Peck and Peck JC

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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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KEYWORDS

Peck and Peck JC

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