B. Prahl-Andersen

Masseter muscle thickness in growing individuals and its relation to facial morphology

It is widely accepted that an interaction exists between masticatory muscle function and craniofacial growth. In adults, correlations have been found between facial dimensions and jaw-muscle cross-sectional area, and between facial dimensions and masseter muscle thickness. Little is known about growth of the human masticatory muscles and its relation with facial dimensions at different ages. In 329 Greek individuals, aged 7-22 yr, masseter muscle thickness was measured by ultrasonography. Muscle thickness was related to age, stature and weight, and to facial dimensions, measured by means of anthropological calipers. Muscle thickness was statistically assessed by univariate analysis of variance, after the males and females had been divided into three age groups. Facial dimensions were assessed by multivariate analysis of variance, age being considered as a covariate. The relation between muscle thickness and facial dimensions was subjected to stepwise multiple regression analysis. Masseter muscle thickness increased with age in both sexes. No differences were found between the left- and right-hand side. For each age group (and corrected for stature and weight), males had significantly thicker masseters than females (p < 0.01). Variation in muscle size and facial dimensions mainly coincided with variation in age, stature and weight. Apart from these, muscle thickness showed a significantly negative relation with anterior facial height and mandibular length, and a significantly positive relation with intergonial width and bizygomatic facial width.

Comparison of Jaw-muscle Bite-force Cross-sections Obtained by Means of Magnetic Resonance Imaging and High-resolution CT Scanning

Cross-sectional areas of the jaw muscles were determined by means of magnetic resonance imaging (MRI) in 12 healthy adult male subjects. These findings were compared with the cross-sectional areas of the jaw muscles of the same subjects, obtained by means of computer tomography (CT) in a previous study (Weijs and Hillen, 1985). Significant correlations (r>0.7) were found between the CT and MRI cross-sections of the masseter, medial pterygoid, and temporalis muscles. The low correlation between the CT and MRI cross-sections of the lateral pterygoid muscle could be explained by the different imaging techniques (slice thickness) of MRI and CT scanning. CT and MRI cross-sectional areas of the masseter and medial pterygoid muscle (but not the temporalis muscle) showed highly positive and significant correlations with the maximal voluntary bite force. In living subjects, the cross-sections of the masseter and medial pterygoid muscles can be visualized with CT and MRI. Compared with CT, MRI has some advantages, such as the absence of adverse effects (no radiation) and the excellent soft-tissue imaging. Furthermore, a series of frontal, horizontal, sagittal, and angulated MRI scans can be made without modification of the patients position, facilitating reconstruction of the jaw muscles.

A Comparison of Jaw Muscle Cross-sections of Long-face and Normal Adults

Long-face subjects have smaller maximum molar bite forces than do normal individuals. This has been attributed both to differences in moment arms and size of the jaw muscles. In this study, a comparison was made between the mid-belly cross-sectional areas of the jaw muscles of 13 long-face and 35 normal adults by means of serial MRI scans. The subjects were selected on the basis of anterior lower face height as a percentage of anterior total face height. These and other cephalometric variables were measured from lateral radiographs. In the long-face group, the cross-sectional areas of the masseter, medial pterygoid, and anterior temporal muscles were, respectively, 30%, 22%, and 15% smaller than in the control group. By a discriminant analysis and a multivariate analysis of variance, these differences were found to be significant (p < 0.001). The findings of this study hint that differences in the sizes of the jaw muscles of long-face and normal subjects might explain, in part, the observed differences in maximum molar bite force.

Quantification of facial morphology using stereophotogrammetry - demonstration of a new concept

OBJECTIVES The purpose of this study was to introduce stereophotogrammetry as a three-dimensional registration method for quantifying facial morphology and detecting changes in facial morphology during growth and development. METHODS Using stereophotogrammetry, three-dimensional (3-D) co-ordinates for the bilateral landmarks Exocanthion and Cheilion and the midsagittal landmark Pronasale were determined in 10 subjects to ascertain the reproducibility of the method, and in 59 children to detect changes in facial morphology due to growth and development. Linear and angular measurements were calculated by means of the 3-D co-ordinates in order to quantify facial morphology. Significant differences were determined by means of analyses of variance (MANOVA). RESULTS During the observation period, significant (P < 0.01) changes in facial morphology were determined for the linear measurements. Advantages and disadvantages of current registration methods are discussed. CONCLUSION It is concluded that stereophotogrammetry is a suitable 3-D registration method for quantifying and detecting developmental changes in facial morphology.

A COMPARISON OF HUMAN MASSETER MUSCLE THICKNESS MEASURED BY ULTRASONOGRAPHY AND MAGNETIC RESONANCE IMAGING

Non-invasive imaging techniques such as computerized tomography, magnetic resonance imaging (MRI), and ultrasonography enable measurements of the cross-section and thickness of human jaw muscles in vivo, providing an indication of the maximal force a muscle can exert. In 15 adult Caucasian men the thickness of the masseter muscle was registered bilaterally on three different levels by ultrasonography. Scans were made on the contracted and relaxed muscle. A comparison was then made with measurements from serial MRI scans, using univariate analysis of variance for repeated measurements and Pearsons correlation coefficients. Variances of the repeated measurements were calculated for the different scanning levels and the different muscle conditions and tested for homogeneity. For both the ultrasound and MRI measurements there was no difference in thickness between the left and right muscle. The registration level with highest reproducibility was halfway between the origin and insertion. Measurements from the contracted muscle were more reproducible than those from the relaxed muscle. The relaxed muscle thickness measured by ultrasonography was smaller than that measured by MRI. The correlation between ultrasound and MRI was significant for the upper and middle level of scanning (p < 0.001). The highest correlation was found between MRI (relaxed) and ultrasound (contracted) at the middle level (R = 0.83, p < 10(-6)). The conclusion is that ultrasonography is an accurate and reproducible method for measuring the thickness of the masseter in vivo. It allows for large-scale longitudinal study of changes in jaw-muscle thickness during growth in relation to change in biomechanical properties of masticatory muscles.

Method for quantifying facial asymmetry in three dimensions using stereophotogrammetry

A three-dimensional method to quantify facial asymmetry is introduced. Stereophotogrammetry was applied to determine three-dimensional (3-D) coordinates for eight pairs of surface landmarks of 106 individuals, including 16 with an operated complete unilateral cleft lip and palate. Facial asymmetry was quantified from four different reference planes that were defined perpendicular to and bisecting lines between pairs of bilateral landmarks related to the eyes, nose and mouth. Significant differences (P < 0.01) between these four planes were determined using multivariate analyses of variance (MANOVA). It is concluded that the best reference plane to select in studies of facial asymmetry is formed by the one which is perpendicular to and bisects the line that connects the landmarks Exocanthion. Reproducibility and validity of the method is demonstrated.

Jaw Muscle Orientation and Moment Arms of Long-face and Normal Adults

Long-face subjects have strongly reduced bite forces relative to normal subjects. This difference cannot be fully explained by the reduced cross-sectional area of the jaw muscles. In this study, we investigated whether the orientation and moment arms of the jaw muscles of normal and long-face subjects are different, and if so, to what extent these differences contribute to the observed differences in maximum molar bite-force levels. Three MRI scan series with different orientations were made of the jaw muscles of 30 normal and 13 long-face subjects. These served as the basis for computer reconstructions of the external shape of the muscles. The spatial orientation of the jaw muscles was defined by the regression line through the centroids of the muscular cross-sections. The moment arms of the jaw muscles and the bite point of the first mandibular molar were measured with respect to the center of the ipsilateral condyle. The muscular variables-including angles, moment arms, and mechanical advantage-were analyzed with a discriminant analysis and a multivariate analysis of variance (MANOVA). Differences in the spatial orientation of the temporalis muscle and the anterior digastric muscle contributed most to the distinction of the normal and long-face group. With MANOVA, it was shown that the normal and long-face group did not significantly differ with respect to the jaw muscle moment arms and mechanical advantage data. Only small differences were found between the sagittal muscle angles of the masseter and anterior digastric muscles in the two groups. In both the normal and long-face group, the orientation and moment arm data of the right and left muscles differed significantly. It was concluded that the variation of the spatial orientation of the jaw muscles is small and does not significantly contribute to the explanation of the different molar bite-force levels of long-face and normal subjects. Therefore, it is tempting to assume that the jaw muscles of normal and long-face subjects are different with respect to the maximum force they can exert per unit of cross-sectional area.

Longitudinal analysis of radiographic trabecular pattern by image processing.

To describe structural and textural changes in bone structure, the radiographic trabecular pattern of children, aged 4-14 years, was examined using a digital image processing system. This investigation is based on data from the Nijmegen Growth Study, a mixed-longitudinal growth study comprised of three birth cohorts, which were observed for five years. Of 3075 left hand-forearm radiographs of 426 children, a standard area of 10 x 10 mm of the radius was digitized and the trabecular pattern was described by ten image features. It is demonstrated that all image features show significant changes during the observation period (multivariate analysis of variance of p < 0.01). The age of the children correlates significantly with the image features (Spearmans Rho = 0.4; p < 0.05). Because of cohort effects, the three cohorts were studied separately. Highest correlations between age and trabecular pattern were shown in the cohort of the youngest children. The changes of the image features during the observation period seems to indicate that the fine trabecular pattern of young children changes into a coarser adult pattern. It is concluded that digital quantification of the radiographic trabecular pattern is appropriate to describe changes in trabecular bone.

Longitudinal study on three-dimensional changes of facial asymmetry in children between 4 to 12 years of age with unilateral cleft lip and palate.

The purpose of the present study was to describe three-dimensional developmental changes of facial asymmetry in children with an operated complete unilateral cleft lip and palate (UCLP) and in children without craniofacial anomalies (controls). Using stereophotogrammetry, three-dimensional coordinates for 16 bilateral and 10 midsagittal facial landmarks were determined for the UCLP group (n = 33) and the control group (n = 63) on two occasions. In this mixed-longitudinal study, the children were 4 to 12 years of age. Facial asymmetry and left-right dominance was measured and resolved for transverse, vertical, and sagittal components. Significant effects were analyzed with multivariate analyses of variance (MANOVA). We concluded that individuals with complete unilateral cleft lip and palate, as well as individuals without craniofacial anomalies, show an increase, during growth, in the amount of facial asymmetry in the basal region of the nose. In the region that is related to the cleft, children with complete unilateral cleft lip and palate do not show changes in the amount of facial asymmetry between the occasions. Regarding facial left-right dominance and variation in dominance, no demonstrable growth changes take place in individuals with complete cleft lip and palate, nor in individuals without craniofacial anomalies.

Intermittent compression stimulates cartilage mineralization.

The effects of intermittent hydrostatic compressive force (ICF; 13 kPa applied at 0.3 Hz frequency), as a substitute for moderate loading in vivo, on ossifying bone organ cultures, were evaluated by means of (histo)-morphometry. In earlier studies, biochemical tests have shown an increased 45Ca intake and an increased alkaline phosphatase activity in bone organ cultures that received ICF, suggesting that ICF promoted matrix mineralization. The purpose of this study was to examine whether an effect of ICF on mineralization can be described by means of histomorphometrical analysis. Fetal mouse metatarsal bone rudiments were cultured for 5 days in serum-free medium, with (experimental) or without (control) ICF. Linear measurements taken during culture demonstrated that the dark zone in the center of the rudiment, representing mineralized hypertrophic cartilage, became significantly longer in the group that received ICF when compared with the control group. This finding was in conformation with the former studies. Histological sections of the rudiments, stained with Goldners trichrome method were used to study changes at the cellular level and to describe the position and relative amount of mineralizing cartilage matrix (defined as Goldner-positive matrix [GPM]). Histomorphometry demonstrated that ICF treatment significantly increased the length of the hypertrophic cartilaginous zone and enhanced the amount of GPM between the mineralizing hypertrophic chondrocytes. However, the total length of the zone containing GPM was not increased, nor was the future bone collar, consisting of a thin osteoid seam, lengthened by ICF. These data indicate that the cellular processes involved in chrondrocyte hypertrophy were accelerated by ICF, as well as the extracellular processes leading to matrix mineralization. The study supports the earlier conclusion that embryonic bone rudiments are sensitive to mechanical stimulation and that moderate loading promotes their ossification in vitro.