A comparison of jaw muscle cross-sections of long-face and normal adults

Relationship between masseter muscle size and maxillary morphology

The aim of this study was to investigate the relationship between masseter muscle size and craniofacial morphology, focusing on the maxilla. Twenty-four patients (11 males and 13 females; mean age 27.6 ± 5.6 years) underwent cephalometric analyses. Ultrasonography was used to measure the cross-sectional area (CSA) of the masseter muscle and bite force was measured using pressure sensitive film. The results showed that CSA-relaxed was positively correlated with upper anterior face height (UAFH)/total anterior face height (TAFH) and negatively with lower anterior face height (LAFH)/TAFH and LAFH (P < 0.05). CSA-clenched was correlated positively with SN-palatal, FH-palatal, UAFH/TAFH, and lower posterior face height (LPFH)/total posterior face height (TPFH) and negatively with LAFH/TAFH, LAFH, upper posterior face height (UPFH)/TPFH, and UPFH (P < 0.05). Bite force was positively correlated with LPFH/TPFH and negatively with UPFH/TPFH (P < 0.05). As the masseter became larger, the anterior maxillary region tended to shift downwards relative to the cranial base, whereas the posterior region tended to shift upwards. The decrease in LAFH/TAFH and increase in LPFH/TPFH as the size of the masseter muscle increases may be influenced not only by the inclination of the mandibular plane but also by the clockwise rotation of the maxilla.

Mastication, EMG activity and occlusal contact area in subjects with different facial types

Dentofacial morphology may affect orofacial functions, therefore the aim of the current study was to evaluate the influence of craniofacial morphology on masticatory function, occlusal contact area (OCA), and masticatory muscles activity. Seventy-eight (78) subjects were divided into three groups according to vertical facial pattern: 1. mesofacial; 2. brachyfacial; and 3. dolichofacial. Artificial material and the sieving method were used to access masticatory efficiency (ME). OCA was determined by registration of posterior teeth. Electromyographic (EMG) activity of the masseter and anterior temporal (AT) muscles was accessed bilaterally at rest and at maximal vertical clenching (MVC). ME (%) was significantly higher in brachyfacial and lower in dolichofacial subjects. Brachyfacials presented the highest OCA (mm2) followed by meso and dolichofacial subjects. The EMG of the masseter and AT at rest and at MVC showed that dolichofacial subjects presented the lowest activity values, while brachyfacial subjects presented significantly higher measurements. Craniofacial morphology affected masticatory function, OCA, and EMG activity of the masticatory muscles.

Morphological observation of the medial pterygoid muscle by the superimposition of images obtained by lateral cephalogram and MRI

OBJECTIVE

To observe the morphological relationship between the maxillofacial skeleton and medial pterygoid muscle by superimposing images constructed by MRI on a cephalogram.

DESIGN

Cross-sectional study.

SETTING

The Departments of Maxillofacial Orthognathics and Orthodontics, Tokyo Medical and Dental University.

PARTICIPANTS

Sixteen patients (5 males and 11 females, aged between 13.5 and 27.5 years) with various craniofacial skeletal patterns, who were about to start orthodontic treatment.

METHODS

Lateral cephalometric radiographs and MRI scans were obtained and their images uploaded to a computer using a digitizer. The area of the medial pterygoid muscle was selected by binarization from the MRI. The mid-sagittal-plane MRI with a complete superimposed sagittal image of the medial pterygoid muscle was superimposed on the cephalogram using anatomical structures situated in the mid-sagittal plane of the head and shapes that could be identified from both the radiograph and the MRI image.

RESULTS

These combined images showed various shapes of the medial pterygoid muscle. The inclination axis of the medial pterygoid muscle was correlated with various cephalometric variables including SNB (r=0.658), Facial angle (r=0.601), ramus inclination (r=0.676) and Ba-Po% (r=0.585). The volume of the medial pterygoid muscle was also correlated with cephalometric variables such as ramus inclination (r=0.453), Ba-Nmm (r=0.676), Ba-Po% (depth) (r=0.447), Ar-Go% (depth) (r=0.444) and Ar-Go% (actual length) (r=0.532).

CONCLUSIONS

Morphometric analysis using a superimposed image of the medial pterygoid muscle produced from a cephalogram and MRI may help explain the influence of the medial pterygoid muscle inclination axis and volume on the shape of the mandibular bone, especially the shape of the ramus.

The adaptive response of jaw muscles to varying functional demands

Jaw muscles are versatile entities that are able to adapt their anatomical characteristics, such as size, cross-sectional area, and fibre properties, to altered functional demands. The dynamic nature of muscle fibres allows them to change their phenotype to optimize the required contractile function while minimizing energy use. Changes in these anatomical parameters are associated with changes in neuromuscular activity as the pattern of muscle activation by the central nervous system plays an important role in the modulation of muscle properties. This review summarizes the adaptive response of jaw muscles to various stimuli or perturbations in the orofacial system and addresses general changes in muscles as they adapt, specific adaptive changes in jaw muscles under various physiologic and pathologic conditions, and their adaptive response to non-surgical and surgical therapeutic interventions. Although the jaw muscles are used concertedly in the masticatory system, their adaptive changes are not always uniform and vary with the nature, intensity, and duration of the stimulus. In general, stretch, increases neuromuscular activity, and resistance training result in hypertrophy, elicits increases in mitochondrial content and cross-sectional area of the fibres, and may change the fibre-type composition of the muscle towards a larger percentage of slow-type fibres. In contrast, changes in the opposite direction occur when neuromuscular activity is reduced, the muscle is immobilized in a shortened position, or paralysed. The broad range of stimuli that affect the properties of jaw muscles might help explain the large variability in the anatomical and physiological characteristics found among individuals, muscles, and muscle portions.

Cephalometric changes produced by locally applied anabolic steroid in Wistar rats

BACKGROUND AND OBJECTIVE

There is a strong relationship between neuromuscular activity and facial skeletal morphogenesis. The systemic use of anabolic steroid accelerates craniofacial growth and induces high functional activity of rats' masticatory muscles. The present study examines the effect of an anabolic steroid locally applied in the masseter muscle of growing rats with the purpose of verifying possible craniofacial changes with minimum drug side effects, using cephalometric analysis.

MATERIALS AND METHODS

Forty-three 3-week-old Wistar rats were evaluated and divided into two groups: (1) NAN group (local bilateral masseter injection of nandrolone decanoate - 0.05 microg/3 microl/bi-weekly) and (2) CON group (local bilateral masseter injection of sterile oil - 3 microl). Tukey and Wilcoxon tests were used.

RESULTS

In NAN group the cephalometric measures showed horizontalized facial growth (reduce of FMA angle -p<0.01), increase of posterior facial height, decrease of anterior facial height, reduction of gonial angle (p<0.05), increase of mandibular height (p<0.01). There was no variation in the length of the mandible, in the skull and animals' weight gain, thus demonstrating that no important systemic effect occurred.

CONCLUSION

The local injection of nandrolone in the masseter muscle of rats was capable to alter the growth direction and the morphology of the craniofacial complex in Wistar rats.

Masseter muscle thickness and mechanical advantage in relation to vertical craniofacial morphology in children

OBJECTIVE

To investigate the relationship between vertical craniofacial morphology and masseter muscle thickness and mechanical advantage in children.

MATERIAL AND METHODS

The sample comprised 72 children (36 F, 36 M), 8.5-9.5 years of age, with various malocclusions and no previous orthodontic treatment. The thickness of the masseter was measured bilaterally by means of ultrasonography, and the recordings were performed both in relaxation and under contraction. Mechanical advantage was measured on the lateral cephalograms as the ratio between the masseter moment and the bite force moment arms. Two linear ratios and three angular measurements were used to describe vertical craniofacial morphology.

RESULTS

The mean masseter thickness was greater in the male group (p<0.05) in both relaxed and contracted conditions. There were no significant sex differences for the mechanical advantage or for the measurements of vertical craniofacial morphology. In females, there is a positive association between masseter muscle thickness and its mechanical advantage. Multiple regression analysis showed a positive association between posterior to anterior facial height ratio in both genders and a negative association between masseter thickness and the intermaxillary angle in females.

CONCLUSIONS

There is a significant association between posterior to anterior facial height and the masseter muscle in children. The importance of the masseter muscle is more evident in the vertical facial morphology of females.

Mandibular muscle morphology in children with different vertical facial patterns: A 3-dimensional computed tomography study

INTRODUCTION

This study was undertaken to assess whether 3-dimensional computed tomography (CT) can be used to evaluate the relationships between the mandibular muscles and craniofacial morphology in children with different underlying vertical facial patterns.

METHODS

Twenty children (mean age, 11.9 +/- 1.6 years) underwent cranial CT examination. Three-dimensional CT images were reconstructed for the evaluation of the cross-sectional size, volume, and spatial orientation of the masseter, medial pterygoid, and lateral pterygoid muscles. These muscle factors were also assessed in relation to vertical and transverse craniofacial form.

RESULTS

Positive correlations were found between the muscles' cross-sectional area and volume, and between muscle size and transverse facial width. Despite the limited sample size, differences were also found in the orientation of the masseter and medial pterygoid muscles in growing patients with different underlying vertical facial patterns.

CONCLUSIONS

Three-dimensional CT can be used for the assessment of soft- and hard-tissue dentofacial forms. Clinicians should note the potential differences in muscle cross-sectional area, volume, and orientation in subjects with different underlying vertical facial patterns.

A comparison of human jaw muscle cross-sectional area and volume in long- and short-face subjects, using MRI

OBJECTIVE

In humans, the vertical craniofacial dimensions vary significantly with the size of the jaw muscles, which are regarded as important controlling factors of craniofacial growth. The functional relevance of the maximum cross-sectional area (CSA), indicating maximum muscle strength, is questionable since peak forces are generated only a fraction of the day. Alternatively, muscle volume (indicating the generated loads) might be a more meaningful functional variable. Therefore, the aim of this study was to investigate if jaw muscle volume is stronger related with vertical craniofacial dimensions than with jaw muscle CSA.

DESIGN

Thirty-one adult healthy subjects with varying vertical craniofacial morphology participated in this study. Axial magnetic resonance imaging (MRI) scans were used for segmentation of the masseter (Mas) and medial pterygoid muscles (MPM). This enabled measurements of the muscle CSA and volume. Cephalometric analysis was performed using lateral radiographs. With factor analysis, the number of cephalometric variables was reduced into two factors that represented the anterior face height and the posterior face height (PFH), respectively. Subsequently, mutual relationships between these factors and muscular variables were assessed using a multiple regression analysis.

RESULTS

It was found that vertical craniofacial dimensions were significantly better (up to 12%) related with muscle volume rather than with CSA. Furthermore, it was shown that especially the PFH factor was significantly correlated with the Mas and MPM.

CONCLUSION

Vertical craniofacial dimensions are stronger related with jaw muscle volume than with CSA. Tentatively, it can be assumed that the generated muscle loads, rather than maximum forces, influence vertical craniofacial growth.

Relationship Between the Morphologies of the Masseter Muscle and the Ramus and Occlusal Force in Patients With Mandibular Prognathism

PURPOSE

The purpose of this study was to examine the relationship between the morphologies of the masseter muscle and the ramus and occlusal force in patients with mandibular prognathism.

PATIENTS AND METHODS

The study group consisted of 71 patients with mandibular prognathism. They were divided into 2 groups, consisting of prognathism with or without symmetry, determined by frontal cephalogram analysis. All patients underwent 3-dimensional (3D) computed tomography (CT) and occlusal force was recorded with pressure-sensitive sheets.

RESULTS

In the cross-sectional area of masseter muscle, there were no significant differences between the right and left sides in the symmetry and asymmetry groups. In occlusal force, there was no significant difference between the symmetry and asymmetry groups. Occlusal force was not significantly correlated to the cross-sectional area of the ramus, but it was significantly positively correlated to the cross-sectional area of the masseter muscle (P < .05).

CONCLUSION

Occlusal force was associated with the ipsilateral cross-sectional area of masseter muscle in patients with prognathism; however, it was not associated significantly with the degree of mandibular deviation.

The mandibular muscles and their importance in orthodontics: a contemporary review

It is widely accepted that the orthodontist should have a thorough understanding of the craniofacial musculature and its association with the growth and development of the dentofacial complex. There is still much controversy regarding the influence of the mandibular muscles on normal growth and development, and on orthodontic treatment and stability. This review presents an outline of the mandibular muscles and the vertical facial pattern. The different methods by which the mandibular muscles have been investigated are discussed. The potential influence of these muscles on normal morphologic variation in different people is also discussed, along with the implications for contemporary orthodontic treatment and stability.

Developmental changes in craniofacial morphology in subjects with Duchenne muscular dystrophy

Lateral cephalometric radiographs of 35 Japanese male patients suffering from Duchenne muscular dystrophy (DMD) were taken longitudinally from 10 to 20 years of age. Eighteen landmarks were placed and 15 angles and four linear distances calculated. Profile diagrams (profilograms) were produced to analyse changes in craniofacial morphological growth in the DMD subjects. The measurements were then compared with Japanese standards. In young patients with DMD, compared with the controls, the following were observed: a large gonial angle; clockwise rotation of the mandible; short sagittal length of the cranial base and protrusion of the upper incisors. In adult patients, the maxillary alveolus and the upper incisors were protruded, compared with the controls. Overbite in DMD subjects also showed a tendency to decrease. In the controls, mandibular growth direction tended to be straight down and forward, while in patients with DMD, the growth direction was down until approximately 16 years of age and, after that, a forward vector of growth was apparent. As a result, the tendency towards a clockwise rotation of the mandible in the adults was less than in the young patients. These findings showed that DMD significantly affects craniofacial morphology.

Fiber-type composition of the human jaw muscles--(part 2) role of hybrid fibers and factors responsible for inter-individual variation

This is the second of two articles about fiber-type composition of the human jaw muscles. It reviews the functional relationship of hybrid fibers and the adaptive properties of jaw-muscle fibers. In addition, to explain inter-individual variation in fiber-type composition, we discuss these adaptive properties in relation to environmental stimuli or perturbations. The fiber-type composition of the human jaw muscles is very different from that of limb and trunk muscles. Apart from the presence of the usual type I, IIA, and IIX myosin heavy-chains (MyHC), human jaw-muscle fibers contain MyHCs that are typical for developing or cardiac muscle. In addition, much more frequently than in limb and trunk muscles, jaw-muscle fibers are hybrid, i.e., they contain more than one type of MyHC isoform. Since these fibers have contractile properties that differ from those of pure fibers, this relatively large quantity of hybrid fibers provides a mechanism that produces a very fine gradation of force and movement. The presence of hybrid fibers might also reflect the adaptive capacity of jaw-muscle fibers. The capacity for adaptation also explains the observed large inter-individual variability in fiber-type composition. Besides local influences, like the amount of muscle activation and/or stretch, more general influences, like aging and gender, also play a role in the composition of fiber types.

3D CT evaluation of masseter muscle morphology after setback osteotomy for mandibular prognathism

OBJECTIVE

Following mandibular setback osteotomy, changes in the direction, length, and cross-sectional area of the masseter muscle were studied by means of computerized tomography (CT) images generated with a 3-dimensional (3D) reconstructive technique.

STUDY DESIGN

Pre- and postoperative CT examinations were performed on 17 prognathic patients treated by sagittal split ramus osteotomy with rigid osteosynthesis and 13 patients treated by intraoral vertical ramus osteotomy without osteosynthesis. The pre- and postoperative masseter muscle direction and length were evaluated using 3D CT images observed from a lateral viewing angle. The cross-sectional area of the masseter muscle was first measured on an axial CT image of a selected slice level, following which the right-angle cross-sectional area of the muscle was revised using the measured area from the axial image.

RESULTS

Postoperatively, anterior tilting of the masseter muscle was observed; however, masseter muscle length was unchanged. Three months postoperatively, a significant reduction in the cross-sectional area of the masseter muscle was seen. A tendency to revert back to the normal dimension was seen between 6 months and 1 year postoperatively. No significant difference was noted between the 2 surgical techniques.

CONCLUSIONS

Three-dimensional computed tomography is an adequate imaging modality for masseter muscle evaluation. The results of this study suggest the masseter muscle may undergo reversible atrophy after mandibular setback osteotomy.

Buccolingual inclinations of posterior teeth in subjects with different facial patterns

This study compared the buccolingual inclination of the posterior teeth in subjects with a horizontal growth pattern (proportionally short lower anterior face height) with that of subjects with a vertical growth pattern (proportionally long lower anterior face height). Seventy subjects with permanent dentition were divided into 2 groups. Group I comprised 35 subjects (16 male, 19 female) with Class II Division 2 malocclusion with a horizontal growth pattern, and group II comprised 35 subjects (15 male, 20 female) with a vertical growth pattern. Buccolingual inclinations of the first molar and second premolar were indirectly assessed on photocopies of buccolingual sections of these teeth by measuring their occlusal surface (represented by an imaginary line connecting the lingual and buccal cusps) inclination. The groups were compared with t tests (P <.05). The maxillary posterior teeth of subjects with a vertical growth pattern had a significantly greater buccal inclination compared with those of subjects with a horizontal growth pattern. However, there were no statistically significant differences in the inclinations of the mandibular posterior teeth between the 2 groups.

The overbite and intraoral mandibular distraction osteogenesis

PURPOSE

The purpose of this study was to quantify the overbite reduction in skeletal Angle Class II malocclusions and discuss the management of the overbite during and after intraoral distraction of the mandible, and during orthodontic treatment with fixed appliances.

MATERIAL AND METHODS

Cephalograms of 26 patients with an Angle Class II malocclusion and orthodontic appliances and distraction osteogenesis of the horizontal part of the mandible before (T0) and at least 1 year after treatment (T1) were evaluated. Mean age of the patients at the time of distraction was 14.6 years (range 12.8-15.9 years) and at the final registration 17.3 years (range 14.6-20.4 years).

RESULTS

The overbite decreased significantly and the SpP/MP value increased significantly. The increase in the value of the Y-axis and the MP/SN angle before and after treatment was statistically insignificant.

CONCLUSION

Opening of the bite during distraction of the mandible can be expected. The use of the 'floating bone' technique did not correct the overbite permanently. The patients in whom an open bite has already been treated are not ideal for mandibular lengthening by means of distraction osteogenesis.

Magnetic resonance spectroscopy of the masseter muscle in different facial morphological patterns

The aims of this study were (1) to develop a reliable noninvasive method to evaluate the masseter muscle metabolism, by using (31)P-magnetic resonance spectroscopy, and (2) to evaluate the metabolic profile of the masseter muscle in subjects with various facial patterns. The maxillary-mandibular relationship, which varied from hypodivergent to hyperdivergent, was measured on lateral cephalograms of 20 adults, 22 to 35 years of age. (31)P-spectra were acquired from the masseter muscle at rest with a custom-made, single-turn, double-tuned, 3 x 5-cm oblong surface coil. The inorganic phosphate to phosphocreatine (Pi/PCr) ratios were measured and compared in relation to vertical and sagittal cephalometric measurements. A statistically significant (R(2) = 0.65, r = 0.81, P = .001) relationship was found between Pi/PCr ratio and the palatal-to-mandibular plane angle. As the maxillary-to-mandibular divergence increased, the Pi/PCr ratio decreased. This correlation suggests that muscles with a higher Pi/PCr ratio have a higher resting metabolic activity than those with a lower Pi/PCr ratio. Consequently, these muscles may keep bone under more tension and influence its growth in a more horizontal direction. Another possible explanation of the results is that the fiber type composition of the masseter muscle varies with facial morphology.

Effects of twin-block therapy on protrusive muscle functions

Protrusive mandibular function, including maximum protrusive force and fatigue time, was investigated in 66 children displaying Class II Division 1 malocclusion. Thirty-two children were treated with the Clark Twin-block appliance and the other 34 children served as untreated controls. The observation period was 6 months. Cross-sectional data based on pretreatment records showed that maximum protrusive force ranged from 18.5 N to 160 N, with a mean of 80.3 +/- 30.7 N. Maximum protrusive force was significantly higher in males than in females (P <.001). The correlation between maximum protrusive force and chronologic age was low (r = 0.20) and did not reach significance. Maximum protrusive force in the group of children with disk displacement was not significantly different from that of the group without disk displacement. Comparison of pretreatment and 6-month records in the untreated control group revealed a significant increase in maximum protrusive force (P <.01) as a result of normal growth, while the measured change in the Twin-block-treated children did not reach significance. Fatiguing the protrusive muscles did not alter mandibular position in the Twin-block group after 6 months of treatment. The present study does not support the lateral pterygoid hypothesis, as there was no evidence of an increase in mandibular protrusive function after treatment with the Twin-block functional appliance.

Masticatory muscle function in patients with spinal muscular atrophy

The purpose of this study was to determine whether spinal muscular atrophy affects masticatory muscle strength and mandibular range of motion. A sample of 15 subjects with spinal muscular atrophy was compared to a sample of age-matched and sex-matched controls. Maximum bite force, masticatory muscle electromyography activity, mandibular ranges of motion and masticatory muscle endurance were evaluated. Results showed that maximum bite forces were one-half as great for the sample with spinal muscular atrophy than for the controls, even though their EMG activity was not significantly different. Slopes of the relationship between electromyography activity and bite force were two to four times steeper for patients with spinal muscular atrophy than controls. Maximum opening and protrusion were reduced to approximately one-half control values. Fatigue times of patients with spinal muscular atrophy were reduced by 30% (17.9 seconds versus 11.1 seconds). We conclude that the masticatory muscles of patients with spinal muscular atrophy are weakened, that their muscles are less efficient, and that they fatigue more quickly than controls. In addition, mandibular movements of these patients take place over a more limited range than unaffected controls.

Ultrasonographic cross-sectional characteristics of muscles of the head and neck

OBJECTIVE

Computed tomography and magnetic resonance imaging are the common techniques for evaluating cross-sectional areas and volumes of human jaw muscles. Because computed tomography has the disadvantage of showing cumulative biological effects and because MRI poses a problem in terms of clinical availability and cost, the purpose of this study was to determine whether ultrasonography could be used to measure local linear cross-sectional dimensions of muscles of the head and neck.

STUDY DESIGN

In 46 patients with signs and symptoms of temporomandibular disorders, the anterior temporalis, anterior masseter, deep masseter, anterior digastric, posterior digastric, and sternocleidomastoid muscles were measured bilaterally by ultrasonography to assess linear local cross-sectional dimensions. Measurements were made in 2 sessions with a time interval of at least 5 minutes. Data were analyzed for reliability and variability through use of the intraclass correlation coefficient (ICC) and the repeatability coefficient (RC). To assess local muscle asymmetry patterns, the absolute asymmetry index was used, with the mean maximum muscle diameters of the respective right and left sides calculated from 3 consecutive measurements.

RESULTS

Satisfactory visualization of muscles was obtained in 93.8% of 1104 imaging procedures. For the ultrasound measurements there was a significant difference in local cross-sectional dimensions between the first and second sessions for the anterior temporalis muscle only (P < .01). Acceptable intrarater reliabilities were obtained for the deep masseter (ICC = 0.92), anterior digastric (ICC = 0.91), and sternocleidomastoid (ICC = 0.86) muscles, whereas evaluation of the posterior digastric (ICC = 0.74), anterior masseter (ICC = 0.72), and anterior temporalis (ICC = 0.70) muscles was associated with moderate reliability. Variability of repeated measurements was found to be acceptable for the anterior temporalis (RC = 0.32 mm) and posterior digastric (RC = 0.48 mm) muscles. Analysis of muscle site-related local cross-sectional dimensions showed a significant difference between the right and left sides for the deep masseter muscle only (P < .05). The study population investigated revealed mean asymmetry indices ranging from 5.3% for the anterior digastric muscle to 8.7% for the deep masseter muscle.

CONCLUSIONS

Ultrasonography may prove to be a reliable diagnostic technique for the evaluation of cross-sectional dimensions and areas of muscles of the head and neck.

Contribution of jaw muscle size and craniofacial morphology to human bite force magnitude

The existence of an interaction among bite force magnitude, jaw muscle size (e.g., cross-sectional area, thickness), and craniofacial morphology is widely accepted. Bite force magnitude depends on the size of the jaw muscles and the lever arm lengths of bite force and muscle forces, which in turn are dictated by craniofacial morphology. In this study, the relative contributions of craniofacial morphology and jaw muscle thickness to the bite force magnitude were studied. In 121 adult individuals, both magnitude and direction of the maximal voluntary bite force were registered. Craniofacial dimensions were measured by anthropometrics and from lateral radiographs. The thicknesses of the masseter, temporal, and digastric muscles were registered by ultrasonography. After a factor analysis was applied to the anthropometric and cephalometric dimensions, the correlation between bite force magnitude, on the one hand, and the "craniofacial factors" and jaw muscle thicknesses, on the other, was assessed by stepwise multiple regression. Fifty-eight percent of the bite force variance could be explained. From the jaw muscles, only the thickness of the masseter muscle correlated significantly with bite force magnitude. Bite force magnitude also correlated significantly positively with vertical and transverse facial dimensions and the inclination of the midface, and significantly negatively with mandibular inclination and occlusal plane inclination. The contribution of the masseter muscle to the variation in bite force magnitude was higher than that of the craniofacial factors.

Determination of the external forces applied to the mandible during various static chewing tasks

The purpose of this study is to determine the external forces that are brought to bear on the mandible during 14 masticatory tasks, exerted in three defined mandibular positions. A static two-dimensional mandibular model is presented, taking into account projections in the sagittal plane of the forces exerted by six muscle groups and the joint reaction force. In calculating these external forces, two working hypotheses are adopted: the existence of a linear relationship between cross-sectional surface of the muscle and its maximum force of contraction, and the existence of a linear relationship between the electrical signal emitted by a muscle and the force the muscle then develops. Our first results are provided and compared with data from the literature. Individual variations recorded in the measurement of the different parameters involved in the equilibrium equations are such that they must be taken into account in the calculation of the forces. Moreover, the functional value of the muscles appears to be closely dependent on the mandibular position with which they are associated, which is not brought out in earlier studies. Finally, intra-joint action proves to be intense, although its direction varies according to the mandibular position and the type of exercise performed.

Comparative study of craniofacial morphology and bite force in Fijians and Japanese

An anthropological survey was conducted in Fiji in 1994 and 1995 to study dental arch form, craniofacial morphology, and bite force of Fijians. Measurements were obtained from dental casts, cephalograms, and thin pressure-sensitive sheets (Dental Prescale®) for bite force analysis. Results were compared with those of Japanese. In every direction, the size of the dental arch in Fijians was larger than in Japanese. Fijians displayed longer palates, longer mandibles, and bimaxillary protrusion. There was no significant difference in upper and lower facial heights. FH to lower incisor angle in Fijians was significantly larger than in Japanese. Fijians were characterized by a small palatal plane angle, occlusal plane angle and mandibular plane angle, and were thus brachyfacial. The Japanese tended to be more dolichofacial. The distances from the Cd line to the pterygoid muscles, masseter muscles, and teeth in Fijians were significantly longer than in Japanese. Occlusal contact areas of Fijians were also greater than those of Japanese. The results indicate that the masticatory muscles and craniofacial morphologies supporting them would be better integrated in Fijians than in Japanese. Am. J. Hum. Biol. 10:63-72, 1998. © 1998 Wiley-Liss, Inc.

Photoelastic analysis of miniplate osteosynthesis for mandibular angle fractures

OBJECTIVE

The purpose of this study was to reassess Champy's findings, which were instrumental in justifying the theory of tension band plating for mandibular angle fractures.

STUDY DESIGN

Ten anatomically correct mandibles were fabricated with a photoelastic resin. In five mandibles, angle fractures were created and fixed with a superior border miniplate; five uncut mandibles served as controls. Each mandible was loaded in a manner that simulated physiologic conditions. The internal stress patterns were preserved within the models by completing a stress freezing cycle.

RESULTS

The stress patterns in the experimental mandibles virtually replicated the patterns seen in the controls. Stress fringes were present surrounding the outer screws, indicating that these screws were subjected to pull-out forces.

CONCLUSIONS

There is greater force on the outer screws that may contribute to fixation failure. The theory of tension band plating for mandibular angle fractures is accurate but Champy's model is oversimplified.

Functional and morphologic changes after combined maxillary intrusion and mandibular advancement surgery

PURPOSE

The purposes of this investigation were 1) to compare the morphology and function of patients with combined vertical maxillary excess (VME) and mandibular retrognathia with that of controls, and 2) to examine how these parameters change after combined maxillary intrusion and mandibular advancement surgery.

PATIENTS AND METHODS

Fifteen female VME/retrognathic patients were compared with 26 female controls before and for up to 3 years after orthognathic surgery. Facial skeletal morphology, mandibular range of motion, maximum isometric bite force, and levels of electromyographic activity (EMG) in selected muscles of mastication were measured on all subjects. Where appropriate, one-way analysis of variance (ANOVA) or t-tests were used to compare the patients with controls. Univariate repeated-measures ANOVA was used to study longitudinal changes.

RESULTS

Preoperatively, patients' morphologic measurements were characteristic of VME compounded by mandibular retrognathia. At surgery, the maxilla was elevated an average of 2.8 mm, and the mandible was lengthened by an average of 7.1 mm. All of the postoperative morphologic measurements were closer to normal values. The patients' masseter mechanical advantage was significantly lower than that of controls both before and after surgery. Surgically induced changes in mechanical advantage were very small. The patients' maximum range of motion and excursion during mastication were all lower than those of controls before surgery. All measurements of mobility decreased immediately after surgery, with a gradual return to preoperative values. However, even 3 years after surgery, all of the motion measurements remained smaller than those of the controls. Before surgery, the patients had maximum isometric bite forces significantly lower than those of controls. Bite forces increased significantly after surgery, approaching normal values within 2 years. The activity levels in the muscles of mastication during isometric bites were not significantly altered by surgery.

CONCLUSIONS

This study confirms that VME/retrognathia patients suffer from substantial deficiencies in their oromotor function. Surgical correction of this particular type of dentofacial deformity improves both the morphologic and functional deficits. Although some changes were not statistically significant, all were toward normalization of the presurgical values.

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.

Functional and morphologic alterations secondary to superior repositioning of the maxilla

PURPOSE

The purpose of this investigation was to 1) compare morphological characteristics and functional performance of a sample of patients with vertical maxillary excess (VME) with controls, and to 2) examine how the patients' oral motor function adapts to surgery.

MATERIALS AND METHODS

Fifteen female VME patients were compared with 26 female controls before and up to 3 years after maxillary intrusion surgery. Measures of skeletal morphology, mandibular range of motion, maximum isometric bite force, and levels of electromyogram (EMG) activity in some of the muscles of mastication were made on all subjects over time. One-way analysis of variance (ANOVA) was used to compare the controls with the patients before and after surgery. Univariate repeated measures ANOVA was used to study longitudinal changes in the patients.

RESULTS

Preoperatively, the patients possessed morphological measurements characteristic of vertical maxillary excess. Superior repositioning of the maxilla averaged 3.3 mm. Concurrently, most skeletal measures were brought closer to normal values. Masseter muscle mechanical advantage was significantly lower in the patients than in controls both before and after surgery (P < or = .05). There was no significant difference between patients and controls for other biomechanical measurements. Mandibular hypomobility was apparent at 6 weeks after surgery, but returned to control values within 6 to 12 months. Before surgery, the patients had maximum isometric bite forces significantly less than those of controls. Bite forces steadily increased after surgery, approaching normal values within 2 years. Before surgery the patients' muscle activity levels per unit of bite forces were equivalent to those of controls or somewhat lower. After surgery some of the patients' muscles had significantly lower levels of muscle activity per unit of bite force than did controls.

CONCLUSIONS

The results of this study suggest that correction of vertical maxillary excess with maxillary intrusion surgery improves some characteristic functional deficits.

Experimental occlusal interferences. Part II. Masseteric EMG responses to an intercuspal interference

In 12 subjects, a rigid unilateral intercuspal interference (minimum mean height of 0.24 mm) was placed on either the right or left mandibular second premolar and first molar (sagittal physiological equilibrium point of the hemimandibular dental arch). During brisk and forceful clenching on the interference, bipolar surface electromyograms were obtained from the right and left masseter muscles. On the side opposite the interference, myoelectric clenching activity was significantly reduced. Correlation analyses showed that the interference elicited a non-linear (complex) co-ordination of the amplitude, but not the duration, of bilateral masseteric clenching activity, i.e. frequently there was significant motor facilitation on the side of the interference, and significant motor inhibition on the side opposite the interference. Theoretical considerations predicted that brief clenching on the interference would easily lead to frontal plane rotatory motions of the mandible which, indeed, occurred clinically.

Masticatory muscle influence on craniofacial growth

The influence of the masticatory muscle function on craniofacial growth has been recorded in a series of animal experimental and clinical studies. The common characteristic of these investigations is that the elevator muscles of the mandible influence the transversal and the vertical dimensions of the face. The increased loading of the jaws due to masticatory muscle hyperfunction may lead to increased sutural growth and bone apposition, resulting in turn in an increased transversal growth of the maxilla and broader bone bases for the dental arches. Furthermore, an increase in the function of the masticatory muscles is associated with anterior growth rotation pattern of the mandible and with well-developed angular, coronoid, and condylar processes.

Craniofacial morphology, occlusal traits, and bite force in persons with advanced occlusal tooth wear

The aim of this study was to investigate the dentofacial structure, the occlusal traits, and the bite force in subjects with advanced occlusal wear. The material comprised 54 adults, 30 men (mean = 40 years, range 16 to 61) and 24 women (mean = 28 years, range 18 to 47), most of whom had a full or near-full complement of natural teeth, and the presence of occlusal wear. Craniofacial structure was studied on lateral cephalograms. Occlusal traits were examined on study casts, these serving also for an evaluation of occlusal wear to be carried out by using an ordinal scale. Bite forces were recorded at differing force levels (maximum biting, "biting as when chewing" and "light biting") and occlusal positions. Although maximum bite force and endurance time did not differ significantly between men and women, the level of bite force was high compared with other samples. The craniofacial structure of the sample was characterized by a deviation in the vertical direction, a small angle between the mandibular-palatal planes and a small gonial angle, as compared with Swedish adult norms. No significant differences were found in anteroposterior relationships between persons with advanced wear and normal standards. The results support the hypothesis that functional hyperactivity of the masticatory system imposed increased stress on the bony structures of the craniofacial complex with possible influences on its structure.

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 Pearson's 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.

Condylar resorption in orthognathic surgery. The role of intermaxillary fixation

Condylar resorption that occurs after orthognathic surgery was investigated in a large sample of patients treated in the Department of Oral and Maxillofacial Surgery of the Free University in Amsterdam, the Netherlands. The findings correspond with previous publications on this subject. In a 1-year follow-up study the role of intermaxillary fixation was investigated radiologically. In a group of 158 patients prone to show occurrence of condylar resorption, 24 (26.4%) of the 91 patients treated with intermaxillary fixation showed signs of condylar resorption. In the group of 67 patients treated without intermaxillary fixation only eight (11.9%) of the patients showed signs of reduced volume of the condyle. Avoidance of intermaxillary fixation seems to reduce the incidence of condylar resorption after orthognathic surgery in patients with a mandibular deficiency with high mandibular plane angle.

Relationship between attachment of the superficial masseter muscle and craniofacial morphology in dentate and edentulous humans

Previous studies have described the importance of the interaction between the masticatory muscles and the craniofacial skeleton in the control of craniofacial growth. This study describes the attachment and orientation of the superficial masseter muscle and its relationship with craniofacial morphology in dentate and edentulous subjects. Data were obtained from lateral cephalometric radiographs of a total of 31 cadavers in which the superficial masseter muscle had been defined with liquid barium. The results provide evidence that the morphology of the superficial masseter muscle in the gonion region differed significantly between dentate and edentulous subjects, with the masseter being 2.7 mm wider and the gonion-anterior muscle border distance being 4.0 mm greater in dentate subjects. The complex relationship between craniofacial morphology and the dimensions and inclination of the superficial masseter muscle were most clearly evident in dentate subjects where the position of the anterior border was related to ramus dimensions and mandibular and occlusal plane angles. The association is much less clear in edentulous subjects where normal function was disturbed. In general, age was not a significant determinant of variation in superficial masseter muscle dimensions and orientation.