Third molar development: measurements versus scores as age predictor

Abstract

Human third molar development is widely used to predict chronological age of sub adult individuals with unknown or doubted age. For these predictions, classically, the radiologically observed third molar growth and maturation is registered using a staging and related scoring technique. Measures of lengths and widths of the developing wisdom tooth and its adjacent second molar can be considered as an alternative registration. The aim of this study was to verify relations between mandibular third molar developmental stages or measurements of mandibular second molar and third molars and age. Age related performance of stages and measurements were compared to assess if measurements added information to age predictions from third molar formation stage. The sample was 340 orthopantomograms (170 females, 170 males) of individuals homogenously distributed in age between 7 and 24 years. Mandibular lower right, third and second molars, were staged following Gleiser and Hunt, length and width measurements were registered, and various ratios of these measurements were calculated. Univariable regression models with age as response and third molar stage, measurements and ratios of second and third molars as predictors, were considered. Multivariable regression models assessed if measurements or ratios added information to age prediction from third molar stage. Coefficients of determination (R²) and root mean squared errors (RMSE) obtained from all regression models were compared. The univariable regression model using stages as predictor yielded most accurate age predictions (males: R² 0.85, RMSE between 0.85 and 1.22 year; females: R² 0.77, RMSE between 1.19 and 2.11 year) compared to all models including measurements and ratios. The multivariable regression models indicated that measurements and ratios added no clinical relevant information to the age prediction from third molar stage. Ratios and measurements of second and third molars are less accurate age predictors than stages of developing third molars.

Introduction

Age estimation methods based on third molar growth are modelled on data registering and classifying a radiologically observed degree of third molar development. Third molar growth starts with the initial mineralisation of a cusp tip and stops at the end of root growth with closure of the apices. The intermediate tooth development can be assessed in different stages of growth. Accordingly multiple tooth staging and related scoring techniques were developed.1, 2, 3, 4, 5, 6, 7, 8 These techniques provide ordinal data and describe anatomic tooth features or predictions of future tooth part dimensions as reference points for identification of the used stages .The “complete calcification of the tooth crown” is an example of an anatomical borderline between two stages, “root half completed” indentifies a reference point without knowing the final root length once the specific tooth has stopped growing [Fig. 1]. The subjective approach in the second example is a drawback to use this scoring technique for age estimations. Furthermore the degree of third molar development between equally scored subjects can differ. The difference is maximally between subjects with features allowing them to be classified with a third molar development just passing the lowest threshold of a specific stage and subjects with a degree of third molar development classified just before the highest threshold of the same stage. Regardless the amount of stages described in the applied technique these differences remain. Both disadvantages could be avoided by measuring the lengths of the developing third molar on the obtained radiographs. These measurements provide continuous data and an objective, precise and highly reproducible tool of registration.9, 10, 11 Moreover these measurements allow to correct certain features. Geometric deformations inherent to radiographical set-ups, could be circumvented by calculating tooth measurement ratios.12, 13 Some deformations due to a tilted cheek position of the measured tooth could be detected and corrected. Taking into account dimensions of the second molar enables to diminish the variability in tooth size between individuals.

The aim of this study was to measure dimensions of third and preceding second molars on orthopantograms and to verify the significance of possible relations between these measurements and age. Furthermore whether measurements add information to age prediction once scoring of third molar development is performed, will be checked.