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A contemporary and evidence-based view of canine protected occlusion

A contemporary and evidence-based view of canine protected occlusion



American Journal of Orthodontics and Dentofacial Orthopedics, 2007-07-01, Volume 132, Issue 1, Pages 90-102, Copyright © 2007 American Association of Orthodontists


Dentists and orthodontists at one time or another have been exposed to the gnathological concept of occlusion. Certainly, a well-known and advocated precept of gnathology is that of “canine (mutually) protected occlusion (CPO).” The basic premise of CPO is that, on laterotrusive movements of the mandible, only the canines (possibly first premolars) contact and therefore protect the remaining dentition from adverse occlusal torsion forces on contacts to and from centric occlusion (and/or centric relation). Furthermore, it is contended that CPO is the optimal (ideal) type of functional occlusion for the natural dentition vis-à-vis dentures and is the functional occlusion type toward which restorative and orthodontic treatments should be directed. It is also argued that orthodontists who do not obtain canine protected functional occlusions are doing a disservice to their patients and possibly not practicing state-of-the-art orthodontics. That is, gnathologists maintain that orthodontists who do not establish a gnathologic finish, including CPO, potentially predispose patients to temporomandibular disorders (TMD) and orthodontic tooth relapse.

However, many past notions in dentistry and orthodontics, particularly related to gnathology, have not withstood the test of time or the rigors of science. And, with the recent emphasis for dentists to practice evidence-based decision making, it makes sense to fully investigate and evaluate the concept of CPO. The purpose of this article is to discuss past and present knowledge and information on the general topic of functional occlusion (particularly regarding CPO and orthodontics) and relate it to logical considerations relevant to science and evidence-based decision making. We provide a provocative and insightful perspective on what constitutes the optimal functional occlusion type for orthodontic treatment. Our goal was to call orthodontists to reconsider their views on functional occlusion—particularly CPO—in light of current knowledge and evidence. Many peripheral topics, issues, and controversies about functional occlusion, such as centric relation, articulators in orthodontics, and the general topic of occlusion, TMD, and orthodontics was addressed in previous studies.

Classic studies by Angle and a later study by Andrews established criteria for the optimal (ideal) morphologic relationship of the human dentition (although there is little evidence of a biological relationship associated with these criteria). However, the optimal functional occlusion type has not been so easily identified and has essentially eluded the dental profession. Ash and Ramjford wrote: “Orthodontic classifications are related more to anatomic and esthetic standards than to neuromuscular harmony and functional stability. It has not been possible to develop a consensus on a numerical index or system of values that applies both to form and function of the masticatory system.” Based primarily on laterotrusive movements from centric occlusion, several functional occlusion types were recognized or advocated: balanced occlusion, CPO, group function occlusion, mixed canine-protected and group function, flat plane (attrition) teeth occlusion, and biologic (multi-varied, physiologic) occlusion.

No single type of functional occlusion has been found to predominate in nature. For example, D’Amico, Ismail and Guevara, and Scaife and Holt all found that CPO predominated, whereas Beyron and MacMillan found predominance of group function occlusion. In addition, the natural occurrence of balanced occlusion (ie, with nonworking contacts) was found in populations studied by Weinberg, Yuodelis and Mann, Ingervall, Gazit and Lieberman, Sadowsky and BeGole, Sadowsky and Polson, Rinchuse and Sassouni, Shefter and McFall, deLaat and van Steenberghe, Ahlgren and Posselt, Egermark-Eriksson et al, and Weinberg and Chastain. Although balanced occlusion for the natural dentition is considered injurious and contravening by gnathologists, it is perhaps the norm rather than the exception with regard to prevalence. Woda et al stated, “Pure canine protection or pure group function rarely exists and balancing contacts seem to be the general rule in the populations of contemporary civilization.”

Literature review

Brief history of CPO

Well over a century ago, Bonwill and Gysi recommended balanced occlusion (bilateral balanced and 3-point contact) for denture construction. The thinking was that, to prevent dislodgement, the denture must have at least 3 points of contact during all possible mandibular movements: “Bilateral balanced and three-point contact has been sponsored chiefly by prosthetists in order to secure a supposed mechanical advantage in stabilization of dentures.” In the 1930s, McLean contended that this concept also applied to the natural dentition. He based his conjecture on his examinations of animals and humans. He further believed that periodontal bone resorption would result from excessive occlusal forces if teeth were not bilaterally balanced. About the same time, MacMillan took a different view and recommended a shift from balanced occlusion (ie, bilateral balanced) to unilateral balanced occlusion for both natural and prosthetically restored dentitions. He believed that bilateral balanced occlusion never existed in nature, either in animals or man. His evidence was based on the evaluation of “various types of masticatory excursions of lower animals.” Arguing in favor of unilateral balanced occlusion over bilateral balanced occlusion, MacMillan stated: “Unilateral balance in molar mastication is beautifully illustrated in comparative anatomy.” He also contended that the analysis of the masticatory process in humans via cinematography demonstrated that the nonworking-side teeth do not come in contact during mastication: “The buccal cusp of the mandibular molar of the idle side never comes in contact with the lingual cusp of the maxillary molar.”

Once balanced occlusion was considered obsolete, with general agreement that nonworking-side contacts (balancing) were to be precluded (this is debated today), the next issue that needed to be addressed was what type of working-side lateral functional occlusion is preferred. Two working-side schemes took precedence—CPO and group function occlusion (unilateral balanced). The requisites for CPO are that only the canines contact (an alternate scheme includes the first premolars) on the working side during eccentric lateral mandibular movements, whereas, on the nonworking side, there are no balancing-side contacts. The advocates of CPO argued that humans innately possess the long and dominant canine that is evident in carnivorous animals. They further argued that the canine is the strongest human tooth type and has the most sensitive proprioceptive fibers. They therefore concluded that the canines are the best teeth to protect the occlusion from eccentric forces that occur on movements to and from centric occlusion (and/or centric occlusion). The CPO enthusiasts also argued that population studies confirm the prevalence of CPO over group function occlusion. They further maintained that the only reason some modern humans do not have CPO is that they eat coarse and abrasive diets that adversely wear down their canines. In a telemetry study (miniaturized radio transmitters placed in temporary bridges and gold inlays), Butler and Zander found that, when the functional occlusions of 2 subjects were periodically changed from CPO to group function occlusion, there were fewer lateral contacts when each subject worn canine protected occlusion (perhaps CPO restricts lateral excursive movements). Also, Ash and Ramjford believed that a steep canine rise on the so-called Michigan occlusal splint can reduce the electromyography (EMG) activities of the masseter and anterior temporalis muscles.

On the other hand, the group function occlusionists argued that equivalent population studies confirm the prevalence of group function occlusion, not CPO. They also indicated that Australian aborigines had group function occlusions. Furthermore, they reasoned that the canine is not necessarily the strongest human tooth (molars have at least 4 roots and offer great support for the dentition). Furthermore, the canines are not necessarily the last teeth lost with age and do not necessarily have more sensitive proprioceptive systems than any other teeth. In addition, Ash and Ramjford argued that prominent canines can adversely “restrain normal lateral movements and the patient may develop chewing motions with a steep path of closure into centric occlusion.” As previously discussed, in addition to CPO and group function occlusion, several other less popular functional occlusion types have also been advocated.

The issue of balanced occlusion

Nine studies published from 1972 to 1991 that included a total of 959 subjects reported the occurrence of balancing contacts ranging from 34% to 89%. Ingervall found that approximately 85% of 100 subjects with normal static occlusions had balanced occlusions. Rinchuse and Sassouni found that 85% of 27 normal static occlusion subjects had balanced occlusions. Sadowsky and BeGole reported that 89% of 75 subjects with various types of Angle malocclusions had balancing contacts. Furthermore, de Laat and van Steenberghe found that 61% of 121 Belgian dental students with various Angle malocclusions had balancing contacts. Shefter and McFall reported that 56% of 66 subjects with Angle malocclusions had balancing contacts. Also, Sadowsky and Polson found that 45% of 111 subjects with Angle malocclusions had balancing contacts. Egermark-Eriksson et al reported that 34.5% of 238 subjects with Angle malocclusions had balancing contacts. In addition, Ahlgren and Posselt found that 34% of l20 subjects with Angle malocclusions had balancing contacts. Finally, Tipton and Rinchuse found that 75% of 101 subjects (52 of 101, or 51.1%, with normal static occlusions) had balanced occlusions.

In the 1970s, orthodontic gnathologists argued that orthodontic patients’ functional occlusions should be finished to CPO. They then alleged that, when orthodontists ignore patients’ functional occlusions and rely on hand-held models rather than articulators, patients would predictably finish with balancing contacts and eventual TMD. These orthodontic gnathologists were partially accurate in their assessment of nongnathologically treated postorthodontic patients; they did have balanced occlusions. However, comparison groups consisting of subjects with ideal static occlusions and Angle malocclusions also had balanced occlusions and to an equivalent extent. In addition, there was no difference in the TMD signs and symptoms between orthodontically treated and untreated subjects. Also, TMD increases with age irrespective of orthodontic treatment.

Several points need to be clarified regarding nonworking-side functional tooth contacts. Two terms are often used synonymously when describing when and where teeth touch, ie, tooth “contacts” vs tooth “interferences.” Although both terms indicate that the teeth touch, there is a semantic difference between an occlusal “contact” and an occlusal “interference.” Occlusal contacts are considered benign compared with occlusal interferences. Ash and Ramjford wrote: “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.” Furthermore, Ash and Ramjford argued against the claim that all lateral forces and stresses on the teeth from balancing contacts are problematic and undesirable: “Lateral stress on the teeth is desirable within physiologic limits; it stimulates the development of a strong fibrous periodontal attachment around the neck of the teeth and lessens the potential for traumatic periodontal injury from incidental or accidental occlusal forces.” Thus, the term occlusal balancing “contact” refers to a condition when the teeth come together usually without incident; ie, without tooth mobility, deflection of the mandible, or effect on the temporomandibular joints.

On the other hand, an occlusal “interference” is generally considered destructive and a harsher condition. Ash and Ramfjord stated: “the term occlusal interference refers to an occlusal contact that interferes in a meaningful way with function or parafunction.” The sixth edition of Glossary of Prosthodontic Terms defines an occlusal interference as any tooth contact that inhibits the remaining occluding surfaces from achieving stable and harmonious contacts. That is, an occlusal interference is a contact that can force the mandible to deviate from a normal pattern of movement. Based mostly on empirical data, gnathologists claimed that occlusal interferences can cause tooth mobility, trauma from occlusion, deflection of the mandible, bruxism, relapse of tooth position, and TMD. The type of balancing side occlusal contacts typically found in young postorthodontic subjects and their matched comparison groups are “contacts” and generally not “interferences.”

The criterion for the acceptance of a functional occlusion type as the optimal or preferred was based on 2 notions: a single functional occlusion type predominates in nature and provides subjects (or is found associated) with the fewest TMD signs and symptoms. As previously discussed, a single type of functional occlusion has not been demonstrated to predominate in nature. Furthermore, electromyographic and intraoral telemetry studies (miniature radio implants embedded in dental prostheses and occlusal contacts monitored outside the mouth similar to the telemetry used for monitoring space flight) of functional occlusions that used prosthetically restored or replaced teeth are equivocal. In addition, the findings from provocation studies, in which functional occlusal interferences (“high crowns”) were produced in subjects, were also inconclusive (ie, symptoms other than TMD found; samples biased because of dental nurses or students).

The superiority of 1 type of static or functional occlusion to ameliorate TMD has not been demonstrated. Data from epidemiologic studies led to the conclusion that morphological or functional occlusion variables play a minor role, or no role, in the multifactorial etiology of TMD. In this regard, Dolwich stated: “Although proposed occlusal factors appear to be mechanically logical, they are based upon empirical, clinical observations and have not been proven by controlled studies.” A cause-and-effect relationship has not been established between occlusion (ie, morphologic or functional) and TMD. At best, there might be an association between these 2 variables. Also, confusion has sometimes arisen because correlation or association research was wrongly interpreted as causality. There are several anecdotal and correlational reports of relationships between TMD signs or symptoms and Angle malocclusions in general, overbite, overjet, Angle Class II, Angle Class III, crossbite, “tilted teeth,” loss of molars, functional occlusal interferences, and centric slides greater than 4 mm. Interestingly, many of these conditions were speculated to be a result of TMD, rather then the cause of TMD.

There is the belief that treating to a “functionally optimal occlusion,” including the attainment of CPO, might be of greater relevance to orthodontic alignment stability than to TMD. In this regard, Roth and Cordray believed this to be particularly true in regard to establishing a correct centric relation position. However, Luther believed that, even if a particular functional occlusion is achieved, it will not be stable. Furthermore, Lopez-Gavito et al found no difference in the long-term stability of mandibular arches between patients with anterior tooth contacts and those with anterior open-bite malocclusions with no canine contact in centric occlusion (and in functional excursions). This leads to the contention that the final assessment of functional occlusion contacts for orthodontic patients is never quite finished because human occlusion is in flux (unless, of course, there is lifetime retention).

Although some EMG data suggest that CPO elicits a better EMG recording than any other type of functional occlusion, other studies contradict or minimize these findings. Some EMG studies that support the superiority of CPO follow. For instance, Williamson and Lundquist found that the EMG activity from the temporalis and masseter muscles was less (better) during lateral excursions in subjects with CPO vs group function occlusion. Their findings were supported by those of McDonald and Hannam and Shupe et al. Belser and Hannam reported that, although CPOs do not significantly alter muscle activity during mastication, they significantly reduce muscle activity during parafunctional clenching. Boero believed that CPO (vs group function) produced the least EMG activity and would therefore have the least occlusal loading. Interestingly, several authors surmised that the canines perhaps have some special proprioceptive function that reduces muscle activity.

Conversely, the validity of EMG analyses was recently questioned: “quantitative electromyography of the masticatory muscles seems to have limited value in diagnostics and in evaluation of individual treatment results. . . . Despite EMG findings . . . it has yet to be shown in clinical, prospective trials that canine protected occlusion has a therapeutic effect—prevents or cures TMD.” A descriptive study of 300 dental students by Bush showed that bilateral canine guidance does not offer protection from facial muscle tenderness. Furthermore, Clark and Evans argued that EMG functional occlusion studies are seriously flawed. That is, there is no proper description of what constitutes normal EMG activity in the masticatory muscles, and the studies lack appropriate control or comparison groups.

Relationship of static occlusion to functional occlusion

Few studies have examined the possible relationship between static occlusion and functional occlusion. Scaife and Holt studied the dentitions of 1200 US military trainees and found that 940 had Angle Class I occlusions. CPO was found to be associated with Angle Class II and then with Angle Class I occlusions and was the least associated with Angle Class III malocclusions. That study was limited in that it did not differentiate between Class I malocclusions and normal (ideal) occlusions and did not identify or describe other functional occlusion types besides CPO. Sadowsky and BeGole examined 75 subjects with various types of Angle malocclusions and found that 91% had balanced occlusions. Tipton and Rinchuse found a trend for 101 subjects to have balanced occlusions more often associated with normal (ideal) static occlusion (or Class I occlusions). It appears that balanced occlusion exists to a far greater extent than gnathologists maintain and that balanced occlusion appears to be more predominant in subjects with normal (ideal) static occlusions (or Class I occlusions) vs Angle malocclusions.

Clarification of balancing contacts

Some clarification and qualification of the balanced occlusions in postorthodontic subjects and their comparison groups need to be addressed. The balancing contacts, for the most part, were contacts and not interferences. Next, most balancing-side contacts (interferences were not found) were on the distal sides of the posterior molars. The exact order of frequency of teeth found to have balancing contacts were the distal aspect of the second molars, the distal aspect of the first molars, and the mesial aspect of the second molars. Of note, all teeth (tooth types in addition to molars—eg, incisors, canines, and premolars) were prone to have balancing-side contacts, although the occurrence was very limited. Furthermore, statistical analysis of the data confirmed that, when there was canine contact alone on the working side, there was a greater probably for a lack of balancing-side contacts on the nonworking side. This is apparently the justification of some orthodontists for the extrusion of canines past their normal contact points or the addition of resin buildups to the worn incisal edges of these teeth to attain CPO. However, one must be mindful of the negative esthetic effect of canine extrusion on the smile arc. A contemporary esthetic treatment objective involves attaining a consonant smile arc whereby the incisal edges of the maxillary incisors and canines should be parallel to the curvature of the lower lip upon smiling. It is argued that the unjustified extrusion of the maxillary canines to obtain CPO creates a nonconsonant smile arc—ie, flattening of the maxillary incisal curvature relative to the curvature of the lower lip.

If one considers the aforementioned and reflects on the relationship between static and functional occlusion, some provocative thoughts come to mind. Notably, there can be little doubt that the typical and normal type of functional occlusions in postorthodontic subjects and their nonorthodontically treated “ideal” static occlusion counterparts is balanced occlusion and not CPO (or even group function occlusion). Importantly, however, not all balanced occlusions are identical. With this in mind and for any who consider it heretical to consider any version of balanced occlusion as normal, it might be just as logical and correct to consider an alternate term, “modified canine protected/group function occlusion,” rather than balanced occlusion. The balancing-side contacts (not interferences) in these groups were generally minor and for the most part on the distal aspects of the most posterior teeth, and, from the perspective of the gnathologist or occlusionist, easily amendable to occlusal equilibration. Therefore, when discussing the predominance of balanced occlusion in orthodontically treated subjects and their matched counterparts, the nature of balanced occlusion must be made clear.

Masticatory chewing patterns

When jaw motions are examined from the frontal plane, 7 patterns have been identified that appear to be sex-specific and related to craniofacial morphology and the interdigitation of teeth. Subjects with normal occlusions tend to have more simple, uncrossed, and elliptical movements than do subjects with malocclusions. The characteristics of the shape of the masticatory cycle are finalized in the second year of life when the deciduous dentition reaches full occlusion and do not vary much throughout life. When malocclusions are corrected with orthodontic treatment, the chewing cycles that were characteristic of the malocclusion generally remain, even with a new normalized occlusion in place. Parenthetically, it is possible that subjects with more vertical chewing patterns (shapes) would best fit a canine protected functional occlusion scheme and those with horizontal chewing patterns would prefer more lateral freedom that would be consistent with balanced or group function occlusions.

Chewing efficiency is closely related to the amount of tooth surface used during the maceration of food. Subjects with normal occlusions have more efficient chewing than those with malocclusions, but no specific masticatory pattern has been identified as the most efficient. The relative efficiency of how the occlusal interdigitations of teeth interact with the different masticatory patterns of jaw movement is still largely unanswered.

Views and concepts of functional occlusion must consider and account for the current knowledge of human mastication and chewing pattern type to establish efficacious guidelines concerning the optimal functional occlusion to achieve for each patient. The potential relationships between chewing pattern type, craniofacial morphology, static occlusion type, and functional occlusion type should be studied and evaluated to ascertain appropriate compatibility matches.

Questionable validity of the functional occlusion data

The validity of the functional data from research studies, as well as those from traditional gnathological functional occlusion recordings, is subject to question. The contrary research findings that show very different occlusal patterns during lateral excursions might reflect more a difference in methodology than the actual results from the studies. Although many functional occlusion recordings can be demonstrated to be reliable, are they valid? For the most part, the recordings and measurements are static and not dynamic. Subjects are not typically asked to chew, swallow, or exercise any parafunctional movements. Subjects are usually asked to move or place their teeth or mandibles in a certain test position, and this static position is then recorded. For instance, to record lateral eccentric jaw movements, a subject is typically asked to slide his or her mandible to a cusp-to-cusp end position (some 3-5 mm laterally), and this stationary border position is then recorded. Whether the subject actually functions in this position appears to be irrelevant. For instance, 1 person with a more vertical chewing pattern might only function laterally 1 mm or so from centric occlusion, whereas another with a more horizontal chewing pattern can actually function in the more extreme lateral cusp-to-cusp border position (some 3-5 mm from centric occlusion). Because the extent of the lateral mandibular movements can vary from person to person, the question is how does a researcher or practitioner know a priori the extent of the lateral mandibular movement for each subject? (The lateral border movement might be 1 mm for 1 subject and 4 mm for another.) The laterotrusive records in research studies assume that all subjects naturally move and exercise mandibular movements similarly. Is this logical?

Another type of recording scheme used in research studies is to have each subject slide his or her mandible so many millimeters (maybe 3-5) to the right or left, rather than to the cusp-to-cusp border end position. Parenthetically, the range of lateral tooth contacts can vary up to 5 mm ; this represents half of the lateral width of an average tooth or the width of a cusp. However, Ingervall et al stated that lateral excursive movements greater than 3 mm are probably rare, and tooth contacts closer to centric occlusion are more relevant.

A further variation is to record several different lateral eccentric movements at several progressive millimeter test positions to the right and then to the left. Also, at times, the entire slide from centric to the laterotrusive cusp-to-cusp position is recorded rather than any particular millimeter movement to the right or left. In addition, studies and findings differ on how the occlusal contacts were recorded based on whether they were directly viewed or aided by an intermediate material such as impression compound, wax, articulating paper, or dental floss. The location and severity of the occlusal contacts in any subject vary throughout the day. A further consideration is whether functional recordings (static) are doctor manipulated or patient governed. In this regard, doctor-manipulated records are considered more reliable, but less valid and physiologic, than patient guided and generated records, and vice versa.

An exception to the above were the recommendations many years ago of Masserman and Masserman et al. Masserman recommended a technique for recording functional occlusion that involved the placement of 30-gauge green wax over half of the occlusal surfaces of the mandibular arch (and repeated for the other side). Then the subject was asked to eat an apple, and the cusp contacts were evaluated by observing the extent of the perforations in the occlusal wax on the nonworking side (opposite the side on which the apple was chewed). Masserman explained: “While in conversation, the patient is asked to chew a section of an apple on the side opposite the wax only. This is done very casually and as the patient chews reflexly, he produces a functional recording of tooth contact in the wax.” He believed that this method was far superior to using an articulator: “In the diagnosis or treatment, an occlusion should be proved on a functional level. . . . [R]egardless of the instrument employed or the technique used, every occlusion must be functionally validated in the mouth.” He further argued that humans can never exactly duplicate on a conscious level functions that are naturally performed on a preconscious level. He stated:

[M]astication is a preconscious act. When patients are asked to record jaw movements on articulation paper, typewriter ribbon, wax, etc., the patient becomes confused in conflict between cortex (conscious) and brain stem (preconscious) function. . . . Stop a man walking, and ask him to show you how he walks. The resulting demonstration will be an awkward imitation of his natural gait. Accordingly, mandibular movements are at best a pantomime or mimicry of true functional movements. The recording is erroneous and results only in fallacious treatment.

In addition, it is claimed that the type of occlusion (static or functional) is not as important as how the subject uses (or misuses) his or her occlusion. Some people have the most perfect occlusions (both static and functional), and yet they have significant TMD, whereas others have the most horrendous static and functional occlusions but no TMD. Furthermore, it is well known that the most destructive of all occlusal forces is that from parafunction (bruxing and clenching). Interestingly, this type of tooth contact is not evaluated in any functional occlusion study dealing with CPO or any gnathologic record used in contemporary clinical practice. Parenthetically, it was estimated that, under normal circumstances (swallowing and mastication), the teeth come in contact as little as 2 to 7 minutes per day in 1 study and 15 to 40 minutes per day in another (possibly 2-6 hours with added parafunction). Considering this, gnathologists can be criticized because they presumably do not record any relevant and meaningful dynamic aspects of mandibular movement—most importantly, patient parafunction.

Discussion

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