Andrew’s six keys of occlusion The first key of normal occlusion: Molar interarch relationship Above: Diagram showing examples of occlusions with poor Class I molar relationships, according to Andrews, caused by insufficient crown angulation of the upper first molar. Bottom left: Diagram showing an improved Class I molar relationship. Bottom right: Diagram showing the correct Class I molar relationship according to Andrew's first key of occlusion. According to Andrew's occlusal concept: - The mesiobuccal cusp of the upper first molar should occlude in the groove between the mesial and medial buccal cusp of the lower first molar. - The mesiolingual cusp of the upper first molar should occlude in the central fossa of the lower 6year molar. - The crown of the upper first molar must be angulated so that its distal marginal ridge occludes with the mesial marginal ridge of the lower second molar (Andrews, 1972). Long axis of the crown Vertical line = Long axis of the clinical crown (= LACC) LA spot = Center of the long axis of the clinical crown Unlike other concepts, it is not the long axis of the tooth which serves as the plane of reference, but rather the long axis of the clinical crown. It passes through the central, vertical ridge of the tooth, i.e. through the most prominent part in the center of the labial or buccal surface. This applies to all teeth, except molars. Mesiodistal angulation of the crown --- Perpendicular to the occlusal plane —— Long axis of the crown The angulation of the crown is defined as the angle which the tooth forms with a line drawn perpendicular to the occlusal plane. Right: Drawing showing the mesiodistal angulation of the crown of the upper left central incisor (Andrews, 1972). Second key of normal occlusion: Mesiodistal crown angulation For the occlusion to be considered normal, the gingival part of the long axis of the crown must be distal to the occlusal part of the axis. The degree of angulation depends on the type of tooth. Right: Diagrammatic view of the mesiodistal angulation of the crowns when the occlusion is normal (Andrews, 1972). Mesiodistal crown angulation for various types of upper teeth Horizontal plane of reference = Line passing through all LA spots (Andrews plane) Vertical plane of reference = Perpendicular to the horizontal plane According to Andrews, in he upper jaw the crowns of the canines exhibit the greatest degree of angulation and the premolars the least. Third key of occlusion: Labiolingual crown inclination --- Tangent on the crown of the tooth —— Perpendiculartotheocclusal plane The third key defines the angle between a tangent to the LACC at its center and a line perpendicular to the occlusal plane. If the gingival area of the crown is more toward the lingual, the result is expressed in positive values; should the opposite apply, the result is negative. Labiolingual crown inclination between upper and lower incisors (crown torque) --- Tooth crown tangent ––– Perpendicular to the occlusal plane –––– Long axes of the incisors The upper incisors form a positive angle with the crown tangent and the line perpendicular to the occlusal plane (+7°) and an angle of 18° between the crown tangent and the long axis of the tooth. The crown torque of the lower incisor is -1° and the angle between its crown tangent and the axis of the incisors is 16°. The interincisal angle between the crown tangents of the upper and lower incisors is 174° for normal occlusions (unlike the interincisal angle between the axes of the incisors which is considered to be, on average, 139°) (Andrews, 1972). Incorrect crown torque and occlusal findings Should the upper anterior teeth be in a too upright position (the labiolingual crown inclinations of the upper incisors have negative values), the occlusion is unstable. The canine guidance is insufficient and there is a risk that the posterior teeth will drift toward the mesial (Andrews, 1972). Anterior and posterior occlusion in case of incorrect crown torque If the posterior occlusion is correct, but the upper incisors are in linguoversion, this can result in interdental spacing of the anterior teeth which then is often incorrectly associated with a discrepancy in the intermaxillary tooth-size (Andrews, 1972). Occlusal changes after orthodontic treatment Clinical picture of the situation schematically Occlusal relations in the postretention stage after orthodontic treatment. The long-term result is a dentally supported bite with lingually inclined upper incisors and a space posterior to the upper right canine. The canines are no longer in Class I relationship. Labiolingual inclination of the posterior teeth in optimal occlusion The tangents on the facial surfaces of the crowns form negative values with the line drawn perpendicular to the occlusal plane, i.e. the gingival portions of the teeth are more pronounced buccally than the occlusal portions. The upper canines and premolars are inclined at virtually the same angle, whereby the molars are tilted slightly more. In the lower arch, the inclination increases progressively from the canine to the second molar. Fourth key of occlusion: Rotations In order to achieve correct occlusion, none of the teeth should be rotated. Rotated molars and premolars occupy more space in the dental arch than normal. Rotated incisors may occupy less space than those correctly alignd. Rotated canines adversely affect esthetics and may lead to occlusal interferences. Fifth key of occlusion: Tight contacts, no spacing If there are no anomalies in the shape of the teeth, or intermaxillary discrepancies in the mesiodistal tooth size, the contact points should abut in normal occlusion. Clinical picture of a poor example, with spaces between the upper teeth and a Class I relationship of the canines. These findings are indicative of a Bolton discrepancy. Sixth key of occlusion: Curve of Spee a- An excessive curve of Spee restricts the amount of space available for the upper teeth, which must then move toward the mesial and distal, thus preventing correct intercuspation. b- A normal occlusion has a flat occlusal plane (according to Andrews, the mandibular curve of Spee should not be deeper than 1.5 mm). c- A reverse curve of Spee creates excessive space in the upper jaw, which prevents development of a normal occlusion (Andrews, 1972). Curve of Spee - Occlusion-Case examplesReverse curve of Spee This panoramic radiograph shows the occlusal relationship resulting from a reverse sagittal compensating curve. When compared to the upper jaw, insufficient space is available in the lower dental arch and the anterior teeth are crowded. The bite is open anteriorly. Flat curve of Spee Flat sagittal compensating curve with good intercuspation around the premolars and molars. This type of curve is considered to be "normal" according to Andrews. Excessive curve of Spee Pronounced sagittal compensating curve with excessive space in the upper dental arch and inadequate space in the lower arch. The lower incisors are crowded and the overbite is increased. Every upper tooth occludes against two opposite teeth except lower centrals. Maxillary midline coincides with mandibular midline. Overbite is 1/3 the crown heights of the lower incisors. In normal adult occlusion ,spaces and crowding are not present. The neighbouring teeth are aligned without Overbite is one third the crown height of the lower incisors and overjet is about the thickness of the incisal edges of the upper incisors in normal Upper third molar occludes against the lower one only in normal adult occlusion. In normal adult occlusion the mid lines are on. Upper arch shape is horseshoe in normal adult occlusion Lower arch shape is parabolic or U-shaped in normal adult occlusion A, ideal intercuspation, buccal view. B, ideal intercuspation, lingual view of normal adult occlusion . Occlusion at 13 years. There are few changes except the tendency to less dental procumbency . Reference Orthodontic Diagnosis - Thomas Rakosi, Irmtrud Jonas, Thomas M. Graber - Thieme, 1993
