Mastication : Set up of Adult Occlusion

-C-   SET UP of  OCCLUSAL SCHEME

The occlusal anatomy of the first molars, their optimal relationships in class 1, and the fact that, in the frontal plane, the axis of dento-alveolar inertia passes through the first pair molars (Treil and Casteigt 2000) helps to understand why these teeth have a fundamental role in the development of adult occlusal scheme and in the child’s ability to bear alone for several years, the swallowing occlusion and chewing balanced without any pathology.

Fig. C6: shearing cycles-in are displayed in red, crushing cycles-out green. The guide rails have a triangular section.

The development and implementation of the adult occlusion on the first couple of molars, mark in six years children, the installation of the occlusal scheme and chewing adults (Lundeen and Gibbs, 1982). Adjacent teeth progressively emerge on the arcades during several years and then integrate gradually, the functional scheme of the first molars, including canines whose appearance is often later than the second molars. The adult functional diagram is already fully developed and balanced when these come late to integrate with the existing chewing envelope. These finding strongly suggest that they do not have a significant role as we have assigned.

set up occluso

Fig. C7a: The occlusal relationship couple 1st molar in Class I, allows a balanced occlusal wedging capable of supporting alone the vertical dimension and balanced chewing function. Fig. C7b: Progressively Bicuspids are integrated at the first molars functional envelope, Followed by second molar. Fig. C7c: Lately cuspid is incorporated at the functional envelope still in place. It Had Not any role in the set-up of occlusion.

Fig. C8 There is one diagonal hand rail and secondary ones on the occlusal face of first molar, that work optimally in class I relationship. The main guidance rail goes from the tip of the distal-buccal cusp to the cusp tip of the mesiopalatal.

36-46 20

Fig. C9 a,b: These two photos were taken from the A1 video. They show the evolution of chewing guidances, on the maxillary and mandibular first molar, 20 years after their initial setting occlusion. The guides are well marked and the rails are matched with a well-preserved guiding capital and good functional efficiency.

P Sheen

Fig. C10. This figure shows the occlusal guidelines on first molars of case C13. Occlusal rails of triangular section are paired with their receiving opposite, V-shaped.
The occlusal surfaces of the rails are incorporated into the occlusal anatomy. These are functional guideways responsible for occlusal stability and efficiency of shearing and crushing. Shearing cycle-in are in red (carnivorous component), crushing cycle-out are in green (herbivorous component).

Rail herbiv

 

 

Fig.C11. case C9. Mandibular rail V is located between the second and third cusp of the first molar. It receives the main rail triangular section of the maxillary first molar.                                                            Fig. C12. In herbivorous models, these triangular rails are usual, as here in a deer. As man is omnivorous, so as herbivorous, it is not surprising to find such rails, at least on our most important couple of molars.

Fig. C13 Optimal guidances and amplitude of cycles in class I, can stimulate the maxillary transversal expansion and during cycle out, mandibular incisors position the maxillary incisors, while stimulating maxilla forward growth  (Bonnet).

The couple of first molars have enough wedging and guiding potential, allowing it to channel alone, without any slippage, the diagonal and transverse kinetic of the chewing cycles and to impose this scheme to the next emerging teeth. Thanks to the presence on their occlusal surfaces, of transverse guiding rails (Le Gall et al 2010, Le Gall and Lauret, 2011, Le Gall 2013), that is  essential anatomical, auto-stabilizing, and dynamic feature.

As soon as they come in occlusion, the first molars become the leading teeth for the posterior guidance, when they are in occlusion of class 1. Because they are optimally paired together, in class 1.

More the early obtaining of a stable wedging and a wide mastication will favor transverse and forward maxilla growth.

It’s in well established class 1 relationship that form and function find the best clinical fitting. In class1 occlusion, the occlusal anatomy of the first maxillary molar is an exact reversed volume of the mandibular one. During closure in M.I.O., a small functional interplay, between cycle out tables, and escaping grooves, is only remaining between .
Video in slow motion and simulation of mastication on thin colored paper, show that all along of the dental kinetics of a cycle, occlusal guidings, and rails are continuously and  finely matched together, with a very reduced but sufficient interplay, to avoid any occlusal blocking.
The other types of occlusion (class II…) have not the same occlusal fitting. They show often, significant underguidances, resulting in cycles incompleted or deformed, and even reduced to a mere shearing.
In child, these occlusal relations are very often associated to abnormalities of tongue position (Deffez 1995) and can be jointly responsible for vertical, transverse or anterior, anomalous growth, resulting in facial insufficiency more or less, progressive and asymmetrical, as well as mandible mis-positioning.
In child, early reeducation of tongue posture and positioning of first right and left molars in class 1, for an extended and alternated mastication, allows to reorient growth and avoid potentially secondary treatments, sometimes heavy (Bonnet 1992,93,99,2010).

In adult, the build-up of an occlusal anatomy, that is functional and working  like class 1 occlusion , with cycle-in and cycle-out well balanced, in a swallowing occlusion relationship and matched with the present articular kinetics, allows to re-establish instantly, without any training, an optimal cycle, fitted with the own patient. Showing that form and function are mutually dependent

Note: During evolution process, the size of cuspid have had a dominant role, because in our simian line her size have played a role in the sexual competition, like the antlers, in cervidae line (Picq, 2010). It’s why, in simian line, there is a dimorphism between the size of the canines of males and females. Fortunately this sexual dimorphism has ceased in human line, since about 2 millions years.
Classical concepts of occlusion have given a leading role to the canines.
But in fact, in this domain, the dominant role during set-up of occlusion and mastication, is held by the couple of first molars

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