to quantify crowding. In the present study, the average maxillary central incisor of males was an isometrically enlarged version of females. The average maxillary lateral incisor of females had a sexually dimorphic crown form characterized by a significantly smaller MD measurement at the level of the CEJ, which translated into more flared lateral incisor crowns. Analyzing the results by Angle classification, the maxillary lateral incisors in Class II division 2 subjects had a distinctive crown form characterized by shorter and narrower crowns. Overall, the maximum mesiodistal tooth width was the single significant predictor of TSASD and of incisor irregularity. The exception was the high predictive value of the width measurement at the level of the CEJ of the lateral incisor for predicting the irregularity index. Based on univariate and multivariate analyses, crown size of the maxillary incisors, rather than crown form, was most predictive of TSASD and of incisor irregularity. We found no suggestion that shape of the crowns were a governing factor of the predisposition to crowding. Larger teeth require more arch space to be well aligned, and in the absence of additional arch space, individuals with larger teeth display greater TSASD and incisor irregularity.