The objective of this study was to assess the relationship between anterior tooth width and the presence or absence of maxillary lateral incisors.
Reference models of 106 subjects, 52 with bilateral agenesis of maxillary lateral incisors and 54 fully dentate controls, were examined. The sample was based on orthodontic patients treated over a 10-year period at Kent and Canterbury Hospital, Canterbury, Kent, United Kingdom. Maximum tooth widths were measured by using digital calipers (Tesa Technology, Renens, Switzerland).
Individual tooth dimensions in the maxillary and mandibular anterior segments were reduced by 0.33 to 0.42 mm, respectively, in subjects with agenesis of the maxillary lateral incisors. Linear regression analysis confirmed that these differences have statistical significance ( P <0.001).
Isolated bilateral absence of maxillary lateral incisors is associated with reduced mesiodistal tooth widths in both the maxillary and mandibular anterior segments.
Hypodontia of the maxillary lateral incisors has a prevalence rate of almost 2%. Bilateral agenesis is believed to be as common as unilateral. A genetic contribution to agenesis is well recognized, with geographic variation and a female predilection established ; candidate genes, including MSX-1, MSX-2, and PAX-9, have also been isolated. Hypodontia is associated with a variety of facial and occlusal traits including bimaxillary retrusion, reduced lower anterior facial height, delayed eruption, ectopic development of teeth, and generalized microdontia.
The relationship between moderate or severe hypodontia and generalized microdontia is well established. An association between mild hypodontia, including isolated aplasia of the second premolars and diminutive maxillary lateral incisors. has also been demonstrated. Similarly, agenesis of the third molars has been linked with generalized microdontia. A gradient for tooth-size reduction with varying degrees of hypodontia has been proposed ; mild hypodontia might be associated with smaller deviations from ideal tooth widths. However, to date, tooth-size alterations with isolated absence of the maxillary lateral incisors have rarely been investigated.
Treatment options for absent maxillary incisors include space recreation for prosthetic replacement and orthodontic space closure with canine substitution. Associated deviations in tooth size can have implications for orthodontic and restorative treatment planning. The aim of this research was therefore to assess the relationship between anterior tooth width and the congenital absence of both maxillary lateral incisors.
Material and methods
This cross-sectional study was carried out at Kent and Canterbury Hospital, Canterbury, Kent, United Kingdom. A total of 104 patients were required to demonstrate a clinically meaningful difference of 0.4 mm per tooth (mean, 9.44 mm; SD, 0.56 mm 6 ) in the tooth widths in the maxillary anterior segment between the respective groups, with a power of 90% and an alpha of 0.01.
A sample of 52 subjects with agenesis of both maxillary lateral incisors (group 2) was selected from the orthodontic patient records taken before orthodontic treatment. The subjects ranged in age from 12 to 18 years when initial diagnostic records and study models were obtained. All impressions were taken with alginate impression material and poured within 24 hours. A further 54 patients were recruited as a fully dentate control group (group 1). The total sample comprised 106 patients, all of white ethnic origin.
The inclusion criteria for those in group 1 were a complete permanent dentition with the anterior teeth from canine to canine fully erupted. The subjects in group 2 had bilateral agenesis of the maxillary lateral incisors without hypodontia remote to the maxillary lateral incisors. Adequate records including complete diagnostic records and treatment notes with pretreatment panoramic radiographs were available for all subjects. Exclusion criteria were craniofacial syndromes including cleft lip and palate, previous orthodontic treatment, previous trauma to the maxillary incisors, ectopic development of the maxillary canines, and previous restorative buildup of the anterior dentition. Panoramic radiographs were used to determine the presence of permanent teeth including the third molars.
Maximum mesiodistal tooth dimensions were measured on pretreatment reference models by using a digital caliper (150 mm ISO 9001 electronic caliper, Tesa Technology, Renens, Switzerland). The teeth were unmarked before measurement. The results were entered into a spreadsheet for analysis.
Analyses were performed with the Statistical Package for Social Sciences (version 13.0, SPSS, Chicago, Ill) with a prespecified level of statistical significance of P <0.01. Descriptive statistics were used to compare tooth sizes between the 2 groups. Due to the missing teeth in group 2, mean values for the maxillary anterior segment were based only on the averages of the maxillary right and left canines and the maxillary right and left central incisors.
Interobserver agreement between 2 sets of measurements of the same teeth was assessed by using the Bland-Altman method. The 95% limits of agreement (within which 95% of all differences between values should occur) were calculated as follows:
mean difference = ±1.96∗(standard deviation of differences).
A Kolmogorov-Smirnov test confirmed that the data were normally distributed. Therefore, linear regression analysis was performed; group and sex were the 2 explanatory factors. The interaction between sex and tooth dimensions was examined. If the interaction between the 2 variables was not statistically significant, this would be omitted from the analysis, and the difference between the groups would be examined for the sexes together.
Tooth dimensions in the anterior segments were measured twice by 1 operator (O.Y.) on 16 study models at an interval of 4 weeks. Agreement between the 2 sets of measurements was assessed by using Bland-Altman limits of agreement. The 95% Bland-Altman limits were –0.53 and 0.54. The small average difference between the repeat measurements indicated that 1 set of scores was not consistently higher than the other, with poor agreement between just 1 pair of observations. The results are displayed graphically ( Fig ), with differences in the pairs of scores plotted against the average of the 2 scores.
Average dimensions for each tooth ( Table I ) and for groups 1 and 2 overall and in the sexes in isolation were calculated ( Table II ). Linear regression was used to examine the effects of sex and the presence of maxillary lateral incisors on the mean maxillary and mandibular values. An association between sex and tooth size was not apparent ( P = 0.61, maxillary arch; P = 0.92, mandibular arch). This suggests that the difference between the 2 groups did not vary between the sexes ( Table III ). Therefore, sex was removed from further analysis. However, there was a significant difference in mean tooth size in both the maxillary and mandibular anterior segments between the groups ( P <0.001). The subjects with missing incisors had smaller teeth than did those who were fully dentate. In the maxillary arch, the mean between-group discrepancy was 0.42 mm per tooth; in the mandibular arch, the corresponding figure was 0.33 mm.