Occlusion is an interesting subject. It is the one subject that everyone should agree on, yet there is no consensus. The standard of care is undefinable, the question of temporomandibular joint (TMJ) liability is unclear, and attempts by academia to explain and understand this subject on a scientific and biologic basis have not been successful. This has created a feeling of apathy by many general practitioners (GPs) toward this subject and a de-emphasis on its teaching in some dental schools. This review/ commentary examines the factors that have created this situation, evaluates the literature in order to provide a reasonable explanation for the current epidemic of TMJ disorders, and suggests treatment based on engineering and physiologic principles that help protect the TMJ.
WHY IS THE SUBJECT OF OCCLUSION SO DIFFICULT TO UNDERSTAND?
It is appropriate to begin with the definition of the word itself. How many times have you heard of a clinical failure blamed on occlusion? What exactly does that mean? Originally it was simple when the sole definition was the closure of teeth.1 If occlusion were suspect in a problem, one would naturally examine the manner in which the teeth touched each other in closure/function. Over the years however, authors felt the need to broaden the definition. The expanded definition:
The relationship between all the components of the masticatory system in normal function, dysfunction, and parafunction, including the morphological and functional features of contacting surfaces of opposing teeth and restorations, occlusal trauma and dysfunction, neuromuscular physiology, the temporomandibular joint and muscle function, swallowing and mastication, psychophysiological status, and the diagnosis, prevention, and treatment of functional disorders of the masticatory system.2
This change caused a lack of understanding of this subject. The word occlusion has only one definition, “the closure of teeth.” It is not a synonym for the stomatognathic system. These are two separate entities and should be dealt with individually. Throughout this review, the word occlusion refers only to the closure of teeth. The stomatognathic system is described separately.
HOW SHOULD THE STOMATOGNATHIC SYSTEM FUNCTION IDEALLY?
Many components comprise the masticatory system. The functional movements of the mandible are of greatest concern. Ideally, the mandible and the muscles of mastication are in a state of physiologic rest most of the time. The mandible has three primary tasks: mastication, swallowing/ closure, and communication. After each task, the mandible returns to a state of physiologic rest. In closure, centric occlusion (CO) equals centric relation (CR) with the condyles positioned anterior — superiorly in their fossae. McNeil confirms, “It must be emphasized that the teeth only come together momentarily during swallowing and occasionally during mastication, and that at all other times the teeth should be apart in the resting stage of the mandible.”3 This description promotes an “open philosophy,” free of parafunction.
IDEAL OCCLUSION IN THE IDEAL ROLE MODEL
How does the anatomical configuration of teeth and the way they touch each other promote ideal function? An nineteen year old patient is presented as an ideal role model. His teeth had no restorations and no attrition from parafunction (Figs. 1-2).4 Models of his dentition in closure were sectioned at 2mm intervals (Figs. 3-6). Examination of the sections reveal that physical contacts are confined to the tips of functional cusps, resulting in an intra-incline space. The intra-incline space, a phenomenon rarely discussed in contemporary dentistry, is defined as the natural space that exists between the incline planes of the cusps with the dentition in closure. This is nature’s ideal occlusion. From an engineering perspective there are several advantages. The occlusal contact confined to the tips of the functional cusps ensures desired vertical loading (Fig. 7) during swallowing/closure, and prevents undesired offloading (Fig 8). Misch and Bidez describe vertical compression forces as normal because they act perpendicular to and maintain the integrity of the alveolar bone.5 They explain that when the incline planes of the cusps are in contact, bending or off-loading is likely during mastication, resulting in destructive shearing forces which act parallel to the alveolar bone.5 Additionally, the intra-incline space allows for neutralization (Fig. 9) which is the desired buccal-lingual position of the teeth by reciprocal action of the muscles of the tongue and cheek6 and it allows unrestricted movement of the mandible, anterior-posteriorly (long centric), as the head changes position from the vertical. Finally, preserving the original sharp configuration of the natural dentition minimizes the force required for mastication.
WHAT HAS BEEN TAUGHT TRADITIONALLY?
In 1926, B.B. McCollum and a group of colleagues founded the Gnathological Society of California. Realizing that there were great diversities in human dentitions, they attempted to identify the best occlusion and the best functional relationship between the maxilla and mandible that would adapt to the many variations.7 They coined the term gnathology, a description of temporomandibular joint movements, and promoted measurements and reproduction of the TMJ as determinants in the diagnosis and treatment of compromised occlusion.8 During this time period, two different opinions evolved to explain occlusal function. One, the open system, was described as a relaxed mandible, free from parafunction, closing lightly when appropriate, then returning to its position of physiologic rest. What did our early investigators of occlusal concepts think about this open philosophy? In Concepts of Occlusion, 1963, Stallard and Stuart discussed it:
“The statement has been made that the teeth in the healthy mouth do everything possible to keep from contacting their opponents. When the jaw is resting, the teeth are discluded. Quiet mouths make occlusal contacts infrequently and then not very firmly. During chewing, the food cushions the contacts. Teeth do their work on the way to occlusion. Occlusion is the end of a chewing stroke. How well teeth are related on the way to occlusion is what makes them fitted for their purpose.”9
Although these ideas exhibited good common sense, Stallard and Stuart, without giving credit to the author, dismissed them as waggish.9 Disregarding the open description with light occlusion, they focused instead on a closed philosophy with rock-solid occlusion to maintain the correct position of the condyles. If they determined that the condyles had migrated, the mandible was repositioned and the occlusion altered to support it by engaging the incline planes of the cusps. This is in sharp contrast to the open concept where the contacts were confined to the tips of the cusps. An early reference to the closed philosophy by Angle, 1899: “So it will be seen that the occlusion of the teeth is maintained first by the occlusal inclined planes of the cusps.”10
WITH TWO OPPOSITE PHILOSOPHIES, WHY DID EARLY INVESTIGATORS FOCUS ON THE STOMATOGNATHIC SYSTEM IN CLOSURE?
The explanation has to do with parafunction- the clenching and/or grinding of one’s teeth. Worn dentition from ancient skulls, as well as numerous references to the gnashing of teeth in the literature bear witness that forceful compressing of one’s teeth is as old as humankind. Our role model is ideal because it is not afflicted with parafunction and is without pathology. The closed viewpoint does not evaluate whether the patient is damaged by parafunction. It concentrates on the position and stability of the TMJ. It does suggest that there be anterior guidance to minimize forceful lateral excursions if grinding. But something was not accounted for. Why did the condyles migrate from their preferred position in the first place? What was hindering the condyles from repositioning themselves without assistance? Does the dentition maintain the condyle’s position? It is the premise of th
is review that milling and compression of ones dentition positions the mandible anteriorly, resulting in deviation of the condyles from their preferred position. Due to the diminished intra-incline space, the incline planes of the cusps are now engaged, and the mandible is unable to return on its own accord during swallowing/closure. The closed philosophy tries to stabilize the TMJ with the dentition, when the best stabilization results with the reduction or elimination of parafunctional forces. Considering the two philosophies, how are dentists restoring their patients?
CONTEMPORARY RESTORATIVE DENTISTRY
Restorative dentistry is practiced on two distinct levels. On one level is full mouth reconstruction (FMR), completed in a limited time frame. On another level is partial reconstruction, continuing over a lifetime. Each has interesting ramifications.
FULL MOUTH RECONSTRUCTIONS (FMR)
Few patients have their mouths totally reconstructed. I would estimate less than three percent. A goal is to verify or re-establish the position and centricity of the condyles, and then create a dentition to support it. Two methodologies are commonly employed. One utilizes a three dimensional articulator; the other employs neuromuscular instrumentation. Both follow the closed philosophy to verify their goals. An editorial in Dentistry Today, “The Occlusion Wars; It’s Time for a Truce,”11 discussed the two methodologies, but the author was of the opinion that one was better than the other. Is one superior to the other? It is like comparing apples and oranges. Both are based on sound principles of natural physiology and good engineering. Followers of each have considerable training and are dedicated to solving their patient’s problems. The differences of opinion are simply justification for the use of one’s particular instrumentation. Should this intellectual friction be shrugged off by the general practitioner (GP)? It offers little insight into answering the important question of temporomandibular disorder (TMD) liability, which is of particular interest to GP’s rather than reconstructionists. GP’s provide the vast majority of treatment to the general public. What are they doing on a daily basis?
PARTIAL RECONSTRUCTIONS
A new patient presenting to a general practitioner undergoes a series of evaluations: general health, periodontal, caries, and restorative. If there are no problems with the TMJ, occlusion, or dysfunction, the GP may focus on restoring the dentition by segments, a project at a time, whether it is a single restoration, multiple units of bridgework, or a removable partial denture. Guidelines for these partial reconstructions are the physical boundaries of the opposing and adjacent dentitions and esthetics. After the teeth are prepared, impressions and bite registrations are taken, and temporaries are placed. The laboratory then mounts the models on a straight-line articulator. What is the occlusal prescription? It is called: “Low and Go.” A GP’s daily schedule is divided into time periods, each of which is assigned a task. There is a desire to complete each task within the allotted time period, without delays. Accordingly, one hopes to spend minimal time adjusting occlusion after cementation. Dental technicians are fully aware of this and have coined the phrase ” Low and Go” to describe a technique creating slight clearance on closure between the new prosthesis and the opposing dentition, to minimize adjustment time. The idea is that following supereruption, everything will be just fine. But will it? What philosophy, open or closed, is the GP using? The average GP is not considering philosophies, but is focused on the restoration of one segment of the patient’s arch. If the patient leaves the office in comfort, why concern oneself with open or closed philosophies? Comfort should not be the sole consideration. It is quite possible that a patient could be affected with parafunction and still be comfortable. That is why every GP should have a thorough understanding of this disorder, in particular, how teeth touch each other ideally in a parafunction free system, with contact confined to the tip of the functional cusp, ensuring intra-incline space for freedom of movement in function. The laboratory should be supplied with an occlusal prescription to limit occlusal contacts to the tips of the functional cusps. An analysis of the occlusion of the dentition not involved in the project is also in order to determine whether an equilibration might be indicated to improve efficiency, comfort, and long-term stability.
PARAFUNCTION, TEMPOROMANDIBULAR DISORDERS (TMD) AND EQUILIBRATION
Although it is recognized that not everyone who clenches and grinds their teeth has TMD, it is the premise of this paper that the majority of these disorders are the result of repetitive motion trauma from chronic parafunction, not unlike carpal tunnel syndrome. There is no disagreement that the heavy compressive forces of parafunction stress the TMJ. If there are signs of parafunction, one must manage this problem first to reduce stress. Side effects include worn flattened teeth, receding gingival tissues and deformations on teeth, bone and restorative materials, all thoroughly documented in, “Dental compression syndrome — a new look at an old disease.”4 Treatment of parafunction consists of education, guard therapy and possibly equilibration. Education lifts the awareness of the condition from the subconscious to the conscious by explaining to the patient what is known about the etiology and the benefits of stress reduction. Patients are asked to monitor themselves for daytime vertical clenching. Guards are recommended when the patient has no conscious control as when sleeping. It is the decision to equilibrate that is difficult and of primary concern. Some reason that the natural bite should not be altered. Most GP’s do not perform equilibrations because this procedure is controversial. Guichet and Landesmann once declared, “Many dentists lack the skills and confidence to provide an equilibration.”12 However, equilibration is a technique easily taught and learned. Many GP’s lack confidence because academia has not adequately explained the rationale or demonstrated a reasonable, simplified technique. Patients hesitate to undergo this procedure because conventional methods of equilibration are tedious and expensive. In addition, the National Institutes of Health (NIH) advise dentists against equilibration for treatment of TMD on the grounds that it is not reversible.13 But reversibility alone is not sufficient justification for this view as very few clinical procedures are. The primary reason for concern is that there is a lack of evidence-based studies to demonstrate that equilibration can give comfort to the TMD patient. It is not a simple question because of the dual etiologies of parafunction. A study could be designed with TMD patients with heavy occlusal contacts. The dentitions could be adjusted according to the open philosophy, but that would not necessarily diminish stress on the TMJ. If a patient with heavy occlusion has discomfort in function, and equilibration reduces TMJ stress, the patient may clench anyway for psychological reasons. So the value of the equilibration and its effects on the TMJ would be difficult to evaluate in a study. Under certain circumstances, however, equilibration alone can reduce a TMD problem. Consider a comfortable patient with no TMJ problem. A gold restoration is placed in tooth #30 with a prematurity on an incline plane displacing the mandible with a side shift on closure. The patient begins to clench, applying unnecessary stress to the TMJ. The prematurity is then discovered, eliminated, and stress to the TMJ is reduced. In this instance occlusal, equilibration can reduce stress to the TMJ and will alleviate discomfort if the prematurity was the only reason the patient clenched. We have been cautioned against imposing occlusal changes based upon the clinicians concept of the ideal.14 However, it is correct to reshape worn, deformed teeth in order to regain their original configuration wi
th intra-incline space. A review of fifteen articles on occlusal equilibrations published in professional journals from 1966 to 1990 reveals generalized agreement on these points:15-29
Prophylactic adjustments in the absence of pathology are not acceptable.
Occlusal adjustment is a misunderstood and under-utilized procedure.
CR should equal CO.
There should be no interferences in lateral excursions.
The height of the buccal cusps should never be shortened except to eliminate interference in lateral excursions.
Traumatic occlusal relationships should be eliminated before restorative procedures.
Cusps should touch loosely in the opposing fossae.
Inclined planes should not touch to ensure axial loading.
Occlusal indicator wax is the most effective way to demonstrate how teeth touch.
There should be no flat plane occlusion in humans.
Cuspid guided occlusion is preferred.
DISCUSSION
All of these recommendations adhere to the open philosophy except one — cuspid guided occlusion. If McNeil is correct that teeth should only come together momentarily during swallowing and occasionally during mastication, why do we need anterior guidance? Patients with class II and III jaw relationships have no anterior guidance and function without difficulty. The concept of anterior guidance was created to minimize parafunctional forces in lateral excursions. The idea has evolved that if a patient had anterior guidance, the remaining dentition was protected. This is simply not true. Parafunction has many forms; grinding in lateral excursions, vertical clenching and anterior thrusting of the mandible. In a majority of cases, protectionism can only come from educating the patient, stress control and the use of a proper guard.
SUMMARY
Fifty years ago, McCollum and Stuart described a subtle pathology of function in the human masticatory system that was difficult to understand.30 That subtle pathology is the damage that results from compression of ones teeth. It is subtle because often the patient is unaware. It is pathologic because it applies untoward stress to the dentition, alveolar bone and the TMJ. It is difficult to understand for many reasons:
Multiple etiology.
Few patient complaints.
Poor understanding of the deformations caused by parafunction.
Dual philosophies of how the stomatognathic system functions.
The role of equilibration during treatment is unclear.
The dissimilar ways it takes its toll.
For proper management of parafunction, the GP should monitor for signs of compression and wear, educate the patient about the problem, and provide proper treatment. While every patient with a flattened dentition should not have their teeth dramatically altered or reconstructed, there should be consensus in the dental profession that natural, sharp morphology of teeth is superior to a flattened dentition and should be preserved throughout one’s lifetime. The GP should design prosthetic replacements to emulate the original configuration of the teeth, with main contact at the tip of the cusp. I believe that the biggest impediment to understanding is that we to often try to explain etiology by the way teeth touch each other, even though the most comfortable patients seldom have their teeth touching.
Dr. McCoy is in private practice in San Francisco, CA and is an honored fellow in the American Academy of Implant Dentists. He lectures nationally and internationally on occlusion and Parafunction.
Oral Health welcomes this original article.
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