Demetrios J. Halazonetis

Sexual dimorphism of the human mandible and its association with dental development

The present study investigates whether the human mandible is sexually dimorphic during early postnatal development and whether early dimorphic features persist during subsequent ontogeny. We also examine whether mandibular dimorphism is linked to dimorphism of dental development. Dense CT-derived mandibular meshes of 84 females and 75 males, ranging from birth to adulthood, were analyzed using geometric morphometric methods. On the basis of the specimens chronological ages and mineralization stages of the deciduous and permanent teeth, we compute dental age as proxy for dental development by the additive conjoint measurement method. By birth, males have, on average, more advanced age-specific shapes than females. However, sex differences decrease quickly as females catch up via a different association between shape and size. This leads to an almost complete reduction of sexual dimorphism between the ages of 4 and 14. From puberty to adulthood, males are characterized by allometric shape changes while the shape of the female mandible continues to change even after size has ceased to increase. Dimorphism of dental maturation becomes visible only at puberty. Sexual dimorphism, concentrated at the ramus and the mental region during the earliest ontogenetic stages and again at adulthood, is not associated with the development of the teeth. At puberty there is a simultaneous peak in size increase, shape development, and dental maturation likely controlled by the surge of sex hormones with a dimorphic onset age. We argue that the infant and adult dimorphism of the mental region may be associated with the development of supralaryngeal structures.

Computer-assisted cephalometric analysis

C o m p u t e r s have entered the orthodontic specialty in two basic areas, practice management and orthodontic diagnosis. The latter area is mainly covered by computer-assisted cephalometric analysis systems. Although the advantages that a cephalometric analysis computer program may offer are probably less substantial than those of a practice management system, they are significant for both the orthodontic practitioner and the researcher1:

Geometric morphometric evaluation of cervical vertebrae shape and its relationship to skeletal maturation

INTRODUCTION Cervical vertebrae shape has been proposed as a diagnostic factor for assessing skeletal maturation in orthodontic patients. However, evaluation of vertebral shape is mainly based on qualitative criteria. Comprehensive quantitative measurements of shape and assessments of its predictive power have not been reported. Our aims were to measure vertebral shape by using the tools of geometric morphometrics and to evaluate the correlation and predictive power of vertebral shape on skeletal maturation. METHODS Pretreatment lateral cephalograms and corresponding hand-wrist radiographs of 98 patients (40 boys, 58 girls; ages, 8.1-17.7 years) were used. Skeletal age was estimated from the hand-wrist radiographs. The first 4 vertebrae were traced, and 187 landmarks (34 fixed and 153 sliding semilandmarks) were used. Sliding semilandmarks were adjusted to minimize bending energy against the average of the sample. Principal components analysis in shape and form spaces was used for evaluating shape patterns. Shape measures, alone and combined with centroid size and age, were assessed as predictors of skeletal maturation. RESULTS Shape alone could not predict skeletal maturation better than chronologic age. The best prediction was achieved with the combination of form space principal components and age, giving 90% prediction intervals of approximately 200 maturation units in the girls and 300 units in the boys. Similar predictive power could be obtained by using centroid size and age. Vertebrae C2, C3, and C4 gave similar results when examined individually or combined. C1 showed lower correlations, signifying lower integration with hand-wrist maturation. CONCLUSIONS Vertebral shape is strongly correlated to skeletal age but does not offer better predictive value than chronologic age.

Cone-beam computed tomography is not the imaging technique of choice for comprehensive orthodontic assessment

It was a pleasure to see that Dr Larson did not take the extreme view of proposing cone-beam computed tomography (CBCT) as a routine diagnostic modality—ie, for every patient, irrespective of malocclusion or other patient-specific factors—as some orthodontic postgraduate programs in the United States seem to do. Even so, he does recommend CBCT as the standard procedure, stating in his conclusions that “CBCT has replaced conventional lateral cephalograms and panoramic images as the most commonly ordered imaging for comprehensive orthodontic patients.” In my Counterpoint, I will try to present arguments against CBCT as the imaging technique of choice for comprehensive orthodontic assessment. Assuming that use for every patient is not advocated, what are the patient selection criteria? The answer should stem from a comprehensive assessment of the benefits and burdens to each patient. This assessment cannot be completely objective, but our decision making should be based on current evidence, which could also serve as the basis to develop general guidelines. Such guidelines already exist. The SEDENTEXCT project of the European Union had as its primary goal “to acquire key information necessary for sound and scientifically based clinical use of CBCT” and “to use this information to develop evidencebased guidelines dealing with justification, optimization and referral criteria for users of dental CBCT.” The guidelines section dealing with orthodontic diagnosis concludes that “large volume CBCT should not be used routinely for orthodontic diagnosis.” The British Orthodontic Society guidelines give a similar recommendation: “routine use of CBCT even for most cases of impaction of teeth . . . cannot yet be recommended.” A similar conclusion was adopted by the American Association of Orthodontists in 2010: “the AAO recognizes that while there may be clinical situations where a cone-beam computed tomography (CBCT) radiograph may be of value, the use of such technology is not routinely required for orthodontic radiography.” If guidelines already exist, what is the purpose of this debate? First, it is an opportunity to make these guidelines well known to the orthodontic community at a time when CBCT use is increasing. The SEDENTEXCT guidelines are based on a systematic review of the literature, thus representing current evidencebased knowledge at a confidence level much higher than this debate can achieve. Most importantly, however, is that these guidelines are not compulsory. The use of ionizing radiation is governed by law in most countries, but all the law requires is clinical justification. The guidelines are designed to assist the clinician in the justification process. I hope that this debate will convince clinicians to follow the guidelines’ recommendations.

Morphological integration between the cranial base and the face in children and adults

The primary aim of the present study was to assess morphological covariation between the face and the basicranium (midline and lateral), and to evaluate patterns of integration at two specific developmental stages. A group of 71 children (6–10 years) was compared with a group of 71 adults (20–35 years). Lateral cephalometric radiographs were digitized and a total of 28 landmarks were placed on three areas; the midline cranial base, the lateral cranial base and the face. Geometric morphometric methods were applied and partial least squares analysis was used to evaluate correlation between the three shape blocks. Morphological integration was tested both with and without removing the effect of allometry. In children, mainly the midline and, to a lesser extent, the lateral cranial base were moderately correlated to the face. In adults, the correlation between the face and the midline cranial base, which ceases development earlier than the lateral base, was reduced. However, the lateral cranial base retained and even strengthened its correlation to the face. This suggests that the duration of common developmental timing is an important factor that influences integration between craniofacial structures. However, despite the apparent switch of primary roles between the cranial bases during development, the patterns of integration remained stable, thereby supporting the role of genetics over function in the establishment and development of craniofacial shape.

Geometric morphometric analysis of craniofacial variation, ontogeny and modularity in a cross-sectional sample of modern humans.

This investigation aimed to quantify craniofacial variation in a sample of modern humans. In all, 187 consecutive orthodontic patients were collected, of which 79 were male (mean age 13.3, SD 3.7, range 7.5–40.8) and 99 were female (mean age 12.3, SD 1.9, range 8.7–19.1). The male and female subgroups were tested for differences in mean shapes and ontogenetic trajectories, and shape variability was characterized using principal component analysis. The hypothesis of modularity was tested for six different modularity scenarios. The results showed that there were subtle but significant differences in the male and female Procrustes mean shapes. Males were significantly larger. Mild sexual ontogenetic allometric divergence was noted. Principal component analysis indicated that, of the four retained biologically interpretable components, the two most important sources of variability were (i) vertical shape variation (i.e. dolichofacial vs. brachyfacial growth patterns) and (ii) sagittal relationships (maxillary prognatism vs. mandibular retrognathism, and vice versa). The mandible and maxilla were found to constitute one module, independent of the skull base. Additionally, we were able to confirm the presence of an anterior and posterior craniofacial columnar module, separated by the pterygomaxillary plane, as proposed by Enlow. These modules can be further subdivided into four sub‐modules, involving the posterior skull base, the ethmomaxillary complex, a pharyngeal module, and the anterior part of the jaws.

Quantitative description of the shape of the mandible

The purpose of this study was to provide quantitative data on the shape of the mandible at the period around the pubertal growth spurt and to test the hypothesis that early mandibular shape may influence the amount and direction of subsequent mandibular growth. Longitudinal data from lateral cephalograms of 55 white female and 39 white male subjects were used. The mandibular outline from articulare to gnathion was analyzed into cosine curves, according to the Fourier equation. The resulting Fourier coefficients, representing mandibular outline shape, were analyzed statistically in relation to age, sex, craniofacial pattern, and mandibular growth rotation. Statistically significant growth changes of the Fourier coefficients were observed, especially during the postpubertal period, indicating a decrease in the gonial angle with age. Sex-related differences in shape were observed at all ages, male subjects showing a more rounded shape of the mandible than female subjects. Mandibular shape, as represented by the Fourier coefficients, was correlated to cephalometric variables, indicating mandibular inclination, but only poorly to cephalometric variables, indicating anteroposterior jaw relation. Total rotation of the mandible during growth could not be predicted by mandibular shape.

Computer experiments using a two-dimensional model of tooth support

The purpose of this investigation was to study the factors that may affect the position of the center of resistance and center of rotation. A two-dimensional computer model of the periodontal ligament was developed. The model permitted the simulation of an isotropic (responding in the same manner regardless of the direction of the applied force) and nonisotropic periodontal ligament and allowed changes in root shape and in position and direction of force application. The center of resistance was found to depend on the distribution of root surface area. For a model of the upper central incisor, it was located at 42% of the root length measured from the alveolar crest. The presence of anisotropy in the periodontal ligament significantly affected the position of the center of resistance, which was in this case also affected by the direction of the applied force. Forces passing through the center of resistance produced translation of the modeled tooth in a direction not necessarily the same as the direction of the applied force. Tipping forces produced much larger stresses than forces causing translation. Simulation of periodontal involvement resulting in loss of attachment increased the stresses exerted on the periodontal ligament. The model permitted easy assessment of various factors that may influence the position of the center of resistance of teeth and revealed a potentially large variability in the position of the center of resistance and center of rotation, caused by variation of the properties of the periodontal ligament.

Novel software for quantitative evaluation and graphical representation of masticatory efficiency

Blending of chewing gums of different colours is used in the clinical setting, as a simple and reliable means for the assessment of chewing efficiency. However, the available software is difficult to use in an everyday clinical setting, and there is no possibility of automated classification of the patients chewing ability in a graph, to facilitate visualisation of the results and to evaluate potential chewing difficulties. The aims of this study were to test the validity of ViewGum - a novel image analysis software for the evaluation of boli derived from a two-colour mixing ability test - and to establish a baseline graph for the representation of the masticatory efficiency in a healthy population. Image analysis demonstrated significant hue variation decrease as the number of chewing cycles increased, indicating a higher degree of colour mixture. Standard deviation of hue (SDHue) was significantly different between all chewing cycles. Regression of the log-transformed values of the medians of SDHue on the number of chewing cycles showed a high statistically significant correlation (r² = 0.94, P < 0.01). ViewGum eliminates drawbacks of previous two-colour chewing gum test methods by the simplicity of its application. The newly developed ViewGum software provides speed, ease of use and immediate extraction of clinically useful conclusions to the already established method of chewing efficiency evaluation and is a valid adjunct for the evaluation of masticatory efficiency with two-colour chewing gum.

Geometric morphometric 3D shape analysis and covariation of human mandibular and maxillary first molars

Dental casts of 160 Greek subjects (80 males, 80 females) were scanned by a structured-light scanner. The upper and lower right first molar occlusal surface 3D meshes were processed using geometric morphometric methods. A total of 265 and 274 curve and surface sliding semilandmarks were placed on the upper and lower molar surfaces, respectively. Principal component analysis and partial least square analysis were performed to assess shape parameters. Molars tended to vary between an elongated and a more square form. The first two principal components (PCs), comprising almost 1/3 of molar shape variation, were related to mesiodistal-buccolingual ratios and relative cusp position. Distal cusps displayed the greatest shape variability. Molars of males were larger than those of females (2.8 and 3.2% for upper and lower molars respectively), but no shape dimorphism was observed. Upper and lower molar sizes were significantly correlated (r(2) = 0.689). Allometry was observed for both teeth. Larger lower molars were associated with shorter cusps, expansion of the distal cusp, and constriction of the mesial cusps (predicted variance 3.25%). Upper molars displayed weaker allometry (predicted variance 1.59%). Upper and lower molar shape covariation proved significant (RV = 17.26%, P < 0.0001). The main parameter of molar covariation in partial least square axis 1, contributing to 30% of total covariation, was cusp height, in contrast to the primary variability traits exhibited by PC1 and PC2. The aim of this study was to evaluate shape variation and covariation, including allometry and sexual dimorphism, of maxillary and mandibular first permanent molar occlusal surfaces.