-A- CHEWING FUNCTION and SWALLOWING, Cornerstones of Occlusion
Since the end of XIXth century, numerous models of dynamic occlusion have been proposed, like for instance, the Bilateral Balanced Occlusion (Gysi 1910, McLean 1938), The Unilateral Group Function (Schuyler 1935), The Canine Protected Occlusion or CPO (d’Amico 1958), The Flat Plane Teeth Occlusion (Begg 1954)…
A common characteristic to these models is to be mainly based on propulsive and lateral movements asked to the patient from centric occlusion, to check occlusal equilibrium. The OPC is currently the most taught and applied model.
As for static occlusion, the Xray introduction, at the beginning of XXth century has allowed to invent a Condyle Centric Relation in the articular fossae. This CR, obtained by manipulation is the founder concept, of the Gnathologic Theory of occlusion (McCollum 1939). CR has became the classical reference for the mandible-maxilla relationship and Centric Occlusion (CR).
However, nowadays, focused on the requirements of evidence based dental medicine, Rinchuse has wrote : “ the terminology, nomenclature, and concept of CPO, as well as group function and balanced occlusions, can be challenged based on its questionable validity “ and above: “ Subjects are not typically asked to chew, swallow, or exercise any para-functional movements “ (Rinchuse et al 2007).
The present knowledges in physiology show that our natural functioning model is founded on mastication (Lauret et Le Gall 1994,1996; Le Gall et col. 1994; Le Gall 2007, Le Gall et Lauret 1998, Le Gall et Lauret 2011; Le Gall 2013) and deglutition (Le Gall et al 2010, Le Gall 2013).
The observation and simulation of chewing function show that cycles, with the recruitment of the elevator masticatory muscles, are in centripetal orientation, with the gradual closing of the posterior teeth up to touch, on the chewing side, that are harmoniously balanced on the whole extent of the occlusal surfaces. (video A1)
While the reverse centrifugal movement, with the recruitment of the opposite lower part of the lateral pterygoid muscle, that is a lowering and lateralization muscle, don’t allow to see and balance them (Le Gall et col.1994), because the border functional envelope, is only partly described, and in reverse orientation.
The only balancing occlusion in CPO is unfinished, and leaves on the occlusal faces, not-coordinated functional guidances (overguidances or underguidances), capable of disrupting or forbidding chewing function, or in case of implants:
– to be amplified by the reduced mobility of implants,
– to not be detected, then non avoided, due to the lack of proprioception,
– and to become potentially dangerous for the long term lasting of implants.
Mastication must be checked and balanced at the prosthetic insertion. It’s still a compulsory rule, because the usual articulators can’t simulate correctly this kinetics (Le Gall et Lauret 2011). The digital 3D modeling will probably be soon able to propose a solution to this problem.
In addition, for more than 95% of the patients (Posselt 1968,69; Joerger 2005, Joerger et al 2012), Central Occlusion in CR (variable depending of the manipulation, and operator dependent) is not accorded with, the natural, Maximum Interdental Occlusion (Ingervall 1964; Sicher et Dubrul 1975, Romerowski 2006). The occlusal contacts during deglutition are naturally situated in a (MIO). they must be paired together (Le Gall et al 2010, Le Gall 2013).
To well understand the natural functioning model, it has been necessary to take into account an essential feature, of the human adult teeth. When they replace the deciduous ones, it’s for the life, without any possibility of regeneration, or of continuous growth (like the herbivorous) or by a replacement by a new tooth bud (like elephant). They have been selected several millions years ago (Coppens, Picq 2000), when the life expectancy was very short compared to nowadays. They probably have not been selected for a such long life, because they are progressively worn-out, losing gradually their initial functional anatomy, and their efficiency. Among others, by attrition, bio-corrosion (Grippo et al 2012) and para-functional habits. These miscellaneous wear are personalized and don’t allow to determine a functioning model, because they result of the destruction of the initial model. For instance it’s like you chose a lame man as a reference to describe physiologic walking.
Clinical observations and mastication recordings, on Replicator® (Lundeeen et Gibbs 1982), then on Sironatograph®, have first shown the kinetics of cycles and posterior guidances and have pointed out the opposite orientation, of the movements currently performed to check occlusion.
Mastication must be checked and balanced at the prosthetic insertion, It’s still a compulsory rule, because the usual articulators can’t simulate correctly that kinetics (Le Gall et Lauret 2011). The digital 3D modeling will probably be soon able to propose a solution to this problem.
Fig A2,: Note the double guidances of cycle-in. The cusps are acute and cutting (like carnivorous) responsible for hacking (or shearing) the food. The cusps are tapered and cutting, (like carnivorous) responsible for shearing the food.
Fig A3: A chewing cycle is composed of a preparatory phase and a dental phase, whose border envelope is limited by the occlusal anatomy of posterior teeth, on the masticating side. See video A1
Fig A4 Cycle-out is responsible for the dynamic crushing of the bolus, between more flat slopes (like herbivorous). These slopes are wrongly called non-working…
But for a good decrypting of the functional model, it has been necessary to observe and understand how, the arrival in occlusion of the couple of first molars, in the child mouth, allows the set up of the adult occlusal scheme. Following, these data have been completed, in the mouth of young adults with all of their guiding potential. In this way it has been possible to establish a close correlation between form and function in a neuromuscular equilibrium and to give a functional coherence to occlusion.
This important step had allowed to find the individual data of occlusal rebuilding, based on the real function of each patient. Without using standard descriptive data, based on reverse movements and taking in count average values that have not any significance for a particular patient.
In addition, we will show, how it’s possible to change the shape of an incomplete or misshapen cycle, by modifying the anatomy of the posterior occlusal faces, generally by addition, this will allow to finely restore the coordination, between occlusal guidances, and the TMJ functional kinetic.
Mouth preparation of bolus is realized in this sequence: – Incision – Mastication – Deglutition. The occlusal balancing is realized in reverse order: – Deglutition – Mastication – Incision.
All these aspects feature in a new Organo-Functional theory of the occlusion, described as a conceptual frame, aiming at restoring or preserving the physiology of the manducatory tract. (Le Gall 2010). It is based on our current knowledges of chewing and swallowing. If these knowledges are to be supplemented or if other data, still unknown today, can enrich them, they will naturally find their place in this frame.
To reach this result, techniques and clinical protocols have been developed. They will certainly evolve, with future technical advances in computing and other fields. If these new contributions allow to reach more easily the initial goal, they will naturally be taken into account.
This frame and the ensuing developped techniques, although still perfectibles, allow us to be closer,than ever, of the holy grail, for the occlusodontology, i.e the understanding of the manducatory apparatus and the restoring of its harmonious functioning.
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