R. Tanguay

Sexual Dimorphism in the Development, Emergence, and Agenesis of the Mandibular Third Molar

The development, alveolar emergence, and agenesis of the mandibular third molars, in a homogeneous group of French-Canadian children and young adults ranging in age from seven to 25 yr, were evaluated from 4640 panoramic radiographs using the method of Demirjian et al. 8 Clinical emergence was recorded from dental casts of children between 15 and 19 yr. The occurrence of bilateral agenesis was about 9%, without significant sexual difference. The left and right third molars had the same pattern of crown and root development and emergence. Girls were ahead of boys up to the second half of crown formation, but this sexual difference disappeared at the first stage of root formation. The root development course was faster in males than in females. At the apex closure, the sexual difference was much more marked for retarded cases (about 2.7 yr) than for median (1.5 yr) or advanced cases (0.4 yr). For alveolar and clinical emergence, males were about six mo ahead of females at the median level.

Effects of Food Texture and Sample Thickness on Mandibular Movement and Hardness Assessment during Biting in Man

This study was designed to investigate the relationship among jaw movements, physical characteristics of food, and sensory perception of hardness in man. Vertical movements of the mandible were recorded with an infrared tracking device in humans during biting on two test foods, carrot and cheese. Samples of standard length (2 cm) and width (2 cm) were prepared in three different thicknesses (0.5, 1.0, and 1.5 cm). Nine subjects were asked to perform two types of bite with their incisor teeth. In the first, they cut through the food, then stopped and spat out the pieces (bite alone); in the second, biting was followed by mastication and swallowing (bite+chew). The 12 conditions (thickness x3, food x2, and bite x2) were presented in a random order within each block, and blocks were repeated five times (60 trials per subject). Subjects also estimated the hardness of the samples twice for each condition on visual analogue scales (VAS) 100 mm long. The duration, vertical amplitude, and maximum vertical velocity of the mandible during biting were calculated by computer for the three phases of the movements (opening, and fast and slow closing). Multilevel statistical models were used for data analysis. The estimated hardness scores associated with the first bite of thin carrot (59.0 VAS units) was significantly greater than for cheese (16.8 VAS units). The type of bite had no significant effect on these scores, but the estimate of hardness was significantly greater for the thickest sample (+13.3 VAS units). Food type had its strongest effect on the slow-closing phase. In particular, the peak velocity that followed the fracturing of the food sample was much greater for carrot than for cheese (thin, 34.1 mm.s-1 vs. 26.6 mm.s-1), and the difference between foods increased with thickness. The amplitude of opening was significantly greater for the thickest sample than for the other two. There were no significant relationships between VAS scores and the movement parameters. These results suggest that, when humans bite food: (1) changing the thickness of food has a greater effect on movement parameters than changing from soft to hard food, (2) the parameters of biting change little if biting is followed by mastication, (3) hardness perception is dependent on the thickness of food, (4) hardness perception is not different when food is removed from the mouth than when it is chewed and swallowed, and (5) there is no relationship between any of the parameters of movement that change with food type and the perceived hardness of food.

A polynomial approach to craniofacial growth: Description and comparison of adolescent males with normal occlusion and those with untreated Class II malocclusion

Orthogonal polynomials are used to model the craniofacial growth of adolescent boys, aged 11 through 14 years, and to evaluate variation between normal occlusion and untreated Class II malocclusion. The results show linear growth (velocity) for the maxillary measures; their angular relationships to the cranial base remain stable throughout the age range. Mandibular measures show growth velocity and acceleration, indicating the adolescent growth spurt. For the majority (80%) of measures, boys with normal occlusion and those with Class II malocclusion were not significantly different. Mean growth velocity of basion-nasion is significantly greater for subjects with Class II, Division 1 malocclusion. Total mandibular length and the ANB angle display significant mean size (constant) differences between boys with normal occlusion and boys with untreated Class II malocclusion. The groups are comparable in growth velocity and acceleration, indicating that the size differences are established before 11 years of age and maintained during adolescence.

Sexual Dimorphism in the Emergence of the Deciduous Teeth

Exact ages of emergence of the twenty deciduous teeth have been investigated on a longitudinal sample of 195 French-Canadian children (113 boys and 82 girls). A multivariate analysis of variance yielded a significant overall sex difference (p<O. 0001). It has been found that the emergence of the deciduous dentition in boys is more precocious than that in girls by about one month; this difference is present at the emergence of the upper central incisor and is maintained henceforth. The only notable exception to this pattern is the emergence of the first molars. Results of the present study, combined with those of previous research, suggest that sexual dimorphism in the emergence of the deciduous teeth could be mediated through ethnic factors.

Radiopacity of glass ionomer dental materials

The radiopacity of glass ionomer dental materials is quite variable. The use of a poorly radiopaque material as a base under other restorative materials can mislead the dentist to a diagnosis of recurrent decay. This study investigates the radiopacity of these materials and proposes a minimal radiopacity under which a material should not be used as a base or liner. All base, liner, and core formulations of glass ionomer under investigation were more radiopaque than dentin. All restorative and luting formulations of glass ionomer under investigation were less radiopaque than dentin and therefore should be avoided as bases or liners.

Modeling longitudinal mandibular growth: Percentiles for gnathion from 6 to 15 years of age in girls

Growth of the cephalometric landmark gnathion is modeled mathematically with multilevel statistical techniques. The findings, pertaining to a mixed longitudinal sample (N = 772) of 105 girls, 6 to 15 years of age, provide the most accurate descriptions of longitudinal mandibular growth presently available. Polar, rather than rectangular, coordinates are used to better distinguish between the amount and direction of growth. The velocity curve for sella-gnathion includes growth spurts during childhood (7.5 years) and adolescence (12.7 years). Growth direction of gnathion changes regularly throughout the age range, indicating a relative increase of vertical over horizontal growth. These reference standards serve as a basis for comparing and better understanding abnormal growth patterns.

Multilevel models for repeated measures: a flexible approach for studying dental arch morphology

Traditional OLS and recently developed multilevel IGLS statistical techniques for analyzing repeated measures are compared and contrasted. Comparisons are based on arch width measurements for a mixed‐longitudinal sample of 166 boys and girls. The multilevel IGLS procedures produce estimates that are more stable and meaningful than OLS estimates; sex differences are consistent for both the deciduous and permanent dentitions, maxillary widths are related to chronological age, and standard errors are consistently smaller based on the IGLS procedures. The multilevel procedures are also able to estimate variances and covariances—even when all measurements are not available for each subject—which can be used for predicting arch length.

Adjusting Craniofacial Correlations for Technical Error

Reliability estimates provide a means of adjusting observed correlations for technical error. The results show that the true correlations among 11 craniofacial landmarks are consistently higher than observed values. Consequently, the regression slopes defining these relationships are also increased. The less reliable two measures are, and the closer their joint reliability approximates the observed correlation, the greater the expected change in true correlation. Adjusting craniofacial relationships for technical error may substantially increase the proportion of variation explained, and thereby alter statistical inferences drawn from results.