Andrew identified 6 keys to normal occlusion based on a study of non-orthodontic models. The 6 keys are: 1. Proper molar relationship between the upper first molar and lower second molar. 2. Positive crown angulation for all teeth. 3. Negative crown inclination for most teeth, except upper front teeth. 4. Teeth should be free from undesirable rotations. 5. Tight contact points between all teeth. 6. An occlusal plane that is relatively flat, rather than a deep or reverse curve of Spee. When these 6 keys are present, it results in optimal intercuspation and occlusion. Deviations

1. Andrew’s six keys of
normal occlusion
Prof Dr Maher Fouda
Mansours Egypt

2. Andrew’s Six Keys to Normal
Occlusion
Lawrence Andrew, in
1972,12 outlined six
keys to normal
occlusion after
studying 120 non-
orthodontic models and
comparing them with
the best 1150 finished
orthodontic cases.
Normal occlusion

3. Andrew’s Six Keys to
Normal Occlusion
The established six keys
where not only
purposeful due to its
presence in all 120
orthodontic normals, but
also due to the fact that
in treated models, the
absence of one of the six
was able to predict
defective incomplete end
result.
Normal occlusion

4. Key I: Molar Relationship
The first of the six keys is
molar relationship.
1. The distal surface of the
distobuccal cusp of the
upper first permanent
molar occluded with the
mesial surface of the
mesiobuccal cusp of the
lower second molar .
Key I molar relation. (A) Improper molar relationship.
(B) Improved molar relationship. (C) More improved
molar relationship. (D) Proper molar relationship

5. Key I: Molar Relationship
The first of the six keys is molar
relationship.
1. It is possible for the
mesiobuccal cusp of the
upper first year molar to
occlude in the groove
between the mesial and
middle cusps of the lower
first permanent molar, while
leaving a situation
unreceptive to normal
occlusion.
Key I molar relation. (A) Improper molar relationship. (B)
Improved molar relationship. (C) More improved molar
relationship. (D) Proper molar relationship

6. Key I: Molar Relationship
The first of the six keys is molar
relationship.
1. The closer the distal
surface of the distobuccal
cusp of the upper first
permanent molar
approaches the mesial
surfaces of the mesiobuccal
cusp of the lower second
molar, the better the
opportunity for normal
occlusion.
Key I molar relation. (A) Improper molar relationship. (B)
Improved molar relationship. (C) More improved molar
relationship. (D) Proper molar relationship

7. 2. The mesiobuccal cusp of
the upper first permanent
molar fell within the groove
between the mesial and
middle cusps of the lower first
permanent molar.
3. The canines and premolars
enjoyed a cusp–embrasure
relationship buccally, and a
cusp–fossa relationship
lingually.
Key I: Molar Relationship
The first of the six keys is molar
relationship.
Key I molar relation. (A) Improper molar relationship. (B)
Improved molar relationship. (C) More improved molar
relationship. (D) Proper molar relationship

8. Key II crown
angulation
or tip: Long
axis of
crown
measured
from line 90°
to occlusal
plane
Key II: Crown
Angulation, The
Mesiodistal Tip
• The term crown
angulation refers to
angulation (or tip) of
the long axis of the
crown, not to
angulation of the long
axis of the entire
tooth.
.

9. Key II
crown
angulation
or tip:
Long axis
of crown
measured
from line
90° to
occlusal
plane
Key II: Crown
Angulation, The
Mesiodistal Tip
• • The gingival
portion of the long
axis of each crown
was distal to the
incisal portion,
varying with the
individual tooth
type.

10. Key II: Crown
Angulation, The
Mesiodistal Tip
• The long axis of the
crown for all teeth,
except molars, is judged
to be the mid-
developmental ridge,
which is the most
prominent and innermost
vertical portion of the
labial or buccal surface
of the crown.

11. Key II: Crown
Angulation, The
Mesiodistal Tip
The long axis of
the molar crown is
identified by the
dominant vertical
groove on the
buccal surface of
the crown.

12. Key II crown angulation or tip:
Long axis of crown measured
from line 90° to occlusal plane
Key II: Crown Angulation, The
Mesiodistal Tip
Crown tip is expressed
in degrees, plus or
minus. The degree of
crown tip is the angle
between the long axis
of the crown (as viewed
from the labial or
buccal surface) and a
line bearing 90° from
the occlusal plane.

13. Key II: Crown Angulation, The
Mesiodistal Tip
A ‘plus reading’ is
assigned when the
gingival portion of the
long axis of the crown
is distal to the incisal
portion and a ‘minus
reading’ when the
gingival portion of the
long axis of the crown
is mesial to the
incisal portion.

14. Normal occlusion
is dependent
upon proper
distal crown tip,
especially of the
upper anterior
teeth since they
have the longest
crowns.
Key II: Crown Angulation,
The Mesiodistal Tip

15. The degree of the tip
of incisors
determines the
amount of
mesiodistal space
they consume and,
therefore, has a
considerable effect
on posterior
occlusion as well as
anterior esthetics .
Key II: Crown Angulation,
The Mesiodistal Tip

16. Key II: Crown
Angulation, The
Mesiodistal Tip
In normal
occlusion, the
crown
angulation was
positive for all
teeth

17. Key II: Crown Angulation, The Mesiodistal
Tip according to Andrew

18. Key III: Crown Inclination
(Labiolingual or Buccolingual
Inclination)
Crown inclination refers
to the labiolingual or
buccolingual inclination
of the long axis of the
crown, not to the
inclination of the long
axis of the entire tooth .
The inclination of all the
crowns had a consistent
scheme.
Key III crown inclination is determined
by the resulting angle between a line 90°
to the occlusal plane and a line tangent
to the middle of the labial or buccal
clinical crown. (A) shows tooth with
positive crown torque and (B) shows
tooth with negative torque .

19. Key III: Crown Inclination (Labiolingual
or Buccolingual Inclination)
Anterior teeth (central and lateral
incisors): Upper and lower anterior
crown inclination was sufficient to
resist overeruption of anterior
teeth and also to allow proper
distal positioning of the contact
points of the upper teeth in their
relationship to the lower teeth,
permitting proper occlusion of the
posterior crowns.

20. Key III: Crown Inclination (Labiolingual or
Buccolingual Inclination)
A, Improperly inclined anterior crowns result in all upper contact points being mesial,
leading to improper occlusion. B, Demonstration, on an overlay, that when the anterior
crowns are properly inclined the contact points move distally, allowing for normal
occlusion.

21. Key III: Crown Inclination (Labiolingual or
Buccolingual Inclination)
Spaces resulting
from normally
occluded posterior
teeth and
insufficiently
inclined anterior
teeth are often
falsely blamed on
tooth size
descrepancy.

22. Key III: Crown Inclination (Labiolingual
or Buccolingual Inclination)
In normal
occlusion, the
crown inclination
for all teeth was
negative except
maxillary central
and lateral incisors

23. Key III: Crown Inclination
(Labiolingual or Buccolingual
Inclination)
Upper posterior teeth
(canines through
molars): A lingual crown
inclination existed in the
upper posterior crowns.
It was constant and
similar to the canines
through the second
premolars and was
slightly more
pronounced in the
molars.

24. Key III: Crown Inclination
(Labiolingual or Buccolingual
Inclination)
Lower posterior
(canines through
molars): The
lingual crown
inclination in the
lower posterior
teeth progressively
increased from the
canines through
the second molars.

25. Tip and Torque
The clinical
implication of
the tip and
torque is that
they collectively
affect the upper
anterior crowns
and total
occlusion.
Andrew’s wagon
wheel concept. (A, B)
Unbent rectangular
archwire with vertical
wires soldered at 90°,
spaced to represent
the upper central and
lateral incisors. (C–E)
As the anterior
portion of the
archwire is torqued
lingually, the vertical
wires begin to
converge until they
become the spokes
of a wheel when the
archwire is torqued
90° progressively.

26. Tip and Torque
In lingual crown
torque, for every 4˚,
there is 1˚ mesial
convergence of
central and lateral
incisor crowns, at the
gingival portion. The
ratio is approximately
4:1. Andrew described
this phenomenon as
the ‘wagon wheel
concept’
Andrew’s wagon wheel
concept. (A, B) Unbent
rectangular archwire
with vertical wires
soldered at 90°, spaced
to represent the upper
central and lateral
incisors. (C–E) As the
anterior portion of the
archwire is torqued
lingually, the vertical
wires begin to converge
until they become the
spokes of a wheel when
the archwire is torqued
90° progressively.

27. Key IV:
Rotations
• The fourth
key to normal
occlusion is
that the teeth
should be
free from
undesirable
rotations.

28. Key IV: Rotations
• The molar, as for
example, if rotated,
would occupy more
space than normal,
creating a situation
unreceptive to normal
occlusion. A rotated
incisor on the other
hand occupies less
space.

29. CLINICAL SIGNIFICANCE
Rotated tooth
• By correcting a
rotated tooth,
space can be
gained in posterior
segment as a
rotated posterior
tooth occupies
more space.
Key IV: Rotations

30. CLINICAL SIGNIFICANCE
Rotated tooth
• For correction of
an anterior tooth,
space is required
as rotated anterior
tooth occupies less
space.
Key IV: Rotations

31. Key V: Tight Contacts
• The fifth key is that the
contact points should be
tight (no spaces).
• Persons who have
genuine tooth-size
discrepancies pose
special problems, but in
the absence of such
abnormalities tight contact
should exist.

32. Key V: Tight Contacts
Without exception,
the contact points
on the
nonorthodontic
normals were tight.

33. Key VI: Occlusal Plane
• The planes of
occlusion found on
the normal models
ranged from flat to
slight curves of
Spee.
• A flat plane should
be a treatment goal
as a form of
overtreatment.
A, A deep curve of Spee results in a more confined area for
the upper teeth, creating spillage of the upper teeth
progressively mesially and distally. B, A flat plane of occlusion
is most receptive to normal occlusion. C, A reverse curve of
Spee results in excessive room for the upper teeth

34. Key VI: Occlusal Plane
• Intercuspation of teeth
is best when the plane of
occlusion is relatively
flat .
• A deep curve of Spee
results in a more
contained area for the
upper teeth, making
normal occlusion
impossible.
A deep curve of Spee results in a more confined area
for the upper teeth, creating spillage of the upper
teeth progressively mesially and distally.
A flat plane of occlusion is most receptive to normal
occlusion.

35. Key VI: Occlusal Plane
The remaining upper
teeth, anterior and
posterior to the first
premolar, are
progressively in error.
• A reverse curve of Spee
is an extreme form of
overtreatment, allowing
excessive space for each
tooth to be intercuspally
placed .
A reverse curve of Spee
results in excessive room for
the upper teeth

36. • There is a natural tendency for the
curve of Spee to deepen with time, for
the lower jaw’s growth downward and
forward sometimes is faster and
continues longer
Key VI: Occlusal Plane
than that of the upper jaw, and this
causes the lower anterior teeth, which
are confined by the upper anterior teeth
and lips, to be forced back and up,
resulting in crowded lower anterior teeth
and/or a deeper overbite and deeper
curve of Spee.
A, A deep curve of Spee results in a more
confined area for the upper teeth, creating
spillage of the upper teeth progressively
mesially and distally. B, A flat plane of
occlusion is most receptive to normal
occlusion. C, A reverse curve of Spee results in
excessive room for the upper teeth

37. References