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I was invited by Dr. Sushil Koirala, Founder and Coordinator – MiCD Global Academy, Chief- Editor of MiCD Clinical Journal, as one of many globally renowned clinicians, academicians and researchers to answer 15 exclusive interview question for their clinical e-journal sharing my experience, clinical and research findings on the subjective of “TMDs – Confusion & Consensus: Expert’s Advice“. The following is the seventh of 15 responses to their questions.
7. What is your view regarding Centric Relation (CR) and its biological significance in the diagnosis and management of TMDs?
Centric relation is a concept that focuses on the position of the condyles, a hinge axis, to properly align the condyle-disk assemblies against the eminentia, irrespective of tooth position or vertical dimension, most superior/self-centering, a “seated and loaded” position within the temporomandibular joints.
Dentists have been searching for a reliable reference starting position in which to establish a mandibular relationship that is repeatable and reliable. Although the centric relation (CR) has been often referenced by leaders and teachers as a starting position to establish a mandibular to maxillary relationship when restoring a patient’s occlusion, dentists have recognized for years that the definition of “Centric Relation (CR)” has been constantly changing. Which definition is the correct definition since the Glossary of Prosthodontic terms has listed for years multiple definitions depending on one’s focus and perspective?
The Glossary of Prosthodontic Terms in the Journal of Prosthetic Dentistry, July 2005 stated a number of different definitions:
- “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-3, Definition 2).
- “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-1, Definition 3).
- “A maxilla to mandible relationship in which the condyles and disks are thought to be in the midmost, uppermost position”. (Definition 5).
- “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.” (GPT-5, Definition 1).
A question I ask our profession is Centric Relation a functional and therapeutic position? Is it unstrained and physiologic?
We know from experience the retruded mandibular position is not physiologic. Studies show muscle EMGs increase in a retruded position. Clinicians around the world also know from sleep studies that improved oral pharyngeal airway flow occurs when the mandible is positioned more anteriorly and vertically from its habitual centric occlusal (CO) position. Improved tongue posture and normalized tongue function occurs when the mandible is anterior and inferior of the classical CR position. Restrictions occur when the mandible postures in a posterior and superior direction.
Definition number 6 of centric relation (GPT-8) states: “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.”
A question we must ask is, do our patients have masticatory dysfunctions? If so, then implementing the CR position would not be appropriate according to the definition. Definition number 7 also states: “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. Occlusion in prosthodontics. J Prosthet Dent 1953;3:633-56. Ash MM. Personal communication, July 1993. Lang BR, Kelsey CC. International prosthodontic workshop on complete denture occlusion. Ann Arbor: The University of Michigan School of Dentistry; 1973. Ramsfjord SP. Personal communication, July 1993.
Question we must ask ourselves is, do our patients have pain or joint derangement? If so, then implementing CR would not be clinically appropriate according to these definitions and teachings.
In May of 2017 all former definitions of Centric Relation were removed and updated in the Prosthodontics Glossary of Terms using the term “physiologic” in the new CR definition (GPT-9): “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”. Note: for the first time the term, “physiologic” has been used in redefining Centric Relation. Is it possible that centric relation is moving closer to a neuromuscular understanding of Myocentric?
It is clear, an evolutionary change is occurring within the dental profession toward a more bio-physiologic awareness of muscles, joints and teeth. Based on the use of the term “physiologic” in this newer definition one would assume that dentists routinely measure the status of their patient’s muscle and jaw conditions, especially when evaluating, diagnosing and managing TMD problems. But we all know the truth and reality of this matter. They do not. Leaders within the dental profession are learning that a manually manipulated mandibular position does not adequately fulfill the bio-physiologic requirements of physiologic health. Dentists are also learning that creating an even balanced occlusion when the mandible is functioning posterior of a more optimal physiologic (myo-trajectory does not constitute physiologic health. Bio physiologic EMG and CMS measurements scientifically confirm this fact. Thus, a good reason why the teachers and leaders within the profession are now acknowledging a needed to update and change their definition and understanding of centric relation.
Based on science and objective measurements one can realize that the definition of centric relation is moving closer to the definition of myo-centric occlusion, a term Dr. Bernard Jankelson coined years ago. “Myocentric Occlusion; synonymous with Neuromuscular Occlusion and Myocentric Position. The terminal point along the neuromuscular trajectory at which occlusal contact occurs. This position allows maximum function with minimal expenditure of energy prior to reaching the functional position”. R. R. Jankelson: Neuromuscular Dental Diagnosis and Treatment, Ishiyaku EuroAmerica, Inc. Publishers, 1990.
Myocentric adj: That terminal point in space in which, with the mandible in rest position, subsequent colonic 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: Myotronics Research Inc, 1984:52. The Glossary of Prosthodontic Terms, Eigth Edition (GPT-8) Volume 94, Number 1, July 2005.
A GNM occlusal understanding does not end with the classical neuromuscular myocentric concepts, but goes even further in establishing sound techniques that identify and physiologically confirm an optimal mandibular position with science and completes the functional and therapeutic position that enhances the meaning of what occlusal stability is really about.
Clayton A. Chan, D.D.S. – Founder/Director
Telephone: (702) 271-2950
Leader in Gneuromuscular Dentistry