Occlusion is perceived to be a subject that describes the morphologic and functional relationships among all components of the masticatory system (the system). It is a branch of knowledge that should explain in detail how the system normally functions and how to make corrections when it does not. Unfortunately, it does not meet these criteria. It is fraught with confusion and frustration because there is no unified theory and no definitive approach to teaching it.
There is no agreement on the occlusal design of teeth, the most ideal way they should touch each other in closure, or the trajectory of the mandible in function, and there are no sound theories on the relationship between the dentition and the temporomandibular joint (TMJ). These discordant views have resulted in a lack of understanding of occlusal problems and conflicting philosophies on how to solve them. If we do not agree on these very basic tenants of everyday dentistry, how is everyone doing their restorative dentistry and is what we don't know contributing to problems with the TMJ? In a conversation with Inside Dentistry, John Kois made the statement “Occlusion is the next frontier that is left virtually untapped by most of the profession, because most dentists, including me, are so highly trained that we have priced ourselves out of what most people can afford.”1 With all due respect, I do not agree that an understanding of occlusion can only be achieved by countless hours of training. Biological systems may be complex in detail but are simplistic as a whole in form and function, the result of a comfortable combination of anatomy and physiology. It is the purpose of this commentary, based on 50 years of clinical observations and experience, to examine this conundrum of occlusion with a fresh point of view that says that, yes, there is a single philosophy that works for all patients despite variations in the jaws and teeth, and that our quest should not be for an ideal occlusion but to achieve a masticatory system that is efficient and comfortable.
The biggest obstacle we have is trying to define our goals by using the word occlusion.
What Is Occlusion?
Occlusion is a word that means closure, whether it be a door or a heart valve. Dental occlusion is the contact of the maxillary and the mandibular teeth when closed. It is 1 of 3 principle parts of the masticatory system (Figure 1), which Ramjford and Ash2 described as a functional unit composed of teeth; their supporting structures, the jaws; the TMJ, muscles of mastication, and tongue; and the innervation and vascularity supplying all of them.
In 1898, Dorland's Medical Dictionary correctly defined occlusion as “the closure of teeth” as did all other dictionaries except one, Jablonski's.3 Why? Mohl,4 like many in the dental profession, felt that the definition of occlusion as closure was too narrow and should include in its definition the concept of a multifactorial functional relationship between teeth and other components of the masticatory system, so he cited Jablonski's definition, which he thought was a more comprehensive interpretation: “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 restoration, 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.”3 So now, the word occlusion, with a simple meaning of closure, had an additional interpretation: a 60-word description of the masticatory system. How did this happen? Jablonski's Dictionary of Dentistry3 uses an encyclopedic approach where certain descriptive definitions are more extensive and in-depth than is customary in standard dictionaries. When he felt that authoritative sources differed about the meaning of a term, he reflected minor differences in a single definition and major differences such as occlusion with more extensive explanations. Where did he derive his information? From the dental profession of course. Jablonski used 31 consultants, 13 of which were dentists and 19 others who were credentialed authorities in dental academia.3
These consultants influenced Jablonski to add a different sense to the word occlusion that was a description of the masticatory system and then quoted him as the authority. One has to ask; what was the point? Occlusion is part of the masticatory system, not a synonym for it. Unfortunately, the dental profession embraced the change, and the study of the masticatory system became known as occlusion. All contemporary texts entitled occlusion are in actuality the author's detailed analysis of the masticatory system. The American Association of Dental Schools then developed “Curricular Guidelines in Occlusion,” which begins by declaring; “The study of occlusion involves the entire masticatory system.”4
The results were disastrous! The American Dental Association's Council on Dental Education had recommended that dental schools establish coordinated teaching programs in occlusion, and from 1980 to 1981, 56% of the 51 US dental schools had separate departments dedicated to this subject.4 To my knowledge, not a single one exists today. Ironically, despite the change, Mohl4 continued to use the term masticatory system in which he detailed 9 mandibular positions, 16 types of mandibular movement, and 9 different occlusal concepts. If occlusion is a term that describes the system, how can we have 9 different occlusal concepts? It is reassuring to note that in the eighth edition of the Glossary of Prosthodontic Terms,5 the definition of occlusion is limited to the closure of teeth, whereas masticatory system correctly describes all the components of the system and the associated neurologic complex. The dual interpretation of the word is responsible for much of the confusion we are seeing today.
How Do We Achieve a Masticatory System That Is Comfortable and Efficient?
Let us begin with a description of how the system functions normally. McNeil6 reminds us: “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.” This is normal function that exists in the absence of parafunction. This is our goal: to achieve and maintain a masticatory system that is comfortable, efficient, and free from the harmful effects of parafunction. We must recognize the signs, understand its etiology, and excel in its management. There are 2 types of parafunction: the grinding of one's teeth called bruxism and clenching of teeth, which has been termed dental compression syndrome (DCS).7
The flattened teeth in the skulls of our ancestors are dramatic evidence that this untoward condition has been in existence since the beginning of time. It was first reported in modern literature by Marie and Pietriewicz,8 who first termed the disorder “la bruxomanie.” Since that time, 1907, it has had many aliases. Management is simple. Because the grinding, with few exceptions, occurs only while sleeping, a comfortable guard should be prescribed.
DCS is a syndrome because the compression produces a set of signs and symptoms that are excellent diagnostic tools that characterize the disorder.7 They include abfractions, occlusal dimples or cupolas, exostosis, lines of Luder in metallic restorations, and material fatigue in acrylic prostheses.
Often mistaken as areas of ablation from overzealous tooth brushing, abfractions (Figures 2 through 4) are, in reality, wedge-shaped examples of hard tissue fatigue caused by compression failure; these were first reported by the author in 1982.9 Fatigue is a weakened state brought by repeated stressing. Gordon described it as “One of the most insidious causes of loss of strength in a structure.”10
The highly polished appearance of these unique concavities may be caused by the exit of positive ions generated by apatite compression during clenching (Figures 5 and 6).7 It is common to observe the rounded depressions confined to the first molars, which take the brunt of clenching in 7-year-olds, only to be relieved by the emergence of the remaining dentition.
Articles on torus palatinus and torus mandibularis have appeared in the literature as early 1814. Although there is no consensus as to their etiology, many associate their occurrence with TMJ and DCS (Figures 7 and 8). The author believes the negative ions generated from the compression of apatite crystals are responsible for the aggregates of new bone growth (piezo-electric effect).7
Lines of Luder
Fatigue easily manifests itself in prostheses and restorative materials such as amalgam and acrylic. In engineering, these wavy patterns are termed lines of Luder or molecular slip bands (Figures 9 and 10). The explanation is that molecules in the alloy rearrange themselves under the influence of compressive strain.7 One can demonstrate the effect by bending a metal coat hanger back and forth to produce a similar effect.
Material fatigue in acrylic
Luder lines occur only in metal. The Luder-like lines in acrylic are termed sets, the result of repeated compressions. These deformations are excellent diagnostic tools for DCS.7
Management of Parafunction
Management of Bruxism is simple. Because the grinding, with few exceptions, occurs only while sleeping, a comfortable guard should be prescribed and perhaps stress counseling. Management of DCS, however, may be problematic because the affected patient may be chronically damaging themselves, yet be quite comfortable, so the need for interception may not be obvious. I would estimate that 50% of the new patients that I have seen on a daily basis had signs of DCS, but only half of those were cognizant and would deny that they clenched their teeth, and they may be correct. It is possible that the observed signs may have occurred during a prior stressful time in their lives, but more than likely, the real reason is that DCS works well within the subconscious, because returning patients would often remark “Since it was brought to my attention, I catch myself all the time.” Management begins with awareness and monitoring. It is suggested that the patient place a small card at their workstation with a drawing of a tooth or a simple question mark as a reminder to relax their mandibles. If the DCS is limited to clenching while awake, it is the patient's responsibility to monitor themselves and relax their mandibles. However, if the DCS is more intense, resulting in pain and discomfort, a thorough consultation is prudent. Is the etiology life stress, exercise, medication, or an uncomfortable bite? The dentist must also weigh if an equilibration might benefit the patient or possibly a guard?
The Question of an Equilibration
Ideally, how should teeth touch other in closure? Does it really matter if the patient is comfortable? Over the years, I've seen of hundreds of patients with unusual jaw relationships such as class II or III, many with crooked dentitions, and those who had anterior open bites occluding only on first and second molars, and they were comfortable because they were not affected with parafunction. However, if a patient were to clench, what would be the most ideal occlusion to embrace these powerful vertical forces (Figure 13)?
Aside from positional deviations, occlusal contacts are usually located on the crest of the buccal-cuspal ridge of the mandibular molars to occlude with the central fossa of the maxillary molars. It is the perfect alignment to direct occlusal forces vertically down the long axis of both upper and lower teeth to be distributed circumferentially via the root system to the alveolar bone. This design should not change when opposing a worn flattened antagonist. The second noteworthy observation is the generous space between the incline planes of the cusps termed the intra-incline space, which is vital to the unrestricted function of the mandible. The benefits of this ample space are that it minimizes collisions during mastication, eliminates frictional resistance that hinders the mandible/condyles from returning to their optimal position during swallowing, and ensures vertical loading on closure.
There is an ongoing dogma that centric occlusion (CO) should coincide with centric relation (CR). Is this true? Not necessarily, not for normal swallowing contractions that are variable depending on the posture of the body and harmless due to their low compressive forces, which are approximately 58–67 lb/in2 . However, it is important that CO coincides with CR during bouts of clenching when the condyles are best positioned in their fossae to receive these powerful compressive forces, which can exceed 500 lb/in2 . The intra-incline space is vital to facilitate this merger.
When the dentition is subjected to wear and the intra-incline space is lost, there is an increase of physical stress at the dentinal enamel junction (DEJ), causing sensitivity to temperature extremes and intense flavors. From an engineering point of view, these teeth would benefit from an equilibration (reducing the incline planes of the functional cusps to recreate the intra-incline space), but it may be detrimental to the patient. In my personal practice, I have performed equilibrations for patients with uncomfortable occlusions and often found comfort to be obtained within minutes; however, for some patients, it aggravated the problem. The difference was that certain patients, despite a flattened inefficient dentition, were quite comfortable, and the equilibration caused a change in their comfort zone, and they were not pleased. In my opinion, these patients should be left alone, and guard therapy should be discussed. What about equilibration as a treatment for TMJ disorders? If a newly placed restoration is uncomfortable and initiates parafunction, which in turn creates discomfort in the joints, it is good common sense to correct the morphology of that restoration. However, the National Institutes of Health has advised against equilibration as a treatment to treat TMJ, and they are correct in their prudence, as methods and goals for equilibration might vary depending on each dentist's philosophy.11
There is an opinion that any type of guard helps, but can there be detrimental consequences? Soft guards are frequently used because they are comfortable but often will encourage patients to clench even more because of their resiliency. A popular guard over the years has been the anterior hard guard, covering teeth #6 to #11, because it reduces occlusal forces by 75%. However, critics have cautioned that the use of this guard would cause the posterior teeth to supra-erupt. This is simply not true if worn only while sleeping; consider the mouth breather. However, there may be a liability. It is possible that clenching could cause the teeth supporting the guard to intrude, creating an anterior open bite, followed by tongue thrusting, giving the impression that the posterior teeth have supra-erupted. It is my opinion that the safest guard would be a full arch, hard, either upper or lower with minimal bulk, and light simultaneous contacts.
Ensuring the Longevity of Implants
Engineering principles for endosteal implants should be the same as natural teeth regardless of the absence of a periodontal membrane. Because there may be compromises in the placement of implants due the availability of alveolar bone, it is extremely important that the restorative dentist manipulate the occlusal surfaces of the prostheses to avoid incline plane contacts. This would ensure vertical loading on closure and avoid harmful lateral influences on clenching. Recently, a colleague associated with full-mouth implant reconstructions expressed concern that there seemed to be a percentage of breakdown after 5 years for no apparent reason. A reasonable explanation is offered; in the restoration of a semiedentulous patient with implants, deformations or wear of the patient's remaining dentition should inspire a dialog as to the awareness and management of parafunctional activities. However, if the patient is totally edentulous, the focus may concentrate solely on the reconstruction and less on parafunction, namely clenching, which chronically and painlessly takes its toll. One must consider that DCS may have been responsible for the demise of the patient's dentition in the first place. Unfortunately, a percentage of implantologists believe that implant protected occlusion,12 which is derived from mutually protected articulation5 (MPA), will ensure longevity. With all due respect, we have to rethink this. What MPA is implying is that parafunction affecting one part of the dentition is acceptable because it will alleviate trauma to another part of the dentition. It defies common sense. If this were a valid concept, there would be no need for a guard. Longevity only comes with an educated patient, impeccable hygiene, and a proper guard.
The principle objective in the understanding of how the masticatory system works is to provide practitioners with knowledge necessary for patient care. Unfortunately, the multiple definitions of the word occlusion have diverted our objective and have been a persistent cause of frustration and confusion. These sources of confusion have resulted in a number of different concepts as to how the system should function. Mohl explains that this variety of concepts is acceptable because “No one anatomical or functional description can apply to every human masticatory system.”4 Malcmacher agrees: “It is even ridiculous to suggest that there is one occlusal philosophy that fits all patients.”14 With all due respect, this is arguable. Therefore, let's be ridiculous and suggest that the one philosophy that would apply to all patients would be the elimination or at least management of parafunction. It is doubtful that our biological architects would have expected a masticatory system to work differently for different people because their original design was the same for everyone. Although there are different ways to rehabilitate or reconstruct a broken-down dentition, there is only one methodology for the general practictioner to maintain a healthy, efficient masticatory system and that is to maintain the original design of the patient's dentition with intra-incline space and focus on the prevention and management of parafunction. Teeth are beautifully designed biological cutting instruments. It should be our mandate to preserve their design throughout our lifetimes. We have a unique problem on our hands: part engineering and part psychological. It is our responsibility to first get the engineering right and then thoroughly investigate and manage other reasons one might initiate various forms of parafunction.