Andrews 6 keys

Andrews’
Six Keys
of
Normal Occlusion
Andrews’
Six Keys
of
Normal Occlusion
www.indiandentalacademy.com

Introduction
 There is no of diagnostic way to measure or
accurately estimate malocclusion, nor to
decide how closely trt. has approached good
results, until we first know what normal (good)
occlusion is.

Introduction
 Six significant occlusal characteristics identified
& first reported in 1972 by Lawrence F. Andrews
“The six keys to normal occlusion”
 These six keys were found to be consistently
present in a collection of 120 models of teeth
with natural excellent occlusion (“nonorthdontic
normal” models)

Introduction
 The keys tell about the characteristic of static
occlusion
 Ronald H. Roth declared six keys to be
“Consistent with desirable function occlusal
goals, provided that the occlusal interdigitation
occurs with the mandible in centric relation”
(Five-Year Clinical Evaluation of the Andrews Straight-
Wire Appliance,1976)

These keys believed by Andrews
& others to be :
 Consistent with nature’s plan esthetically &
functionally.
 Attainable in trt. of the great majority of N
American orthodontic patients – the
estimated 90% or more who present no trt.
limiting abnormalities.

Study
 A gathering of data (1960 to 1964)
 120 nonorthodontic normal models
 With the cooperation of
Some Orthodontists & general dentists in San Diego
University of Illinois (Dr. A..G Brodie )

Teeth of selected models
 Never had orthodontic treatment
 Were straight and pleasing in appearance
 Had a bite which looked generally correct
 Would not benefit from orthodontic treatment.

The crowns of Sample
 Angle's molar cusp groove concept was
validated still.
 The molar relationship in these healthy
normal models exhibited two qualities when
viewed buccally
 Angulation (MD tip) and inclination
(buccolingual inclination) showed predictable
natures as related to individual tooth types.www.indiandentalacademy.com

The crowns of Sample
 Teeth had no rotations.
 No spaces between teeth.
 The occlusal plane was not identical in all
models but with a limited a range of
variation.
Tentative conclusions were reached,
and six characteristics were formulated in
general terms.www.indiandentalacademy.com

The six keys
 The six keys to normal occlusion contribute
individually and collectively to the total scheme
of occlusion and, therefore, are viewed as
essential to successful orthodontic treatment.
 Lack of even one of the six is a defect
predictive of an incomplete end result in treated
models.

Key I. Interarch relationships
The nonorthodontic normal models consistently
demonstrated that
 The distal surface of the distal marginal ridge
of the upper first permanent molar contacts
and occludes with the mesial surface of the
mesial marginal ridge of the lower second
molar.

 The mesio-buccal cusp of the upper first
permanent molar falls within the groove
between the mesial and middle cusps of the
lower first permanent molar.
 The mesio-lingual cusp of the upper first
molar seats in the central fossa of the lower
first molar.

 The premolars enjoy a cusp-embrasure
relationship buccally, and a cusp fossa
relationship lingually.
 Max. canine has a cusp-embrasure
relationship with mand. canine & 1st
PM. The
cusp tip is slightly mesial to embrasure
 Max. incisors overlap mand. Incisors &
midlines of arches match

Key II. Crown angulation (tip)
Facial axis of the clinical crown (FACC)
 Best viewed from the labial or buccal perspective
 For all teeth except molars, is located at the mid-
developmental ridge that runs vertically and is the most
prominent portion in the central area of the labial or
buccal surface.
 The facial axis of molar crowns is identified by the
dominant vertical groove on the buccal surface.

 Viewed from mesial or distal perspective, the FACC
is represented by a line that is parallel to the mid-
developmental ridge (or with molars, the dominant
groove), and tangent to the middle of the clinical
crown on the labial or buccal surface

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.
 As orthodontists, we work specifically with the
crowns of teeth and, therefore, crowns should
be our communication base or referent.

Crown Angulation or Crown tip
 The degree of crown tip is the angle formed by
the FACC and a line perpendicular to the
occlusal plane.
 A “+ reading" when the gingival portion of the
FACC is distal to the incisal portion.
 A “- reading" when the gingival portion of the
FACC is mesial to the incisal portion.www.indiandentalacademy.com

Each normal model had a distal inclination of the
gingival portion of each crown, It varied with each
tooth type, but within each type the tip pattern was
consistent from individual to individual.

 Normal occlusion is dependent upon proper
distal crown tip, especially of the upper ant.
teeth ( longest crowns).
 Degree of tip of incisors, determines the
amount of MD space they consume & has a
considerable effect on post.
occlusion as well as ant.
esthetics.

Key III. Crown inclination (torque)
 Crown inclination angle formed by a line which
bears 90°to the occlusal plane and FACC (as
viewed from the mesial or distal).
 A + reading is given if the gingival portion of the
tangent line (or of the crown) is lingual to the
incisal portion,
 A - reading is recorded when the gingival portion
of the tangent line (or of the crown) is labial to
the incisal portionwww.indiandentalacademy.com

ANTERIOR CROWN INCLINATION.
In upper incisors + crown inclination.
In lower incisors - crown inclination
The average inter-incisal crown
angle - 174°.

 Properly inclined anterior crowns contribute to
normal overbite and posterior occlusion,
 when too straight-up and -down they lose
their functional harmony and overeruption
results.

 If the inclination of the anterior crowns is
not sufficient, space, in treated cases, is
often incorrectly blamed on tooth size
discrepancy.

POSTERIOR CROWN INCLINATION— UPPER.
 A minus crown inclination for each crown from the
U canine through the U-2nd
PM.
 A slightly more negative crown inclination existed
in the U-1st
& 2nd
molars

POSTERIOR CROWN INCLINATION— LOWER.
A progressively greater "minus" crown
inclination existed from the lower canines
through the lower second molars

Tip & torque
 As the anterior portion of an upper rectangular
arch wire is lingually torqued, a proportional
amount of mesial tip of the anterior crowns
occurs.
 The ratio is approximately 4:1. For every 4° of
lingual crown torque, there is 1 ° of mesial
convergence of the gingival portion of the
central and lateral crowns.

Tip & torque
Wagon wheel

Key IV. Rotations
 Teeth should be free of undesirable rotations.
Rotated molar, would occupy
more space than normal,
creating a situation unreceptive
to normal occlusion.

Key V. Tight contacts
 Contact points should be tight (no spaces).
 Persons who have genuine tooth-size
discrepancies pose special problems.
 Serious tooth-size discrepancies should be
corrected with jackets or crowns, so the
orthodontist will not have to close spaces at the
expense of good occlusion.www.indiandentalacademy.com

Key VI. Occlusal plane (curve of spee)
 Depth of curve of spee ranges from flat plane
to slight concave surface (0- 2.5 mm)
 A flat plane should be a treatment goal as a
form of over treatment.
 There is a natural tendency for the curve of
Spee to deepen with time.

 L jaw's growth downward and forward
sometimes is faster and continues longer than
that of the U jaw.
 This causes the L ant. teeth,, to be forced back
and up,  crowded lower anterior teeth and/or a
deeper overbite and deeper curve of Spee.

 At the molar end of the lower dentition, the 3rd
molars are pushing forward, even after growth has
stopped, creating essentially the same results.
 If the lower anterior teeth can be held until after
growth has stopped and the 3rd
molar threat has
been eliminated by eruption or extraction, All
should remain stable, assuming that treatment has
otherwise been proper.

 Intercuspation of teeth is best when the plane of
occlusion is relatively flat
There is a tendency for the c.o.s to deepen after trt.
 Trt.. the plane of occlusion until it is somewhat flat
or reverse to allow for this tendency.

 A deep curve of Spee results in a more contained
area for the U teeth, making normal occlusion
impossible.
 Only the U- 1st
PM is properly intercuspally placed.
 The remaining upper teeth, ant. & post. to the 1st
PM, are progressively in error.

Key VI. Occlusal plane (curve
of spee)
 A reverse c.o.s is an extreme form of over
treatment, allowing excessive space for each
tooth to be intercuspally placed

Key VII. Correct tooth size
 Bennett & McLaughlin
 If Andrews’ non orthodontic models have
shown tooth size discrepancy, it would have
resulted in either spacing or crowding in either
of arches, until compensated by tip & torque in
ant. segment.
 Prior to trt. by Bolton analysiswww.indiandentalacademy.com

Key VII. Correct tooth size
 Discrepancy may exist prior to trt. but frequently
not noticed until the finishing stage
 The potential need for interproximal reduction to
↓ tooth size in one arch or restorative procedure
to ↑tooth size in opp. arch should be discussed
with patient/parents before trt.www.indiandentalacademy.com

Treated occlusion
 1150 treated cases of nation's most skilled
orthodontists (on display at national meetings)
were studied, from 1965 to 1971.
 To learn to what degree the 6 keys were
present
 Whether the absence of any one permitted
prediction of other error factors, such as the
existence of spaces or of poor posterior
occlusal relations.

Treated occlusion
 Key I :- In 932 (80 % ) cases distal surface of
the U-6 did not occlude with the mesial surface of
the mesiobuccal cusp of the L-7.
 Key II :- 91% of cases (if angulation of FACC varies
more than ± 2°from optimum for that tooth type)
– improper MD tip faulty contacts, or post. occ.,
or spaces. U-2 ,U-3, U-7 frequently undertipped.www.indiandentalacademy.com

Treated occlusion
 Key III :- 78% of cases (if inclination of FACC varies
more than ±2°from optimum for that tooth type)
Insuff. torqued ant.  improper post. occ., or
spaces, or ant. overbite
Upper pos. also insuff. torqued
 Key IV :-67% of cases (if line connecting contact
point of crown varies >± 2°)
U-6 & teeth adjacent to extn. frequently rotated.www.indiandentalacademy.com

Treated occlusion
 Key V :- 43% of cases
Extn. Site space due to insuff. closure & spaces
due to improper ant. torque most common
 Key VI :- 56% of cases excessive curve of
spee(> 2.5 mm)

Treated occlusion
 A comparison of the best in treatment results
(the 1,150 treated cases) and the best in
nature (the 120 nonorthodontic normals)
revealed differences
 This could provide significant insight on how
we could improve ourselves orthodontically.

Conclusion
 The 120 nonorthodontic normal models
differed in some respects, but all shared the
six characteristics.
 Compromise treatment is acceptable when
patient cooperation or genetics demands it,
but should not be acceptable when treatment
limitations do not exist.www.indiandentalacademy.com

 When possible, six keys should be our measure
of the static relationship of successful orthodontic
treatment.
 Achieving the final desired occlusion is the
purpose of the six keys to normal occlusion.

Andrews 6 keys

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Andrews 6 keys