Relationships between the size and spatial morphology of human masseter and medial pterygoid muscles, the craniofacial skeleton, and jaw biomechanics

Abstract

The relationship between human craniofacial morphology and the biomechanical efficiency of bite force generation in widely varying muscular and skeletal types is unknown. To address this problem, we selected 22 subjects with different facial morphologies and used magnetic resonance imaging, cephalometric radiography, and data from dental casts to reconstruct their craniofacial tissues in three dimensions. Conventional cephalometric analyses were carried out, and the cross-sectional sizes of the masseter and medial pterygoid muscles were measured from reconstituted sections. The potential abilities of the muscles to generate bite forces at the molar teeth and mandibular condyles were calculated according to static equilibrium theory using muscle, first molar, and condylar moment arms. On average, the masseter muscle was about 66% larger in cross section than the medial pterygoid and was inclined more anteriorly relative to the functional occlusal plane. There was a significant positive correlation (P less than 0.01) between the cross-sectional areas of the masseter and medial pterygoid muscles (r = 0.75) and between the bizygomatic arch width and masseter cross-sectional area (r = 0.56) and medial pterygoid cross-sectional area (r = 0.69). The masseter muscle was always a more efficient producer of vertically oriented bite force than the medial pterygoid. Putative bite force from the medial pterygoid muscle alone correlated positively with mandibular length and inversely with upper face height. When muscle and tooth moment arms were considered together, a system efficient at producing force on the first molar was statistically associated with a face having a large intergonial width, small intercondylar width, narrow dental arch, forward maxilla, and forward mandible. There was no significant correlation between muscle cross-sectional areas and their respective putative bite forces. This suggests that there is no simple relationship between the tension-generating capacity of the muscles and their mechanical efficiency as described by their spatial arrangement. The study shows that in a modern human population so many combinations of biomechanically relevant variables are possible that subjects cannot easily be placed into ideal or nonideal categories for producing molar force. Our findings also confirm the impression that similar bite-force efficiencies can be found in subjects with disparate facial features.

Fiber-type differences in masseter muscle associated with different facial morphologies

“…4 These force differences might not be due to intrinsic muscle differences but, rather, to mechanical advantage loss in obliquely applied force. 5 Others have imaged muscle to determine overall size and orientation using a variety of techniques, including cephalometrics, 5 computed tomography, 6 ultrasound, 7 and magnetic resonance imaging (MRI), 8,9 but they do not agree with regard to muscle size, orientation, and craniofacial form.…”

Caracterização eletromiográfica e ultrassonográfica da função mastigatória em indivíduos com oclusão normal

“…One of the important contributions of this study was the repeated measure of the thickness of the masseter muscle that ensured a lower variation in the error and higher reliability of the measure of the actual variation in the muscle thickness. To control the measurement error variable, the position of the transducer was standardized (1)(2)(3)29) . The variables related to body posture, interocclusal relationship and rest time between voluntary dental clenching tasks were also controlled.…”

“…Among the complementary exams are surface electromyography (sEMG), computed tomography, magnetic resonance imaging, masticatory efficiency test (with food, chewing gum or beads) and ultrasound (USD). All of these methods present advantages and disadvantages that may occasionally necessitate the performance of more than one method for proper diagnosis (1)(2)(3)(4) . The sEMG and USD have increasingly been foci of research on changes in mastication (4)(5)(6) .…”

“…Although there are questions about the selectivity, reliability and interpretation of the electromyographic signal, efforts have been made to standardize this method (7) . Some authors have described sEMG as a valuable method for the characterization and diagnosis of patients with occlusal and orofacial motricity changes (1,(4)(5)(6) . The sEMG provides information on the muscle, or, in the case of mastication, on the muscle groups, regarding the contraction time and amplitude of their electrical activity (8,9) .…”

“…Studies involving patients with different types of dentofacial deformities indicate a low performance of the masticatory muscles, reduced maximal force of contraction, reduced maximal potential recorded during the sEMG and reduced electrical potentials recorded during mastication, compared to control groups (14,15) . Some authors have determined the sections and volumes of the masticatory muscles by computed tomography and magnetic resonance imaging (1,2) . The optical sections from these imaging techniques have shown a correlation with the maximal muscle strength (maximal isometric contraction) and with other parameters related to functionality and age (3,6) .…”