The practical evaluation of the study model is an important step during the diagnosis and treatment planning of an orthodontic case. This includes observing the model in three different views: lateral, frontal and horizontal.
Lateral View: We can observe from this view the following:

Angle's classification

Incisal classification

Overjet (horizontal relationship)

Overbite (vertical relationship), lateral overbite or supraeruption

Curve of Spee

Inclination of the front teeth, primary evaluation (best done on cephalometrics)
Frontal View: The following can be seen:

The midline, upper and lower. We can determine the palatal midline by using a symmetroscope

Deviating axial inclination, meaning the mesial, distal buccal or lingual tipping of the front teeth

Ant or post Crossbite, unilateral or bilateral, including one tooth or a group of teeth

Scissors bite, also unilateral or bilateral, individual or a group of teeth

Diastemas, we should determine the amount in millimeters
Horizontal View: Determine the following:

Eruption stage, deciduous/mixed permanent

Width of the alveolar process

Shape of the dental arch, ellipsoid/parabolic

Width of the dental arch, the intercanine and intermolar distance

Deviation in tooth morphology, ex. Peg shape lateral/fusion

Space condition, Moyer’s analysis, Nance Analysis/Bolton’s Analysis
One of the most important aspects when viewing the study models is to observe the amount of space required for the eruption of teeth, also termed the space condition, as mentioned above. In order to estimate if there is any arch discrepancy and space available, and whether we need to extract, the following analysis “Model Analysis” has been developed:
Plaster Model Analysis:
The most common analysis used are: Mixed dentition analysis (Moyer’s Analysis), Arch length analysis (Nance Analysis), Tooth size analysis (Bolton’s Analysis).

Mixed Dentition Analysis, “Moyer’s Analysis”
This analysis is based on measurement of the mandibular permanent incisor. A quantitative assessment of crowding may be obtained by this mixed dentition analysis. The space available in each dental arch is measured on the study models and the sum of the mesiodistal dimension of the unerupted teeth is determined by measuring the mesiodistal dimensions of the four erupted mandibular permanent incisors (fig. 44, ad). Thus, predicting the combined sizes of the unerupted canine and premolars from the table. The following diagrams show the method used step by step:


a. Measuring mesiodistal tooth size of incisors

b. Transferring sizes to sheet



c. Calculating sum of mandibular incisors

d. Prediction of unerupted canine and premolars

Fig. 44
How to apply Moyer’s Analysis

Determine the maximum mesiodistal width of each of the four lower permanent incisors in the study model. Calculate their sum.

From the incisors value determine the predicting size for unilateral upper 3, 4 and 5 (cuspid, first and second bicuspid). This can be found from the probability charts on the following page. The upper half of the chart is for the upper teeth, and the lower half is for the lower teeth, this value is termed the space required.
The predicting size for unilateral lower 3, 4 and 5 is taken from the lower probability chart (this value is termed the space required).

Calculate the space available after alignment of upper and lower incisors each arch separately. This value determines the space available needed to accommodate 3, 4 and 5.
Space available – space required = will give the space adequacy or inadequacy for the nonerupted 3, 4 and 5.
431 PDS
Space Analysis in Mixed Dentition
Name: ___________________________________

Computer No.: _________________________


Maximum Mesiodistal width of lower incisors

=


=


mm
 
Predicted size of upper unilateral 3, 4 and 5

=




 
Predicted size of lower unilateral 3, 4 and 5

=





UPPER ARCH


Left

Right
 
Mesiodistal width of 1 and 2

=


mm


mm
 
Space needed for alignment of 1 and 2

=


mm


mm
 
Space left after alignment of 1 and 2

=


mm


mm

(space available – space required)

=


mm


mm





















LOWER ARCH


Left

Right
 
Mesiodistal width of 1 and 2

=


mm


mm
 
Space needed for alignment of 1 and 2

=


mm


mm
 
Space left after alignment of 1 and 2

=


mm


mm
 
Space adequacy or inadequacy
(space available – space required)

=


mm


mm

ANALYSIS OF DENTITION AND OCCLUSION
Probability chart for predicting the sum of the widths of upper 345 from 2 112
12 12

19.5

20.0

20.5

21.0

21.5

22.0

22.5

23.0

23.5

24.0

24.5

25.0

95%

21.6

21.8

22.1

22.4

22.7

22.9

23.2

23.5

23.8

24.0

24.3

24.6

85%

21.0

21.3

21.5

21.8

22.1

22.4

22.6

22.9

23.2

23.5

23.7

24.0

75%

20.6

20.9

21.2

21.5

21.8

22.0

22.3

22.6

22.9

23.1

23.4

23.7

65%

20.4

20.6

20.9

21.2

21.5

21.8

22.0

22.3

22.6

22.8

23.1

23.4

50%

20.0

20.3

20.6

20.8

21.1

21.4

21.7

21.9

22.2

22.5

22.8

23.0

35%

19.6

19.9

20.2

20.5

20.8

21.0

21.3

21.6

21.9

22.1

22.4

22.7

25%

19.4

19.7

19.9

20.2

20.5

20.8

21.0

21.3

21.6

21.9

22.1

22.4

15%

19.0

19.3

19.6

19.9

20.2

20.4

20.7

21.0

21.3

21.5

21.8

22.1

5%

18.5

18.8

19.0

19.3

19.6

19.9

20.1

20.4

20.7

21.0

21.2

21.5

Probability chart for predicting the sum of the widths of lower 345 from 2112
12 12

19.5

20.0

20.5

21.0

21.5

22.0

22.5

23.0

23.5

24.0

24.5

25.0

95%

21.1

21.4

21.7

22.0

22.3

22.6

22.9

23.2

23.5

23.8

24.1

24.4

85%

20.5

20.8

21.1

21.4

21.7

22.0

22.3

22.6

22.9

23.2

23.5

23.8

75%

20.1

20.4

20.7

21.0

21.3

21.6

21.9

22.2

22.5

22.8

23.1

23.4

65%

19.8

20.1

20.4

20.7

21.0

21.3

21.6

21.9

22.2

22.5

22.8

23.1

50%

19.4

19.7

20.0

20.3

20.6

20.9

21.2

21.5

21.8

22.1

22.4

22.7

35%

19.0

19.3

19.6

19.9

20.2

20.5

20.8

21.1

21.4

21.7

22.0

22.3

25%

18.7

19.0

19.3

19.6

19.9

20.2

20.5

20.8

21.1

21.4

21.7

22.0

15%

18.4

18.7

19.0

19.3

19.6

19.8

20.1

20.4

20.7

21.0

21.3

21.6

5%

17.7

18.0

18.3

18.6

18.9

19.2

19.5

19.8

20.1

20.4

20.7

21.0

Probability charts for computing the size of unerupted cuspids and bicuspids. The top chart is for the upper arch. The bottom chart is for the lower arch.

Measure and obtain the mesiodistal widths of the 4 permanent mandibular incisors.

Find that value in the top horizontal column.

Reading downward in the appropriate vertical column, obtain the values for expected width of the cuspids and premolars corresponding to the level of probability you wish to choose.
Ordinarily, the 75% level of probability is used. Note that the mandibular incisors are used for the prediction of both the mandibular and maxillary cuspid and premolar widths.

Arch Length Analysis, “Nance Method”:
The amount of space available is determined by adapting a length of 0.025 inch diameter brass wire to fit from the mesial marginal ridge of the left first permanent molar around the arch, to the mesial marginal ridge of the right mandibular first permanent molar. The brass wire should pass over the imagined correct position of the cuspid, the center of the occlusal surfaces of the bicuspids, and the incisal edge of the most labial of the incisor teeth. The wire should be a smooth arch, free from kinks and should simulate the desired arch form. Adjustment to the arch form should be made if a mandibular buccal or lingual crossbite is present.
The length of the brass wire, determined in millimeter, is regarded as the available space for the total complement of the dentition. This consists of the 1^{st} and 2^{nd} bicuspids, cuspids and lateral and central incisors of both the right and left sides of the mandibular arch.
It is important to recognize that the available space may or may not be adequate for the proper alignment of the teeth. The required space is determined by measuring the mesiodistal width of each tooth from the right 2^{nd} bicuspid to the left 2^{nd} bicuspid, then calculating the sum;
The space available – space required = will give us the space adequacy or inadequacy to accommodate the teeth.
431 PDS
Space Analysis in the Permanent Dentition
Name: ___________________________________

Computer No.: _________________________

Upper Jaw

R

F

L


Space Available




Space Required




Difference




Lower Jaw

R

F

L


Space Available




Space Required




Difference





Tooth Size Analysis, “Bolton Analysis”:
The determination of tooth size ratios between the maxillary and mandibular teeth is essential for proper orthodontic diagnosis, treatment planning and result prediction. This relation determines:

Teeth interdigitation

Excessive overbite

Overjet

Spacing between teeth
The desirable ratio is necessary to attain an optimum interarch relationship. If the analysis indicates a marked deviation, it can give an insight into the required pattern of treatment and extraction. The Bolton procedure is used in this case to determine the overall ratios. It is as follows:

The sum of the mesiodistal diameter of the 12 maxillary teeth and the sum of the mesiodistal diameter of the 12 mandibular teeth including the first molar is calculated, this is called the overall ratio:
Overall ratio =

Sum of 12 mandibular teeth

x 100 = 91.3%

Sum of 12 maxillary teeth

If the overall ratio is less than 91.3%, then the maxillary tooth material is excessive. We can determine from the table the desired size of the mandibular 12 teeth, appropriate for the actual size of the maxillary 12 teeth. The value represents the excessive amount of mandibular tooth material.
We can use the same equation for the anterior 6 teeth only from canine to canine. This called the Anterior Ratio:
Anterior ratio =

Sum of 6 mandibular teeth

x 100 = 77.2%

Sum of 6 maxillary teeth

If the ratio is less than 77.2%, the maxillary teeth are excessive
How to apply the Bolton’s Analysis:
If the overall ratio of the 12 mandibular and 12 maxillary teeth is more than 91.3%, then the teeth that are at fault are the 12 mandibular teeth, meaning that they are excess in size. From the table in the following page, we determine what the corrected sum of the 12 mandibular teeth should be (this is achieved by locating our actual sum of the 12 maxillary teeth which we have already the chart, this is termed the corrected mandibular.
If the overall ratio is less than 91.3%, then the teeth that are at fault are the 12 maxillary teeth, meaning that they are excess in size. The same procedure is done, but here we take the actual sum of the 12 mandibular teeth instead, and locate our corresponding maxillary value from the chart.
When determining the anterior ratio, the same procedure as above is used, calculations are done when the amount is more than 77.2% or less than 77.25.
References

Profitt, WR and Fields, HW. Contemporary Orthodontics. Second edition, Mosby Yearbook Inc., St. Louis Missouri, 1993.

Thilander, B and Ronning, O. Introduction to Orthodontics. Fifth edition, printed by Minab/Gotab, Stockholm, 1985.

Walther, DP and Houston, WJ. Orthodontic Notes. Fifth edition, ButterworthHeinemann Ltd., Oxford 1994.

Wisth, P. Introduction to the Edgewise Technique: A Technical Manual, University of Bergen, Norway, 1985.

American Board of Orthodontics. Specific Instructions for Candidates, American Board of Orthodontics, St. Louis, 1990.
