Human mandibular shape is associated with masticatory muscle force

Abstract

Understanding how and to what extent forces applied to the mandible by the masticatory muscles influence its form, is of considerable importance from clinical, anthropological and evolutionary perspectives. This study investigates these questions. Head CT scans of 382 adults were utilized to measure masseter and temporalis muscle cross-sectional areas (CSA) as a surrogate for muscle force, and 17 mandibular anthropometric measurements. Sixty-two mandibles of young individuals (20–40 years) whose scans were without artefacts (e.g., due to tooth filling) were segmented and landmarked for geometric morphometric analysis. The association between shape and muscle CSA (controlled for size) was assessed using two-block partial least squares analysis. Correlations were computed between mandibular variables and muscle CSAs (all controlled for size). A significant association was found between mandibular shape and muscle CSAs, i.e. larger CSAs are associated with a wider more trapezoidal ramus, more massive coronoid, more rectangular body and a more curved basal arch. Linear measurements yielded low correlations with muscle CSAs. In conclusion, this study demonstrates an association between mandibular muscle force and mandibular shape, which is not as readily identified from linear measurements. Retrodiction of masticatory muscle force and so of mandibular loading is therefore best based on overall mandibular shape.

Dental malocclusions are not just about small and weak bones: assessing the morphology of the mandible with cross-section analysis and geometric morphometrics

“…the shape of class II mandibles is dependent of their small size and the shape of class II mandibles, of their large size. Same as genetics, masticatory muscle force has shown to have an effect on mandibular shape in modern individuals [15]. But large shape variation leading to the extreme features shown by individuals with malocclusions could be linked to the effect of other factors, such as nutrition and basal metabolism in modern humans.…”

“…Moreover, inducing masseter muscle paralysis in adult mice leads to an increased activity of the bone resorption marker RANKL, followed by a reduction of bone tissue per area and trabecular thickness of the condyle head [13]. Using medical data from modern populations, it has been observed that muscle force is only weakly related to cranial shape [14], but more so to the shape of the mandible [15].…”

Changes in Dietary Consistency and the Epidemiological Occlusal Transition

“…As a result of the dietary change, children's mandibles rotated anteriorly, facilitated by increased gonial angle and stronger chewing muscles (Ingervall and Bitsanis 1987). These changes have also been observed in more recent studies of mandibular metrics and masticatory muscles (Sella-Tunis et al 2018). Based on the studies outlined above, dietary consistency has a clear impact on masticatory stimulation, specifically in relation to gonial angle and facial form.…”

“…Permanent dentition is almost fully erupted by late adolescence, with the variable exception of third molars. The jaw has developed into what will most likely be its adult form, with no significant morphological or metric changes likely to occur until senescence (Scheuer and Black 2004). The idea of functional integration can be used to explain this.…”

“…These characteristics could include metric measurements and qualitative analysis of bite alignment. While this would require additional testing in order to substantiate, there is reason to believe that one can infer information about the diet consumed during development based on characteristics such as these in combination with other dietary reconstruction methods (Sella-Tunis et al 2018). Because it is not always abundantly clear how a group subsisted, this has use in substantiating claims regarding diet and food processing made on the basis of stable isotope analysis and archaeological interpretations.…”