Orthodontic Model Analysis: Bolton, Pont's and More Guide

Orthodontic model analysis is the systematic measurement of study models, the plaster or digital replicas of a patient's dental arches, to answer three questions in numbers: how much space is missing, whether upper and lower teeth match in size, and how wide the arches should be. Instead of eyeballing crooked teeth, the orthodontist measures every tooth and arch dimension.

This matters because treatment decisions hinge on millimetres. Whether to extract teeth, expand the arch or simply align depends on exactly how much crowding exists and where. Model analysis also catches tooth-size mismatches that would leave a bad bite even after perfect alignment.

Key Concepts

• Arch perimeter analysis: space available along the arch versus the sum of tooth widths; the difference in millimetres is the crowding or spacing.
• Bolton analysis: compares total tooth widths upper vs lower; the overall ratio should be about 91.3% and the front-six ratio about 77.2%.
• Pont's index: estimates ideal arch width from the four upper incisors (SI): premolar width = SI × 100 / 80, molar width = SI × 100 / 64.
• Ashley Howe's analysis: compares total tooth material with premolar basal arch width; a large excess over basal bone indicates extractions.
• Moyers analysis: predicts unerupted canine and premolar sizes from the lower incisors present, using probability tables at the 75% level.

Formulas

Crowding = space available − space required, both in millimetres; negative means crowding, positive means spacing. Bolton overall ratio = (sum of 12 lower widths ÷ sum of 12 upper widths) × 100, norm 91.3%; the anterior ratio uses the front 6 teeth, norm 77.2%. Above the norm means relatively larger lower teeth, below means upper excess.

Pont's: interpremolar width = SI × 100 / 80; intermolar width = SI × 100 / 64, where SI is the sum of the four maxillary incisor widths. Assumptions: teeth are intact and unworn, models are accurate, and norms are population averages, so local norms are preferred where available.

Worked Example

Lower arch: space available = 68 mm; sum of lower tooth widths = 73 mm. Crowding = 68 − 73 = −5 mm, i.e. 5 mm of crowding. Bolton: 12 upper widths sum to 92 mm, 12 lower to 85 mm; overall ratio = (85 ÷ 92) × 100 = 92.4%, about 1.1% above the 91.3% norm, a mild lower excess of roughly 1 mm. Pont's: four upper incisors sum to 30 mm; expected interpremolar width = 30 × 100 / 80 = 37.5 mm, but the actual is 34 mm, so the upper arch is 3.5 mm narrower than ideal. The numbers decide the plan: upper expansion plus mild lower slenderizing, not guesswork.

Common Misconceptions

• "Measuring models is outdated now that we have scans." Wrong: the medium changed from plaster to pixels, but the mathematics is identical and just as necessary.
• "A Bolton discrepancy means extraction." Wrong: small discrepancies are fixed by interproximal reduction or composite build-ups.
• "Moyers analysis gives the exact future tooth size." Wrong: it is a probability estimate at the 75% level, deliberately slightly overestimating.

Real-World Applications

Model analysis is performed for virtually every comprehensive orthodontic case and is required documentation for board examinations. In the mixed dentition, Moyers analysis at age 8 to 9 decides between watchful waiting, space maintenance or early expansion.

Orthodontic model analysis measures crowding with arch perimeter analysis, tooth-size harmony with Bolton's ratios, ideal width with Pont's index, and future crowding in children with Moyers analysis. These millimetre-level answers decide expansion versus extraction and catch mismatches that alignment alone could never fix.