-C- FUNCTIONAL DENTAL GUIDANCE
Fig. C14a: Changing the cycles breadth, in a young woman mouth. She is 28 years old and in class 1 occlusion
Note: The axis of the first maxilla molar, in mesial inclination, as well as the constant prominence of the disco-buccal cusp (far from that is shown in dental anatomy books) certainly contributes to the early interception of cycle-in and constitute a cornerstone of functional occlusion, overtaking and reinforcing the description still done by Andrews (1972). View next video C14
Fig. C14b: Different phases of a chewing cycle and terminology
A chewing cycle can be divided into two phases (fig.C15 to C19):
– A preparatory phase, remote from teeth, in the shape of a loop.
– A dental phase situated at the apex of the cycle, that is divided itself into a dental cycle entry (cycle in) and a dental cycle exit (cycle-out), prior to and following M.I.O.
The next description deals with a typical cycle in a young adult having all his guiding capital and concerns one of the last cycles, prior to swallowing (Lauret and Le Gall, 1994,1996). It describes the functional envelope of limit movements, with dental guides. The preceding cycles, with interposed food, are situated inside this envelope of border movements.
The dental cycle-in contacts of the inner guiding slopes of the buccal maxilla cusps(Fig. C15, C17) , glide on opposed lower buccal supports. Simultaneously internal slopes of the mandible lingual cusps glide on opposed palatal supports. These dual slidings,stabilize the mandible and lead to MIO. The dental phase of cycle-out, following the passage through MIO, puts in guiding relationship, the buccal mandibular cusps, and the palatal maxillar cusps, antagonists, on their inner slopes (Fig. C15, C18).
It is not a simple sliding guides between congruent surfaces in the frontal plane.
There are, on the occlusal faces of the first molars, transversal guiding rails in a more or less diagonal orientation and of triangular section (Le Gall and Lauret, 2011). The most important rail – which is also easier to identify – is situated on the maxillary first molar (fig. C8, C9). It goes from the tip of the distal-buccal cusp and it follows the enamel bridge, guiding the movement passing through M.I.O. , and it ends in the distal slope of the mesial-palatal cusp. Its orientation is often modified on the enamel bridge, comma-shaped. The triangular profile of this guiding rail, accurately matches with its antagonist receiving structure, shaped like a V, between the second and third mandibular buccal cusps, where it is channelled. To summarize, in class 1 occlusion, the occlusal surface of a first molar is a 3D mirror picture, of its antagonist, with a small functional interplay and some secondary escape grooves for the bolus. This rail and its counterparts, directly operational in Class 1, enables the couples of first molars, to be canalized in the three planes of space, so providing an excellent stability during the chewing process. They remain active in the adult, at least as long as the occlusal faces can keep their anatomy. Let us remind that, unlike some mammals (Barones, 1966), our teeth are not self-repaired. In the adult, they do not have a continued growth, and they are not naturally replaced by successive teething, when they are worn out. They were selected at a time when the life expectancy was much shorter than today. Normally, the gradual loss of the occlusal anatomy and the attrition of the chewing guides, result in a coordinated wear, dental and joint surfaces (Mongini 1972, 75, 77, 85).
But this adaptability has its limits, in particular when the loss of the occlusal anatomy is fast (biocorrosion) or brutal (avulsions, prostheses, iatrogenic actions…), because the volume loss, resulting therefrom, can cause a complete disruption of function guides and the installation of the joint protection mechanisms.
Furthermore, if lost volumes are not restored by addition, there is a fast decrease in the vertical dimension of the lower part of the face.
How and at what level to start clinically recover these lost volumes?
Fig. C15a Occlusal view of maxillary guides physiology of a chewing cycle on the right side, in a 28-year-old young woman. The limits of the envelope of motion, during mastication, is a function of Coordinated group guidances, which concern the whole extend of occlusal surfaces on the chewing side (see video C14, same case).
Fig. C15b Optimal shape of a cycle (50% of persons, according to Pröschel in 1987), in a patient with well coordinated guides,
and mostly with molar occlusal relationships of Class I
Fig. C15c During cycle-out, there are one or several guidances in the cuspid area, on the nonchewing side (cuspid, first bicuspid or lateral).
Fig. C16 Same case 10 years later
Fig. C17a. In a centripetal orientation, the Cycle-In is characterized by a dual dynamic guide, and stabilizing . The nonchewing side is not in touch. Fig. C17b. Buccal view of the guiding relationship of cycle-in, at the disto-buccal cusp of 16. Note the low position of the cusp, due to the mesial tilting, of the molar axis. (Extracted from video A1).
Fig. C18a. After the MIO, the Cycle-Out carry-on its centripetal movement of crushing.
Fig. C18b. Clinical view of the crushing guides and the occlusal relationship of the both tribosphenic cusps. At this step, there is a balancing touch on the internal face of the contralateral cuspid. (Extracted from video A1).
Fig. C19a. Occlusal view of the maxillary chewing guidances, on the right side. Guides are present on all of the teeth, including the cuspid, in cycle-in, and the palatal slopes of cycle-out (although traditionally regarded as non working). The guiding rails are pointed with arrows (Extracted from video A1).
Fig. C19b. Perspective view of the opposite mandibular chewing guidances. ( video A1).
Fig. C19c. The laterality movement in a reverse orientation is guided by the cuspid. There are no posterior contact
Fig. C19d: The hand rail of the first maxilla molar, passing through the enamel bridge, and a subsidiary one.
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