an Index for Assessing Tooth Shape

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An index for assessing tooth shape

deviations as applied to the

mandibular incisors

Harvey Peck, D.D.S., M.Sc.D.,* and She ldo n Peck, D.D.S., M.Sc.D.*

Boston, Mass.

I has been shown that naturally well-aligned mandibular inciso rs

possess distinctive dimensional characteristics; these teeth are significantly

smaller mesiodista lly and significantly larger faciolingually, when compared

with average population tooth dimensions. 1 It appears, therefore, that tooth

shape (mesiodistal and faciolingual dimensions) is a determining factor in the

presence and absence of lower incisor cr0wding.l These new findings have

stimulated this present effort.

The purpose of this article is to present the scientific basis and the clinical

application of a new method for detecting and evaluating tooth shape deviations

of the mandibular incisors.

The scientific basis

Any consideration of tooth dimensions must to some degree involve

odontometry, the science of measuring the size and proportion of teeth. Many

orthodontists today pract ice some form of odontometry, perhaps unknowingly,

as part of their routine case diagnosis.

Since this article largely concerns

odontometric procedures, it is important that we become totally famil iar with

the nature and scope of odontometry as it relates to orthodontics.

Orthodontic odontometry.

The crown dimension most frequently reported in

dental literature is the mesiodistal (hereafter abbreviated MD) diameter. For

the incisors, it is a measurement easily obtainable from plaster casts with a

reliabi lity comparable to that of measurements taken directly from the mouth

or from skeletal materials.

Presented in part before the twentieth annua l meetin g of the Midd le Atlantic Society

of Orthodont,ists, Washing ton, D. C., Oct. 5, 1971 , and the forty-second ann ual meet-

ing of the Great Lakes S ociety of Orthodontists, Colum bus, Ohio, Oct. 26, 1971 .

*Assistant Professors of Orthodontics,

Boston University School of Graduate

Dentistry.

384


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Volume 61

Number 4

Index for assessing tooth shape deviations 385

The faciolingual (hereafter abbreviated FL) crown diameter is reported in

the literature far less often than the MD dimension. The primary sources of FL

tooth size data for the incisors have been skele tal materia l and extracted teeth,2-7

not plaster casts. In odontometric studies utilizing plaster casts, the FL diameters

of the incisors are usually not reported. Moorreess has justified this omission by

stating :

It cannot be ascertained whether these teeth have erupted sufficiently

to make the greatest labiolingual dimension measurable [on plaster casts].

To this deliberate oversight by odontologists, orthodontists add another,

perhaps greater, reason for the general neglect of FL measurements. One paper

summed it well : [The orthodontist] is mainly interested in mesiodistal widths

of teeth in relation to the available space in the jaws, and would not normally

measure buccolingual diameters.

Tooth size measurements, obtained either from the mouth or from plaster

casts, play an important role in orthodontic diagnosis. Orthodontists use them

primarily in the spatial analysis of existing or potential malocclusions.

Each diagnostic analysis utilizing tooth size data is designed to serve at least

one of three functions:

1. Prediction of unerupt,ed tooth size.

2. Assessment of tooth size-arch size compatibility within the same

arch.

3. Assessment of tooth size compatib ility between the two arches.

The first category includes mixed-dentition analyses, such as those described

by Moyers, lO Nance,ll and others.10-14 Examples of the second group are found

in various arch length discrepancy determinations of the permanent denti-

tion.15-19 The last category includes analyses of maxillomandibular tooth size

relationships, such as those relating the tooth widths of the maxillary anterior

segment with the tooth widths of the mandibular anterior segment in the

permanent dentition.20-22

It is worth noting that all of these orthodontic diagnostic procedures require

only MD tooth measurements in their construction. No currently used clinical

alza lysis employs or even takes into consideration the BL tooth dimension.

In contrast, tooth size indices incorporating both FL and MD dimensions have

been quite useful in physica l anthropology. In 1918 RamstrijmZ3 employed a

breadth-length index in reporting the dimensions of fossil lower molars. Since

then, FL-MD crown indices have been advantageously employed to facilitate

anthropologic communication.

3, 4a?, sl 24-28 In addition, these indices have been

wel l applied in studies of approximal and occlusa l tooth wear.29, 3o

The index. At best, assessments of tooth size deviation presently used in

orthodontic practice are rather empirical. Decisions are often based on experience

and expediency. When a Boley gauge is employed, it is directed only to the MD

crown dimension.

T;Ve now know that both MD and FL dimensions appear to be related to

*The tooth d imension referred to in this article as fac iol ingud is also known as buccol ingwcl

and Zabiol ingtd. The term buccol ingual is commonly used with reference to the posterior teeth,

while ZabdoEingzlaZ is an anterior tooth designation. However, faciolingual, like mesiodistal, is a

term app licable to all the teeth and is therefore preferred by the authors.


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386 Peck and Peck

Am. J. Orthod.

Apri l 1972

Fig. 1. A mandib ular central incisor showing the mesiodistal (MD) and faciolingual (FL)

crown diameters. The MD/FL index (MD/FL X 100) is a numerical expression of the crowns

shape as seen from the incisal aspect. For the incisor shown, the MD diameter approx-

imately equals the FL diameter, yielding an MD/FL index OF 100. If the MD diameter of

this tooth were greater than its FL diameter, the index would be greater than 100.

Similarly, if the MD diameter were less thar. the FL diameter, the index would be less

than 100.

incisor a1ignment.l 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 clinica l orthodontics utilizes an MD/FL

ratio. It is constructed in the following manner :

Index =

Mesiodistal (MD) crown diameter in mm. x 1Oo

Faciolingual (FL) crown diameter in mm.

In this article the use of the MD/FL index; as a numerical expression of

crown shape as viewed incisally is confined to the mandibular incisors (Fig. 1).

Crown shape and snandibular incisor alignment. Employing the MD/FL

index, a study was undertaken to determine the relationship between mandibular

incisor shape and the absence of crowding. The question investigated was: Do

naturally well-aligned mandibular inciso rs possess distinctive crown shape as

expressed by the MD/FL index 1

MATERIALS AND METHODS.

The mandibular inciso rs of two groups of young

female Caucasian adults from the Northeastern region of this country were

studied. One group was designated as the group with perfect mandibular

incisor alignment, while the other was designated as the control population

group. The members of both samples were all within the same age range

(17 to 27 years) and were of European ancestry.

The group with perfect mandibular incisor alignment consisted of forty-five

subjects selected from a clental 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.


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4. The absence of overlapping in the mandibular incisors .

5. Minimal rotational deviation from the ideal arch form in the

mandibular incisors.

An evaluation of rotational deviations according to the method described by

Bjiirk, Krebs, and So10w~~

was orig inally attempted as part of the select ion

procedure. However, this method proved unworkable since the tooth rotations in

question were always very small (less than 10 degrees divergence from the chord

of the ideal incisal arc) and therefore were difficult to measure. Instead, a

subjective evaluation by both investigators independently was utilized, This

procedure proved effective in setting the limit of acceptable tooth rotation for

the perfectly aligned sample.

The control population group consisted of seventy subjects of comparable

age and of European stock but was otherwise unselected.

For each subject in both groups, the maximum mesiodistal (MD) crown

diameter and the maximum faciolingual (FL) crown diameter for each man-

dibular incisor tooth were measured directly in the mouth. A Helios dial

caliper with 0.05 mm. readout was used for all measurements. The caliper t ips

were specially pointed to facilitate accurate measurement, 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 subgingiva lly in most

cases.

In order to quantify the reliabi lity of the measuring technique, the first

twenty-five measurements of each tooth dimension were taken a second time.

The error of the method was then determined statis tieal1y.l The measurement

error in this study proved to be under 1 per cent for all dimensions measured.

The MD/FL index for each tooth measured was then calculated ant1 recorded.

For the stat istical analysis of the data, the right and left teeth of the same

category (central incisors , lateral inciso rs) were pooled within each of the two

groups of subjects. This procedure is in accordance with accepted odontometric

methods.32 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 inc isors in each tooth category.

The means and standard deviations for the MD/FL index were computed.

The differences between the means were evaluated statistically.

FINDINGS.

The mean values of the MD/FL index for two groups of females-

a group with perfect mandibular incisor alignment and a control population

group-are presented in Table I with supplementary stat istical data.

The mandibular central incisors of the group with perfect alignment had a

mean MD/FL index of 88.4 with a standard deviation of 4.3. The same teeth in

the control population group showed a mean index of 94.4 with a standard

deviation of 4.9. The difference between the means (d = 6.0) was very highly

significant (p < 0.001).

The mandibular lateral incisors of the group with perfect alignment had a

mean MD/FL index of 90.4 with a standard deviation of 4.8. The mean index

for the lateral incisors of the control population group was 96.8 with a standard


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388 Peck and Peck

Table

I. Values of the MD/FL index (F x 100) for the mandib ular incisors

Group

/female J

(&ml incisors

Perfect alignment

Control population

Lateral incisors

Perfect alignment

Control population

90 xx.4 4.3 ix.?- 97.i

6.0 9.3n*

130

I j4.4 4.9 80.0-105.3

90 90.4 4.8 i8.0-101.9

6.4 9.27*

130 96.8 5.2 x5.7-112.7

N = Number of teeth.

SD = Standard deviation.

d = Difference between the means.

t = test value.

Very highly significant difference, p < 0.001

tleviat,ion of 5.2. The difference between the means (d = 6.4) was again very

highly significant (p < 0.001).

DISCUSSIOK. These findings indicate that well-aligned mandibular central and

lateral incisors possess remarkably distinctive crown shape, as expressed by the

ND/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 characteris tics becomes evident. Lower incisors

apparently conducive to good alignment hare MD/FL indices significantly

lower t,han the population averages for the same teeth. In fact, we would expect

any lower arch possessing central inciso rs 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

t,o 90.4 to have excellent incisor alignment. There are, however, many factors

other than tooth shape which may lead to lower incisor irregular ity. Therefore,

one may find occasional cases in which mandibular inciso rs are crowded and yet

possess favorable MD/FL indices.

It is also expected that a similar relationship between incisor shape and

incisor position exists in

the

opposite direction: MD/FL indices higher than the

perfect alignment mean values (for the respective mandibular inciso rs) 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.

The estimated population incidence of perfectly aligned mandibular

inciso rs is quite low for AmericanP and Europeans.34 It is probable, therefore,

that the average dentition in the population has a detectable amount of lower

incisor crowding.

In this light, the control population means for the MD/N,

index (cmtral incisor = 94.4, lateral incisor = 96.8) are themselves indicative

of some degree of tooth shape deviation.


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Volume 61

Nvm her 4


Fig. 2. The crown shape typical of natural ly wel l -al igned mand ibular inc isors. This tooth, a

central inc isor, has an MD/FL index of approximately 87. The geometr ic diagram i l lustrates

the kite-shaped crown form which appears esp ecially characteristic of ma ndibu lar in-

cisors with low MD/F L indices (less than 90).

Garn, Lewis, and Kerewsky 26 have reported sex differences in tooth shape

throughout the dentition. Estimates of the mandibular incisor MD/FL indices

for males and females which we constructed from their dataz6 and from

odontometric data of other+ 7, 35s 6

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 per cent of the MD/FL index value for

both central and lateral incisors. Therefore, on the basis of available information,

we may conclude that male-female differences in the MD/FL index are not

significant clinically. We may assume for clinical purposes that our values for

the MD/FL index of females are representative also of the MD/FL index of

males.

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 (Fig. 2). The kites width (side to side) would be

equivalent to the MD crown diameter, and the kite s length (top to bottom)

would be equivalent to the FL crown diameter. A kites width is less than its

length, just as the average lower incisors MD width is less than its FL length.

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 pattern represented

*An

interesting corollary

derives from this observation.

If mandib ular incisor shape in-

fluences mandibu lar incisor position, as our study suggests, then the incisors of males should

actually demonstrate a lower frequency of crowding than the incisors of females, because of

their naturally lower MD/FL index. Only one published study (by Seipelsr) gives the fre-

quency of crowding of the individua l teeth separately for males and females. It indeed shows

the frequency of crowding for the mandibu lar central and lateral incisors to be appreciably

less among ma les than among females, thus confirming this corollary and supporting a causal

relationship between incisor shape and crowding.


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390 Peck nrld Pick

Am. J. Orthod.

April 1972

by a low MD/FL index (less than 90) confers upon the incisor crown ant1 root

an anatomic advantage over the phenomena of tooth rotat,ion and overlap.

The relatively narrowed MD diameter characteristic of well-alignrtl man

tlibular incisors obviously cont,ributes less tooth substance to manclibular arc11

length. This factor, coupled with the chance that a more kite-shaped incisor

would tend to hare flatter, less acute mesial and distal surfaces, less susceptible

mechanically to cornact slippage. may account in part for the incisor shapcb--

alignment relationship.

The cl in ical appl icat ion

The observed relationship between mandibular incisor shape and the presence

and absence of mandibular incisor crowding has significant clinical relevance.

The MD/FL index as previously described and utilized is a numerical expression

of crown shape. As such, it provides an effective clinical method for diagnosing

tooth shape deviations which influence and contribute to mandibular incisor

crowding.

The remainder of this article introcluccs a method of tooth size analysis based

upon the MD/FL index and used by us in clinical diagnosis and treatment

planning.

Clinica l principles. In order to recognize tooth shape deviations, a knowledge

of optimum tooth shape is necessary. For the mandibular incisors, the lower the

MD/FL index, the more favorable the tooth shape relative to good alignment.

0ur studies show that well-aligned mandibular central incisors have an MD/FL

index of 88.4 _+4.3, while well-aligned mandibular lateral inciso rs have an indes

of 90.4 + 4.8.

From these data we have adapted the clinical standards which we use 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

The lower limit of each range represents approximately the mean value of

the MD/FL index of well-aligned teeth. The upper limit of each range is derived

from the lower limit plus one standard deviation.

liower inciso rs wit,hin or below these ranges are considered favorably shapetl.

Any lower incisor w ith an MD/FI, index above these ranges, howe\-er, is con-

sidered to have a crown shape deviation which may influence or contribute to the

crowding phenomenon (Fig. 3).

Of course, this is not always the case. Since we are dealing with four teeth

when we speak of mandibular incisor crowding, good alignment is often present,

with various combinations of favorably and unfavorably shaped teeth. For

instance, lateral inciso rs with an index of 97 may be well aligned in a mandibular

arch with central inciso rs that have an index of 86.

However, an MD/Fli index in excess of 100 for any of the lower incisors

represents a severe shape deviation, characteristic of existing or potential tooth

irregularity. From the previously described cont,rol population sample, we have


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VoZume 61

Number 4


Fig. 3. Variations of mandib ular incisor shape. Pictured are the low

adul Its,

untreated orthodontically. The number lingu al to each tooth i

valu me. from studying the photographs, one may readily gather that (

h igh

I lY

variable, (2) incisor shape and incisor alignment are closely relc

(31 1

O\n I MD/FL index values are characteristic of well- alig ned incisors,

ind e x\ values are characteristic of crowded incisors.

er incisors of four

s its M D/FL i ndex

1) incisor shat

38 is

rted variables, and

while h igh M

D/FL


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392 Peck and Peck

Am . J. Orthod.

AfwiZ1972

Fig. 4. A, Measuring the maximu m mesiodistal (MD) diameter of a lower incisor. A dial

cal iper is employed. The cal iper beaks are posi t ioned near the incisal e dge and are held

perpendicular to the long axis of the tooth. B , Measuring the maximu m faciol ingual (FL)

diameter of a lower incisor. The dial cal iper beaks are s l ipped s l ight ly beneath the gin-

gival margin and are held paral lel to the long axis of the tooth.

deduced that approximately 15 per cent of the population have an MD/FL index

greater than 100 for one or both mandibular central incisors. Simi larly, about

25 per

cent

of the population have mandibular lateral incisors with

an

MD/FL

index in excess of 300. The aT-erage orthodontic practice contains a high con-

centration of these persons. They usually arc the patients exhibiting bimaxilla rJ

crowding.

Patients whose mandibular inciso rs have MD/FL indices above the desired

ranges may wel l bc candidates for the removal of some mesial and/or dista l tooth

substance in conjunction with orthodontic therapy. This procedure is commonly

called stripping. Although part of the orthodontic vernacular, stripping is a

somewhat distasteful term. Articles and texts frequently resort to euphemisms,

such as proximal reduction. We are convinced that purposeful tooth size

alteration will have an increasing ly significant place in the orthodontists

therapeutic armamentarium. It is therefore deserving of a more exacting, more


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MD/FL

Index

88-92 18-92 1

88-928-92

90-950-95

Fig. 5. Table for clinically recording MD and FL crown dimensions. These measurements are

used in computing the MD/FL index for each lower incisor. S ample measurements [in milli-

meters] are written in.

appropriate name. In place of stripp ing we propose reproximation, a word

whose derivation implies (the act of redoing the approximal surfaces. As used

in this article and by definition, tooth reproxinzation is a clinical procedure

involving the reduction, anatomic recontowing, and protection of the mesial

and/or dista l enamel surfaces of a permanent tooth. Protection in this instance

refers to the topical application of cariostatic agents, such as acidulated

phosphate-fluoride.

Clinica l methods. The mesiodistal (MD) and faciolingual (FL) crown diam-

eters of the mandibular incisor teeth are measured directly in the mouth, not on

plaster casts. The maximum MD diameter is usually located at or near the

incisa.1 edge, while the maximum FL diameter is found almost always beneath the

gingival margin, thus precluding the use of plaster casts (Fig, 4).

Measurements are taken with a millimeter caliper calibrated at least to tenths

of a millimeter. Some calipers have a vernier scale (the Boley gauge, for

example) for reading out the measurements, while others have a dial scale. For

tooth measurements, where tenths of a millimeter are important, we prefer a

dial caliper because of its superior readability and precision (Fig. 4). The caliper

tips must be specially sharpened to a knife-edged point to make accurate measur-

ing possible. ,

We take the lower inciso r measurements in a sequence, beginning with the

four MD measurements, right lateral incisor to left 1atera.l incisor, followed by

the four FL measurements, right lateral incisor to left lateral incisor. The

accuracy of each measurement is quick ly checked by comparing the values

recorded for the right lateral incisor with those of the left lateral incisor and

making the same comparisons for the central incisors . Any gross discrepancy

observed between right and left measurements is often a sign that a measurement

error was made, since right and left tooth dimensions usually correspond closely .

In these cases the measurements in question are routinely repeated, even though

gross asymmetries in crown dimension are not uncommon.

The MD and FL crown measurements are recorded in an appropriate table or

grid (Fig. 5). The next step is to compute the MD/FL indices of the four teeth

measured.

This procedure is simplified with the use of a mathematical reference table


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394

Peck

and

Peck

Am . J. Orthod.

Apr i l 1972

Table II. Reference table of values for the MD/FL index*

q,g 89 Pd Q 94 96 98 100 ma 04 106 ma /IO L? KS / I i t9

*Dimensions are in millimeters.

developed for this purpose (Table II). The reference table provides the computed

value of the MD/FL index, given the MD and FL crown dimensions. It contains

only MD/FL index values from 86 to 119. Al l values below 86 are exceedingly

favorable and therefore require no further clinical consideration. Values above

119 are exceedingly unfavorable but occur very rarely.

Using the MD and FL data given in Fig. 5, we shall illustrate the use of the

reference table. The measurements for the mandibular right la teral incisor are

MD = 6.0 and FL = 6.3. With this information, we go to the reference table

which displays MD dimensions horizontally and FL dimensions vertically , in

graduations of 0.1 mm, We first look across the MD dimensions until MD = 6.0

is found. Then we search down the FL dimensions to find FL = 6.3. Where the

selected MD column and FL row intersect lies the appropriate MD/FL index,

which in this case is 95. Simila rly, the MD/FL index for the right central incisor

(MD = 5.4, FL = 6.0) is 90; for the left central incisor (MD = 5.1, FL = 5.8),

88; and for the left lateral hcisor (MD = 6.1, FL = 5.9), 103. In the absence

of the reference table, longhand computations of the MD/FL index are rounded

off to the nearest whole number.


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Fig. 6. Diagnostic Case 1.

The MD/FL bdex i?L

cli~~iccd

diagnosis. To illustrate the clinical application

of the MD/FL index as a means of detecting and evaluating tooth shape devia-

tions of the mandibular incisors , three diagnostic cases will be presented.

DIAGNOSTIC CASE 1 (BIG. 6). Al l four lower inc isors of this patient show ex-

treme tooth shape deviations. The right and left latera l incisors have MD/FL

indices of 119 and 112, respectively, The right and left central incisors have

MD/FL indices of 102 and 103, respectively. There is marked crowding, for

which the untoward shape and size of the lower inc isors-are at least partly

responsible. As part of this patients orthodontic treatment (which in this cake

calls for premolar extractions), reproximation of the four mandibular incisors

is mandatory. Otherwise, recrowding of the lower anterior teeth will surely

follow retention.

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.

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

maxilla ry to mandibular anterior tooth size ratio. Therefore, selective reproxima-

tion of the maxillary inciso rs may also be indicated to maintain a harmonious

anterior intermaxillary relationship. (For example, if the pretreatment anterior

Bolton indexz2 appears satisfactory, but reproximation of the mandibular incisors


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396 Peck and Peck

Am . J. Orthod.

Apri l 1972

MD/FL

MD/FL Index

Tooth MD FL Index Standards

-q 6.6 7.0 94 90-95

7 5.7 6.6 86 88-92

F 5.7 6.5 88 88-92

p 6.6 6.9 96 90-95

Fig. 7. Diagnostic Case 2

is indicated for posttreatment stah ilky, then an equal amount of maxiZZary

anterior reproximation should be planned.)

In summary, this case represents extreme tooth shape deviation requiring

reproximation of the lower incisors as an integral part of orthodontic therapy.

DIAGNOSTIC CASE 2 (FIG. 7). This is a case of bimaxilla ry 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. The central incisors , with indices of 88 and 86, are exceptionally 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. In the posttreatment period for this patient we would not expect any

lower incisor alignment complications or instability because of tooth shape.

It is instructive to compare the photograph of this case (Fig. 7) with that of

Case 1 (Fig. 6). In the two photographs, we notice a distinct difference in the

nature of the lower incisor crowding. While Case 1 is essentially a kotation and

overlap irregularity, Case 2 is primarily a displacement irregularity with the

lateral incisors characteristically displaced lingually and the central incisors

squeezed labially. It is the shape-MD ad FL dimensions-of these incisors,

not their MD widths alone, that explains this observed difference in irregu-

larity.

DIAGNOSTIC CASE 3 (FIG. 8 ) . In this case we observe mild irregularity of the

central incisors only. Inspection of the MD/FL indices reveals that the lower

lateral inciso rs are quite favorably shaped, while the shape of both lower central

incisors is slightly deviant. This is a circumstance in which slight reproximation


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Volume 61

Number 4


I

F

5.3 1 5.6

5.4 5.5

1

5.6

1

6.1

MD/FL

Index

-4

tandards

90-95

88-92 1

88-92

90-95

Fig. 8. Diagnostic Case 3.

of only the central inciso rs is indicated as part of any orthodontic treatment

planned for the lower arch.

Concluding remarks

The substantial evidence that lower incisor shape has significant bearing on

lower incisor alignment may well affect many areas of orthodontic practice.

For one thing, the introduction of a tooth shape index for use in .clinical

orthodontics opens up new channels of communication. Now we have a means

of numerically expressing what perhaps many orthodontists have been sub-

consciously perceiving all along-a fundamental anatomic difference between a

stable-looking orthodontic result and a not-so-stable-looking result. When, for

example, a colleague now speaks of an extraction case with MD/FL indices for

the mandibular incisors all in excess of 100, a significant message has been

succinctly communicated. Besides its use in orthodontic diagnosis and treat-

ment planning, the MD/FL index may prove useful as a parameter in treatment-

priority assessments and epidemiologic surveys of malocclusion.

The observed relationship between lower incisor shape and alignment may

alter some present concepts of retention. Posttreatment retention in orthodontics

is a valuable ingredient of successful therapy. Most orthodontists would agree

that a provision for retention should be included routinely in orthodontic treat-

ment plans.

Perhaps the most worrisome area for the orthodontist during the retention

phase of treatment is the lower inciso r segment of the dentition. Over the years

this has led to wide acceptance of prolonged retention or indefinite retention


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398 Peck and Peck

Am. J. Ov-thod.

April 1972

- 5.4 6.0 90 88-92

12 5.9 6.3 94 90-95

Fig. 9. irregular ma ndib ular incisors with favorable MD/F L indices. Factors other than

tooth shape are respons ible for this crowding.

for these teeth. d canine-to-canine fixed retainer is often used for this purpose.

It is frequently left in for some years as insurance against the indeter-

minable causes of incisor relapse. In light of our findings, however, prolonged

retention seems more a postponement of the problem than a solution. We contend

that most of the cases presently demanding prolonged retention for the lower

incisors probably require instead judicious reproximation because of tooth shape

deviations.

In orthodontic cases requiring premolar estractions because of major tooth

size-arch 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 st ill exaggerated. The mandibular incisors are

often markedly wide and fan shaped, with MD/FL indices approaching or in

excess of 100. Incisors of these proportions are destined to recrowd in time, no

matter how perfect the posttreatment occlusion and alignment may appear

to be.

Lest the MD/FL indes be misconst,rue


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Volume 61

Number 4


lower incisor crown shape (according to their RID/FL index values) but with

noticeable displacement or overlapping of these teeth. (Fig. 9 and see Fig, 7). It

is well known that there are many potential factors surrounding the etiology of

lower incisor crowding. Certainly, other dominant variables, such as occlusion,

habits, supra-alveolar fibers, and early deciduous tooth loss, to name only a few,

are capable of upsetting any alignment stab ility conferred by tooth morphology

alone. Then, too, dental crowding may be a natural aging phenomenon; even

the best-shaped, best-aligned inciso rs may inevitably crowd with age.

Within the framework of clinical orthodontics, however, seldom can any

degree of mandibular incisor alignment or stability be achieved without some

consideration of crown dimensions, tooth shape, and the ;MD/FL index.

Summary and conclusions

This article presents the scientific basis and the clinical application of a new

method for detecting and evaluating tooth shape deviations which influence and

contribute to mandibular incisor crowding.

It has been shown that mesiodistal (MD) and faciolingual (FL) crown

dimensions appear to be related to mandibular incisor alignment.* However, a

survey of odontometric procedures used today by orthodontists reveals that the

FL crown dimension is completely neglected in orthodontic diagnosis and treat-

ment planning.

An index incorporating both dimensions is proposed for clinical orthodontics.

It is constructed in the following manner :

Index =

Mesiodistal (MD) crown diameter in mm.

x 100.

Faciolingual (FL) crown diameter in mm.

In this article the MD/FL index is used as a numerical expression of mandibular

incisor crown shape as viewed incisally.

An odontometric study was undertaken to investigate the question Do

naturally well-aligned mandibular inciso rs possess distinctive crown shape as

expressed by the MD/FL index?

Two samples of young American white women of European ancestry were

utilized. The first group consisted of forty-five subjects carefully selected for

their perfect lower incisor alignment. The second sample was a control popula-

tion group of seventy subjects. MD and FL crown diameters of the mandibular

inciso rs were recorded for each subject in both groups by direct intraoral

measurement. The MD/FL index for each tooth was then calculated, and the data

were processed statistically.

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 re-

markably 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

inciso rs appear to be below clinica l significance.


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Am. J. O hod.

April 1972

The final part of this article introduces a clinical method of lower incisor

toot,11 size analysis based upon the MU/FL index. C linical principles and pro-

cedures related to the use of the MU/FL index are explained. Cases art:

presented to illustrate the use of the index in orthodontic diagnosis and treat-

ment planning. Reproximation (stripping) is described as a clinical procedure

for correcting tooth shape deviations.

A consideration of tooth shape and the MU/FL index appears essential foi

the successful orthodontic management of mandibular incisor irregularities.

Spe cial ackno wledgm ent is expressed to Miss Gail N. Cross, Director of the Den tal As-

sistant Programs at. Boston University Scho ol of Graduate Dentistry and Beth Israel Hospita l,

Boston, and to Miss Barbara Schultz, Assistant Dean at the Forsyth Scho ol for Dent al Hy-

gienists, Boston, for their cooperation in implem enting the odontometric study. The authors

are gratefu l to Miss Sylvia Fleisch, Assistgnt Director, Boston University Com putin g Center,

for her valued assistance.

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Index

for assessing tooth

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