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An index for assessing tooth shape
deviations as applied to the
mandibular incisors- Peck and Peck
Harvey Peck and Sheldon Peck
AJO-DO 1972
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Tooth shape (mesiodistal and faciolingual
dimensions) is a determining factor in thepresence and absence of lower incisor crowding.
These new findings have stimulated this presenteffort.
Purpose : The scientific basis and the clinical
application of a new method for detecting and
evaluating tooth shape deviations of the mandibular
incisors.
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Any consideration of tooth dimensions must to
some degree involve odontometry.
ODONTOMETRYScience of measuring the size and
proportion of teeth.
MESIODISTAL For incisors, easily
obtainable from plaster
casts.
FACIOLINGUAL Reported in the literature
far less often than MD
dimension.
Primary sources
Skeletal material &extracted teeth.
MoorreesIt cannot be ascertained whether these teeth have
erupted sufficiently to make the greatest facio-lingual dimension
measurable [on plaster casts]
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Each diagnostic analysis utilizing tooth size
data is designed to serve at least one of three
functions:
1. Prediction of unerupted tooth size.
2.Assessment of tooth sizearch size
compatibility within the same arch.
3.Assessment of tooth size compatibilitybetween the two arches.
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It is worth noting that all of these orthodonticdiagnostic procedures require only MD tooth
measurements in their construction.
No currently used clinical analysis employs or
even takes into consideration the FL tooth
dimension.
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We now know that both MD and FL
dimensions appear to be related to incisoralignment. Therefore, an index incorporating
both dimensions would seem ideally suited for
orthodontic tooth size analysis, at least of the
lower incisors.
The index proposed in this article for clinical
orthodontics utilizes an MD/FL ratio.
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INDEX= Mesiodistal (MD) crown diameter in
mm X 100Faciolingual (FL) crown diameter in
mm
In this article the use of the MD/FL index as a
numerical expression of crown shape as viewed
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Material and Method
The mandibular incisors of two groups of young femaleCaucasian adults from the Northeastern region of this
country were studied.
Group 1 -group with perfect mandibular incisoralignment
Group 2-control population group
Age
17 to 27 years
European ancestory
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The group with perfect mandibular incisor alignment consisted
of forty-five subjects selected from a dental survey of several
hundred. Selections were based on the following criteria:
1. Complete mandibular dentition (excluding third molars).
2. No orthodontic treatment received.
3. Approximal contact present among the mandibular incisors.
4. The absence of overlapping in the mandibular incisors.
5. Minimal rotational deviation from the ideal arch form in the
mandibular incisors.
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For each subject in both groups, the maximum mesiodistal (MD)
crown diameter and the maximum faciolingual (FL) crown
diameter for each mandibular incisor tooth were measured
directly in the mouth.
Helios dial caliper with 0.05 mm readout.
The maximum MD diameter was usually found at or near the
incisal edge.
To record the maximum FL diameter, however, the caliper tips
had to be placed subgingivallyin most cases.
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For the statistical analysis of the data, the right and left teeth of the
same category (central incisors, lateral incisors) were pooled within
each of the two groups of subjects.
For the perfect alignment group, the number of teeth (N) equaled 90
(45 right + 45 left) for the central and lateral incisors each.
For the control population group, this number equaled 130 (70 right
+ 60 left), rather than 140, because of lack of data for ten left
incisors in each tooth category.
The means and standard deviations for the MD/FL index were
computed. The differences between the means were evaluated
statistically.
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FINDINGS The mean values of the MD/FL index for two groups of
females
a group with perfect mandibular incisoralignment and a control population groupare presented
in Table with supplementary statistical data.
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DISCUSSION
These findings indicate that well-aligned mandibular central
and lateral incisors possess remarkably distinctive crown shape,
as expressed by the MD/FL index.
Since the experimental sample was selected on the basis of
exceptionally good lower incisor alignment, a close association
between the absence of incisor crowding and certain tooth
shape characteristics becomes evident.
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Lower incisors apparently conducive to good
alignment have MD/FL indices significantly lower
than the population averages for the same teeth.
In fact, we would expect any lower arch possessing
central incisors with an MD/FL index of less than
or equal to 88.4 and lateral incisors with an MD/FL
index of less than or equal to 90.4 to have excellent
incisor alignment.
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It is also expected that a similar relationship between
incisor shape and incisor position exists.
MD/FL indices higher than the "perfect alignment" mean
values (for the respective mandibular incisors) should be
characteristic of crowded incisors.
Logically, the higher the index, the greater the tooth shape
deviation and the greater the likelihood and degree of
associated incisor crowding.
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Garn, Lewis, and Kerewskyhave reported sex differences in tooth
shape throughout the dentition.
Estimates of the mandibular incisor MD/FL indices for males
and females which was constructed from their data and from
odontometric data of others generally indicate lower MD/FL
indices for males than for females of the same population.
This difference, however, does not appear marked, roughly
averaging 2% of the MD/FL index value for both central andlateral incisors. Therefore, on the basis of available information,
we may conclude that male-female differences in the MD/FL
index are not significant clinically.
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Comment on the possible mechanisms responsible for the
relationship between mandibular incisor shape and the presence
and absence of crowding can only be conjectural at this stage.
The lower incisor crown, as viewed incisally, resembles a
diamond-shaped kite.
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As this difference between the MD width and the FL length increases, the
MD/FL index decreases, and the mandibular incisor crown form appears
more characteristically "kite shaped." Perhaps the "kite-shaped" patternrepresented by a low MD/FL index (less than 90) confers upon the incisor
crown and root anatomy.
The relatively narrowed MD diameter characteristic of well-aligned
mandibular incisors obviously contributes less tooth substance to
mandibular arch length.
This factor, coupled with the chance that a more "kite-shaped" incisor
would tend to have "flatter," less acute mesial and distal surfaces, less
susceptible mechanically to contact slippage, may account in part for the
incisor shape
alignment relationship.
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Clinical Application
The observed relationship between mandibular incisor shape and the presence
and absence of mandibular incisor crowding had significant clinical relevance.
The MD/FL index provides an effective clinical method for diagnosing tooth
shape deviations which influence and contribute to mandibular incisor
crowding.
This data helps in determining whether a lower incisor is favorably or
unfavorably shaped relative to good alignment.
The following ranges are employed as clinical guidelines for the maximum limit
of desirable MD/FL index values for the lower incisors:
Mandibular central incisor --- 88-92
Mandibular lateral incisor --- 90-95
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Clinical application
Lower incisors within or below these ranges are considered
favorably shaped. Any lower incisor with an MD/FL index abovethese ranges, however, is considered to have a crown shape
deviation which may influence or contribute to the crowding
phenomenon.
However, an MD/FL index in excess of 100 for any of the lowerincisors represents a severe shape deviation, characteristic of
existing or potential tooth irregularity.
Patients whose mandibular incisors have MD/FL indices above the
desired ranges may well be candidates for the removal of some
mesial and/or distal tooth substance in conjunction with orthodontic
therapy "reproximation"
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CLINICAL METHODS
The mesiodistal (MD) and faciolingual (FL) crown
diameters of the mandibular incisor teeth are measured
directly in the mouth.
The maximum MD diameter is usually located at or near
the incisal edge, while the maximum FL diameter is found
almost always beneath the gingival margin.
We take the lower incisor measurements in a sequence,
beginning with the four MD measurements, right lateralincisor to left lateral incisor, followed by the four FL
measurements, right lateral incisor to left lateral incisor.
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The MD and FL crown measurements are
recorded in an appropriate table or grid :
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The next step is to compute the MD/FL indices of
the four teeth measured.
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The MD/FL index in clinical diagnosis
DIAGNOSTIC CASE 1
All four lower incisors of this patient show extreme tooth shape
deviations.
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As part of this patient's orthodontic treatment (which in this
case calls for premolar extractions), reproximation of the four
mandibular incisors is mandatory. Otherwise, re-crowding ofthe lower anterior teeth will surely follow retention.
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The lateral incisors are so severely deviated that reproximation,
limited by the thickness of the mesial and distal enamel, can
only lessen the deviations rather than eliminate them
completely.
For the central incisors, however, we may expect that
reproximation will yield favorable MD/FL indices.
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With tooth shape deviations of the intensity observed in these
incisors, we would expect a total of 2 to 3 mm. of mesiodistal
enamel to be removed by reproximation.
A loss of tooth substance of this magnitude may upset the
maxillary to mandibular anterior tooth size ratio. Therefore,
selective reproximation of the maxillary incisors may also be
indicated to maintain a harmonious anterior intermaxillary
relationship.
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DIAGNOSTIC CASE 2
This is a case of bimaxillary crowding requiring orthodontic
therapy with premolar extractions.
The lower incisors appear grossly irregular. However, the
MD/FL indices of all four incisors are essentially favorable.
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The central incisors, with indices of 88 and 86, areexceptionally well shaped, while the lateral incisors,with indices of 96 and 94, average out at the high end
of our acceptable range.
.Lower incisor reproximation is not indicated.
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CASE 1 CASE 2
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DIAGNOSTIC CASE 3 In this case we observe mild irregularity of the central incisors only.
Inspection of the MD/FL indices reveals that the lower lateral incisors
are quite favorably shaped, while the shape of both lower centralincisors is slightly deviant.
This is a circumstance in which slight reproximation of only the
central incisors is indicated as part of any orthodontic treatment
planned for the lower arch.
S
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SummaryThe substantial evidence that lower incisor shape has significant
bearing on lower incisor alignment may well affect many areas of
orthodontic practice.
the introduction of a tooth shape index for use in clinical
orthodontics opens up new channels of communication.
The observed relationship between lower incisor shape andalignment may alter some present concepts of retention.
Perhaps the most worrisome area for the orthodontist during theretention phase of treatment is the lower incisor segment of the
dentition. Over the years this has led to wide acceptance of
"prolonged retention" or "indefinite retention" for these teeth.
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In orthodontic cases requiring premolar extractions because of
major tooth sizearch size discrepancies (such as
malocclusions of the Class I bimaxillary crowding type), post-
retention lower incisor crowding is often observed, even in the
presence of residual extraction space. This is not idiopathic or
indeterminable but is, rather, a logical consequence.
In these cases it is usually clear that there is a generalized
excess in the mesiodistal dimension of all the teeth. Although
premolar extractions nicely eliminate the arch length
discrepancy, the crown shape of the remaining teeth is still
exaggerated.
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The main conclusions drawn from this study are as follows:
1. A substantial relationship exists between mandibular incisor
shape and the presence and absence of mandibular incisor
crowding.
2. Well-aligned mandibular central and lateral incisors have a
remarkably distinctive crown shape, as expressed by the MD/FL
index.
3. Well-aligned mandibular incisors have MD/FL indices
significantly lower than those-of crowded incisors.
4. Male-female differences in the MD/FL indices for the
mandibular incisors appear to be below clinical significance.
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Conclusion
A consideration of tooth shape and the
MD/FL index appears essential for the
successful orthodontic management ofmandibular incisor irregularities.
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