MODEL ANALYSIS

MODEL ANALYSIS

INTRODUCTION• Model analysis is one of the

essential diagnostic aids in orthodontics.

• The study model provides a three dimensional view of the maxillary and mandibular dental arches in all three planes of space, i.e., sagittal, vertical and transverse planes.

• Model analysis allows us to carefully examine several parameters such as dentition, jaw relationships and make objective measurements for detailed evaluation and treatment planning.

• Helps in detecting midline discrepancies

REQUSITES OF STUDY MODELS

• Should accurately reproduce all the teeth and soft tissues without any distortion.

• Should be trimmed symmetrical on either side.• Posterior surface should be trimmed, such that

when placed on their back they should reproduce the occlusal plane.

• Should reproduce the alveolar process as much as possible.

PARTS OF A STUDY MODEL• Anatomic Portion Anatomic Portion - Consists

of the actual impressions of the dental arch & its surrounding structures.

• Artistic Portion Artistic Portion - Consists of the plaster base that supports the anatomic portion and helps in analysing the occlusion & orientation of the study models.

According to ABO (1990) Guidelines

Lateral view of the study model

1.Base of the maxillary cast is trimmed parallel to the occlusal plane.2. Upper & lower cast base should be parallel.

Maxillary occlusal view of study model

Mandibular occlusal view of study model

REQUIREMENTS TO DO MODEL ANALYSIS

• Well prepared study models• Vernier calipers• Divider • Ruler• 0.033” Soft Brass wire

CLASSIFICATION

PERMANENT DENTITION MODEL ANALYSIS

MIXED DENTITION MODEL ANALYSIS

Arch perimeter analysis Moyer’s Mixed dentition analysis

Carey’s analysis Tanaka and johnston analysis

Ashley Howe’s analysis Nance mixed dentition analysis

Pont’s analysis Huckaba’s mixed dentition analysis (Radiographic method)

Linder Harth analysis

Korkhaus analysis

Bolton’s analysis

PONT’S ANALYSIS• Pont in 1909, proposed a method of determining the ideal

dental arch width in premolar and first molar area based on the sum total of mesio-distal widths of maxillary incisors

Pont suggested that :• The ratio of the combined upper incisor width to transverse

arch width was ideally 0.80 in the premolar area and 0.64 in the molar area.

• He also suggested that the maxillary dental arch should be expanded 1-2 millimeters more during treatment than that found in normal occlusion to allow for relapse.

PONT’S ANALYSIS HELPS IN

• Determining whether the dental arch is narrow or is normal in the premolar and molar region for a given sum of widths of incisors.

• Determining the need for lateral arch expansion.

• Determining how much expansion is possible at the premolar and molar regions.

ANALYSIS

• DETERMINATION OF SUM OF INCISORS (SI)

• DETERMINATION OF MEASURED PREMOLAR VALUE (MPV)

• DETERMINATION OF MEASURED MOLAR VALUE (MMV):

• CALCULATED PREMOLAR VALUE (CPV): or the expected arch width in the premolar region is determined by: CPV = SI X 100

80

• CALCULATED MOLAR VALUE (CMV): or expected arch width in the molar region is determined by: CMV = SI X 100

64

INFERENCE

•If the measured value is less than the calculated value, then the arch is narrow for the given sum of incisors width and expansion can be done.•If the measured value is greater than the calculated value, the arch is wider and there is no scope for expansion.

DRAWBACKS

• Maxillary laterals are the teeth most commonly missing from the oral cavity.

• Peg-shaped laterals can be seen.• The analysis was done from the casts of

French population• It does not take skeletal mal-relationships

into consideration.

LINDER HARTH ANALYSIS• Linder Harth proposed an index very similar to that of Pont’s

analysis.• He made variation in the formula to determine the calculated

premolar and molar values• The calculated premolar value (CPV):

CPV = SI X 100 85 • The calculated molar value (CMV):

CMV = SI X 100 64

KORKHAUS ANALYSIS

• This analysis is similar to Pont’s analysis.

• Only difference is that it make use of Linder Harth’s formula to determine the ideal width in the premolar & molar regions.

• According to Korkhaus for a given width of the upper incisors a specific value of the perpendicular distance between the mid point of the inter premolar line to the point between the two maxillary central incisors should exist.

• An increase in this measurement denotes proclination of the upper anterior teeth, while a decrease in this value denotes retroclined upper anteriors.

Korkhaus’ Measurements

ASHLEY HOWE ANALYSIS

• Ashley Howe considered the crowding of teeth to be the result of deficiency in arch width rather than arch length.

• He found the relationship between the twelve teeth anterior to the permanent second molars and the width of the dental arch in first premolar region.

• This is usually done in the upper arch.

ANALYSIS• DETERMINATION OF TOOTH MATERIAL (TTM): mesodistal

width of all the teeth anterior to the permanent second molars are measured with the help of callipers and all the values are summed up.

• DETERMINATION OF PREMOLAR DIAMETER (PMD): it refers to the distance or arch width from the tip of the buccal cusp of one first premolar to the tip of the buccal cusp of opposite first premolar.

• DETERMINATION OF PREMOLAR BASAL ARCH WIDTH (PMBAW): measurement of width from canine fossa of one side to another gives us the width of the dental arch at the apical base or junction between the basal bone and the alveolar process.

INFERENCE I

• The PMBAW and PMD are compared.• If the PMBAW is greater than the PMD,

then it is indicated that arch expansion is possible.

• If on the other hand, the PMBAW is less than PMD, then arch expansion is not possible.

INFERENCE II

• According to Howe, to achieve a normal occlusion with a full complement of teeth, the basal arch width at the premolar region (PMBAW) should be 44% of the sum of the mesiodistal widths of all the teeth mesial to the second molar (TTM)

INFERENCE

PMBAW % CONCLUSION

37% or less It indicates a need for extraction

37-44% Borderline case

44% or more Possibly non extraction if other factors are favorable

BOLTON’S ANALYSIS• Wayne Bolton considered the ratio of the tooth material

of the maxillary arch to the mandibular arch i.e M-D widths of upper & lower teeth by nature have predetermined proportions to maintain normal occlusal relaionship.

• An alteration in this balance will lead to improper intercuspation, overjet or spacing

• Bolton said that extraction of one/several tooth should be done acc. to the ratio of tooth material b/w upper & lower arch to get ideal overjet & overbite .

Measurements

• Sum of maxillary 12• Sum of mandibular 12• Sum of maxillary 6 • Sum of mandibular 6• Overall ratio• Anterior ratio

• Overall Ratio = Sum of mandibular 12 *100 Sum of maxillary 12• For establishing ideal overjet & overbite overall ratio should be

91.3%• If the overall ratio is less than 91.3%, it indicates maxillary tooth

material excess.• The amount of maxillary tooth material excess is determined by

using the formula

• The amount of mandibular tooth excess is determined by:

Determining Overall Ratio

• The sum of M-D widths of the mandibular anteriors to the M-D width of the max. anteriors should be 77.2%

• The anterior ratio is determined using the following formula:

• If < 77.2 , maxillary anterior excess• If >77.2 , mandibular anterior excess

Determination of Anterior Ratio :

ARCH PERIMETER ANALYSIS

• Many malocclusions occur due to discrepancy between arch length & tooth material.

• It is done in the upper arch.• Two measurements are required for intra-maxillary

analysis of space requirement:1. Calculation of space required2. Calculation of space available.

• Arch perimeter is the geometrical dental arc formed by teeth at their incisal / cuspal edges.

PROCEDURE• DETERMINATION OF SPACE REQUIRED:

• measure the mesiodistal dimension of all the teeth mesial to the first molar (54321│12345)

• DETERMINATION OF SPACE AVAILABLE:1. measure the arch perimeter using brass wire. From mesiobuccal

line angle of maxillary right first molar , pass the wire along the buccal cusp and incisal edges in the anterior region, ‘pass the wire on the left quadrant like a mirror image till the mesiobuccal line angle of the left maxillary first molar.

2. Mark the wire and measure the wire, which gives the space available.

• In case of proclined incisors, pass the brass wire in the cingulum region, and if the anterior teeth are retroclined, pass the wire labial to them like a smooth curve.

DETERMINATION OF THE DISCREPANCY

•The difference between the space required and space available gives the arch discrepancy or excess.•If the tooth material is more than the arch length, the space available for alignment is not sufficient results in crowding.•If the tooth material is less than the space then there can be spacing.

CAREY’S ANALYSIS

• The arch length-tooth material discrepancy is the main cause for most malocclusions.

• This discrepancy can be calculated with the help of Carey’s analysis.

• The analysis is carried out in the lower arch.

INTERPRETATION OF CAREY’S ANALYSIS

ARCH LENGTH DISCREPANCY INFERENCE

0 to 2.5 mm Proximal stripping can be carried out to reduce the minimal tooth material

excess

2.5 to 5 mmExtraction of second premolar is

indicated

Greater than 5mmExtraction of first premolar is usually

required

MIXED DENTITION ANALYSIS

AIM• The purpose of mixed dentition analysis is to evaluate

the amount of space available in the arch for succeeding permanent teeth and necessary occlusal adjustments.

• Methods of analysis of arch length during mixed dentition i. Those in which the sizes of unerupted cuspids and premolars are

estimated from radiographic images ii. Those in which the sizes of cuspids & premolars are derived from

the knowledge of already erupted permanent tooth in the mouth.(Probability Tables)

iii.Combination of the above two method

HUCKABA’S MIXED DENTITION ANALYSIS (RADIOGRAPHIC METHOD)

• This analysis makes use of a radiograph and study cast to determine the width of unerupted teeth.

• Advantages: Easy, practical & relatively accurate.• Disadvantage: Chances of distortion of

radiographic image.• IOPAR are preferred over Panoramic Images as

they are more accurate.

PRINCIPLE• It is based on the principle that if we measure an object,

which can be seen both in radiograph as well as on a cast, then we can compensate for the enlargement of the radiographic image

• A simple proportional relationship can be established as follows:

SPACE AVAILABLE: the arch is divided into segments which are approximately straight lines. The dimensions in each of the segments is measured and added up.

• SPACE REQUIRED: for the un-erupted teeth is calculated from the radiographs. The discrepancy is calculated segment wise

• It is based on the premise that there is a reasonably good correlation b/w the size of erupted permanent incisors and the unerupted canines & premolars

• This is because a person with large teeth in one part of the mouth will have large teeth elsewhere also, as their development is controlled by the same genetic mechanism.

• Here the lower permanent incisors are measured and the mesio-distal widths of unerupted permanent upper and lower canines and premolars is derived from the probability chart.

• The mandibular incisors are chosen for measuring as they are the first teeth to erupt in the mixed dentition period.

MOYER’S MIXED DENTITION ANALYSIS

PROCEDURE• STEP I: SPACE REQUIRED- measure the

mesio-distal dimension of all four lower incisors and sum it up. Using the Moyer’s probability chart find the total mesio-distal width of upper and lower canine and premolars from the upper and lower charts at 75% probability for the given lower incisor dimension.

Moyer’s Probability Tables

STEP II: SPACE AVAILABLE- measure the distance between the distal surface of permanent lateral incisors and mesial surface of permanent first molar. Determine the amount of space required for the proper alignment of mandibular incisors. The amount of space left behind gives the space available.

TANAKA AND JOHNSTON ANALYSIS (1974)

• They developed a method to predict the width of un-erupted canine and premolar using the width of lower incisors.

• This methods has good accuracy despite a small bias towards over- estimating the un-erupted tooth size.

• They have simplified Moyer’s 75% prediction table into a formulas• Predicted width of maxillary canine & premolars =

Sum Of Mandibular Incisors + 11 2

• Predicted width of mandibular canine & premolars = Sum of Mandibular Incisors + 10.5 2

CONCLUSION

Mixed dentition analysis forms an integral aspect of orthodontic diagnosis to determine whether the treatment plan is going to involve serial extraction, space maintenance, space gaining or simply periodic observation of the patient.

RECENT ADVANCES IN MODEL ANALYSIS

Cast analysis by E Models/ 3D Digital Models

ADVANTAGES OF COMPUTERIZED ANALYSIS

• More Accurate• Easy• More information:

– Arch form – Determine asymmetric Arch – Space analysis– Prediction