Occlusal philosophy: investigating the reasons orthodontists have for occlusion preference

Drs. Colin M. Webb and Donald J. Rinchuse delve into functional occlusal schemes

Abstract

Objective: The aim of this study was to survey orthodontists to investigate if and why they preferred certain functional occlusal schemes.

Methods:   An email invitation from the American Association of Orthodontists, AAO Partners in Education, was sent to a random sample of the AAO members in the United States and Canada (n = 2,300), requesting participation in a 14-question online survey (Survey Monkey). There was a total of 111 orthodontists who participated in the survey. 

Results:  It was found that 68% of orthodontists do not believe that there is one functional occlusal scheme that is ideal for all patients. The majority (71%) of orthodontists disagree or strongly disagree that occlusion is the primary cause of temporomandibular disorders (TMD). The overwhelming majority (94%) of orthodontists believe that TMD and genetics, plus psychosocial factors, were either strongly correlated or moderately correlated. The vast majority (82%) of orthodontists believe that if they did not share the same occlusal philosophy as their referring dentists, then the referring dentist would be less likely to refer patients to them. 

Conclusions: This survey demonstrated that the majority of orthodontists believe that there is no functional occlusal scheme that is more ideal than another, and that referring dentists do play a role in orthodontists’ decisions on occlusion.

Introduction

There is much controversy in the orthodontic community as to which functional occlusal scheme is optimal for patients. When determining which scheme to use, there are many different factors that practitioners can consider. Esthetically, orthodontists can examine the facial type (mesofacial, brachyfacial, dolichofacial) and soft tissue (smile arc) to determine which occlusal scheme appears to have the most harmonious form with the face.¹ Functionally, orthodontists can examine the chewing kinematics to determine which functional occlusal scheme a patient would most benefit from.¹ Historically, the orthodontic profession has taken an approach that believed any occlusion that deviated away from the “ideal” mesiobuccal cusp of the maxillary first molar in the buccal groove of the mandibular first molar, minimal overjet and overbite, and canine protected occlusion to be considered non-optimal or “diseased.”^2,3 

Many graduating dentists are taught that canine protected occlusion is the ideal functional occlusal scheme, and that all patients should possess this occlusal scheme.⁴ In 1958, D’Amico⁵ found that when canines were in contact, there was an immediate interruption of the tension of the temporal and masseter muscles, and therefore, the magnitude of force was reduced. In 1985, Schneikert⁶ found what he believed was evidence that canine teeth are designed to be “guardian teeth.” He cited their corner position, their large size, the length of their roots, and the fact that they are the last primary tooth lost as evidence proving that they were designed to guard the rest of the occlusion.⁶  Similar to D’Amico⁵, Schneikert⁶ also found that because the canine is located far away from the “hinge” of the temporomandibular joint (TMJ), the canine is in a more favorable position to bear lateral forces. 

The argument for group function tends to be that distributing lateral force through three or more teeth lessens the amount of force on any one tooth and avoids subjecting the canine to the entire brunt of the force.¹ In studies performed by O’Leary, Shanley, and Drake⁷ and McAdam,⁸ it was found that teeth in group function showed less mobility than teeth in canine protected occlusion. In the 1970s, Isaacson⁹ introduced a biological concept of occlusion that focused on determining what types of occlusions were most beneficial to individual patients — i.e., patients with anterior teeth that were periodontally compromised would benefit best from an occlusion that removed forces from these teeth. So, based on Issacson’s⁹ view, patients with periodontally compromised canines should probably not be set up to have canines protect their occlusion.

Historically, the balanced occlusion scheme has had much less support in the dental community, although this may not be true today. While most would agree that balancing side interferences can be detrimental to the dentition and the TMJs, all balancing side contacts do not have to be interferences. According to Ash¹⁰, “A balancing side contact is not a balancing side interference if it does not interfere with function nor cause dysfunction … or … injury to any of the components of the masticatory system.” There seems to be no concrete evidence that indicates that a balanced occlusal scheme without interferences is not suitable for most people.^11,12 Some researchers have even found that the balanced occlusal scheme may give an advantage to TMD patients.¹³ In 1990, Minagi¹³ evaluated 430 dental students and observed a highly significant correlation between the absence of contacts on the non-working side and the increase of joint sounds with age. There are even epidemiological data that have demonstrated that balanced occlusion may be the most prevalent functional occlusal scheme in Class I normal occlusions.^19-31

The orthodontic literature is equivocal with regard to which functional occlusion predominates in nature. D’Amico,⁵  Ismail and Guevara,¹⁴ and Scaife and Holt¹⁵ all found that canine protected occlusion (CPO) was more common, while MacMillan,¹¹ Shuyler,¹⁶ Alexander,¹⁷ and Beyron¹⁸ found predominance of group function occlusion. However, the natural occurrence of balanced occlusion (i.e. with nonworking contacts) was found by: Weinberg,¹⁹ Yuodelis and Mann,²⁰ Ingervall,²¹ Gazit and Lieberman,²² Sadowsky and BeGole,²³ Sadowsky and Polson,²⁴ Rinchuse and Sassouni,²⁵ Shefter and McFall,²⁶ de Laat and van Steenberghe,²⁷ Ahlgren and Posselt,²⁸ and Egermark-Eriksson.²⁹ If there was one functional occlusion scheme that clearly predominated in nature, then this made lead us to believe that humans were meant to function optimally in that occlusion, but that is not the case.

One of the main problems with the current functional occlusion paradigm is the recordings and criteria used to classify the type of functional occlusion. In 2001, Clark and Evans³² stated, “The criteria that denote an ideal functional occlusion have not been conclusively established.” It is very difficult to study and test different occlusal schemes when the criteria and standards for testing are not universal and agreed upon. Many CPO studies examine tooth contact in one single lateral position (edge-to-edge) after a lateral movement.³² However, this does not adequately reflect the natural pattern of lateral movement performed by individual patients. Studies of this nature do not evaluate the functional zone between centric and the extreme lateral edge-to-edge position.  In normal function, most individuals do not even make the extreme lateral edge-to-edge movement.² Even in parafunction, the edge-to-edge purely laterotrusive movement is rarely used; most often there is a protrusive and lateral component to parafunctional shifting of the mandible.¹ It’s been shown that 99% of all people lack cuspid protected occlusion in protrusion⁵, which implies that CPO does nothing to protect from protrusive parafunction. In 1987, Yaffe, et al.,³³ found that only one of 69 subjects (with Class I occlusions) demonstrated pure CPO at the first stage of lateral movement. Yaffe, et al.,³³ also found that the lateral glide movement is a complex movement in which the nature of tooth contact changes in location, direction, and number of teeth participating. It has also been found, using basic principles of engineering mechanics, that widely held notions of tooth loading during excursions are no longer valid, and this includes the notion that CPO can reduce or eliminate harmful lateral forces during excursions.³⁴ 

When orthodontists attempt to consider which occlusal scheme would be best for chewing function, they must also consider individual variation. Not including the different chewing-pattern differences between children and adults, there are about seven different adult chewing patterns that are believed to be gender specific and related to cranio-facial morphology.¹ The characteristics of the shape of the masticatory cycle are completed in the 2^nd year of life when the primary dentition reaches full occlusion and does not vary much throughout life.² Subjects with normal occlusions tend to have more simple elliptical movements. It is possible that an individual with a more vertical chewing-pattern shape may adapt better to CPO, while another individual with a more horizontal chewing-pattern shape may function best with group function or a balanced occlusion (without interferences).¹ 

Perhaps most importantly, esthetics of these different schemes must also be considered. As a profession we have begun to strive for a consonant smile arc. This can be easily achieved with a group function or balanced occlusion scheme. However, in the presence of pure canine protected occlusion, the canines must be extruded enough to disocclude the posterior segments in most patients.¹ This extrusion of the canines could preclude a consonant smile arc and give patients a “vampire” teeth look. 

The aim of this study was to gather information of the general consensus among orthodontists with regard to functional occlusion. This investigation also sought to understand the reasoning behind their preferred occlusal scheme and whether or not orthodontists’ referring dentists play a role.

Materials and methods

A 14-question survey was emailed to a random sample of orthodontists across the United States via email through the American Association of Orthodontics (AAO). Orthodontic residents and faculty from Seton Hill University worked concomitantly to develop the 14-question survey. The original survey was comprised of 20 questions. It was first sent to the mentors of this project and other faculty to be evaluated, and then four questions were eliminated. The existing 16-question survey was pre-tested with fellow residents and two more questions were eliminated. After the pre-test, the language in some of the questions was also modified. In the finalized 14-question survey, the first 11 questions focused on the beliefs of the orthodontist, and the last three were respondent demographic questions.  A finalized copy of the 14-question survey was then entered into a template using Survey Monkey. The survey was then sent to the AAO for distribution to a random sample of 2,300 orthodontists who were members of the AAO. The survey was first available to the participants on July 9, 2013. Parenthetically, the AAO sends research surveys to only a percentage of their members so that all of the members aren’t receiving weekly emails soliciting their participation in a research study, and the response rate is expected to be no more than 10%. A motivational tool to encourage response rate was the opportunity to win a raffle prize upon completion of the survey. On August 9, 2013, a reminder was sent to all participants in an attempt to increase the response rate. A total of 111 orthodontists participated in the survey. All data were tabulated and analyzed within the survey instrument (Survey Monkey) or hand tabulated via an Excel spreadsheet. 

Results

Of 2,300 emails sent to a random sample of orthodontists who are members of the American Association of Orthodontics, 111 participated in the survey; there were 109 responses after the first mailing and two additional responses after the reminder email. The survey was completely anonymous. The important findings from the survey are shown in Table 1: Functional occlusion survey summary results. 

Some of the more important findings were that 68% of orthodontists do not believe that there is one functional occlusal that is ideal for all patients. The vast majority (88%) of orthodontists believe that a group function occlusal scheme could be ideal for some patients. The vast majority (79%) of orthodontists believe that maxillary first premolars could function similarly as maxillary canines in cases with agenesis of canines, extraction of canines, or canine substitution. The vast majority (71%) of orthodontists disagree or strongly disagree that occlusion is the primary cause of TMD. The majority (63%) of orthodontists believe that occlusion and TMD were either not correlated, minimally correlated, or neutral. The overwhelming majority (94%) of orthodontists believe that TMD and genetics, plus psychosocial factors, were either strongly correlated or moderately correlated. The majority (60%) of orthodontists reported that most of their referring dentists prefer that they treat patients they have referred to a certain functional occlusal scheme, at least to some extent. The vast majority (82%) of orthodontists believe that if they did not share the same occlusal philosophy as their referring dentists, then that dentist would be less likely to refer to them in the future. 

Respondents who indicated that they had completed a continuing education course that emphasized a certain occlusal scheme were more likely to believe that CPO was the ideal occlusion for all patients than respondents who did not complete such CE courses (26.7% compared to 9.7%). The respondents who completed these courses were also more likely to indicate that they believed occlusion to be the primary cause of TMD with 16.6% reporting that they agree or strongly agree compared to 6.9%. Lastly, 56.6% of these respondents that had completed a continuing education course that emphasized a certain occlusal scheme indicated that they agree or strongly agree that an understanding of the exact position of centric relation was important in diagnosing and treating their orthodontic patients. All (100%) respondents who indicated that they were mainly involved in “academia” indicated that  they disagree or strongly disagree that occlusion was the primary cause of TMD, and that occlusion and TMD were minimally correlated. With regard to how long the respondents have been practicing, there was very little difference in the response percentages.

Discussion

Because no similar studies exist, the results of this study could not be compared to the results of other survey studies investigating similar questions. Therefore, the results of this study were compared to the current literature on the topic of functional occlusion. 

The first result to be discussed is that the majority (68%) of orthodontists agree that there is not one functional occlusion scheme that is optimal for every patient. It seems that orthodontists understand that each individual is unique, and therefore, has his or her own unique optimal occlusal scheme. This may mean that while some patients will function best in a canine protected occlusion, others would benefit most from a group function occlusal scheme. This correlates well with what Isaacson⁹ found decades ago when he introduced his “biologic concept of occlusion.” CPO as the optimal type of functional occlusion is a claim that is unsupported by the majority of the high-level evidence.¹ Based on the literature, CPO could be one possible occlusal scheme for patients, but there are several others that orthodontists could treat patients toward as well.¹ Group function and balanced occlusion with no interferences appear to be perfectly acceptable functional occlusion schemes, and in some cases, depending on unique patient characteristics, it could be argued that they are superior.^1,7,13 When orthodontists consider a patient’s chewing cycle kinematics, craniofacial morphology, static occlusion type, current oral health status, and parafunctional habits, important and relevant information concerning the most suitable functional occlusal scheme for each patient may be gleaned.¹ Orthodontists must always remember that the type of occlusion a patient possesses is not nearly as important as how he or she uses the occlusion.¹ The same functional occlusion scheme could be very different between a patient with parafunctional habits and a patient without parafunctional habits.¹ Subjects with perfect occlusion have TMD, and subjects with awful occlusion are TMD-free.^1,13,27 The arbitrary selection of one functional occlusal scheme for all patients ignores the value and importance of the variation between individual patients. Each person has a unique stomatognathic and neuromuscular functional status, and orthodontists must continually consider this while diagnosing, treatment planning, and treating their patients.¹

Another result of this survey was that most orthodontists believed occlusion to be TMD-neutral, or not correlated at all. In fact, the overwhelming percentage of orthodontists (94%) believed genetic and psychosocial factors to be moderately or strongly correlated with TMD. While occlusion cannot be ruled out completely, there is little to no evidence that demonstrates that it is the primary cause of TMD.¹ TMD is a generic disorder with six subclasses, and each of those subclasses has its very own multifactorial etiology,¹ and most orthodontists seem to understand this.

With regard to referring dentists, 82% of orthodontists believe that if they didn’t share the same occlusal philosophy as one of their referring dentists, then they would lose that referrer. This should be very troubling. This could lead to orthodontists treating patients in ways not supported by the best evidence to simply keep a referring dentist happy. With the increasingly overwhelming pressure to produce revenue put on by the burden of student loans, new practitioners could be succumbing to the pressure from referring dentists in order to keep their new patient numbers high.

Another finding of this survey was that respondents who indicated that they had completed a continuing education course that emphasized a certain occlusal scheme were more likely to believe that CPO was the optimal occlusion for all patients than respondents who did not complete such CE courses (26.7% compared to 9.7%). The respondents that completed these courses were also more likely to indicate that they believed occlusion to be the primary cause of TMD with 16.6% reporting that they agree or strongly agree compared to 6.9%. Questions that these data raised follow: Did these respondents trust the CE course lecturer so much that they did not refer to the evidence for themselves? Do these respondents consider the beliefs of these courses to be “above” the evidence? Do orthodontists understand what evidence-based clinical orthodontics is, and how they should apply it to their practice?

There were several limitations of this survey study.  The data from this study were based on the assumption that answers were an accurate and honest representation of orthodontists across the United States and Canada.  As noted earlier, the survey results were based on the responses of 111 of 2,300 orthodontists surveyed.  Per the AAO, this response rate was somewhat expected. In retrospect, it may have been beneficial to have a more attractive title to intrigue and motivate orthodontists to respond to the questionnaire. Another possibility would be to offer the questionnaire in a different way.  The researcher could have brought the questionnaire to an AAO meeting or other orthodontic conference and had the questionnaire completed by orthodontists onsite.

The next course of action in regard to research could be having this study repeated, but comparing the responses of ABO-Boarded versus non-Boarded orthodontists. Another study could survey dentists rather than orthodontists and see how they respond to similar questions. 

Conclusions

Despite the almost certain diversity in educational experiences within the sample group, responses were consistent with current literature concerning functional occlusion and TMD. However, with historical philosophies still being taught in dental schools, residencies, and continuing education, orthodontists still fear they are at the mercy of their referring dentist.

• 68% of orthodontists do not believe that there is one functional occlusal scheme that is ideal for all patients. 
• 79% of orthodontists believe that maxillary first premolars could function similarly as maxillary canines in cases with agenesis of canines, extraction of canines, or canine substitution. 
• 71% of orthodontists disagree or strongly disagree that occlusion is the primary cause of TMD. 
• 63% of orthodontists believe that occlusion and TMD were either not correlated, minimally correlated, or neutral. 
• 94% of orthodontists believe that TMD and genetics, plus psychosocial factors, were either strongly correlated or moderately correlated. 
• 82% of orthodontists believe that if they did not share the same occlusal philosophy as their referring dentists, then that dentist would be less likely to refer to them in the future.
• Respondents who completed a continuing education course that emphasized a certain occlusal scheme were more likely to believe that CPO was the ideal scheme and that occlusion was more strongly correlated with TMD.

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