-B- CLASSICAL CONCEPTS
Following the introduction of dental radiology, from the 1920 years, researches begun in California, by Mac Collum, on the first tomographic X-rays of TMJ, have led him to take as inter-maxilla reference for edentulous patients, a Centric Relation of the mandibular articular heads in the temporal fossae. Afterwards, this Centric Relation has became the founder concept of the Gnathology School, that was created with Stallard (Mac Collum 1939), in the company of California Gnathology.
Fig. B11 :A manipulation was a convenient way to find intermaxillary relationship in edentulous patients. It worked because the potential recording errors was compensated by a light displacement of the prosthetic bases on the soft mucosa. This is totally different in patients whose fixed teeth have high proprioception.
That reference is always utilized for full removable denture, wherein the potential movement of the mandible, are balanced by a slight moving of the prosthetic base on the flexible supporting mucosa. Unlike toothed patients whose teeth position is steady and where mandible and teeth mis-positioning have result in numerous clinical disappointments that have been controversial and have leaded to several different definitions of C.R. The rule imposing a tripod stabilization of occlusal contacts in CR has also been modified by additional concepts like the «short centric», «long centric», «wide centric» and at least the «freedom in centric», reflecting a conceptual and clinical unease.
These controversies are far from be extinguished today, because the concept of correspondence between CR and Maximum Interdental Occlusion (MIO) is not validated by the physiologic data of deglutition, where the mandibular posture is mainly determined by the tongue and the hyoid muscles, and chewing cycle in, that needs a posterior functional play to work.
Anatomic data (Sicher H, Dubrul 1975) and several studies have shown that central occlusion is not placed in CR (Ingervall 1964), but is more anterior, for more than 96% of the patients (Posselt 1968, Joerger 2005), proving so that CR, even in its last definition (CNO 2001), can not be regarded as a systematic clinical reference for the patients. For instance, a study, in children (Melikian et al 1993),finds an average difference of 3mm between CR and MIO, with 12 mm for the maximum difference measured. In addition, these figures indicate that manipulations are not relevant, when TMJ are immature.
In the usual occlusal concepts, the inter-maxilla relationship (IMR) is placed in an articular position of Centric Relation (CR) obtained by manipulation (Mac Collum 1939;Weinberg 1972; Dawson 1985). Maximal interdental occlusion (MIO) is then accorded to CR.
The swallowing, however, the natural reference for MIO is not taken into account by these techniques (Fontenelle and Woda, 1993; Le Gall et al., 2010; Le Gall and Lauret, 2011; Joerger et al., 2012).
Fig. B12: The articular functional surfaces, are placed on the anterior faces of the articular heads. The manipulations have searched the reference backward…
Fig. B13: Depending on the operator, the articular resilience, the strength, the kind of manipulation : the resultant position of the mandible is more or less posterior, to the natural occlusion position, during swallowing. A too posterior position of the mandible, risk away the articular surfaces, much the concept of CR does not take into account the functional posterior play, necessary for the proper positioning of the mandible during cycle-in, in a lateral and posterior position than Maximum Intercuspation.
Evolution of Protocols Depending of the operator, of the articular resilience of the patient and of the type of manipulation and strength: the resulting position is more or less behind the occlusion position, compatible with swallowing. Clinical failures with toothed patients, when CR is too posterior, have imposed several evolutions of protocols, toward more anterior positions, obtained by not forced manipulations, still broadly depending of the operator (but is it still a CR reference ?).
To eliminate the operator dependence, the use of an anterior jig has been proposed by Lucia (1964, 1983), to provoke a mandible backward, by a sliding of lower incisors on the palatal face of the jig, and then record CR without any manipulation. This approach has not been conclusive, because it is the concept of CR itself, which is problematic.
The concept of a myo-centric occlusion has been proposed (Jeanmonod 1978) as an alternative, but without solving the problem of the repetitivity of the clinical recordings
In toothed mouths, a concept of jig slightly different with a specific clinical protocol (Le Guern 1987), has allowed to obtain a more acute inter-maxilla relationship, with a more physiologic approach of MIO. But even if the recordings are easy and repetitive, deglutition data and tongue posture are not taken into account. This topic has been developed above.
Even so, the more used reference concept is still CR. It ’s a problem, because a too posterior CR can be problematic, whereas a not forced anterior CR (but is it still a CR), comes closer to the natural reference, however without reach it.
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