## Moyer’S Mixed Dentition Analysis

Moyer’s concept is similar to that of Tanaka and Johnston.

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According to Moyer, there exists a correlation between the widths of permanent lower incisors, permanent canines, and premolars.

If the mesiodistal width of permanent lower incisors is known, the sum of widths of unerupted canine and premolars can be predicted from Moyer’s prediction chart.

## Space Analysis In Orthodontics

The chart gives different sets of predicted sizes for a given width of lower incisors. The chart also gives a percentage value to each of the sets.

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**Read And Learn More: Paediatric Dentistry Notes**

This means that the prediction value is applicable to 75% of the population.

The 75% prediction value set is generally used for predicting the sizes of unerupted teeth, with the hope that the reliability is higher.

In any case, it is better to overestimate and be forewarned of higher demand than be at the comfort of smaller demand only to get perturbed later upon realizing that the demand had actually been more.

## Space Analysis In Orthodontics

**Method:**

The sum of the mesiodistal widths of lower incisors is assumed as ‘X’.

From the prediction chart, the corresponding value for the sum of widths of permanent canines and premolars for maxillary and mandibular arches is found.

**Radiographic Method Recommended By Huckaba:**

Huckaba used intraoral radiographs to predict the width of unerupted teeth.

An intraoral radiograph of the posterior region of the dental arch is taken (Right angle technique) at around 8 years of age.

This reveals the image of unerupted canine and premolars and the erupted molar in the same vicinity.

The actual width of the erupted molar can be measured from the study model, while its radio graphic width can be measured from the radiograph.

## Space Analysis In Orthodontics

The difference between the two measurements is the magnification factor.

This factor will be the same for the erupted molar and the unerupted canines and premolars in the same firm.

Hence, the ratio in dimensional change will also be the same.

The width of the unerupted teeth can be predicted from their radiographic image in this way.

**Method:**

The space available is calculated as explained above.

The true or actual width of the erupted tooth is measured from the model cast.

The radiographic or apparent width of the same tooth is measured from a periapical view radiograph.

The radiographic width of the unerupted teeth is also measured from the same periapical radiograph.

The idea of this proportional change is used to find the true width of the unerupted tooth.

This is a fairly reliable technique, which uses a model and a radiograph.

It can be used for maxillary and mandibular arches and on all groups of the population.

The radiographic image has to be clear. It is essential that the images are measured from the same film depicting both the unerupted teeth and the erupted tooth.

Obtaining an undistorted image of a canine is difficult due to its position.

This makes the measurement of its radiographic width difficult. Also, rotated teeth cannot be properly measured in radiographs

## Hixon And Oldfather Prediction Graph

Obtaining an undistorted radiographic image of saline and therefore prediction of its width from the radiograph was difficult.

Hixon and Oldfather predicted the relative width of unerupted permanent lower canines using a dental model, periapical radiographs, and a prediction graph.

The method was later revised by Staley and Kerber.

## Space Analysis In Orthodontics

The X-axis of the prediction graph depicted the sum of the widths of the permanent lower incisors and lower fist and second premolars.

The widths lower canine and premolars are depicted on the Y-axis.

The width of the lower permanent incisors is measured from the study model.

The width of unerupted premolars is predicted from the periapical radiograph.

With these measurements, the width of the lower permanent canine can be deciphered from the prediction graph formulated by Hixon and Oldfather.

In the graph, the line connecting the red dots is the prediction line.

The given sum of lower incisors and premolars on the X-axis is projected onto this prediction line and the corresponding Y-axis value is read as the sum of the lower permanent canines and premolars, from which the size of the permanent canine is deciphered.

The values are applicable to the northern European population, with questionable reliability for other ethnic groups.

## Tweed’s Method

Tweed suggested a modification in obtaining the space available (SA) for space analysis.

According to Tweed, the axial inclination of lower incisors on the mandibular basal bone plays a major role in treatment planning and that space analysis should include space needed for correction of inclination.

**Method:**

Tweed introduced a triangle widely known as ‘Tweed’s triangle’ formed by three planes, namely the Frankfurt horizontal plane (passing through portion to orbitale), the mandibular plane (passing through gonion and mention), and the mandibular incisor plane (long axis of the lower incisor).

Th angles formed are called Frankfurt-mandibularangle(FMA), incisormandibular plane angle (IMPA) and Frankfurt mandibular incisor angle.

**The established relationships derived from Tweed’s research are as follows:**

1. When FMA is between 21° and 29°, the FMIA should be 68°

2. When FMA is 30° or higher, the FMIA should be 65°

3. When FMA is 20° or lesser, the IMPA should be 92° Patient’s lateral cephalogram is traced.

The axial inclination of the lower incisor is checked and if found to not correspond with the relationship as said by Tweed, then an objective lower incisor axial line is drawn according to the patient’s FMA, as suggested.

## Space Analysis In Orthodontics

The distance between the actual axial line of the lower incisor and the objective line is measured along the occlusal plane.

Double this distance is required for correction of incisor proclination (for the right and left sides of the arch) and is reduced from the space available.

This method not only took the teeth into consideration but also the facial balance and stability in planning treatment.

## Total Space Analysis

Total space analysis of the lower arch was developed by Levern Merrifield.

All the space analyses discussed previously in this chapter measure the magnitude of space discrepancy in the dental arch.

The total space analysis measures the magnitude of space discrepancy and also reveals the exact location of discrepancy in the arch.

The dental arch is divided into anterior, middle, and posterior areas.

The analysis is done separately for each of these areas to localize the specific area of space discrepancy in the lower arch.

For further information about this ‘total space analysis’, the reader is advised to refer to appropriate orthodontic books.

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