The influence of jaw-muscle fibre-type phenotypes on estimating maximum muscle and bite forces in primates

Numerous anthropological studies have been aimed at estimating jaw-adductor muscle forces, which, in turn, are used to estimate bite force. While primate jaw adductors show considerable intra- and intermuscular heterogeneity in fibre types, studies generally model jaw-muscle forces by treating the jaw adductors as either homogeneously slow or homogeneously fast muscles. Here, we provide a novel extension of such studies by integrating fibre architecture, fibre types and fibre-specific tensions to estimate maximum muscle forces in the masseter and temporalis of five anthropoid primates: Sapajus apella ( N = 3), Cercocebus atys ( N = 4), Macaca fascicularis ( N = 3), Gorilla gorilla ( N = 1) and Pan troglodytes ( N = 2). We calculated maximum muscle forces by proportionally adjusting muscle physiological cross-sectional areas by their fibre types and associated specific tensions. Our results show that the jaw adductors of our sample ubiquitously express MHC α-cardiac, which has low specific tension, and hybrid fibres. We find that treating the jaw adductors as either homogeneously slow or fast muscles potentially overestimates average maximum muscle forces by as much as approximately 44%. Including fibre types and their specific tensions is thus likely to improve jaw-muscle and bite force estimates in primates.

Signs and Symptoms of Temporomandibular Disorder in Male Narghile Smokers versus Male Cigarette Smokers.

Objective: Narghile smoking is highly prevalent in Egypt. The act of narghile smoking involves strong contraction of the jaw muscles, which would putatively cause overloading of the temporomandibular joint; thereby leading to internal derangement of the joint. The current study compared symptoms and signs related to temporomandibular disorder in male narghile smokers versus male cigarette smokers. Methods: This prospective clinical study involved 233 male narghile smokers and 233 male cigarette smokers. A questionnaire and examination findings protocol was applied for each participant. Results: There was a significant increase in incidence of signs of internal derangement in the temporomandibular joints of narghile smokers versus those of cigarette smokers (p=0.001). Tympanic membrane retractions were also more common in narghile smokers versus cigarette smokers(p=0.001), reflecting the significant effects of jaw muscle mechanical efforts on the middle ear system, associated with narghile smoking. Conclusion: Narghile smoking is a traumatizing habit as regards its effects on the temporomandibular joints and ear structures.

The reciprocal jaw-muscle reflexes elicited by anterior- and back-tooth-contacts—a perspective to explain the control of the masticatory muscles

Aims Tooth-contact sensations are considered essential to boost jaw adductor muscles during mastication. However, no previous studies have explained the importance of the inhibitory reflex of human anterior-tooth (ANT)-contacts in mastication. Here I present the “reciprocal reflex-control-hypothesis” of mammalian mastication. Subjects and setting of the study I demonstrate the hypothesis with the live kinematics of free jaw-closures as inferred from T-Scan recordings of dental patients. Results The jaw-closures started with negligible force, predominantly with ANT-contacts (the AF-bites). The first ANT-contact inhibited the first kinematic tilt of the mandible, whereas the bites starting from a back-tooth (BAT)-contact (the BF-bites) accelerated the first tilt. The second tilt established a low-force static tripod of the ANT- and bilateral BAT-contacts for a fixed mandible-maxilla relation. Thereafter, semi-static bite force increased rapidly, relatively more in the BAT-area. Discussion and Conclusions In the vertical-closure phase of chewing, the primate joint-fulcrum (class 3 lever) conflicts with the food-bolus-fulcrum in the BAT-area (class 1 lever). The resilient class 3 and 1 lever systems are superseded by an almost static mechanically more advantageous class 2 lever with a more rigid fulcrum at the most anterior ANT-contact. For humans, the class 2 levered delivery of force also enables forceful horizontal food grinding to be extended widely to the BAT-area.