Masticatory Biomechanics

This overview paper presents the leading theories which attempt to explain the emergence of one of the prominent features specific to our species – the protruding chin. Established theories discussing this phenomenon include the effect of masticatory stress theory, a sexual selection theory and theories relating the human chin to the emergence of spoken language. However, these explanations have proved inconclusive and the processes responsible for the emergence of mentum osseum in the Late Pleistocene remain unclear.

The reduction of occlusal dimensions in early Homo is often proposed to be a functional adaptation to diet. With their smaller occlusal surfaces, species of early Homo are suggested to have reduced food-processing abilities, particularly for foods with high material properties (e.g., increased toughness). Here, we employ chewing efficiency as a measure of masticatory performance to test the relationships between masticatory function and food properties. We predicted that humans are more efficient when processing foods of lower toughness and Young's modulus values, and that subjects with larger occlusal surfaces will be less efficient when processing foods with higher toughness and Young's modulus, as the greater area spreads out the overall bite force applied to food particles. Chewing efficiency was measured in 26 adults using high-speed motion capture and surface electromyography. The dentition of each subject was cast and the occlusal surface was quantified using dental topographic analysis. Toughness and displacement-limited index were negatively correlated with chewing efficiency, but Young's modulus was not. Increased occlusal two-dimensional area and surface area were positively correlated with chewing efficiency for all foods. Thus, larger occlusal surface areas were more efficient when processing foods of greater toughness. These results suggest that the reduction in occlusal area in early Homo was associated with a reduction in chewing efficiency, particularly for foods with greater toughness. Further, the larger occlusal surfaces of earlier hominins such as Australopithecus would have likely increased chewing efficiency and increased the probability of fracture when processing tough foods.

The temporomandibular joint (TMJ) is a morphologically and functionally complex component of the skull. Temporomandibular joint shape varies considerably across mammals and within primates, and some aspects of the TMJ have been linked to differences in feeding behavior. However, a broad comparative context describing TMJ variation across primates is lacking. This dissertation therefore evaluated TMJ shape variation in the context of biomechanical hypotheses regarding TMJ function, and in light of phylogenetic and body size variation across anthropoid primates.

Three-dimensional geometric morphometrics were used to quantify TMJ shape across a broad sample of 48 anthropoid primates, and more narrowly among small groups of closely related taxa with documented dietary differences. Linear measurements of the TMJ (e.g., glenoid length) were subsequently calculated and compared among taxa. Results of the dietary analyses indicate that taxa with more resistant diets tend to have larger joint surface areas, as well as mediolaterally wider and anteroposteriorly shorter TMJs. Strong correlations were found between glenoid length and measures of gape, suggesting that one way increased gape is achieved is through increased translation at the TMJ. Analyses of scaling in the TMJ found that many variables scaled with positive allometry against cranial and body size, although differences in scaling patterns among platyrrhines, cercopithecoids, and hominoids were identified. In the phylogenetic analysis, genetic and morphological phylogenies were compared and not found to be particularly congruent. This congruence varied across clades, however, and in many instances dietary and body size variation were correlated with morphology, suggesting that TMJ morphology is adaptive. These data highlight the myriad ways in which multiple factors may influence TMJ shape, which may or may not be congruent with known genetic relationships among taxa.

Although the TMJ is only a small portion of the skeleton, the morphology of this joint can provide valuable information with which to infer or reconstruct the biology of primate taxa. Ultimately, these data will help to provide a framework for future analyses of primate, and particularly fossil hominin, TMJ variation, and more generally to contribute to the growing body of literature regarding form and function in the primate masticatory apparatus.

Introduction Kinesio tape (KT) application is a treatment option for patients with musculoskeletal problems. High levels of muscle tenderness in trapezius muscle were correlated with high levels of jaw dysfunction. Aim The aim of the present study was to evaluate the influence of Kinesio tape application on the masticatory muscles activity using surface electromyography. Material and methods Thirty healthy adults (10 men and 20 women; mean age 25±6) were examined. The electrical activity of the temporalis anterior (TA) and masseter muscle (MM) of the right and left side before and after KT application on trapezius muscle was recorded during two conditions: resting mandibular position and isometric clenching. The electromyograph BioEMG III and BioPAK Measurement System were used for the recording. Result The electrical activity of the TA and MM muscles of the right and left side during clenching isometric activity after KT application was significance lower

One of the most prominent features of the cranial component of the temporomandibular joint (TMJ) is the articular eminence (AE). This bar of bone is the primary surface upon which the condyle translates and rotates during movements of the mandible, and is therefore the primary point at which forces are transmitted from the mandible to the cranium during loading of the masticatory apparatus. The shape of the articular eminence is highly variable across primates, and the raised eminence of humans has often been considered a defining feature of the human TMJ, yet few data exist to address whether this variation is functionally significant.

This study used a broad interspecific sample of anthropoid primates to elaborate upon and test the predictions of a previously proposed model of AE function. This model suggests that AE inclination acts to resist non-normal forces at the TMJ thereby maximizing bite forces. Articular eminence inclination was predicted to covary with two specific features of the masticatory apparatus: height of the TMJ above the occlusal plane and inclination of the masticatory muscles. A correlate of this model is that taxa utilizing more resistant food objects should also exhibit relatively more inclined AEs. Results of the correlation analyses found that AE inclination is strongly correlated with height of the TMJ above the occlusal plane, but less so with inclination of the masticatory muscles. Furthermore, pairwise comparisons of closely related taxa with documented dietary differences found that the AE is consistently more inclined in taxa that utilize more resistant food items. These data preliminarily suggest that variation in AE morphology across anthropoid primates is functionally related to maximizing bite forces, and add to the growing dataset of masticatory morphologies linked to feeding behavior.

The fossils from Malapa cave, South Africa, attributed to Australopithecus sediba, include two partial skeletons-MH1, a subadult, and MH2, an adult. Previous research noted differences in the mandibular rami of these individuals. This study tests three hypotheses that could explain these differences. The first two state that the differences are due to ontogenetic variation and sexual dimorphism, respectively. The third hypothesis, which is relevant to arguments suggesting that MH1 belongs in the genus Australopithecus and MH2 in Homo, is that the differences are due to the two individuals representing more than one taxon. To test these hypotheses, we digitized two-dimensional sliding semilandmarks in samples of Gorilla, Pan, Pongo, and Homo, as well as MH1 and MH2. We document large amounts of shape variation within all extant species, which is related neither to ontogeny nor sexual dimorphism. Extant species nevertheless form clusters in shape space, albeit with some overlap. The shape differences in extant taxa between individuals in the relevant age categories are minimal, indicating that it is unlikely that ontogeny explains the differences between MH1 and MH2. Similarly, the pattern of differences between MH1 and MH2 is inconsistent with those found between males and females in the extant sample, suggesting that it is unlikely that sexual dimorphism explains these differences. While the difference between MH1 and MH2 is large relative to within-species comparisons, it does not generally fall outside of the confidence intervals for extant intraspecific variation. However, the MH1-MH2 distance also does not plot outside and below the between-species confidence intervals. Based on these results, as well as the contextual and depositional evidence, we conclude that MH1 and MH2 represent a single species and that the relatively large degree of variation in this species is due to neither ontogeny nor sexual dimorphism.

"Haitian species of the extinct ground sloth genus Neocnus (Mammalia: Pilosa: Megalonychidae) have previously been hypothesized to have a much reduced jugal bone and a correspondingly reduced masseter musculature but a paucity of specimens has prevented further investigation of this hypothesis. Recent discovery of jugal bones belonging to Haitian specimens of Neocnus within the University of Florida Museum
collections enables the element to be more accurately described. The discovery also makes it possible to explore mastication in these sloths. Osteological characters related to feeding were examined, along with comparative estimations of bite force with the extant tree sloths, Bradypus and Choloepus, and their known dietary habits as a means to infer aspects of the paleodiet of Neocnus. There is a significant difference in moment arm calculations for m. masseter between predicted and actual jugals, but the overall significance for bite force is lost
and hampered by small sample size. Neocnus demonstrates a variety of characters that are similar to those of Bradypus and not to Choloepus, which is a close phylogenetic relative. The masticatory musculature of Neocnus enabled a chewing cycle emphasizing a grinding combination of mesiodistal and  linguobuccal movements of the molariform dentition. The orientations of m. masseter and m. temporalis are estimated to produce relatively high bite force ratios that imply a masticatory system with stronger versus faster components. Because of the
similarity of bite forces and jaw mechanics to those of Bradypus, in addition to a number of osteological adaptations indicative of herbivorous grazers (elevated mandibular condyle, large and complex masseter, and robust angular process), the Haitian forms of Neocnus are considered to have been selective feeders with a folivorous
diet."

Maximum jaw gape is a performance variable related to feeding and non-feeding oral behaviors, such as canine gape displays, and is influenced by several factors including jaw-muscle fiber architecture, muscle position on the skull, and jaw morphology. Maximum gape, jaw length, and canine height are strongly correlated across catarrhine primates, but relationships between gape and other aspects of masticatory apparatus morphology are less clear. We examine the effects of jaw-adductor fiber architecture, jaw-muscle leverage, and jaw form on gape in an intraspecific sample of sexually dimorphic crab-eating macaques (Macaca fascicularis). As M. fascicularis males have relatively larger maximum gapes than females, we predict that males will have muscle and jaw morphologies that facilitate large gape, but these morphologies may come at some expense to bite force. Male crab-eating macaques have relatively longer jaw-muscle fibers, masseters with decreased leverage, and temporomandibular jo...

Aim: This retrospective cohort study assessed the differences between the masticatory performance of individuals with and without orthodontic treatment need. Methods: Masticatory performances of 120 participants with equal numbers of males and females (n = 60 each) with and without treatment need (n = 60 each, determined using the index of the complexity, outcome, and need [ICON]) matched by age and sex were assessed. Participants chewed on test foods at sequences of 20, 30, 40, 40, 30, and 20 masticatory cycles. After sieving the chewed particles, the masticatory parameters (median particle size [MPS], masticatory frequency [MF], and broadness of particle distribution [BPD]) were calculated. The associations between treatment need and the MPS with age, weight, and stature, as well as the relationship between sex and MPS and MF, were assessed. Results: According to the independent samples t test, MPS of subjects with and without treatment need differed significantly (P < .05) while MF did not. The differences between BPD values reached the level of significance only after 40 cycles of mastication (P < .05). Females showed significantly greater MPS values. Only at the 40-cycle sequence did females in need of treatment chew significantly slower. According to the repeated-measures ANOVA, no significant differences existed (P > .05) between each of the MPS, MF, and BPD values measured at different sequences. No significant correlations were found between MPS and age, sex, or stature (P > .05 [Spearman correlation coefficient]). Conclusion: The masticatory performance of individuals with and without treatment need differed for MPS and BPD (at the 40-cycle sequence). Age, weight, and stature did not affect the treatment need and MPS. Females showed poorer results regarding MPS-only at the 40-cycle sequence did females with malocclusion chew significantly slower.

Kinematic analyses of mandibular movement in humans demonstrate that the mandibular instantaneous center of rotation (ICoR) is commonly located near the level of the occlusal plane and varies in its position during a chewing sequence. Few data are available regarding the location of the ICoR in non-human primates and it remains unclear how the position of the ICoR varies in association with mastication and/or gape behaviors. ICoR was quantified throughout the gape cycle in five species of non-human primates (Macaca mulatta, Cebus apella, Chlorocebus aethiops, Eulemur fulvus, Varecia variegata). The ICoR is commonly located below the mandibular condyle close to the occlusal plane and varies considerably both superoinferiorly and anteroposteriorly through the gape cycle. The path of the ICoR, and by inference condylar movement, in Macaca and Chlorocebus differs from humans whereas movement in Cebus resembles that of humans. Similarities between humans and Cebus in articular eminence and occlusal morphology may explain these resemblances. Food material properties had little influence on ICoR movement parameters.

Introduction: Modern literature indicates an uptrend in the occurrence of sleep disorders. Currently, about 30-40% of adults are suffering from sleep disorders, which makes them one of the most common health problems of the population. The aim of the study: The determination of the quality of sleep on the muscle tension of the masticatory apparatus. Methods and materials: 27 adults volunteered to take part in the study. After the application of the exclusion criteria, 12 women (with an average age of 23 ± 1 year) have qualified. They were subsequently divided into two equal groups, in accordance with the guidelines of the Pittsburgh questionnaire: six women with a below 6 point result - high quality of sleep and six women with an over 6 point result (or higher) - low quality of sleep. Electromyographic activity of the anterior temporalis (TA) and masseter muscle (MM) was evaluated in resting mandibular position. Based on the electromyographic results, the values of the asymmetry indexes of the muscles of the masticatory apparatus muscles were calculated. The U Mann-Whitney test has been used for the statistical analysis, with the significance level of 0,05. Results: There were no significant differences in resting activity of the anterior temporalis, masseter muscles and the asymmetry index of the anterior temporalis between groups (p>0,05). There was the significant difference in the asymmetry index of masseter muscles between groups (p<0,05). Conclusions: The quality of sleep has no significant influence on the resting activity of masticatory muscles. The influence of the quality of sleep on the asymmetry index should be the object of further research. In order to verify this conclusion, the research should be carried out on a larger group of people.

This overview paper presents the leading theories which attempt to explain the emergence of one of the prominent features specific to our species – the protruding chin. Established theories discussing this phenomenon include the effect of masticatory stress theory, a sexual selection theory and theories relating the human chin to the emergence of spoken language. However, these explanations have proved inconclusive and the processes responsible for the emergence of mentum osseum in the Late Pleistocene remain unclear.