Peck and Peck JC

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  • 8/21/2019 Peck and Peck JC


  • 8/21/2019 Peck and Peck JC


    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


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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


    FACIOLINGUAL Reported in the literature

    far less often than MD


    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


    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


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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


    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


    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


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


    a group with perfect mandibular incisoralignment and a control population groupare presented

    in Table with supplementary statistical data.

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    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


    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


    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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    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


    All four lower incisors of this patient show extreme tooth shape


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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


    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


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    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.


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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


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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


    2. Well-aligned mandibular central and lateral incisors have a

    remarkably distinctive crown shape, as expressed by the MD/FL


    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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    A consideration of tooth shape and the

    MD/FL index appears essential for the

    successful orthodontic management ofmandibular incisor irregularities.

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