– C- MASTICATION and INCISION: ROLE of MUSCLES
Chewing function includes all neuromuscular, dental and joint interactions that allow the oral preparation of the food before swallowing.
The incision and chewing are the first steps in the eating
Fig. C2ab: The dental phase of a cycle is composed of a cycle-input and a cycle-output, prior to and following MIO. Elevators muscles: Temporal, Masseter, Medial Pterygoid are mainly responsible for food bolus preparation. Lower lateral Pterygoid is partially responsible for the transverse displacement of the mandible and the following opening. Upper lateral ptrerygoid maintains a good relationship between TMJ surfaces..
Chewing function is an alternate unilateral act, characterized by a series of successive cycles of the mandible allowing the oral processing of the food bolus before it is swallowed. The chewing cycles have a centripetal direction (Gibbs 1981, Lundeen and Gibbs1982) and are arranged around the patient’s M.I.O. (Joerger et al., 2012).
Fig. C3: This diagram compares the muscle recruitment during a right cycle-in and that of the lateral movement on the same side, in a mouth in occlusion of class I of Angle, with all her guiding potential. During cycle-in, the recruitment of elevator muscles of chewing side, helps explain the gradual closing of the posterior teeth, up to touch, at the last cycles before swallowing. During sideways movement with only anterior cuspid guidance, the hand recruited muscle, is the lower fascicle of the contralateral lateral pterygoid, which is lateralizer and depressor. This is why the posterior teeth are well separated of their opposite. The situation is different when the teeth are worn and moving towards a flat occlusion with personalized adaptive modes and the complete loss of the characteristics of the original model.
The recruitment of elevators muscles, of the mandible (Masseter, medial Pterygoid and Masseter) is responsible for their kinetics, as to the crushing of food. During the preparation of the bolus, the occlusal surfaces of posterior teeth move closer to the contact in the last cycles before swallowing. As revealed during chewing simulation.
The lateral movement, usually asked patient, has a reverse orientation and is usually guided by the cuspid (D’Amico, 1958, 1961). The main muscle responsible for this movement is the lower fascicle of contralateral pterygoid. This muscle is lateralizer and depressor, and thereby explain the early disocclusion of posterior teeth, observed during this centrifugal movement (Lauret and Le Gall, 1994, 1996; Le Gall et al., 1994).
Fig. C4 This diagram compares the muscle recruitment during a right cycle-out and that of a lateral movement movement toward contralateral side, in a class I patient with all of its potential guide. Both movements have the same orientation, but the muscle recruitment are different. During cycle-out, the gradual closing of the posterior teeth up to touch,is obtained by contraction of elevator muscles, especially the pterygoid medial. While lateral movement, when the main muscle recruited, is the lower head of the lateral pterygoid, depressor and lateralizer, without lifting action. Therefore posterior teeth are well separated right side, with only a cuspid guidance on the contralateral side. The situation is different when the teeth are worn, with the complete loss of the characteristics of the original model.
During the protrusion movement, the main muscles involved symmetrically are the lower heads of the lateral pterygoid muscles, that are propulsor and depressor muscles, and provoke a postero-anterior movement of the mandible, with a bilateral posterior disocclusion. Simulating not loaded, protrusion movement shows anterior leading guidances only, caused by the slight and symmetric recruitment of masseter superficial and temporal anterior.
During incision it’s different, the main muscles recruited are the elevators, but only the fascicles, who are elevator and/or retropulsor (deep and medium Masseter, posterior and medium Temporal). Muscular action, during movement of incision, result in antero-posterior movement of the jaw, with leading anterior guidances early accompanied by light posterior contacts and guidances, that are symmetrical.
Fig. C5: During the protrusion movement, in sagittal orientation, lower fascicles of the two symmetrical muscles lateral pterygoid are recruited simultaneously, they are then depressor and propulsor. The fascicles of the temporal anterior and masseter superficial, that are elevator and propulsor, are recruited enough to maintain anterior contacts. There is no posterior touch (unless gap …). During the incision, the elevator and retropulsor fascicles of the masseter (superficial, medium) and temporal (posterior, middle) are simultaneously contracted, to introduce the bolus in the mouth. When empty, the anterior guidances are leading, they are accompanied by posterior subsidiary ones
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