VIEWPOINT

Angle classification revisited 2: A modified Angle classification Morton I. Katz, DDS*

Washhlgton, D.C. Edward Angle, in his classification of malocclusions, appears to have made Class I a range of abnormality, not a point of ideal occlusion. Current goals of orthodontic treatment, however, strive for the designation "Class I occlusion" to be synonymous with the point of ideal intermeshing and not a broad range. If contemporary orthodontists are to continue to use Class I as a goal, then it is appropriate that Dr. Angle's century-old classification, be modified to be more precise. (AM J ORTHOD DENTOFAC ORTHOP 1992;102:277-84.)

H a v i n g observed inconsistency in the application of the Angle classification of malocclusion as a postgraduate orthodontic teacher, I developed a questionnaire to test whether the observed inconsistency is merely anecdotal or whether orthodontists across the spectrum of ages, training, and geographic location truly have problems applying the Angle classification. The questionnaire presented 10 buccal views of study models: three ideal interdigitations and seven with varying degrees o f Class II tendency (two of which were mutilated). A sample was selected of 347 orthodontists (approximately 5% of active practitioners listed in the 1987 American Association of Orthodontists list of members) selected from every state and distributed among orthodontists trained in the 1960s and before, the 1970s, and the 1980s, as well as all chairpersons of departments of dental school orthodontic programs. The 77.8% response rate on the single mailing demonstrated an intense interest in the subject of classification reliability. Unfortunately, the statistics showed a n extremely p o o r reliability of Angle classification when applied by contemporary orthodontists. This study illustrated that a problem exists with the current use of Angle's valuable classification. ~ I have further attempted to supplement the Angle method to address the weaknesses observed, while maintaining the strengths of the Angle method. Why is the classification of malocclusion important? First, classification is an essential communication tool between dental school professor and student, between practitioners, and between practitioner and insurance company or government bureaucracy. It is essential that

*Associate Professor, Postgraduate Clinical Director, Department of Orthodontics. Howard University College of Dentistry, Washington. D.C. 8/1/29405

everyone "speak the same language." No one has difficulty classifying the extremes of a full Class II and a full Class III. However, the gray area of Class I is subject to much confusion and inconsistency. Second, classification has a significant effect on patient treatment. Once a patient is classified, the practitioner will tend, almost unconsciously, to apply treatment mechanics appropriate to that classification. Even though model analysis, especially buccal interdigitation, is but a small part of the complete orthodontic diagnosis, the decision to use Class II or Class III mechanics invariably is strongly influenced by the perceived Angle classification of the patient. Third, if one of the goals in the treatment of a malocclusion is to achieve Class I, there must be a consensus among orthodontists as to what constitutes ideal occlusion, and Class I must be redefined to agree with the prototype standard. If every orthodontist has a different idea of what ideal buccal interdigitation means, then the dental specialty of orthodontics has no standardized method by which to evaluate successful or unsuccessful treatment. Angle developed his classification 100 years ago to eliminate the anarchy that existed in the specialty. Considering the current level of disagreement as to what constitutes Class I, and what describes ideal occlusion, is the Angle classification inherently faulty, or are contemporary orthodontists not properly applying it, as designed by Dr. Angle? Rather than supplanting Angle, supplementing the Angle classification might make it more descriptive and precise. In an era of piezo-electric and magnet-accelerated tooth movement, titanium alloy wires, esthetic orthodontic appliances, state of the art magnetic and x-ray imaging, and diagnosis and treatment planning by computers, it seems anachronistic to communicate with a classification scheme that has not been significantly modified or upgraded in 83 years. Our respect for tra277

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Fig. 1. Angle's prototype ideal occlusion. All teeth (except lower centrals and upper third molars) have two antagonists. Note proper mesial tilt of the upper first molar.

}Fig. 2. Traditional Angle Class I molar (upper mesiobuccal cusp in lower molar groove). However, teeth have only o n e antagonist. Not ideal Angle intermeshing because upper molar is too upright, forcing an anterior displacement of the upper premolars, canines, and incisors.

dition must be balanced with the need to define more precisely our patients' occlusal problems. DISCUSSION It is all too easy to dismiss the Angle classification as an archaic remnant of orthodontic past and develop a complex, multifactorial classification to represent the Cutting edge of current thinking. If we can accept that Dr. Angle, one of the specialty's most seminal thinkers, put as much effort into his classification as he did his other innovations, then it would be appropriate to reread his writings before judging him too harshly. Every dental student learns the Angle "mesiobuccal cusp of the upper first molar fits into the buccal groove of the lower first molar" truism by rote. However, since few ever open the now yellowed pages of Angle's articles and books, many miss his complete "occlusion concepts. Angle 24 described in minute detail each contacting cusp incline to prove his point that in ideal

occlusion every tooth (except the lower centrals and upper third molars) should have t w o antagonists (Fig. 1). In other words, even if a patient has the mesiobuccal cusp of the upper first molar fitting perfectly into the lower molar buccal groove (Fig. 2), the patient does n o t possess p r o p e r occlusion according to Angle, 24 and later Strang, 5 Stoller 6 and Andrews, 7 unless the upper first molar also has a mesial crown tilt that allows the distal incline of the distal cusp of the upper first molar to occlude with the mesial incline of the mesial cusp of the lower second molar. The practitioner must, therefore, look at more than first molars. Proper cuspal incline contacts of a l l teeth should be noted. Angle emphasized the importance of each premolar and canine contacting two occluding teeth. An occlusion where the first molars classically fit the criteria of the upper mesiobuccal cusp to lower molar groove, but the premolars and canine contact only one opponent tooth each (Fig. 2), would be considered Class I by Angle (because Class I is a premolar-width range of abnormality). However, Angle would not have considered the occlusion as having met his standards for "ideal" occlusion of a well-treated case. Therefore all "ideal" occlusions are Class I, but not all Class I occlusions are "ideal." Orthodontists are goal oriented. Can contemporary orthodontists make attainment of Class I a goal of treatment? The answer is no, after one reviews Angle's definitions. The original classification by Angle, 2"3 had Class II as a full premolar-width distoclusion and Class III as a full premolar-width mesioclusion. Assuming an average premolar width of 7.5 mm, then Class I ranged from 7 mm mesioclusion to 7 mm distoclusion, for a total range of Class I of 14 mm. This range was far too broad, and so in 1907, Angle 4 revised his definition, making Class II more than half of a cusp distoclusion and Class III more than half of a cusp mesioclusion. Angle's modification reduced the range from 14 mm to

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a 7 mm range. However, 7 mm is still too broad a range to act as a treatment goal if an orthodontist is to treat with precision. It is my contention that Angle never intended his classification to depict Class I as a treatment goal or "ideal," but as a range of abnormality between the extremes of full Class II and Class III. Angle would not have described, in the most minute detail, the precise intermeshing of every cuspal incline and then hypocritically imply that teeth can be arranged almost cusp tip to cusp tip and be acceptable. Class I is not a goal or an "ideal," as Angle envisioned it, but current orthodontists speak with misguided pride of having achieved the treatment goal of Class I. Contemporary orthodontists should become educated as to Angle's complete occlusion criteria, in addition to first molar relation. Also, some supplemental information should be added to Angle's classification to better quantify deviation from Angle's description of proper occlusal intermeshing. EXPLORING THE POSSIBILITIES Using MOLARS to evaluate ideal buccal occlusion Angle 24 considered the upper first molar as the most reliable point of reference from which to compare other teeth because of its constancy in taking a correct position relative to the bony skeleton's jugal buttress (also known as the key ridge). The key ridge is a ridge of bone descending downward and forward from th e zygoma, which is the anterior edge of the jugal or molar buttress of the maxillary bone and which marks the union of the anterior and the posterior buccal walls of the antrum of Highmore. Contemporary orthodontists, however, do not consider the anatomic interrelationship of the upper molar to the cranium as significant. Angle 8 acknowledged that the first molar might erupt in an altered position when influenced by the malpositions of other teeth or the loss or nondevelopment of deciduous and permanent teeth anterior to the first molar. Therefore Angle recommended visualizing the upper first molar into its proper position relative to the jugal buttress before classifying the malocclusion. There are two problems with this concept. First, visualizing the "correct" position of the upper first molar to the jugal buttress and lining up the remaining dental units relative to it is a very subjective pursuit. It is quite probable that no two orthodontists would exactly visualize the same "correct" position. And second, modem orthodontists are more concerned with the proper position of the incisors relative to the profile for esthe!!c and stability concerns and are willing to adjust first molar position and even sacrifice teeth to better align the incisors (concepts Angle would never have ac-

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cepted). Modern orthodontists advance molars in extraction treatments or distalize molars in nonextraction treatments with little concern for the immutable relationship of the upper first molar to the bony landmarks, such as the key ridge, as promulgated by Angle. In addition, Friel 9 and later Arya et al. ~~ demonstrated the dynamic nature of the position of the first molar in the mixed dentition, changing as the occlusion matures into the permanent dentition because of jaw growth and second deciduous molar leeway space considerations. This natural "adjustment" makes a molardefined classification awkward in young patients. Using CANINES to evaluate ideal buccal occlusion Maxillary canines are among the most stable of dental units because they are the longest rooted of all teeth and therefore very well anchored to the alveolar bone. The canine is the "keystone" tooth in the dental arch, and like the keystone of a stone archway, it provides a buttressing support for the incisors, as well as the posterior teeth. Also, canines provide a vital protective function in lateral excursive movements. Historically, Simon 'L'2 attempted a canine-focused classification. His Law of the Canine considered the orbital plane (a line drawn from orbitale perpendicular to Frankfort horizontal) as coincident with the distal third of the maxillary canine in ideal occlusion. While modern orthodontists no longer consider Simon's law valid, the strategic position occupied by the canine makes it a favored tooth to reference for classification. However, the principal objection to a canine-derived classification relates to tooth anatomy. The maxillary canine exhibits a mesial incisal ridge that is shorter and less severely sloped than its distal incisal ridge. As a result, the central axis of the maxillary canine does not bisect the cusp tip. ~3 Tooth sizes and shapes vary, but the cusp tip averages 1 to 1.5 mm mesial to the center axis. Therefore the cusp tip of the maxillary canine does not directly fit into the embrasure formed b y t h e mandibular canine and the first premolar, but rides up on the distal slope of the mandibular canine (Fig. 3). Also, the cusp tip of the maxillary canine does not work well as a landmark because occlusal wear frequently alters the cusp tip from a point to a flat facet, and the modified architecture of its incisal edge obscures the true cuspal form. Although not ideal, one could use the imaginary center axis of the maxillary canine as a reference point, since it lines up with the mandibular canine-first premolar embrasure (Fig. 3). Another objection to relying on the canine for classification rests with the time of its eruption. The maxillary canine is one of the last teeth to erupt (other than third molars). This holds up classification efforts until

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y Fig. 3. Central axis of upper canines bisects distal embrasure of lower canines in ideal Class I. The cusp tip of upper canine, however, is mesial to the axis.

the patient is 12 years, or older in slowly erupting patients. The deciduous canine offers little assistance with classification since it is smaller in mesiodistal width than its permanent successor, resulting in a center axis that is not coincident with the center axis of its future permanent replacement. Using PREMOLARS to evaluate ideal buccal occlusion

Favorably, premolars usually present a sharply defined cusp tip, which is centered on the central axis of the premolar crown and which fits precisely into the opposing embrasure. Also, the cuspal inclines are steeper and deeper than molar cusps, which makes a more positive fit. From the negative perspective, orthodontists traditionally have not had high regard for premolars as functional dental units and have selected premolars most often of all tooth types for sacrifice in an extraction treatment. Also, premolars may have anomalous tooth size or shape. Furthermore, some judgment is required when less than a full complement of premolars are present. However, orthodontists have always been forced to contend with the frequently missing first molar, so this problem is not unique to premolars. A MODIFIED ANGLE CLASSIFICATION A goal-directed classification

The Class I designation was used by Angle to describe a range of abnormality (malocclusions in the middle ground between Class II and Class III). Unfortunately, many modem orthodontists use the ~ 1 ~ : I designation as a goal of successful treatment. Therefore, to more closely match current use, it is necessary

Fig.4. One upper premolar (t3) opposing two lower premolars (B) in modified Class I. Note traditional Angle only classifies molars and would call this excellent occlusion a Class IH

to redefine Class I for the purpose of this modified Angle classification as the pohu of ideal intermeshing. Angle 2'3 described in detail the "ideal" occlusal relationship, and his conception of ideal occlusion has withstood the test of time and must be retained. However, the large 7 mm range of Class I has been discarded in this modified version, and all the teeth visible from a buccal view (except the upper third molars) must occlude with two antagonists as Angle demanded for an ideal occlusion (Fig. 1). By defining Class I more narrowly, the designation "Class I" will match the occlusion goal modem orthodontists aspire to achieve. A premolar-derived classification

After considering the advantages and disadvantages of the possible choices, o n l y the premolar occlusion offers sufficient precision of intermeshing to be a valid standard. Class I, in the modified Angle classification is defined: The most anterior upper premolar fits exactly into the embrasure created by the distal contact of the most anterior lower premolar. This definition applies whether a full complement of premolars are present, whether one upper premolar opposes two lower premolars (Fig. 4), whether two upper premolars oppose one lower premolar (Fig. 5), or whether only one premolar is present in each quadrant (Fig. 6). When this relationship is achieved, the canines will also relate correctly, as will the incisors. We are unconcerned about molar relation. The occlusion functions and intermeshes properly and is considered Class I, even though when one upper premolar correctly opposes two lower premolars, the molars are full Class II (Fig. 4), and when two upper premolars oppose one

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Fig. 5. Two upper premolars (t3) opposing one lower premolar (B) in modified Class I. Note traditional Angle only classifies

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Fig, 7. Mixed dentition: Central axis of upper first deciduous molar bisects embrasure between two lower deciduous molars in modified Class I.

molars and would call this excellent occlusion a Class II1!

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Fig. 8. Mixed dentition, with early loss or extraction of upper Iirst deciduous molar: Central axis of upper edentulous space bisects embrasure between two lower deciduous molars in modified Class I,

Fig. 6. One premolar (13) in each quadrant.

lower premolar, the molars are full Class 1II (Fig. 5), according to Angle. Modem orthodontists do extract teeth, whereas Angle did not, and we need to have an occlusion classification that works when teeth are extracted in only one arch. In the rare instance where no premolar exists in a quadrant, then the center axis of the upper canine crown (not the cusp tip) should be used as a reference to the distal contact of the lower canine. Deciduous and mixed dentition classification

The deciduous or mixed dentition was not directly addressed by Angle in his classification. In the modified classification, the center axis of the upper first deciduous molar should split the embrasure between both lower deciduous molars (Fig. 7). Although it is possible to use the center axis of the upper second deciduous" molars, they are less accurate than using first deciduous molars because of the leeway space, which adds im-

precision. However, in the event that an upper first deciduous molar is prematurely lost, a line drawn through the center axis of the edentulous space should bisect the embrasure between the two lower deciduous molars (Fig. 8). Since orthodontists are currently treating patients earlier, when dentitions are mixed, it is essential that a classification be used which eliminates the first molar focus, as Friel 8 and Arya et el. 9 demonstrated, the relationship between the upper and lower first molars changes considerably through life. Just as Angle required the orthodontist to mentally reposition a first molar that had drifted mesially because of early tooth loss or ectopic tooth eruption, with this proposed modified Angle classification, the orthodontist may be confronted with an unusual situation requiring some imaginative solution to classify the patient accurately. Given the large diversity of the human dentition, no simplistic classification system will always apply. This supplemental classification does not eliminate the need for professional judgment in the special situation, but it is hoped that this classification approach will prove to be more consistent with treatment goals and the mechanics necessary to obtain these goals.

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Fig. 9. Right side is ideal intermeshing = 0 deviation. Left side is 2 mm Class II = + 2 deviation.

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Right side is half cusp Class II = + 4 deviation. Left side is full Class II = + 8 deviation.

Quantifying the classification Another major concern regarding the Angle classification is the lack of a numerical quantification of the degree of Class II or Class III. Techniques that use millimeter measurements to quantify Class II or Class III tendency have been used by Ricketts and others t3 for Rocky Mountain Data Systems (with a cephalometric film to evaluate canines and molars), and by Elsasser 14 in a complicated, multifactorial analysis. This proposed modified classification designates ideal cusp-embrasure occlusion (as described by Angle) as zero (0). A plus sign ( + ) designates Class II direction and a minus sign ( - ) designates Class III tendency. In this article the fight side is evaluated first, then the left side. Ideal occlusion on both right and left side~'i~ therefore, (0,0). For example, if a patient presents with ideal inter-

meshing on the right side, but a 2 mm Class II tendency on the left side, then the modified classification would read ( 0 , + 2 ) (Fig. 9). However, if another patient presents half a cusp Class II (cusp-to-cusp occlusion) on the right side and a full cusp Class II on the left side, then the modified classification would read ( + 4, + 8) (Fig. 10). A third patient who is 1.5 mm Class II on the right and 3.5 mm Class III on the left side would be classified ( + 1 . 5 , - 3 . 5 ) (Fig. I1). Note that any occlusion numbers ranging from + 3.5 to - 3 . 5 would have fallen within the range of a Class I malocclusion in the traditional Angle classification. And so, the occlusion in (Fig. 11), which requires both Class II and Class III mechanics to correct to ideal, would be considered Class I on both sides by Angle's definition. However, in the proposed modified Angle classification, only (0,0) would meet the requirements of the ideal Class I, and could be considered to

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Fig. 11. Right side is 1.5 rnm Class II = + 1.5 deviation. Left side is 3.5 mm Class III = - 3.5 deviation. Note traditional Angle can not classify a patient with both Class II and Class III sides.

satisfy the goals of excellent occlusion described by Angle. This simple modified Angle classification has several advantages over other classification efforts. First, this new system establishes a treatment goal that is a specific cusp-embrasure point rather than a range of 7 mm (half of a cusp each way) as in the traditional Angle classification,of malocclusion. Second, this supplemental classification quantifies the degree of occlusion error of a malocclusion precisely in millimeters and for each side separately. Third, this method allows the orthodontist to classify those patients who have Class II tendency on one side and Class III tendency on the other side. Angle 2-~'8 mentioned that this possibility existed, but he did not include a provision for this situation in his classification because he said it was a rare occurrence. Although it is extremely rare to find a full Class II coexisting with a full Class III, partial Class II does occur on one side and partial Class III on the other side with some frequency. This new approach covers that eventuality. Fourth, this numerical system allows computer input to the facilitated and the severity of the malocclusion to be easily rated and compared for statistical and research purposes. Fifth, this new system addresses the need for deciduous and mixed dentition classification to assist early treatment analysis. Because dentists are familiar with the term Class I, it has been retained in the modified Angle classification, but it has been more narrowly redefined to coincide with Angle's description of the "ideal" occlusion, allowing Class I to be used as a goal of treatment in keeping with contemporary usage. Another significant issue is the long-standing tradition of orthodontists to evaluate the occlusion in the mouth, as well as to trim plaster study models, with

the teeth in maximum intercuspation (centric occlusion). Maximum intercuspation frequently places the jaws in quite a different relationship than the orthodontist would see if the condyles were properly located and if the neuromuscular system was in balance and comfort. Therefore the ideal time to classify a patient, either before treatment or during treatment, would be after the patient has been "deprogrammed" so as not to see the tooth-deflected false bite of maximum interdigitation. Optimally, classification can be done on models properly mounted on a fully adjustable articulator with a correct bite registration. This modified Angle classification works especially well on articulated study models. Ideal Class I has not been achieved if the patient slides on tooth prematurities into a nice "fit" that looks good but is not in harmony with the comfort position of the condyles and the relaxed, unstrained postural rest position of the muscles. In conclusion, it is hoped that this effort to modify classification technique, without discarding the impressive contributions of Angle and others, will make communication regarding malocclusions more precise in teaching, research, and treatment. REFERENCES

1. Katz Ml. Angle classificationrevisited 1: is current usage reliable? AM J ORTHOD DENTOFACORTHOP [in press]. 2. Angle EH. Classification of malocclusion. Dent Cosmos 1899;41:248-64, 350-7. 3. Angle EH. Treatmentof malocclusionof the teeth and fractures of the maxillae. Angle system. 6th ed. Philadelphia: SS White Manufacturing, 1900:6-8, 37-44. 4. Angle EH. Treatmentof malocclusionof the teeth and fractures of the maxillae. Angle system. 7th ed. Philadelphia: SS White Manufacturing, 1907:44-59. 5. Strang RHW. A text-bookof orthodontia, 3rd ed. Philadelphia: Lea & Febiger, 1950:107-34.

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6. Stoller AE. The normal position of the maxillary first permanent molar. A,',t J OR'IttOD 1954;40:259-71. 7. Andrews LF. Six keys to normal occlusion. AM J ORTIXOD 1972;62:296-309. 8. Angle Ell. The latest and best in orthodontic mechanisms. Dent Cosmos 1928;70:1143-58. 9. Friel S. Occlusion. Observations on its development from infancy to old age. l:,,rrJ ORTHOD 1927;13:322-34. 10. Arya BS, Sarava BS, Thomas DR. Prediction of the first molar occlusion. AM J OR'rHOD 1973;63:610-21. 11. Simon PW. On gnathostatic diagnosis in orthodontics. INI J OwnloD 1924;10:755-8. 12. Simon PW. The simplified gnathostatic method. I,'r J OR'rHOD 1932;18:1081-7.

13. Kraus B, Jordan R, Abrams L. Dental anatomy and occlusion. Baltimore: Williams & Wilkins, 1969:33-41. 14. Ricketts RM, Roth RH,Chaconas SJ, Schulhof RJ, Engel GA. Orthodontic diagnosis and planning. Vol. 1. Denver: Rocky Mountain/Orthodontics, 1982:127. 15. Elsasser WA. Orthodontic assessment by the numbers. J Clin Orthod 1978;12:116-22. Reprint requests to: Dr. Morton I. Katz 3435 Philips Dr. Baltimore, MD 21208

AAO MEETING CALENDAR

1993mToronto, Canada, May 15 to 19, Metropolitan Toronto Convention Center 1994--Orlando, Fla., May 1 to 4, Orange County Convent!on and Civic Center 1995mSan Francisco, Calif., May 7 to 10, Moscone Convention Center

(International Orthodontic Congress) 1996--Denver, Colo., May 12 to 16, Colorado Convention Center 1997--Philadelphia, Pa., May.3to .7, Philadelphia Convention Center 1998--Dallas, Texas, May 16 to 20, Dallas Convention Center 1999--San Diego, Calif., May 15 to 19, San Diego Convention Center