Andrews six keys of occlusion / certified fixed orthodontics courses in india

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INTRODUCTIONINTRODUCTION
 For over 100 years orthodontists have always followed a
classic guideline given by Angle in diagnosis - The mesiobuccal
cusp of the upper first permanent molar should rest on
mesiobuccal groove of the lower first permanent molar.
Even though it was genuine & accurate, how sufficient it was? It
has always remained a question over these years. Even though
classification have been made about malocclusion, no one gave
the criteria for ideal occlusion.

In the early 60s Lawrence.F.Andrews noted that too many
post treated models had obvious inadequacies, despite the
acceptable molar relationship as described by Angle. Recognizing
conditions in treated cases that were obviously less than ideal was
not difficult, but neither it was sufficient.
Occlusal analysis is a vital part of diagnosis and a
necessary procedure in assessing treatment progress or results. The lack
of a specific but uncomplicated procedure for judging occlusion has long
been a handicap . There was no diagnostic way to measure or to
accurately estimate malocclusion , nor to decide how closely treatment has
approached good end results , until we have first what good occlusion is ?
INTRODUCTION…

A proposed solution was reported by Andrews in 1972 in which six
significant characteristics, which were found to be consistently present
in collection of 120 models of teeth with naturally excellent occlusion in
his research findings.
The evidence showed that only few post treatment results met the
six key standards. So a quarter century of research done by Andrew,
devoted to naturally optimal & treated occlusions has yielded not only
the six keys, but also several principles fundamental to the concept of
fully programmed appliance.

Principles:
They are:-
Each normal tooth type is similar in shape from one individual to
another.
The size of normal crowns within a dentition has no effect on their
optimal angulation, inclination or prominence of their facial surface.
Most individuals have normal teeth regardless of whether their
occlusion is flawed or optimal.
Jaws must be normal & correctly related to permit the teeth to be
correctly positioned & related.
Dentitions with normal teeth and in jaws that are or can be correctly
related can be brought to optimal occlusal standards.

IMPORTRANCE OF THE SIX KEYS.
1. They are a complete set of indicators of optimal occlusion .
2. They can be judged from tangible landmarks .
3. They can be adjudged from the facial and occlusal surfaces of the
crown , reducing the need for a lingual view .
4. Proper Diagnosis and treatment planning .
5. Consistent with nature’s plan aesthetically and functionally.

Key 1 : Interarch relationship
This key consists of seven points.
1. The mesiobuccal cusp of permanent
maxillary first molar occludes in the
groove between the mesial and middle
buccal cusps of the permanent
mandibular first molar as explained by
Angle.
2. The distal marginal ridge of the
maxillary first molar occludes with the
mesial marginal ridge of the mandibular
second molar.
3. The mesiolingual cusp of the maxillary
first molar occludes in the central fossa of
the mandibular first molar.
SIX KEYS TO OPTIMAL OCCLUSION

4. The buccal cusps of the maxillary
premolars have a cusp-embrasure
relationship with the mandibular
premolars.
5. The lingual cusps of the maxillary
premolars have a cusp-fossa
relationship with the mandibular
premolars.
6. The maxillary canine has a cusp-
embrasure relationship with the
mandibular canine and first premolar.
The tip of its cusp is slightly mesial
to the embrasure.
7. The maxillary incisors overlap the
mandibular incisors, and the midlines
of the arches match

MOLAR RELATIONSHIP
When Angle described about molar relationship he stated
that The mesiobuccal cusp of the upper first permanent molar
should rest on mesiobuccal groove of the lower first permanent
molar. But non orthodontic models consistently demonstrated
that “Distal surface of the distobuccal cusp of the upper
first molar occluded with the mesial surface of the
mesiobuccal cusp of the lower second molar”.

As seen in this diagram,
closer the distobuccal cusp of
the upper first molar
occluded with mesial surface
of the mesiobuccal cusp of
the lower second molar the
better the opportunity for
normal occlusion .

Key 2 : CROWN ANGULATION
(the Mesiodistal tip of the crown)
The gingival portion of the long axes of all crowns are more distal
than the incisal portion
crown tip is expressed in degrees, plus or minus.

The degree of crown tip is the angle
between the long axis of the crown &
a line drawn 90 degrees from occlusal
plane.
“Positive”- when gingival portion is
distal to incisal portion.
“Negative”- when gingival portion is
mesial to incisal portion.
All the non orthodontic models had
a distal inclination of the gingival
portion of the crown. ie Positive
reading

Key 3 : CROWN INCLINATION
Labiolingual or Buccolingual inclination.
crown inclination is determined from the mesial or distal perspective.
Angle formed by a line 90 degree to
occlusal plane & a line tangent to bracket
site.
“Positive” when gingival portion of
tangent line is lingual.
“Negative” when gingival portion of
tangent line is labial or buccal.

In upper incisors ,the
gingival portion of the crowns
labial surface is lingual to the
incisal portion.
Most maxillary incisors
have a positive inclination and
are more positive than
laterals.
In all other crowns ,
including lower incisors , the
gingival portion of the labial or
buccal surface is labial or
buccal to the incisal or
occlusal portion.

The pattern of upper posterior
crown inclination was consistent
in non orthodontic normal
models
. A minus crown inclination
existed in each crown from
upper canine through the upper
second premolar.
A slightly more negative crown
inclination existed in upper 1st &
2nd molar.
In the lower teeth, a
progressively greater minus
crown inclination existed from
lower canines through the lower
second molars.

When the upper anterior crown are
insufficiently inclined, upper posterior
crowns are forward of their normal
position.
But when the anterior crowns are
properly inclined, one can see how the
posterior teeth are encouraged into
their normal positions.

Key 4 : ROTATIONS
The fourth key to normal occlusion
is that the teeth should be free of
rotations,
Because if molars rotated, would
occupy more space than normal. So
it will create a situation unreceptive to
normal occlusion.
If incisors rotated less space.

Key 5 : TIGHT CONTACTS
Contact points should abut
unless there is a tooth size
discrepancy in mesiodistal
crown diameter

Key 6 : OCCLUSAL PLANE
The planes of occlusion found in
the non orthodontic models ranged
from flat to slight curve of spee.
Andrew believes that a flat plane
should be treatment goal as a form
of over treatment since there is
natural tendency for the curve of
spee to deepen with time.

Bolton stated that a correct maxillary and mandibular mesiodistal tooth
size relationship is important to the achievement of proper occlusal
interdigitation in the finishing stages of orthodontic treatment. He
computed the specific ratios of the mesiodistal widths that must exist
between maxillary and mandibular teeth from both canine-canine and first
molar-first molar so as to obtain optimum occlusion. He concluded that
without a proper mesiodistal tooth size ratio between maxillary and
mandibular teeth, proper coordination of arches would be difficult.
Key7: Boltons analysis

Before we go further, we will see some of the landmarks given by
Andrew which would help more to understand further details
1. Andrews Plane
The surface or a plane on which
the mid transverse plane of every
crown in an arch will fall when
teeth are optimally positioned.
2. Clinical crown
The amount of visible crown in the late
mixed dentition with gingiva that is
healthy and not recessed. Orban defined
clinical crown as anatomical crown minus
1.8 mm. www.indiandentalacademy.com

3. Facial Axis of the Clinical Crown
[FACC]
For all the teeth except molars, the most
prominent portion of the central lobe on each
crown’s facial surface. For molars, buccal
groove that separates the 2 large facial
cusps.
4. FA Point
The point on the facial surface that
separates the gingival half of the
clinical crown from the occlusal
half www.indiandentalacademy.com

5. Midsagittal Plane
An imaginary line that separates the crown mesio-distally at the facial
axis of the clinical crown (FACC).
6. Mid Transverse Plane
An imaginary line that separates occlusal half of the crown from the
gingival half of the crown.

MEASUREMENTS
After determining the 6 keys of occlusions Andrew made certain
measurements in the non orthodontic models which helped in the
development of a fully programmed appliance. The purpose was to learn
the extent to which positions & in certain ways shape were constant within
each tooth type & how relative size was consistent within an arch. The
measurement were made with Protractor with adjustable readout arms,
Boley gauge & template of circles.

a. Bracket area
b. Vertical contour
c. Angulation
d. Inclination
e. Maxillary molar
Offset
f. Horizontal contour
g. Facial prominence
h. Curve of spee
The measurements made are:

METHODS OF MEASUREMENT.
A. Bracket area.
On the facial aspect of each crown the height
and the width of potential bracket area were
measured with Boley gauge
The smallest crowns of each normal tooth
determined the occlusogingival height and
mesiodistal width limits for each bracket base.

B. Vertical crown contour
The vertical contour of the facial surface
of each crown in the bracket area was
ascertained by superimposing a series of
arcs from the template of circles .
This information was used to design the
vertical anatomy of each bracket base.

C.Crown angulation.
Arch shaped Plastic template was positioned over
occlusal surfaces
The base of the protractor was placed on plastic
template parallel to a line that would connect the
contact points .
The protractors read out arm was adjusted to
parallel the crowns FACC
The angulation of the crown was read, from where
the center line of the read out arm fell on the
protractors scale.

Average maxillary Crown angulation.
The measurements were used to determine how much the slots
needed to be angled within each bracket.

4. Crown inclination.
The plastic templates and protractor was
similarly used like in the case of angulation.
The protractors readout arm was adjusted to
be parallel and tangent to the FACC at the FA
point and the inclination of the crown was read
on the protractors scale.

The crown inclination when incorporated into each bracket
for each tooth type would reduce or eliminate the need for the
third order bends for attaining the correct inclination of the
teeth .
Average maxillary crown inclination.

5.Offset of maxillary molars.
A straight section of wire was placed with the
trimmed occlusal surface , connecting the facial cusps.
The angle between the wire and the embrasure line
is the offset angle.
This measurement is used as design data to reduce
or eliminate molar offset bends that are needed with
less sophisticated edge wise brackets.
No offset was needed for mandibular molar
because the middle & mesiobuccal cusps are
equal in prominence.

6.Horizontal crown contour.
The horizontal radius of each cusp was
measured at the junction of the crowns facial
surface .
These measurements revealed the constancy
of the horizontal contour of each tooth types
facial surface .
Horizontal crown contour data were used to
establish the horizontal contour for each bracket
base .

7.Crown facial prominence.
The distance from the embrasure line to each
crowns most prominent facial point was
measured
These measurements established the relative
constancy of crown prominence with in a arch
This information was applied inversely for
designing bracket prominence.

The average findings for the maxillary teeth are

8 .Depth of curve of spee.
The template was placed over the
occlusal surface of mandibular arch .
Measured in millimeters , the
distance from the side of the template
facing the teeth to the buccal cusp tip
farthest from it.
The depth of curve is important to
bracket design because the slots of a
fully programmed appliance must
collectively be on a surface that is
nearly parallel to the occlusal surface
of an arch
The curve of spee ranged from flat to 2.5 mm

This study revealed consistencies in the position,
morphology & relative facial prominence for the crown of each tooth type
within an arch except for incisor inclination. The differences in the incisor
inclination were attributed to interjaw disharmony. Thus special
considerations must be given in the bracket design to correlate the
inclination of incisors with interjaw relationship.
Conclusion of the study

After making the measurements, Andrew compared the 120 non
orthodontic models with the treated 1150 models from the post treatment
dental casts displayed at American association of orthodontics, the
Tweed foundation and the Angle society.
ie best in nature Vs best in treatment.
Comparison were made particularly in relation to the 6 keys of optimal
occlusion:-
NATURALLY OPTIMAL VERSUS TREATED OCCLUSION

KEY I – Inter-arch relationship
The traditional method for measuring only the cusp groove relationship of
the first molars may misinterpret the true nature of the interarch
relationship.
Eg :
1. Premolars and canines that are 3mm class II but first molars are class I
2. Second deciduous molars cause a temporary end to end relationship of
the permanent molars
 The interarch relationship of a side can be correctly interpreted only
when the effects of deciduous molars and incorrect angulations are
compensated .

The interarch relationship for each
molar,premolar,and canine is measured
and totaled and averaged.
In 80% of the treated models, Distal
marginal ridge of maxillary molar I molar
did not occlude with mesial marginal ridge
of mandibular II molar.
Premolars and canines did not have a
cusp-embrasure relationships.
Whereas in the non orthodontic models,
that relationship was optimal.

KEY II – Angulation
 If the angulation of the FACC varies more than plus or minus 20
from the
optimal it is considered incorrect.
 In 91% of the treated models, The crowns had one or more teeth whose
angulation differed from those of optimal sample.
 The maxillary lateral incisor and canine angulation were frequently
negative ,in contrast to positive in the optimal sample.
 Maxillary molar angulation often was negative or insufficiently positive in
nonextraction treatment and excessively positive in extraction treatment.

KEY III – Inclination
 If the inclination of the FACC varies more than plus or minus 2o
from
the optimal it is incorrect.
Inter incisal FACC
In post treatment casts– more than 180 degrees in 78%
In optimal sample.- Less than 180 degrees in 81.5%
Maxillary posterior crown inclination was not always negative.

KEY IV – Rotation
If a line connecting the contact
points of a crown varies more than
2o
from parallel to a line
representing the arch form , the
tooth is rotated.
Rotations were evident in 67% of
post treated cases.
Most of the rotated teeth were
those that had required translation
during treatment. www.indiandentalacademy.com

KEY V – Tight contacts
 Barring missing teeth or tooth size discrepancy , interdental space
indicates incorrect maxillary incisor angulation,or incisor inclination or
mesiodistal faciolingual position of a tooth.
 Spaces were seen in 43% of post treatment casts.

KEY VI – Curve of spee
The curve of spee is incorrect if it is less than
0 or more than 2.5mm.
More than 2.5 mm in 56% of post treatment
casts..

NORMALITY – ABNORMALITY & OPTIMALITY
In this point of time Andrew spoke about what exactly
Normality, Abnormality & Optimality means.
Abnormality : Individuals with abnormal jaws & teeth fall into
this category. For example cleft palate, peg laterals, etc. treatment
for this group always needs a multi disciplinary approach. ie only
orthodontists can’t solve the problem.
Naturally Optimal : Group with normal teeth & jaws who require
no orthodontic treatment. 120 non orthodontic models can be
included in this group.

So both the naturally optimal & the normal malocclusion fall
under a same category where they are differentiated only through 6
keys of optimal occlusion.
Normal malocclusion: Group with malocclusion which
can be treated to Optimal standards.

IMPLEMENTING THE SIX KEYS
Implementing key I
(Interarch relationships)
A deficient distal marginal ridge occlusion of the
maxillary first molar that is otherwise class I is due
to the abandonment of the molars that have been
tipped distally
The problem must be solved by correcting the
final molar angulation before completing the
treatment.
Head gear can promote tipping rather than
translation of maxillary molars.

Corrected by prescribing brackets with the face
bow tube gingival to the arch wire slot
With high –pull head gear , angulated slots and wire
bends redirect the extra oral force closer to the tooth's
center of resistance to encourage translation than
tipping.
In seating of the mesiolingual cusps of the maxillary molars in
the central fossa of the mandibular molars .it requires maxillary
molars mesiolingual cusps to be more occlusally prominent.

Cusps of teeth that encroach in the freeway space will generally not
correct themselves and must be dealt with by orthodontic treatment ,
equilibration , reconstruction or surgery.
Completing active treatment with molars slightly intruded is preferable to
extrusion , because usually self correction will occur.
A concave curve installed in a maxillary rectangular arch wire , or a
convex curve installed in mandibular rectangular arch wire will deal with
occlusogingival tooth displacement.
The key I standards for molars are applied to all permanent dentition.

IMPLEMENTING KEY II (Crown angulation)
 The extent of angulation varies according to tooth type.
 Using the FACC is easier and more accurate, this land mark is visible
,tangible and close to the bracket for the determination of the angulation.
 To achieve optimal results it may be necessary to overcorrect before
completing treatment, for eg :when translation is required ,it is advisable to
exaggerate angulation in anticipation of the rebound that will bring about the
correct position.
 Open extraction sites which were frequently present in the sample of post
treatment dental casts can be avoided by completely closing the space and
overcorrecting the angulation of teeth requiring translation before completing
treatment, rebound would be towards the space .

 When the treatment goal includes functional occlusion , the merit of
correct canine angulation becomes evident.
 In the protrusive excursion ,the distal incline of the maxillary canine
should ride on the mesial incline of the mandibular first premolars buccal
cusp.

IMPLEMENTING KEY III. (Crown inclination)
For optimal static and functional occlusion all crowns require proper
inclination.
In cephalometrical analysis the long axis of the central incisor are
used for measuring the inclination; however for bracket selection the
inclination of the incisors facial axes ,relative to the occlusal plane is the
more important measurement.
The average angular difference between the FACC and the long axis
is helpful information for wire bending and wire forming and bracket
selection.
Inter incisal crown angle and inter incisal
tooth angle

IMPLEMENTING KEY IV.(Rotations)
Routinely including permanent second molar in treatment ensures that
their positioning is not left to chance.
Naturally rotated teeth are usually dealt with early in treatment, small
diameter wires are deflected , when engaged in the bracket, to the extent
of the rotation for each individual tooth
Rotation springs are also used when the wire is too stiff to flex.
Often teeth requiring translation are found to be rotated at the conclusion
of the treatment.
To ensure an ultimately correct position for a translated tooth, treatment
must include over rotation to compensate for post treatment rebound.www.indiandentalacademy.com

IMPEMENTING KEY V.(Tight contacts)
Post treatment spaces signify incomplete treatment ,crown – size
discrepancy or treatment limitation.
Except for teeth with mesiodistal size discrepancy , adjacent teeth
should touch at the contact point.
Closing the mesio distal discrepancy by orthodontist may compromise
the occlusion
Crowns that are large are no problem they can be made narrower
If small crowns generally enlarging with composites or jacket crown is
the best solution to the problem

IMPLEMENTING KEY VI.(Curve of spee)
A flat curve of spee fully bares the occlusal surfaces of the lower
teeth, providing to the upper teeth the whole surfaces and the largest
total area to which to occlude
A deep curve of spee presents a more confined area with which to
occlude.

Successful orthodontic treatment involves many disciplines, not all of
which are always within our control. Compromise treatment is
acceptable when patient cooperation or genetics demands it.
Compromise treatment should not be acceptable when treatment
limitations do not exist. In that nature's non-orthodontic normal
models provide such a beautiful and consistent guideline, it seems
that we should, when possible, let these guidelines be our measure
of the static relationship of successful orthodontic treatment.
Achieving the final desired occlusion is the purpose of attending to
the six keys to normal occlusion.
CONCLUSION

Andrews six keys of occlusion / certified fixed orthodontics courses in india

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