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Centric Relation (CR) emerged as a search for a reproducible mandibular position to facilitate prosthodontic rehabilitation. Over the past century, more than 26 definitions of CR have been proposed. The term CR has become thoroughly confusing because of many conflicting definitions. Unfortunately definition of CR changed repeatedly over past ten decades. Unfortunately, consensus remains elusive due to conflicting viewpoints for the past 93 years. Palaskar, Murali and Bansal reported, “Numerous definitions for CR have been given, however, no consensus exists and the definition given by a current glossary of prosthodontic terms is confusing. It relates CR to many clinically invisible parts and cannot guide a dental surgeon to record the CR following its description” (Centric Relation Definition: A Historical and Contemporary Prosthodontic Perspective, Indian Prosthodontic Society, October 2012).

MYOCENTRIC

In May 2017 the definition of “centric relation” changed once again removing all the previous known definitions of CR in the Glossary of Prosthodontic Terms and now has been updated according to the Glossary of Prosthodontic Terms (Ninth Edition, Volume 117 Issue 5S, May 2017).

NEW CENTRIC RELATION DEFINITION: May 2017

centric relation \sĕn΄trĭk rĭ-la΄shun\: acronym is CR; “a maxillomandibular relationship, independent of tooth contact, in which the condyles articulate in the anterior-superior position against the posterior slopes of the articular eminences; in this position, the mandible is restricted to a purely rotary movement; from this unstrained, physiologic, maxillomandibular relationship, the patient can make vertical, lateral or protrusive movements; it is a clinically useful, repeatable reference position”.

Question: How does one objectively determine the “unstrained” and “physiologic” maxillomandibular relationship?

It is interesting to note that after 93 + years of occlusal debates the great leaders and teachers within the dental profession who prominently have promoted and advocated the CR concepts are now having a greater awareness. Many dental students have been trained and influenced by CR minded teachers in dental school universities and post graduate continuing dental education that the centric relation concept was based on “science” or evidence-based. But after further investigation and clinical experience they soon realized the science did not exist neither was able to support their teachings as scientific. Whether CR was clinically applied by a bimanual manipulated techniques or chin point techniques the question continued to haunt the serious student as to how does one arrive at a repeatable centric relation position.

History has proven that this term “centric relation” (theoritically) has been an evolution of thought and beliefs. Definitions have been constantly changing to fit the occlusion dogmas. But as time proves, truth about how the physiology and anatomical structures of the neuromuscular system (the teeth, muscles and temporomandibular joints) becomes clear with the advent of scientific objective measurement and technology (Myotronics J5 dental TENS and K7x kineseograph jaw tracking, electromyography and electrosonography tools) that the mandible and joint relationships can be determined in an unstrained and truly physiologic manner without manual intervention by the operator.

Many Centric Relation definitions have been published and proposed as the guiding light, or the north star to establishing a jaw relationship in prosthetics, all with good intent, but none could quantify with objective measured proof they were promoting as truly scientific, but rather handed down generational teachings and techniques.

The Shifting Definitions of Centric Relation

GPT-5, Definition 1: “The maxillomandibular relationship in which the condyles articulate with the thinnest avascular portion of their respective disks with the complex in the anterior-superior position against the shapes of the articular eminencies. This position is independent of tooth contact. This position is clinically discernible when the mandible is directed superior and anteriorly.” (July, 2005).

GPT-3, Definition 2: “The most retruded physiologic relation of the mandible to the maxillae to and from which the individual can make lateral movements. It is a condition that can exist at various degrees of jaw separation. It occurs around the terminal hinge axis.”

GPT-1, Definition 3: “The most retruded relation of the mandible to the maxillae when the condyles are in the most posterior unstrained position in the glenoid fossae from which lateral movement can be made at any given degree of jaw separation.”

GPT-4, Definition 4: defined CR as “‘”the jaw relation when the condyles are in the most posterior, unstrained position in the glenoid fossa at any given degree of jaw separation from which lateral movements can be made” [1977]. Academy of Denture Prosthetics (1977) Glossary of prosth-
odontic terms. J Prosthet Dent 38:66–109.

Definition 5: “A maxilla to mandible relationship in which the condyles and disks are thought to be in the midmost, uppermost position”.

Main Thoughts of Centric Relation:

• Thought to be an “unchangeable reference” in one’s life time.
• Considered to be a highly repeatable position.
• Considered to be an initial reference point in determining a mandibular position.
• A focus on the position of the condyles – Hinge axis
• Properly aligned condyle-disk assemblies
• Against the eminentia
• Irrespective of tooth position or vertical dimension
• Most superior/ self-centering
• Coordinated muscle action “seats and loads” the TMJs

More Centric Relation Definitions:

Definition 6: “The relation of the mandible to the maxillae when the condyles are in the uppermost and rearmost position in the glenoid fossae. This position may not be able to be recorded in the presence of dysfunction of the masticatory system”.

Definition 7: “A clinically determined position of the mandible placing both condyles into their anterior uppermost position. This can be determined in patients without pain or derangement in the TMJ.” (Ramsfjord) Boucher CO).

TODAY – A NEW AWAKENING WITHIN THE DENTAL PROFESSION

With an increase and ever growing awareness of patient’s neuromuscular upper airway obstruction and sleep apnea issues the dentists now realize that the oral pharYngeal/airway needs space. Condyles are shifted in a down and forward manner (decompressed) within the glenoid fossa, especially with patients who presented with overclosed bites, retracted mandibles with posterior vertical deficiencies, all benefited with an improved mandibular to maxillary jaw relationship that would support improved tongue swallow and patent upper airway. Sleep appliances and the awareness of sleep apnea could no longer depend on a CR jaw relationship. No longer is a “Centric Relation” method to determine the appropriate jaw position for a sleep appliance can be scientifically promoted. Why? Because the profession has finally recognized the musculature and joints are not happy in a manipulated CR position. The airway and tongue restrictions want to be free of any strain relationship and prefers to be in a more “physiologic” homeostatic/neutral relationship that can be measured objectively in a more anterior unstrained position relative to a habitual centric occlusal position.

Today, dentists have come to a conclusion that CR is not the most retruded position of the heads of the condyles, but rather the most anterior and superior position. Whether the patient has achieved this position clinically while recording CR or not can only be simply checking the opening of the temporomandibular joint and visualizing the disc complexes with tomographic imaging.

MYOCENTRIC: A Neuromuscular Paradigm that is Based on Objective Measurements

myocentric \mı΄o sĕn΄trĭk\ adj: “that terminal point in space in which, with the mandible in the rest position, subsequent clonic muscle contraction will raise the mandible through the interocclusal
space along the myocentric (muscle balanced) trajectory; also described as the initial occlusal contact along the myocentric trajectory (isotonic closure of the mandible from rest position)”. Jankelson B. Dent Clin North Am 1979;23:157-68. Jankelson BR, Polley ML. Electromyography in clinical dentistry.
Seattle: Myotronica Research; 1984:52.

physiologic \fĭz΄e-a-lŏj΄ĭk\ adj (1814): 1. characteristic of or conforming to the innate function of a tissue or organ; 2. pertaining to organic processes or to functions in an organism or in any of its parts; 3. the opposite of pathologic; syn, physiological \fĭz΄e-a-lŏj΄ĭ-
kal\ adj.

physiologic rest position \fĭz΄΄e-a-lŏj΄ĭk rĕst pa-zĭsh΄un\: syn, REST VERTICAL DIMENSION, VERTICAL DIMENSION OF REST.

physiologically balanced occlusion \fĭz΄e-a-lŏj΄ĭ-ka-le bal΄ansd a-kloo΄zhun\ obs: a balanced occlusion that is in harmony with the temporomandibular joints and the neuromuscular system (GPT-4).

Questions to all the Dentists:

Is Centric Relation a therapeutic and functional position? If your awareness of CR is changing than most likely you can appreciate Myocentric. Occlusal paradigms have shifted with the advent of digital computerized technologies that brings the clinician to a greater awareness how it is possible for the teeth (occlusion), muscles and jaw joints can function and relate to one in an unstrained and truly physiologic manner – stable/homeostasis.

Clayton A. Chan, D.D.S. – Founder/Director

www.occlusionconnections.com

Telephone: (702) 271-2950

Leader in Gneuromuscular Dentistry