Occlusion Fails

The subject of occlusion has to be presented as a body of knowledge that is consistent with the norms of healthy physiology, good engineering, and common sense. But unfortunately, occlusion demonstrably fails to meet these norms, and this is causing problems.

Freelance writer Jackie Syrop recently discussed these problems in an article for Inside Dentistry.¹ After interviewing a group of dentists who were knowledgeable about the role of occlusion in dental practice, she concluded that “because frustration and confusion reign over this complicated subject, only a small percentage of dentists have significant knowledge to use occlusal guidelines in their practices.” The lack of clarity regarding occlusion has plagued the dental profession for more than 100 years, hindering us from providing our patients with the very best dental health service. The purpose of this article is to discuss these problems and to offer plausible explanations and solutions.

PROBLEM NO. 1
Defining Occlusion
In the author’s opinion, we are using the word occlusion in the wrong context. The word simply means “shut up” or “close up,” from the Latin occludere. If we want to check a patient’s occlusion, we examine how the teeth touch in closure, but as Syrop1 points out, referencing Türb et al:²

Most dentists who have studied occlusion in depth concur that any definition of occlusion should not be limited to tooth-contact relationships, but rather should take into account the dynamic morphologic and functional relationships among all components of the masticatory system—not just teeth and supportive tissues but also the neuromuscular system, temporomandibular joints (TMJs), and the cranioskeleton.

I agree, but the understanding of those relationships should not be called occlusion. Occlusion is not a homonym for the masticatory system, it’s just a word that means closure. Occlusion (the way teeth touch each other in closure) and the masticatory system are 2 separate entities and should be described separately to remove the present ambiguity.

PROBLEM NO. 2
Two Views of How the Masticatory System Should Function
Another problem that contributes to the confusion is how the mandible functions. Normally, the mandible functions vertically, whether for eating, talking, or swallowing. Yet a popular paradigm contradicts normal vertical function and directs us to harmonize the patient’s teeth to accommodate horizontal excursions. But this is not a normal function; this is a parafunction, also known as grinding, bruxism, and dental compression syndrome (DCS).

This confusion all started in 1880 when a German anatomist, F. Graf von Spee,³ presented a paper, “Die Verschiebungsbahn des Unterkiefers am Schadel Arch” (“The Condylar Path of the Mandible in the Glenoid Fossa”), in Kiel, Germany. In that paper, von Spee³ described what he thought was the most efficient pattern for chewing, which was a horizontal rubbing of food between teeth, the course of which is determined by the anatomy of the TMJs and the occlusal surfaces of the teeth. He³ even suggested shortening the cuspids to prolong the horizontal rubbing. This was the birth of the Balanced Occlusion Theory, which met no opposition and was popular for many years. Gysi⁴ wrote in 1910: “Balanced occlusion is considered essential to any organized dentition.”

However, the study of the evolution of the primate family does not support von Spee’s observations³ or the Balanced Occlusion Theory. Our dentition is designed for a vertical shearing/cutting action, not the horizontal grinding action of the herbivores or ruminants. There were a few committed opponents to the theory, such as Nagao,⁵ who refuted von Spee’s observations,³ and Shaw,⁶ a dentist and anthropologist, who interpreted balanced occlusion as a malocclusion. Nevertheless, the theory remained popular as late as the 1950s, until 3 California dentists—Stuart, Stallard, and McCollum—abandoned it, citing clinical failures.⁷

But the majority of the dental profession was still committed to guiding patients into laterotrusive movements without questioning the purpose this served, which confronted them with the subtle pathology of bruxism. The result was that the condyles were distracted, teeth were wearing down, and the muscles of mastication were in tetany due to the accumulation of lactic acid. Faced with this damaging compressive power, Schuyler⁸ sought to minimize that force by distributing it to the bicuspids and molars, creating a group function. The group function philosophy created physiologic wear, which was viewed as a compensatory adaptive change. This was thought to be natural and beneficial because it resembled the vigorous function that primitive man exhibited, but it did not solve the grinding problem.

In an effort to further reduce these destructive lateral forces, the concept of canine guidance was introduced. The idea was that the vertical and horizontal overlap of the canine teeth disengages the posterior teeth in the excursive movements of the mandible. It is widely believed that this idea originally came from Nagao,⁵ but the clinical application of canine guidance was actually initiated by Stuart and Stallard⁹ and by McCollum and Stuart¹⁰ because canine guidance had the slowest rate of wear. This concept was also heavily endorsed by D’Amico¹¹ in his treatise on canine protected occlusion.

Next came the concept of mutually protected articulation, which, according to The Glossary of Prosthodontic Terms, was “an occlusal scheme in which the posterior teeth prevent excessive contact of the anterior teeth in maximum intercuspation, and the anterior teeth disengage the posterior teeth in all mandibular excursive movements.” All this confusion came from von Spee’s erroneous idea³ that we eat horizontally. Is it possible that von Spee had flat teeth from bruxism?

For 120 years, the dental profession has been unsuccessfully trying to diminish the destructive power of parafunction by designating certain teeth as targets for damage control. Instead, we need to educate our patients about the preventative aspects of parafunction by monitoring themselves during waking hours for clenching and using a nightguard if appropriate. Furthermore, the dentist needs to evaluate the teeth to determine the need for equilibration.

PROBLEM NO. 3
Lack of Consensus on the Morphology of Teeth
At this point in time, it is hard to believe that there is no consensus about the morphology of teeth. In the fabrication of removable dentures, we have a choice of 7 different designs for the posterior teeth, from 33° to flat plane occlusion. It makes no sense to provide flat plane occlusion for a denture patient who has little or no alveolar ridge and a diminished mastication force of 75% and expect that patient to eat his or her food efficiently. This is another unfortunate legacy of balanced occlusion.

There are design principles that appear to govern the structure-function relationship in organisms; that is, there is an interface between mechanical engineering and biology. Biological materials and structures are designed for specific functions. The job of teeth is to cut food, and of sharp teeth to cut food efficiently. It should be our mandate to preserve that original design. Unfortunately, some people believe that our evolutionary blueprint has programmed us for zero-degree occlusion. However, it is poor speculation to declare that we are predestined to have a flattened dentition, since there are many seniors who maintain naturally sharp teeth simply by not grinding them.

PROBLEM NO. 4
The Philosophical Problem of Occlusion
Restorative dentistry takes place on 2 distinct levels: complex and maintenance. Complex restorations are described as either full-mouth reconstructions or rehabilitations. Because there is no unified theory on how these complex cases should be done, Syrop¹ explains that dentists have divided themselves into “camps” of different occlusal philosophies such as centric relation, joint-based, and neuromuscular. Let’s pause to analyze this confusion because, again, dentists use the word occlusion incorrectly. Occlusion is just a word that means “closure”; there is no philosophy associated with it. However, there is a set of ideas relating to the discipline of how the masticatory system is supposed to work. This is the philosophy we want and need to understand. Centric relation, joint-based, and neuromuscular are not occlusal concepts nor are they philosophies; they are reference points to aid in the rehabilitation, as are the teeth themselves. Is one reference point better than another? No, because they all have to work in harmony with each other.

Maintenance Dentistry
Maintenance dentistry is the way that 99% of the restorative work is being accomplished throughout the world. Unfortunately, there are no guidelines. It is a “copy-what-you-see” technique. If, in the fabrication of a crown, the opposing tooth is occlusally flat, so will be the new crown. If the opposing tooth is sharp, the new crown will match accordingly. The general consensus is that this approach allows patients to function with whatever bite they currently have, which assumes that they have no problems. However, the caveat is that they may have chronic parafunction (clenching/grinding), but may also be quite comfortable.

PROBLEM NO. 5
TMJ Problems
It should be within the purview of general dentists to manage and prevent problems of the TMJs. We know that temporomandibular disorders (TMDs) can stem from developmental deformities, from a disease process such as osteoarthritis, or from trauma, but the majority of problems that we see with TMJs are simply repetitive motion trauma from clenching and grinding, not unlike carpal tunnel syndrome. In the 40-plus years that I have been in practice, I have not seen a single TMD patient who did not have one or more signs of parafunction—such as flattened teeth, enlarged masseters, abfractions, occlusal dimples, and exostosis. Therefore, if patients with TMJ problems are clenching and/or grinding, the dentist should address the parafunction first to take the pressure off the TMJ.

CLOSING COMMENTS
The concept of occlusion, as we know it, is scientifically baseless. It fails to provide a simplified understanding of how to maintain the masticatory system in a healthy, comfortable state. The concept has been so confusing that it has been a major distraction from addressing the more serious problem of parafunction. So, the dental schools are correct in not teaching occlusion as a subject because it’s not a subject at all; it’s just a word that means closure.

We have a unique problem on our hands that is part engineering-related and part stress-related. It is important for dentists to understand and fulfill the engineering requirement before concentrating on the psychological part of the problem. There are 2 major flaws in our perception of how the masticatory system works: first, there is no apparent consensus on the morphology of teeth; and second, we are accommodating parafunction with our focus on lateral excursions.

Biological systems are designed to be mechanically efficient. Flat teeth and lateral excursions do not support this principle.

References

1. Syrop J. Understanding occlusion. Inside Dentistry. 2013;9:46-58.
2. Türp JC, Greene CS, Strub JR. Dental occlusion: a critical reflection on past, present and future concepts. J Oral Rehabil. 2008;35:446-453.
3. Spee FG. Die verschiebungsbahn des unterkiefers am schadel arch. Arch Anat Physiol. 1890;16:285-294.
4. Gysi A. The problem of articulation. Dent Cosmos. 1910;52:1-19.
5. Nagao M. Comparative studies on the curve of Spee in mammals, with a discussion of its relation to the form of the fossa mandibularis. J Dent Res. 1919;1:159-202.
6. Shaw DM. Form and function in teeth: a relational unifying principle applied to interpretation. Int J Orthod. 1924;10:703.
7. Thornton LJ. Anterior guidance: group function/canine guidance. A literature review. J Prosthet Dent. 1990;64:479-482.
8. Schuyler CH. Correction of occlusal disharmony of the natural dentition. N Y State Dent J. 1947;13:445-462.
9. Stuart CE, Stallard H. Diagnosis and treatment of occlusal relations of the teeth. In: Stuart CE, Stallard H, eds. A Syllabus on Oral Rehabilitation and Occlusion. San Francisco, CA: University of California, San Francisco; 1959.
10. McCollum BB, Stuart CE. A Research Report. South Pasadena, CA: Scientific Press; 1955.
11. D’Amico A. The canine teeth—normal functional relation of the natural teeth in man. J South Calif Dent Assoc. 1958;26:6-23, 49-60, 127-142, 175-182, 194-208, 239-241.
12. McCoy G. Dental compression syndrome: a new look at an old disease. J Oral Implantol. 1999;25:35-49.
13. McCoy G. Occlusion confusion. Gen Dent. 2013;61:69-75.

Dr. McCoy has a private practice in San Francisco that is limited to the treatment of dental compression syndrome. He conducts equilibration workshops for study groups throughout the United States and internationally, where he is a frequent guest lecturer at Peking University in Bejing and Tsurumi University in Yokohama. He has published more than 30 articles on occlusion confusion, bruxism, and TMJ disorders. He can be reached at genemccoydds@sbcglobal.net or toothcrunch.com.

Disclosure: Dr. McCoy reports no disclosures.