The 100-year dilemma: what is a normal occlusion, and how ... · Fig 4 Stages of occlusion trorr 3 years to old age, (Re-printed from Friel,^'} since perfection is not the rule in

Special Report

The 100-year dilemma: what is a normal occlusion,and how is malocelusion classified?Morton I. Katz* / Jeanne C. Sinkford** / Charles F Sanders, Jr***

Orthodontic treatment cannot properly begin until the malocclusion is de.\rribed.Angle's classification of malocclusion, developed a century ago, is still the subjectof debate and controversy. A review of the literature is presented and the differingviewpoints are di.Kussed. (Quintessence Int 1990:21:407-414.)

Introduction

Classification is a vital element in the diagnosis of amaloeclusion and in treatment planning for ortho-dontic care. After countless numbers of individualmalocclusions have been segregated into groups con-taining common factors, conclusions pertaining toetiology, prevention, proper mechanics, prognosis,and retention that aid the practitioner in resolving theproblems can be made. Classification also facilitatescommunication between professionals since it pro-vides dentists with a common descriptive language.When two dentists discuss a patient's orthodonticprohlem, it is axiomatic thai Angle's classification ofmalocelusion probably will be mentioned within thefirst sentence, or surely within the second. So pervasiveis acceptance of Angle's classification, that, nearly acentury after its introduction, it is virtually the onlyuniversally accepted classification of malocelusion.His concept, whieh has stood the test of time in therapidly evolving profession of dentistry for 100 years,has been eminently useful — but is it precise?

The authors, as academicians and chnicians, havefound it difficult to apply Angle's classification to the

* Assistant Professor, DeparlraenE of Orthodontics, HowardUniversity, College of Dentistry, WashmgLQii DC 20059.

* ' Dean. Howard University, College of Dentistry.*** Assistant Professor and Acling Chairman. Department of Or-

thodontics. Howard University.

Address all correspondence to Dr Morton I. Katz, 3435 PhilipsDrive, Baltimore. Maryland 21208.

diversity of malocclusions seen in the population. Wequestion the concept of Class I as a treatment goal.Are these concerns merely anecdotal, and based onpersonal biases, or have others also observed weak-nesses in classification methods?

Etiology versus morphology

Basically, there are only two approaches to classifyinga maloeclusion. The first method considers etiology.Kingsley. in 1880. focused on causitive factors in hispioneering work.' However, since the etiology of amalocelusion is frequently multifaclorial and often notdiscernible, his classification was not widely accepted.

By far. the preponderance of orthodontic classifi-cations utilize morphology. Morphologic classifica-tions are comphcated by the wide variations found inhuman occlusions and anatomic forms, the variationsin severity of malocclusions, and the frequent over-lapping of numerous problems in a patient. Angle, inthe 1890s, utilized the presumed constancy of the po-sition of the maxillary first molar to develop a mor-phologic standard.

Angle's classification

In 19ÜÜ Angle--̂ stated, "The key to occlusion is therelative positions of the first molars. In normal occiu-sion the mesio-buccal cusp of the upper first molar isreceived in the buccal groove of the lower first mo-lar . . . . The mesial inchne of the upper cuspid oc-cludes with the distal incline ofthe lower cuspid, the[cuspid's] distal incline occluding with the mesial in-cline of the buccal cusp of the lower first bicuspid . . .and the distal incline of the distal eusp of the upper

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Fig 1 A drawing by Angle of ¡deal occlusion. (Reprintedfrom Angle.')

first molar occluding with the mesial inchne of themesial cusp of the lower second molar It will thusbe seen that each of the teeth in both jaws has twoantagonists or supports in the opposite jaw, except thelower central and the upper third molar" (Fig 1).

In his 1900, sixth edition text, Angle defmed Class11 malocclusion as an abnormal mesiodistal relationof jaws and dental arches in which all mandibularteeth occlude distally to normal by the width of onebicuspid. Class III malocclusion was defined as anabnormal relation in which the mandibular teeth arepositioned mesial to normal by the width of one bi-cuspid.

However, in 1907, when Angle'' published his seventhedition work, he made a significant modification inthe definition of Class II and Class III malocclusions.Angle altered the amount of deviation from idealClass I to either Class II or Class III malocclusionfrom "the width of a bicuspid'" to "a deviation fromideal of more than one-half the width of one cusp."

Controversy

For a concept of occlusion that was destined to van-quish all others. Angle's principles met with immedi-ate criticism. Crycr,̂ in 1904, took Angle to task foridealizing the straight white profile of an Apollo Bel-vedere (Fig 2}, while selecting as his example of anideal dentition Broomell's "'Old Glory'" skull of aprognathic black male (Fig 3), These two ideals couldnever have coexisted in one individual. Cryer̂ believedthat, in contrast to Angle's teachings, the outline ofthe face in all of its individual variations, the appear-ance of the teeth when the lips arc open, and theimportance of occlusion in regard to vocalization andmastication should all be considered. For these rea-sons, Cryer' considered extraction of teeth as an ac-ceptable solution to malocclusion, in direct contra-diction to Angle's strict prohibition against extractionfor orthodontic purposes.

Case*̂ * also criticized Angle's belief that, when thedentures are placed in normal occlusion and the max-illary first molars are related properly to the cranium,the facial outhne will take care of itself Case' said,"Failure to extract teeth where demanded [by facialprofile considerations] is quite as much malpractice asthe extraction of teeth where not demanded.'" Casemade plaster masks of faces to illustrate facial typesand esthetic concerns. He considered Angle's classi-fication imprecise. Case's classification divides ClassI into a division 1 with seven subtypes of dental ir-regularities and a division 2 for facial considerations.His Class II has a division 1 for cases in which themandible is retrusive (with two subtypes) and a divi-sion 2 for cases in which the maxilla is protrusive (withfour subtypes). His Class III has four divisions thatdescribe combinations of maxillary retrusions andmandibular protrtisions. That Angle's classificationremains the standard, while Case's superior effort nev-er became popular, was an unfortunate result of thepersonality cult that surrounded early orthodonticgiants.

In 1915 Van Loon' publisbed criticism of Angle'sclassification and proposed an improved technique fortrimming plaster casts. He also oriented casts of teethinto plaster facial masks so that the true relationshipof the teeth to the soft tissue of the face could beevaluated.

Dewey,'" an early student of Angle and then fellowteacher who philosophically parted company with hismentor, wrote in 1915, "I have great respect for thefirst molar as an organ of mastication, and they are

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Fig 2 Apollo Belvedere, drawn by Angie. (Reprinted fromAngle.')

Fig 3 Broomeli's skull, known in the dentai literature as"Old Glory." (Reprinted Irom Angle.')

very necessary in producing normal occlusion, but asa basis of classification, when taken alone, I have notmueh respect for them, as they are as liable as anyother teeth to assume an abnormal position undercertain conditions." Dewey'" recommended that clas-sification be based on the anteroposterior relation ofthe arches as a whole rather than only the first molars.Dewey'" snbclassified Class I into three types, andAnderson" later added two more.

In 1920 Hellman'-" stated, "In all its simplicity, theAngle classification does not convey exactly the sameidea to everyone. This is, perhaps, due to the fact thatits definition is not sufficiently accurate," Hellmanillustrated his contention with photographs of castsabout which considerable difference of opinion couldoccur regarding their classification. Hellman'^'' stud-ied comparative anatomy and the evolution of pri-mitive landmarks of occlusion. He recommendedelimination of the buccal cusp relationship as the focusof classification. The mesiolingual cusp of the max-illary first molar occludes in the central fossa of themandibular molar. Hellman'^" considered this lingualeusp to be the only reliable distinguishing feature, be-cause it was the most primitive landmark of occlusionstill retained by modern man. He observed in a high

percentage of malocclusions that the maxillary firstmolar had rotated on the longitudinal axis of the lin-gual cusp, and the buccal cusps had rotated mesially.The mesially rotated maxillary buccal cusps falselyindicated a Class II occlusion, while the more "pri-mitive" maxillary hngual cusp sat in its ideai Class Iposition.

What is normal and what is ideal?

Orthodontic practitioners wrestled with the practicaland philosophical tniplications of the concept of idealocclusion as a goal in treatment. Johnson,'"' m a seriesof lectures delivered at the University of Pennsylvaniaand then pubhshed in 1923, pondered this conundrum.He asked, '-What is nonnal occlusion? Does it [nor-mal] mean an ideal, a goal to be sought after but neverfound?" If normal is synonymous with typical or av-erage, can normal be taken as a standard because ofa high frequency of occurrence? Or does normal meannatural? Some believed that teeth moved to normalocclusion will be stable. If normal is defined as ae-eording to, or not deviating from, an establishednorm, then norm is a rule or authoritative standard.Normal cannot be synonymous with ideal or natural.

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Stages of occlusion trom threp yeEirs of age to old age, [A]. At ttirceyears of age the triangular ndge of tfie medio-buccal cus]) ol uppersecond deciduous molar occlndes in the buccat groove ot tiie lfAVOrsecond deciduous molai ¡B) At five and a half years of age ttie towerarch fias moved forwards, so that the buccal groove of the lowersecond deddiious molai is media! to the triangutar ridge of the uppersecond deciduous molar. (C), At eight years of̂ age the lower arch hasmoved slightly more forwards in retation to the upper arch. The per-manent incisors and first permanent molars havt? erupted The pointof tbe aisp of the upper first permanent motar is slightly medial tothe buccal groove of the tower first permanent molar, p). Young adult.All the permanent teeth are in occtusion. The point of She cusp of theupper first permanentmolaris opposite the buccat groove of the lowerfirst permanent molai. (E). Old age. Teeth worn. Lower teeth in moreforward relation to upper teeth. Buccal groove of tower first molar ismadiaf to medio-buccal cusp point of upper fiist motar. Disto-buccalcusp of upper first molar is in contact with medio-buccal cusp of lowersecond molar. Incisors meet edge to edge.

Fig 4 Stages of occlusion trorr 3 years to old age, (Re-printed from Friel, '̂}

since perfection is not the rule in animal organization.Johnson''' discussed Hellrnan's concept of variationfrom normal. He agreed with Helhnan that normalocclusion, as conceived by dentists to represent 100%perfection, was a myth.

felt that norms cannot be used as treat-rnent goals, beeause the anatomic variation in size atidshape of teeth among individuals is so large thatiiorrns cannot distinguish excellent occlusion fromtiialocciusion,

Horowitz and Hixon' considered normal, meaningtypical or usual, not atrccptable as a goal in treatment,hccause malocclusion is the most typically seen situ-ation. Ideal occlusion, being very rare in a populationis, in trnth. abnormal. Ideal occlusion is a theoreticalmodel, a manufactured convenience, not a biologicreality or necessity.

Further occlusion research

Bennett'" was a precursor of Simon when, in 1912, herecommended classifying malocclusions in sagittal,transverse, and vertical planes of space, Bennett,"' anEnglishtnan, developed a classification that also usesClass I, il, and III, but he divides malocclusions byetiology rather than morphology.

In the 1920s Simon""'^ of Berlin developed a gna-thostatic system using an elaborate facehow andimpression-orienting apparatus to create study caststhat were ccphalometrically oriented to the patient'sorbital plane (a line drawn from orbitale perpendic-uiar to Frankfort horizontal). According to Simon'slaw of the canine, the orbital plane is coincident withthe distal third of the maxillary canines, Simon'̂ ""related the dentition to the craniutn in three dimen-sions of space: the median sagittal plane (using themidpalatal raphe) at right angles to the Frankfort hor-izontal plane, and at right angles to the orbital plane.While advanced for his time, the complexity of theequipment and the preeision required to produce ac-curately oriefited sttidy casts obviated ready accept-ance of his gnathostatic models. Further research hasproved Simon's law of the canine to be in error, buthis concept of three-dirnensionai orientation of thedentition to the cranium was a forerunner of modemday gnathology,

in 1927 Friei""'-' published his stages of occlusionfrorn 3 years of age to old age. He demonstrated notonly numerous changes in the primary dentition,which Angle did not address, but also the variabilityof first molar position in a normal occlusion as itundergoes its metamorphosis from the transitionalmixed dentition to the worn teeth of old age (Fig 4),Friel-" -' thus illustrated the difficulty of trying to forceAngle's static ideal on a changing mechanism.

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Aogle answers critics

In 1928 Atigle-- reiterated his befiefthat the rnaxillaryfirst molar was the most rehable point of referencefrom which to compare other teeth, becanse of itsconstancy in taking correct position on eruption.However, Angle acknowledged that the tirst molarsmay erupt in modified positions when inflnenced bythe malposition of other teeth, either pnmary or per-manent, or by the loss or nondevelopment of teethanterior to the first molars. Therefore, Angle recom-mended visuahzing the first molars into their properpositions relative to the jugal buttress (key ridge) be-fore classifying the maloccluston {Fig 5). Angle- said,"Notwithstanding the efforts of the prejudiced and thesuperficially informed to disprove them, lhe first per-manetit molars are now and must continue to be rec-ognised by all who know the growth, physiology andmechanics of the denture, as the chief reliance, notonly as a basis for diagnosis, but also as a basis fromwhich to note changes both favorable and unfavorablein the positions of the other teeth throughout thewhole process of treatment and retention." Angle wasnot a man given to equivocation or timidity in pre-senting an inflexible position, even in the face of con-trary evidence.

The debate continues

In 1938 Strang-' felt strongly that the process of clas-sification should include inclined plane relationships,the axial inclination of each tooth, a frontal analysisof midline deviations and possible asymmetric con-dylar positions, rotations of posterior teeth, and facialand skeletal considerations using photographs and ce-phalometric radiographs. In effect, Strang-' advocatedmaking a complete diagnosis, using all available rec-crds, before deciding on the true classification of acase, rather than basing classification on the dentalocclusion alone. He believed that Angle's reliance onthe constancy ofthe maxillary first molars to the cran-ium led to unreliable classification results. Strang'stextbook-'' gave elaborate examples of ten cases andhis method for obtaining Angle's classificafion. Hisdirections required mentally repositioning the firstmolars to their "proper" locafion using determina-tions such as ideal axial positions of other teeth, es-pecially canines. The fallacy in this technique, how-ever, lies in the inconsistent application of subjectivemeasures by different clinicians.

In 1939 Atkinson" defended Angle's hypothesis of

Fig 5 Turners si<uli. An iliustration ot ideal occlusion. (Re-printed Irom Angle.')

a relationship between the maxillary first molar andthe cranium. However, by showing that these relation-ships change with age and among different racialtypes, he neutralized Angle's absolutist dogma.

In 1951 Massier-^ remarked that, even when dentistsare carefully trained, applying Angle's classificationmeets with considerable subjective judgment and dif-ferent results with different examiners.

In ¡954 Stoller" integrated concepts from Angle,Strang, and others. He observed that the mesiobuccalcusp of the maxillary first molar should ideally beslightly distal to the mandibular molar buccal groove,contrary to usual practice. Cases in whieh the max-illary mesiobuccal cusp fit exactly in the mandibularbuccal groove did not exhibit proper interdigitationof the premolars and canines. Also, Stoiier-' reiteratedStrang's observation that the maxillary first molarcrown should be tipped mesially (or, expressed anotherway, that the roots tip distally) in ideal occlusion (Fig6). This places the distobuccal cusp lower (more oc-clusally) than the mesiobuccal cusp. Therefore, thedistobuccal cusp of the maxillary first molar shouldrest well down into the embrasure between the man-dibular first and second molars. StoUer-̂ noted that,when the maxillary first molar is not tipped the max-illary premolars and canines mesh shghtly mesial tothe embrasures of the mandibular teeth- The maxillary

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Fig 6a The molars may appear to be in ideal Class 1 oc-clusion (mesiobuccal cusp ot the maxillary first molar fittingin the buccal groove ol the mandibular first molar), but thepremolars and canines do not interdigitate properly be-cause the maxillary first molar is too upright.

Fig 6b An ideal premolar and canine interdigitation canbe achieved by tipping the crown of the maxiliary first molarmesially and by placing the mesiobuccal cusp of the max-iilary first molar slightly distai to the groove of the mandib-ular first moiar.

canine is, therefore, riding up on the mandibular can-ine, a situation that causes mandibular anterior col-lapse in postretention. In addition, Stoller'' thoughtthat if the maxillary first molar, instead of properbuccal outset from the second premolar, is rotatedmesiolingually so that its buccal cusps line up withthe premolar, the maxillary molar occupies more space(because of its trapezoidal shape), and thereby causesan anterior positioning of all teeth mesial to the max-illary first molar.

Ricketts and others,-* working with Rocky Moun-tain Data Systems in the late 1960s, designed a com-puterized cephalometric analysis that classified mal-occlusions from a cephalometric radiograph ratherthan from casts. Measuring the distance along occlu-sal plane between the distal surfaces of the mandibularand maxillary molars, it was determined that in ClassI the mandibular molar is 3 mm forward, in Class 11the maxillary molar is forward of even, and in Classin the mandibular moiar is more than 6 mm forward.Using the canines as an additional anteroposteriorclassification aid, they determined that in Class I themaxillary canine cusp tip is 2 mm distal to the man-dibular canine cusp tip, in Class II the maxillary cusptip is 1 mm or more forward, and in Class Hi themaxillary cusp tip is more than 5 mm distal to themandibular cusp.

In 1969 Ackerman and Proffit^' acknowledging the¡imitations of Angle's classification, proposed a new

classification scheme that combines five descriptivecharacteristics for malocclusions: alignment in occlu-sal view, profile and soft tissue, transverse plane de-viations (crossbites). sagittal plane deviations (antero-posterior) using Angle's classification, and verticalproblems of bite depth. The five characteristics caneach be found alone or in overlapping combinationfor a total of nine classification groups. This classi-fication lends itself to computerization.

In 1972 Andrews^" published six keys to normal oc-clusion. Molar relation was discussed in key 1. First,the distal surface of the distobuccal cusp of the max-illary first permanent molar should occlude with themesial surface of the mesiobuccal cusp of the man-dibular second molar. Second, the mesiobuccal cuspof the maxillary first molar should fall in the groovebetween the mesial and middle cusps of the mandib-ular first molar, as per Angle. Andrews'" thought thatthis relationship alone could be insufficient, becauseit is possible to have Angle's Class I and to have themaxillary second premolar not fit properly in the em-brasure between the mandibular premolar and molar.Third, the mesiolingual cusp of the maxillary first mo-lar must be seated in the central fossa of the mandib-ular first molar. Keys 2 through 6 also contain objec-tives that must be met for the occlusion to be consid-ered correct.

In 1973 Arya et al" published a paper that studiedthe relationship between the terminal piano of the sec-

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ond primary molars and the subsequent occlusion ofllie permanent first molar. In cases in which the per-manent first molars erupted initially into cusp to cuspocclusion, 70% became Class I and the remaindereventually became Class II. Because of the inconsis-tency of molar occlusion over lime in an individual,a classification technique that helps bridge the tran-sition from primary through permanent dentition isneeded.

Graber and Swain'- noted that Angle's classificationfails to separate maiocclusions with analogous antero-posterior relationships but with other characteristicsthat are dissimilar and thus require different treatmentplans. They warned that the tendency to treat mal-occlusions of the same classification in a similar man-ner is detrimental to patients who do not have ho-mologous malocclusions.

Integrating occlusioD to physiology

Graber''-'"' stressed that modern concepts of normalocclusion require three main areas of health: the oc-dusal position of teeth in contact, in harmony withthe postural resting position as determined by themusculature, and the temporomandibular joint mor-phology. A fourth element could be added: the antero-posterior development of the maxilla and mandible.Each of these elements must be healthy, individuallyand in combination, for optimal function and com-fort-

Roth"" stated that form and function are not al-ways coincidental. Excellent occlusion and morpho-logic form may not always be in harmony with thetemporomandibular joints. The patient's neuromus-cuiar protective meehanism could cause him or her toavoid occluding when interferences are present in thepatient's terminal hinge centric relation path of clo-sure as dictated by the temporomandibular joint. Toavoid contacting interferences, the mandible slides in-to a maximal intercuspation, which looks beautiful ina hand-held set of plaster casts. Unfortunately, thecondyles have translated away from the healthy centricrelation position (condyle most centered and superior)into an unhealthy position, with the condyles m theirfossae. When the discrepancy between the dictates ofthe joint and the dictates of occlusion becomes toogreat, muscles go into spasm and it becomes virtuallyimpossible to find the ideal path of closure and theoffending occlusal interferences. Roth's'' criteria for agood functional occlusion include proper condylarand disk position upon closure and movement, even

occlusal contad bilaterally, holding centric contacts,harmonious anterior guidance, and posterior clear-ance on excursions.

Elsasser'* published a numeric classification in 1978.Using millimeter measurements of incisai overbitc,overjet, maxillary midline, mandibular midline, rightcanine, left canine, right first molar (mesiodistal),right first molar (buccolingual), left first molar(mesiodistal), and left first molar (buccolingual),he developed a ten-number classification. A patient'smalocclusion is compared to the ideal goal of2200-00-0202.

The enormous challenge involved in developing aclassification method that is "all inclusive" has, so far,defeated dentistry's greatest minds.

The present and Ihe future

Roth" recommends centric relation recording tech-niques in conjunction with fully adjustable articula-tors and detailed facebow transfer methods to obtainmounted casts that are properly related to the joints.Clinicians of the 1990s find no fault with the veracityof his recommendations. However, for many practi-tioners, economics and the demands of the market-place make adherence lo Roth's complex methodologydifficult to follow for routine orthodontic treatment.The challenge of developing more simplified systemsfor estabhshing accurate recording of occlusion-temporomandibular joint interrelationships may restwith the next generation of dentists.

Koski'̂ wrote concerning the difficulty of estabhsh-ing a diagnostic norm for the dentition, as well as atherapeutic norm to serve as a basis for treatment. BuiKoski'^ optimistically noted, "However, the general or-derliness of nature (naturally allowing for commonvariations) and the rather close structural-functionalsynchronization of the different parts of an organismseem to suggest that there may esist certain basic reg-ularities or norms also within the dento-facial com-plex, which might be discovered through more system-atic and diversified effort than heretofore."

We witness at the end of the twentieth century therecent return of the personality cult and the dogmaof absolutist extraction theory that plagued the earlytwentieth century, with Angle against his peers. His-tory has a way of coming full circle.

One hundred years of occlusion research has ended.What advances can we expect in the next century? Thesearch continues.

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