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A Brief Review of a Healthy Temporomandibular Joint (TMJ)

Inspiration for this article:

Recently we received the following comment on the blog article, "The Importance of Load Testing" by Dr. Leonard Hess. 


I am a past attendee of the Pankey and Dawson curriculums ( long ago in the 1980's) and am flabbergasted that you still preach this anachronistic content which has long been disproven in the evidence based literature and debunked in dental education. I am a dental educator and long ago stopped teaching that every patient had to be in centric relation, that the TMJ bore the main load of occlusion and that anterior guidance and posterior interferences had a significant relationship to TMD. I see patients with harmful irreversible changes caused by excessive and unnecessary equilibration, and TMD patients grossly overtreated with unneeded prosthetics, orthodontics and orthognathic surgery. I am hoping that you will post this and respond. Although I have great respect for Dr. Dawson as a pioneer in dentistry the Dawson Academy needs to greatly modify its curriculum content to come into the 21st century. !

Harold F. Menchel DMD

You can find responses by both Dr. Hess and Dr. Dawson to Dr. Menchel's comments at the bottom of that article. Below is my response.

As dental educators, it is our duty to seek a total understanding of the foundational concepts and the associated clinical relevancy in order to teach our students and positively impact patients.  Dr. Dawson’s very concept of complete dentistry, and what is taught by the Dawson Academy, is to identify any signs or symptoms that impede anatomic and functional harmonyIt is our goal to provide the least invasive, most conservative treatment to help our patients achieve optimal health that is maintainably comfortable and beautiful.  This we agree upon. 

The “anachronistic content” you describe stems from an incomplete understanding of the anatomy and function of the masticatory system as well as the erroneous application of that misinformation.  Much controversy and clinically skewed research stems from a lack of a common classification system for joint position, occlusion and temporomandibular disorder (TMD) diagnostic criteria. Sadly, such has perpetuated our profession and the opportunity to clarify is welcomed.

Let’s begin with a brief review of a healthy temporomandibular joint (TMJ):

TM JointThe TMJ is a synovial joint. A biconcave fibro-cartilaginous disk fits between the two convexities of the condyle and articular eminence.  The inelastic collagen of the posterior ligament and the elastic fibers of the superior elastic stratum tether the disk to the condyle distally preventing anterior disk displacement.  Collateral diskal ligaments attach to the medial and lateral aspects of the condyle to maintain alignment of the disk as the condyle moves along the articular eminence.  This assembly, housed in a fibrous capsule, receives vital nutrients via synovial fluid and through an arteriovenous shunt phenomena of the retrodiskal tissues each time the jaw opens and closes via release and contraction (respectively) of the superior belly of the lateral ptyergoid muscle.  Elevator muscles contract to seat the TMJ into the most anterior-superior medial position against the eminence, which is medially braced by thick bone. 

A review of the biology and function of the system clearly illustrates the TMJ is load bearing. 

Fibrocartilage, as found in the disk, only exists in the body where high tremendous mechanical stress occurs.  It is made of dense type I collagen fibers that resist deformation under that high stress.  Ligaments stabilize the joint when it is not loaded and when elevator muscles load the TMJ in a vector that is anterior-superior and medially through the avascular, aneural disk that remains centered on the condyle in a stable system. 

So why all the fuss? 

The inability to attain quality in vivo data forced researchers to use a classic biomechanical model which describes the TMJ as the fulcrum in a lever system.  This simplistic linear description of a non-linear system resulted in some controversy it did not complete the full picture.  Newer models, such as the biotensegrity model for example, have described a non linear model for other load bearing joints and further supports the TMJ as a load bearing joint. 

Accepting that the TMJ is a load bearing joint and that the elevator muscles seat the condyle anterior-superior and medially, we find Dr. Dawson’s definition of centric relation (CR) to be the most orthopedically stable position.  Centric relation is 100% comfortable, anatomical, and reproducible.  If it is not 100% comfortable, it is not in centric relation. 

CR is defined as the maxillomandibular relationship where a properly aligned condyle disk assembly is in the most anterior-superior position against the articular eminence and it is totally independent of tooth contact.  Posselt’s envelopes of mandibular movement and gothic arch tracings all confirm the joints seat as Dr. Dawson describes.   It is a fundamental fact that any occlusion analysis must begin at the joint level.  Verification of healthy temporomandibular joints that can accept firm loading is an imperative component of the complete examination.  This is done via an orthopedic incremental load test as Dr. Hess described so well.  If either jaw joint cannot accept firm loading without being 100% comfortable, further evaluation is necessary.  Appropriate diagnosis of the condition and stability of the TMJ is mandatory. 

Accurate joint diagnosis is another area where the term “TMD”, and consequently the literature, fail us.  

Temporomandibular disorder makes no distinction between the significantly varying etiologies of dysfunction and orofacial pain.  Occlusal muscle disorders, internal derangements, neurologic pain, dental pain, pathology and more have been grouped together routinely in the literature.  It is absurd to believe one treatment modality of any kind would successfully treat all those sources of TMD.  It is thus equally absurd to accept much of the literature as valid without further clarification and classification of the etiology. 

EMG studies that have spanned literal decades have and continue to show that occlusal interferences increase muscle activity and results in condylar position change.  Anterior guidance that provides immediate posterior disclusion while in harmony with the envelope of function has also been repeatedly shown to reduce muscle activity.  We must become a physician of the masticatory system to truly understand all the possibilities of “TMD” and gather all the appropriate diagnostic information to make an accurate diagnose.  It is crucial that a uniformly accepted classification system be utilized in order for research and literature to be truly evidence based. 

I began my journey with the The Dawson Academy in 2002.   Never in my 14 years has it been taught that every person must be treated to CR.  It is through the process of a complete examination that the dentist and patient co-discover signs and symptoms that the masticatory system is not adequate in form and function.  When signs of instability are present, further analysis and diagnosis is required.  Let’s be clear that irreversible, definitive occlusal treatment is never performed without an appropriate diagnosis and thorough understanding of the etiology.  TMJs must be 100% comfortable with firm loading. 

The Dawson Academy teaches a process for detailed, systematic treatment planning with photography and properly mounted diagnostic casts to arrive at the best, least invasive, restorative option for the patient.  I would strongly urge you to come back and go through our curriculum.  I believe you will find our philosophy and processes are in no way anachronistic. 

Is Anterior Guidance Important?


Picture of Dr. Shannon Johnson

Shannon Johnson, DMD is an associate faculty member at the Dawson Academy. She also leads two Dawson Study Clubs and teaches the Core Curriculum. Dr. Johnson owns and operates her practice in Louisville, KY.