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Andrew’s sixAndrew’s six
keys ofkeys of
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 6-
- 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
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
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
--- 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
--- 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°)
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
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
The upper canines and
premolars are inclined at
virtually the same angle,
whereby the molars are tilted
In the lower arch, the
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
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
b- A normal occlusion has a flat
occlusal plane (according to
Andrews, the mandibular curve of
Spee should not be deeper than 1.5
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 examples-
Reverse 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
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
Lower arch shape is
parabolic or U-shaped
in normal adult
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 .
Orthodontic Diagnosis - Thomas
Rakosi, Irmtrud Jonas, Thomas M.
Graber - Thieme, 1993