Six Key of Normal Occlusion 2011

The purpose of the present lecture is to describe normal occlusion in adult subjects. In the present lecture, the following topics will be discussed.

A. Prevalence of normal occlusion in Egypt.

B. Static occlusion "Larry Andrews' six keys to normal occlusion".

C. Functional occlusion " Ronald Roth' concepts".

Physiologically speaking, the word "normal" designates a range ratherthan a single point. Literally, the word "occlusion" means closing up (oc=up,and clusion = closing).The description of normal occlusion at different ages was previouslylectured (i.e., normal growth and development of the dental arches andocclusion).

A. Prevalence of Normal Occlusion in EgyptPublished research has shown that the prevalence of normal occlusionamong adult Egyptian is 34.3%. It is significantly more common in females(36.7%) than males (31.6%).The prevalence of normal occlusion among Egyptians is lower thanthat of the Indians. However, it is higher than that of the Americans, Swedish,

Dr.Khaled AboulAzm ,2011

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DESCRIPTION OF NORMAL OCCLUSION INADULTS

Lawrence F. Andrews studied dental casts of 120 nonorthodonticsubjects having normal occlusion. Following his study, he described sixd y n a m i c a l l y - i n t e r r e l a t e d characteristics of normal occlusion designed bynature without any orthodontic intervention. He called these characteristicsthe "six keys". These keys are: molar relationship, crown angulation, crowninclination, rotations, tight contacts, and occlusal plane.

Key 1. Molar Relationship

The maxillary first permanent molar displayed a solid three- pointcontact with opposing teeth.

a. The distal surface of the distal marginal ridge of the maxillarypermanent first molar made contact and occluded with the mesial surfaceof the mesial marginal ridge of the mandibular permanent second molar.b. The mesiobuccal cusp of the maxillary permanent fist molar fell within the buccal groove located between the mesial and middle cusps of the mandibular permanent first molar- as indicated by Angle .c. The mesiolingual cusp of the maxillary permanent first molar wasseated in the central fossa of the mandibular permanent first molar. (The molars and premolars enjoyed a cusp-embrasure relationship buccally,and a cusp-fossa relationship lingually. The tips of the maxillary canines


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B. Static Occlusion :Larry Andrews' Six keys to normal occlusion

were slightly mesial to the mandibular canine-premolar embrasure- whichis consistent with the canine-rise concept).Key II. Crown Angulation (Mesiodistal Tip)

Fig. 3. Crown angluation (mesiodistal tip). P: perpendicular to the occlusalplane. A: crown angulation. M: mesial. D: distal. OP: occlusal plane.

By definition, the crown angulation is the mesiodistal tip of the long axisof the crown. It is the angle formed between the long axis of the crown (as

viewed from a facial perspective) and a perpendicular line erected from theocclusal plane .It is positive when the gingival portion of the long axis of the crown is distal to the incisal portion , and vice versa. In normal occlusion, the crown angulation was positive for all the teeth.

Key III. Crown Inclination (Faciolingual Torque)

Fig. 4. Crown inclination (faciolingual torque). B y d e f i n i t i o n , t h e c r o w n inclination is the faciolingual torque of the longaxis of the crown. It is the angle formed between the facial long axis of the


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crown (as viewed from a proximal perspective) and a perpendicular lineerected from the occlusal plane . It is positive when the gingival portion of the long aixs of the crown is lingual to the incisal portion, and vice versa In normal occlusion, the crown inclination was negative for all theteeth except the maxillary central and lateral incisors. Further, the lingual crown inclination was similar for the maxillary canines through the max i l l a ry second premolars, and was slightly more pronounced in the maxillary molars.Furthermore, the l ingual crown inclination progressively increased from themandibular canines through the mandibular second molars.

Key IV. Rotations

Fig. 5. Rotation In normal occlusion, the dentition should be free from undesirablerotation . If a molar is rotated, it would occupy more space than normalwithin the dental arch . The reverse is true for the anterior teeth.


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Key VI. Occlusal Plane

Fig. 6. Occlusal plane and curve of spec. By definition, the occlusal plane is the imaginary plane on which the teeth meet in occlusion. (It is indeed a curved compound surface which is commonly approximated by a plane- straight lines on lateral views-based onspecific reference points within the dental arches). In normal occlusion, theocclusal plane should be flat or nearly flat (Fig. 6). A deep curve of Spee

results in a more confined area for the maxillary teeth, creating spillage of themaxillary teeth mesially and distally . On the other hand, a reverse curve of Spee results in excessive room for the maxillary teeth . In other words, if the curve of Spee is not relatively flat teeth in one arch will tend to be crowded while those in the other will be spaced .


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Ronald Roth applied gnathological concepts to orthodontics. He has shown that functional occlusion can be orthodontically achieved even aftertherapeut ic extract ions . Roth described the mutually-protective occlusal scheme in which maximum dental interdigitation occurs when the mandible is in its rearmost, midmost, and uppermost position. The occlusal stops are equally distributed among the posterior teeth on their centric cusps. The anterior teeth are not in positive contact but may elicit a mark with an articulating paper. However, when the posterior teeth are in occlusion, a thin mylar strip will not be held by the anterior teeth.

Fig.7. Mutually – protective occlusalscheme

INCISAL GUIDANCEProtrusive movement

In protrusive movements, the maxillary anterior teeth articulate


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C. Functional Occlusion Ronald Roth's Concepts

equally and evenly with the mandibular anterior teeth and the first premolars (or the second premolars in first premolars extract ion cases) to gently disclude the posterior teeth.

In lateral movements

, the maxi l lary anterior teeth articulate with the same mandibular teeth to on any movement out of centric. The canines serve as the main guiding inclines. The rate of canine-rise is very gentle and harmonious with the lateral excursions

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Six Key of Normal Occlusion 2011