CENTENNIAL SPECIAL ARTICLE

Evolution of occlusion and temporomandibulardisorder in orthodontics: Past, present, and future

Professor and chairmanment of Oral HealthDentistry, University ofThe author has completential Conflicts of InteAddress correspondencence, College of Dentise-mail, okeson@uky.eSubmitted, revised and0889-5406/$36.00Copyright � 2015 by thttp://dx.doi.org/10.10

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Jeffrey P. OkesonLexington, Ky

Occlusion has been an important consideration in orthodontics since the beginning of the discipline. Earlyemphasis was placed on the alignment of the teeth, the stability of the intercuspal position, and the esthetic valueof proper tooth positioning. These factors remain important to orthodontists, but orthopedic principles associatedwithmasticatory functionsmust also be considered. Orthopedic stability in themasticatory structures should be aroutine treatment goal to help reduce risk factors associated with developing temporomandibular disorders. (AmJ Orthod Dentofacial Orthop 2015;147:S216-23)

The role of occlusion and its impact on functionaldisorders of the masticatory system continues tobe a resounding issue in orthodontics. This interest

is appropriate because orthodontists routinely andcompletely change a patient's occlusal conditions duringtherapy. Orthodontic therapy can be likened to a full-mouth reconstruction by a prosthodontist; however,this therapy is accomplished in the natural dentition.Adding to this issue is the fact that most of thesechanges occur in young, healthy adults, so this is unlikeany other dental specialty. It would therefore behooveorthodontists to be cognizant of the effects of thesechanges, since they will influence masticatory functionsfor each patient's lifetime.

Over the years, the role of occlusion on temporoman-dibular disorder (TMD) has been extensively debated,leading to many opinions and much controversy. Thepurpose of this article is to review the history of occlu-sion and TMD as it relates to orthodontics. Occlusaltreatment goals will be reviewed as they relate to jointfunction. As a nonorthodontist, I am pleased to havethis opportunity to share some thoughts on this subject.

, Provost's Distinguished Service Professorship, Depart-Science; director, Orofacial Pain Program, College ofKentucky, Lexington, Ky.ted and submitted the ICMJE Form for Disclosure of Po-rest, and none were reported.e to: Jeffrey P. Okeson, Department of Oral Health Sci-try, University of Kentucky, Lexington, KY 40536-0297;du.accepted, February 2015.

he American Association of Orthodontists.16/j.ajodo.2015.02.007

Perhaps an outside voice may have a different perspec-tive on this subject.

This article is divided into 5 sections: (1) the history ofocclusion and TMD in orthodontics, (2) the role of ortho-dontic therapy in TMD, (3) current functional treatmentgoals for orthodontic therapy, (4) future considerationsof occlusion for the orthodontist, and (5) conclusions.

HISTORY OF OCCLUSION AND TMD INORTHODONTICS

The history of orthodontics must begin with the workof Dr Edward Angle, considered the father of this spe-cialty.1 He founded the Angle School of Orthodontia inSt Louis, Missouri, in 1900. Dr Angle introduced theterm “malocclusion” to the dental profession as any ab-normality in the dental configuration. He developed aclassification of malocclusions that is still used today.2

He generally divided the occlusion into 3 types: normal,or Angle Class I; a retrognathic jaw, or Angle Class II; anda prognathic jaw, or Angle Class III. These classificationswere useful for communications between professionalsand for research purposes.

At that time, interest in occlusion was primarily asso-ciated with esthetics. Sound occlusal stability withacceptable tooth angulations and centered midlineswere needed to establish successful esthetics. Andrews3

proposed 6 basic keys to establishing a sound AngleClass I occlusion; these became well-accepted ortho-dontic treatment guidelines for finalizing the dentalocclusion. Although these guidelines were useful, theyhad no reference to the joint position. Instead, the or-thodontic specialty focused more on various treatment

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philosophies, such as fixed vs removal appliances, func-tional appliances to affect growth, and extraction vsnonextraction treatment. At that time, most orthodon-tists were taking their patients' casts, occluding theteeth, and grinding the backs of the casts on a modeltrimmer. This was done so that the backs of the castscould be placed on a table, and the teeth could bebrought together in the maximum intercuspal position.The orthodontist could then evaluate the occlusion, butthere was no reference to the patient's joint positions.There was little concern for jaw function.

By the mid-1970s and early 1980s, some orthodon-tists began to consider the importance of developing asound occlusal position at the same time that the con-dyles were in a stable joint position. This concept hadbeen considered for years by prosthodontists, whorealized that a stable joint position was essential for asuccessful prosthodontic reconstruction. At that time,Dr Ronald Roth began to write a series of articles inthe orthodontic literature suggesting the importanceof joint positions in orthodontic therapy.4-7 Accordingto Roth, orthodontic treatment goals can be dividedinto 5 categories: facial esthetics, dental esthetics,functional occlusion, periodontal health, and stability.8

The uniqueness of Dr Roth's goals was the inclusion offunction. One of his suggestions was to use a dentalarticulator to better evaluate the relationship of theocclusal position to the joint position. He insisted thatorthodontists needed to use a dental articulator fortreatment planning and managing orthodontic patients.This became a debated and controversial concept. At thetime, orthodontists were not routinely using articulators,and they all thought that they were successful with theirpatients. Why add this technique to improve an alreadysuccessful treatment?

History suggests to us that sometimes outside forcescan alter professional directions. This was true in 1987,when a lawsuit was filed by a patient claiming that theorthodontist caused her to suffer with a TMD.9,10 Tothe surprise of the scientific community, the patientwon the lawsuit and received a sizable financialcompensation. This successful lawsuit created muchanxiety in the orthodontic community. Funds werethen generated by orthodontic organizations forresearch needed to more completely understand therelationship, if any, between orthodontic therapy andTMD. The results of these studies will be discussed inthe next section.

THE ROLE OF ORTHODONTIC THERAPY IN TMD

As interest in the relationship between TMD andorthodontic therapy grew, speculation also grew. There

American Journal of Orthodontics and Dentofacial Orthoped

were claims of orthodontic therapy's always causingTMD to claims of its never causing TMD. Similar claimswere being made for certain types of orthodontic treat-ment: eg, that extraction of teeth always leads to TMD ornever leads to TMD. The problem was that theseconcepts were based on clinical impressions and noton scientific evidence. The need for evidence wasobvious, so the specialty began to study this relationship.By the mid-1990s, a series of studies became availablethat helped to answer this important question. It is notthe purpose of this article to thoroughly review all thesestudies, but the data did not suggest that orthodontictherapy was a significant risk factor for the developmentof symptoms of TMD.11-17 A review article hashighlighted some of these studies.18 As these studieswere published, orthodontists became more comfortablewith the concept that their treatments were not a signif-icant etiologic factor associated with TMD. This percep-tion lowered the general anxiety about the originallawsuit. However, the question that must be asked ishow these studies should be interpreted. Certainly,most of these studies were well designed, leading readersto conclude that orthodontic therapy is not a risk factorfor TMD. Therefore, one might say that orthodontictherapy is simply unrelated to TMD. Although most or-thodontists would be comfortable accepting thisconcept, such a broad statement is most likely too sim-ple. A second consideration is that all the long-termstudies on the relationship between orthodontic therapyand TMD have been accomplished with well-controlledorthodontic therapies. Almost all the studies were per-formed in university graduate training programs, whereorthodontic therapies are well supervised and controlled.Perhaps poorly completed orthodontic therapies doreveal risk factors for TMD. Another consideration in in-terpreting these results is that many patients receivingthe orthodontic therapy were young, healthy, and adap-tive. Providing orthodontic therapy in a developingmasticatory system may help patients to adapt to theocclusal changes and joint positions, rendering themless likely to have functional problems in the future.This variable has not been well studied and certainly isa consideration when it comes to TMD. Still anotherconsideration in interpreting the results of these studiesis that although orthodontic therapy does change thepatient's occlusion, the occlusion is only one of severalfactors that are associated with TMD. A thorough reviewof the literature shows that there are at least 5 majoretiologic factors that can be associated with TMD: occlu-sion, trauma, emotional stress, deep pain input, and par-afunction.19 In addition to these variables is eachpatient's adaptability, which is still another factor thathas yet to be well investigated.

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The concept of patient adaptability is an importantissue that is presently being explored. It has beendemonstrated that variations in genetic makeup mayhave significant impacts on pain perception.20,21 Thegene that encodes for catechol-O-methyl-transferase,an enzyme associated with pain responsiveness, variesin patients. There appear to be 3 clusters of personswho respond differently to a painful stimulus. Someare more sensitive to pain, whereas others are less sensi-tive. In an interesting prospective cohort study of 186postorthodontic female patients, those who were genet-ically found to be in the pain-sensitive cluster developedmore symptoms of TMD than did those in the pain-insensitive cluster.22 This suggests that perhaps theactual orthodontic therapy itself was not the significantfactor in the developing TMD, but rather it was perform-ing orthodontic therapy in a patient with a geneticallydetermined pain-sensitive haplotype. Perhaps the futurewill help us to determine which patients are more vulner-able for developing pain disorders, and this might affecttreatment options.

Therefore, assuming that orthodontic therapy iscompletely unrelated to TMD is a relatively naïvethought. The question that really needs to be asked ishow orthodontic therapy can be used to minimize anyrisk factors that relate to TMD. Of the known etiologiesof TMD, orthodontic therapy routinely affects only 1factor: occlusion. However, even occlusal factors arenot always related to TMD.18,19 So, where doesorthodontic therapy fit into the big picture of TMD?Orthodontic therapy routinely alters a patient'socclusion. Since occlusal factors may be a potentialsource of TMD in some patients, it would seem logicalthat orthodontists should develop occlusal conditionsthat will minimize any risk factors that might beassociated with TMD. However, developing a soundocclusal relationship does not mean that the patientwill always be free of TMD. At least 4 other etiologiesare outside the control of the orthodontist. Developingan orthopedically stable occlusal condition should bethought of as minimizing a dental risk factor. It seemslogical that since orthodontic therapy will change thepatient's occlusion, emphasis should be placed oncreating an occlusal condition that will provide thebest opportunity for successful masticatory functionfor the patient's lifetime.

CURRENT FUNCTIONAL TREATMENT GOALS FORORTHODONTIC THERAPY

As previously discussed, orthodontists' early concernsregarding occlusionwere related to esthetics and intercus-pal stability. Although these are important considerations

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for successful orthodontic therapy, a greater concern forthe patient's lifetime is developing a stable functioningmasticatory system. A stable masticatory system includesa stable occlusal position in harmony with a stable jointposition. To accomplish this, orthodontists need to appre-ciate basic orthopedic principles that lead to successfulfunction. A lack of harmony between the occlusal positionand the joint positionmay be a risk factor that potentiatesdysfunction of the structures.

In establishing the criteria for the optimum orthope-dically stable joint positions, the anatomic structures ofthe temporomandibular joint must be closely examined.The temporomandibular joint is made up of the condyleresting in the articular fossa with the articular disc inter-posed. The articular disc is composed of dense fibrousconnective tissues, devoid of nerves and blood vessels.23

This allows it to withstand heavy forces without damageor creating a painful stimulus. The purpose of the disc isto separate, protect, and stabilize the condyle in themandibular fossa during functional movements. Thearticular disc, however, does not determine the positionalstability of the joint. As in any other joint, positional sta-bility is determined by the muscles that pull across thejoint and prevent separation of the articular surfaces.The directional forces of thesemuscles determine the op-timum orthopedically stable joint position. This is a basicorthopedic principle that is common to all mobile joints.Every mobile joint has a musculoskeletally stable posi-tion: the position stabilized by the activity of the musclesthat pull across it. The musculoskeletally stable positionis the most orthopedically stable position for the jointand can be identified by observing the directional forcesapplied by the stabilizing muscles.

The major muscles that stabilize temporomandibularjoints are the elevators: temporalis, masseter, and medialpterygoid muscles. The force placed on the condyles bythe temporalis muscles is predominantly in a superior di-rection. The temporalis muscles have some fibers that areoriented horizontally; however, because these fibersmust traverse the root of the zygomatic arch, most ofthe fibers elevate the condyles in a straight superior di-rection.24 The masseter and medial pterygoid musclesprovide forces in a superoanterior direction, which seatsthe condyles superiorly and anteriorly against the poste-rior slopes of the articular eminences. These 3 musclesare primarily responsible for joint position and stability,but the lateral pterygoid muscles also contribute to jointstability by stabilizing the condyles against the posteriorslopes of the articular eminences.

In the postural position, without influence from theocclusal condition, the condyles are stabilized by themuscle tonus of the elevator and the inferior lateralpterygoid muscles. The temporalis muscles position the

Journal of Orthodontics and Dentofacial Orthopedics

Fig 1. The directional forces applied to the condyles (upper thick arrow) by the temporal muscles are toseat the condyles in a superior position in the fossae. The directional forces applied to the condyles bythemasseter and themedial pterygoidmuscles (lower thick arrow) are to seat the condyles in a superioranterior position in the fossae (thin arrow). When these forces are combined with the lateral pterygoidmuscle (not shown), the condyles are seated into their superior and anterior positions in the fossaeagainst the posterior slopes of the articular eminences. Reprintedwith permission fromOkeson,19 p. 74.

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condyles superiorly in the fossae. The masseter and themedial pterygoid muscles position the condyles super-oanteriorly. Tonus in the inferior lateral pterygoid mus-cles positions the condyles anteriorly against theposterior slopes of the articular eminences (Fig 1).

Therefore, the most orthopedically stable joint posi-tion as dictated by the muscles is where the condylesare located in their most superoanterior positions inthe articular fossae, resting against the posterior slopesof the articular eminences. This description is not com-plete, however, until the positions of the articular discsare considered. Optimum joint relationships are achievedonly when the articular discs are properly interposed be-tween the condyles and the articular fossae. Therefore,the complete definition of the most orthopedically stablejoint position is when the condyles are in their mostsuperoanterior positions in the articular fossae, restingagainst the posterior slopes of the articular eminences,with the articular discs properly interposed. This positionis the most musculoskeletally stable position of themandible.

American Journal of Orthodontics and Dentofacial Orthoped

An easy and effective method of locating the muscu-loskeletally stable position is the bilateral manualmanipulation technique.19,25 This technique beginswith the patient lying back and the chin pointedupward (Fig 2, A). Lifting the chin upward places thehead in an easier position to locate the condyles nearthe musculoskeletally stable position. The dentist sitsbehind the patient and places the 4 fingers of eachhand on the lower border of the mandible at the angle.The small finger should be behind the angle, with the re-maining fingers on the inferior border of the mandible.The fingers must be located on the bone and not inthe soft tissues of the neck (Fig 2, B and C). Next, boththumbs are placed over the symphysis of the chin sothat they touch each other between the patient's lowerlip and chin (Fig 2, D and E). When the hands are inthis position, the mandible is guided by upward forceplaced on its lower border and angle with the fingers,while at the same time the thumbs press downwardand backward on the chin. The overall force on themandible is directed so that the condyles will be seated

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Fig 2. A, Successfully guiding the mandible into the musculoskeletally stable position begins with hav-ing the patient recline and directing the chin upward.B, The 4 fingers of each hand are placed along thelower borders of the mandible. The small finger should be behind the angle, with the remaining fingerson the inferior border of the mandible. An important point is to place the fingers on the bone and not inthe soft tissues of the neck. C, The thumbs meet over the symphysis of the chin. D and E, Downwardforce is applied to the chin, while superior force is applied to the angle of themandible. The overall affectis to set the condyle superiorly and anteriorly in the fossae, as depicted in Figure 1. Reprinted withpermission from Okeson,19 p. 196.

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in their most superoanterior position braced against theposterior slopes of the eminences. Firm but gentle forceis needed to guide the mandible so as not to elicit anyprotective reflexes.

Locating the musculoskeletally stable position beginswith the anterior teeth no more than 10 mm apart toensure that the temporomandibular ligaments havenot forced translation of the condyles. The mandible is

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positioned with a gentle arcing until it freely rotatesaround the musculoskeletally stable position. This arcingconsists of short movements of 2 to 4 mm. Once themandible is rotating around the musculoskeletally stableposition, force is firmly applied by the fingers to seat thecondyles in their most superoanterior positions.

In this superoanterior position, the condyle-disccomplexes are in the proper relationship to accept forces.

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With such a relationship, guiding the mandible to thisposition should not produce pain. If pain is elicited, itis possible that some type of intracapsular disorder ex-ists. When there is pain, an accurate mandibular positionwill probably not be found. Therefore, the reason for thispain needs to be investigated and managed before anyorthodontic therapy.

The most orthopedically stable position justdescribed does not consider the stabilizing effects ofthe structures at the other end of the mandible: theteeth. The occlusal contact pattern of the teeth also in-fluences the stability of the masticatory system. Whenthe condyles are in their most stable positions in thefossae and the mouth is closed, the teeth should occludein their most stable relationship. The most stableocclusal position is the maximal intercuspation of theteeth. This occlusal relationship furnishes maximum sta-bility for the mandible and minimizes the amount offorce placed on each tooth during function.

In summary, the criteria for optimum orthopedic sta-bility in the masticatory system would be to have evenand simultaneous contact of all possible teeth whenthe mandibular condyles are in their most superoanteriorposition, resting against the posterior slopes of the artic-ular eminences, with the discs properly interposed. Inother words, the musculoskeletally stable position ofthe condyles coincides with the maximum intercuspalpositions of the teeth.

One additional consideration in describing theocclusal condition is the fact that the mandible canmove eccentrically, resulting in tooth contacts. Theselateral excursions allow horizontal forces to be appliedto the teeth, and horizontal forces are not generallywell accepted by the dental supportive structures; yet,the complexity of the joints requires some teeth tobear the burden of these less-tolerated forces. Whenall the teeth are examined, it becomes apparent thatthe anterior teeth are better candidates to accept thesehorizontal forces than the posterior teeth because theyare farther from the force vectors; this results in less forceto these teeth. Of all the anterior teeth, the canines arethe best suited to accept the horizontal forces duringeccentric movements.26-28They have the longest andlargest roots and therefore the best crown-to-root ra-tio.29 They are also surrounded by dense compactbone, which tolerates the forces better than does themedullary bone around the posterior teeth.30

The laterotrusive contacts need to provide adequateguidance to immediately disclude the teeth on the oppo-site side of the arch (mediotrusive or nonworking side).When the canine is not available for this guidance, agroup function should be provided. Efforts should bemade to avoid nonworking-side guidance, since this

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may introduce joint instability during certain eccentricparafunctional activities. When the mandible moves for-ward into protrusive contact, the anterior teeth shouldalso provide adequate contact or guidance to disarticu-late the posterior teeth.

The following is a summary of the conditions thatprovide optimum orthopedic stability in the masticatorysystem. This represents orthodontic treatment goals forall patients.

1. When the mouth closes, the condyles should be intheir most superoanterior position (musculoskele-tally stable), resting on the posterior slopes of thearticular eminences with the discs properly inter-posed. In this position, there should be even andsimultaneous contact of all posterior teeth. Theanterior teeth may also contact but more lightlythan the posterior teeth.

2. When the mandible moves into laterotrusive posi-tions, there should be adequate tooth-guided con-tacts on the laterotrusive (working) side toimmediately disclude the mediotrusive (nonworking)side. The canines (canine guidance) provide the mostdesirable guidance.

3. When the mandible moves into a protrusive posi-tion, there should be adequate tooth-guided con-tacts on the anterior teeth to immediately discludeall posterior teeth.

4. When the patient sits upright (in the alert feedingposition) and is asked to bring the posterior teethinto contact, the posterior tooth contacts shouldbe heavier than the anterior tooth contacts.31

When planning treatment for a patient, the muscu-loskeletally stable position should be located, and therelationship of the maxillary and mandibular teethshould be observed in this mandibular position. Sincethe orthodontic treatment goal is to develop themaximum intercuspal position of the teeth in thismandibular position, the orthodontist should select theproper orthodontic strategies that will accomplish thisgoal. In some instances, the orthodontist may find ituseful to mount the patient's casts on an articulator inthe musculoskeletally stable position to better visualizethe occlusal relationship. This can be especially helpfulwith a significant intra-arch discrepancy. I do not believethat it is necessary to mount every orthodontic case onan articulator. In most growing patients, the orthodontictherapy will most likely be completed before the finalmaturation of the condyle-fossa relationship. The ortho-dontist must always be aware of the musculoskeletallystable position of the condyles and finalize the occlusionin relationship to this position. However, the final preci-sion of the position in a developing adolescent is most

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likely accomplished by the physiology of form and func-tion as the young adult matures. In other words, theorthodontist must provide an occlusal condition that iswithin the patient's physiologic tolerance or adapt-ability. In a growing patient, it would be reasonable toassume that this is within several millimeters of themusculoskeletally stable position of the joint. Once theorthodontic therapy is finalized, the patient's individualloading during function will normally assist in stabiliz-ing the masticatory system. The only point in questionis how adaptable the patient's masticatory structuresare. Of course, this is unknown; therefore, the orthodon-tist needs to always strive to develop the occlusal posi-tion as close to the musculoskeletally stable position aspossible. In difficult cases, an articulator may be usefulin achieving this goal. However, an articulator is merelya tool that can assist in achieving the goal, not a magicalinstrument that will ensure success.

In adult patients, it may be more important to pre-cisely locate the orthopedically stable position, sincegrowth and adaptability are less likely. The articulatormay be of greater assistance in these patients, butonce again articulators are not always needed. The clini-cian needs to assess the dental relationships and thendetermine whether an articulator will assist in accom-plishing the treatment goals. The articulator is only asaccurate as the operator who takes the records andmounts the casts.

FUTURE CONSIDERATIONS OF OCCLUSION FORORTHODONTISTS

One might think that the study of occlusion is static.Certainly, much emphasis has already been placed on thestatic relationship of the teeth from esthetic and tooth-contact perspectives. The static relationship of the teethis not greatly related to functional problems such asTMD.19 What has yet to be investigated is how theocclusal-contact patterns of the teeth affect the dynamicfunctions of the masticatory system. There is a need tobetter understand how orthopedic stability or instabilityaffects the dynamic functions of the masticatory system:chewing, swallowing, and speech. Additional questionsarise as to how and when parafunctional activitiesbecome contributors to the breakdown of the system.Over the years, we have made many statements and as-sumptions regarding occlusion, parafunction, and TMD,but most are unsubstantiated by scientific evidence. Weneed to challenge the present concepts with studies thatcan provide evidence about mechanisms so that moreeffective treatments can be provided to our patients.

In addition, there is a great need to investigate the vari-ability of a patient's innate ability to adapt to change.

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Every experienced clinician knows that most patientsseem to successfully adapt to changes in their dentalstructures.However, somepatients donot. Abetter under-standing of this adaptability would lead to a better selec-tion of treatment methods and more accurate predictionsof outcomes, but investigating this concept is certainly notan easy task.Many variables probably contribute to adapt-ability. A few factors may include the patient's biology,learned experiences, and psychologic conditions (ie,obsessive-compulsive disorders). As previously discussed,genetic factors can play a role in TMD and other pain con-ditions,20,32,33 and in some patients these are specificallyassociated with orthodontic therapy.22

CONCLUSIONS

Occlusion has been an important consideration in or-thodontics since the beginning of the discipline. Earlyon, emphasis was placed on the alignment of the teeth,the stability of intercuspal positions, and the estheticvalue of proper tooth positioning. These factors remainimportant to orthodontists, but more recently orthope-dic principles associated with masticatory functionshave emerged as a consideration. Establishing an ortho-pedically stable relationship between the occlusal posi-tion of the teeth and the joint position is important forproper masticatory function throughout the patient'slifetime. Although in most situations orthodontic ther-apy neither causes nor prevents TMD, the orthodontistis in an excellent position to provide orthopedic stabilityin the masticatory structures. Treatment goals directedtoward establishing orthopedic stability in the mastica-tory structures should be a routine part of all orthodontictherapy. Achieving these goals will most likely reduce thepatient's risk factors for developing TMD.

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