Mass properties of the human mandible

Computer simulation of human masticatory dynamics requires specification of the jaw's mass properties. These are difficult to estimate, especially in living subjects. Here, we used calibrated computed tomography (CT) to determine the properties of eight osseous jaw specimens with adult dentitions. When the CT numbers were converted to mineral densities, the mean estimated jaw mass was 13% greater than the mean wet weight. Putative bone marrow accounted for an extra 7% of mass. The mean bone densities for the sample were very consistent (1.72+/-0.02g/cm(3)). The mass and geometric centers were close (mean linear difference 0.43+/-0.18mm), and were always located anteroposteriorly between the second and third molars. The largest moment of inertia (MI) occurred around the jaw's superoinferior axis, and the smallest around its transverse axis. Bone marrow added an extra 7% to the MIs. There were linear relationships between the mandibular length (expressed three dimensionally), the actual and estimated masses, and the moments of inertia. Our study suggests non-invasive imaging (such as magnetic resonance) and even direct linear measurement, may be adequate to estimate jaw mass properties in living humans.     

Biomechanics of torsion in the human mandible

Comparative investigations of mandibular function among primates have relied upon elementary structural models to estimate states of masticatory stress and strain. In these studies, mandibular corpus morphology is idealized as a homogeneous, isotropic symmetrical body of invariant geometry, and this morphological abstraction is used to infer relative levels of stress and strain in the jaw. In reality, none of the limiting conditions assumed by these models is satisfied; consequently, it is prudent to ask whether this “textbook” engineering approach is valid for the inference of biomechanical behavior. In this study, the predictions of various geometric representations of the mandibular corpus are evaluated against strains recorded in a sample of human jaws loaded in torsion. Symmetrical geometrical models (including various “robusticity” shape indices), although convenient, are probably not consistently reliable for predicting the distribution of strains in the corpus. The experimental data suggest that variations in cortical thickness within sections play a significant role in determining the profile of relative strains. For comparative applications, characterization of the corpus as an asymmetrical hollow ellipse (i.e., with differing thickness of medial and lateral cortical plates) may provide a reasonable portrayal of relative strains. Am J Phys Anthropol 105:73–87, 1998. © 1998 Wiley-Liss, Inc.     

Three dimensional model of the human mandible

A new biomechanical three-dimensional (3D) model for the human mandible is proposed. A simple two-dimensional model cannot explain the biomechanics of the human mandible, where muscular forces through occlusion and condylar surfaces are in a state of dynamical 3D equilibrium. All forces are resolved into components according to a selected coordinate system. The muscular forces, which during clenching act on the jaw, along with the necessary force level for chewing, also act as some kind of stabilizers of the mandibular condyles preventing dislocation and loading of nonarticular tissues.     

Postural stability of the human mandible during locomotion

Movements of the head and of the mandible relative to the head were measured in human subjects walking and running on a treadmill at various speeds and inclinations. A miniature magnet and piezo-electric accelerometer assembly was mounted on the mandibular incisors, and a Hall-effect sensor along with a second accelerometer mounted on a maxillary incisor along a common vertical axis. Signals from these sensors provided continuous records of vertical head and mandible acceleration, and relative jaw position. Landing on the heel or on the toe in different forms of locomotion was followed by rapid deceleration of the downward movement of the head and slightly less rapid deceleration of the downward movement of the mandible, i.e., the mandible moved downwards relative to the maxilla, then upwards again to near its normal posture within 200 ms. No tooth contact occurred in any forms of gait at any inclination. The movement of the mandible relative to the maxilla depended on the nature and velocity of the locomotion and their effects on head deceleration. The least deceleration and hence mandibular displacement occurred during toe-landing, for example, during "uphill" running. The maximum displacement of the mandible relative to the head was less than 1mm, even at the fastest running speed. The mechanisms that limit the vertical movements of the jaw within such a narrow range are not known, but are likely to include passive soft-tissue visco-elasticity and stretch reflexes in the jaw-closing muscles.     

Repression of folate synthesis in the logarithmic phase of Euglena gracilis growth

Microbiological assay showed that in Euglena gracilis cultures the amount of cell folates reaches its maximum at the beginning of the culture cycle and rapidly and markedly decreases long before the cells reduce their duplication rate. [³H]Folic acid was a suitable precursor of Euglena folates (a full recovery of growth in sulfanilamide inhibited cultures was obtained by addition of folic acid), and a complete radiochromatographic profile of cell folates was obtained by separation on G-25 columns. This allowed the measurement of the rate of folate degradation, obtained from the rate of radioactivity disappearance in chromatographic patterns of extracts corresponding to increasing times of culture cycles. With the exception of the stationary phase, the process of folate degradation showed a first order kinetics with a rate constant of 5.4 + 10⁻⁴ min⁻¹ and a half-life of 21 h and 12 min. The rate of folate biosynthesis was calculated by adding the amount of degradation to the measured increase (or decrease) in cell folates. The specific rate reached its maximum (8.6 ng of folinic acid equivalent h⁻¹ for 10⁶ cells) as the culture entered the logarithmic phase of growth and rapidly decreased to about 1/20 of this value before leaving it. This indicates that the logarithmic phase of growth corresponds to a phase in which folate biosynthesis is strongly repressed.     

The functional significance of morphological variation of the human mandible and masticatory muscles

A review is given of what is known about the functional significance of variation of the morphology of the human mandible and jaw muscles. First, the mandible is a lever transferring muscular forces to the teeth. The angle between corpus and ramus and the width of the ramus are particularly relevant in this respect as they determine the mechanical advantage of the lever system and the capacity for sagittal (open-close) movement. The stability of the mandible in asymmetric bites is especially affected by the ratio between the intermolar and intercondylar distances. The repertoire of bite forces that can be generated at any tooth and the loading pattern of the temporomandibular joint are strongly dependent on the relative size of the masseter, temporalis and medial pterygoid muscles. Second, executing its function as a lever, the mandible is subjected to shearing, bending and torsional forces. The bony parts harbouring the teeth, joints and muscle attachments serve to counter these forces; additional strength is needed in three areas i.e. in the symphysis, the condylar neck and in the transition area between corpus and ramus. In human populations there are clear-cut patterns of correlation between some facial skeletal traits, jaw joint morphology and strength and line of action of the jaw muscles. As a result, facial morphologies can be distinguished with marked differences in mechanical performance of their masticatory apparatus. It is suggested that they emerge as a result of diverging environmental influences during postnatal growth.     

Lengthening the human mandible by gradual distraction.

Lengthening of the mandible by gradual distraction was performed on four young patients (average age 78 months). The amount of mandibular bone lengthening ranged from 18 to 24 mm; one patient with Nager's syndrome underwent bilateral mandibular expansion. Following the period of expansion, the patients were maintained in external fixation for an average of 9 weeks to allow ossification. The patients were followed for a minimum of 11 months to a maximum of 20 months with clinical and dental examinations as well as photographic and radiographic documentation. The technique holds promise for early reconstruction of craniofacial skeletal defects without the need for bone grafts, blood transfusion, or intermaxillary fixation.