Space Analysis in Orthodontics UK

What space analysis measures

Space analysis compares two figures: the total width of your teeth (measured tooth by tooth) and the length of the arch available to hold them in good alignment. Where the teeth need more room than the arch provides, the difference is called an arch length discrepancy, and it is what shows up clinically as crowding. Where the arch provides more room than the teeth need, the result is spacing. Space analysis puts a number on this rather than relying on a visual impression, which is what allows an orthodontist to plan treatment against an actual measurement.

Little's Irregularity Index for measuring crowding severity

Little's Irregularity Index is a widely used method for scoring how crowded the front six lower teeth are, by measuring the horizontal displacement between adjacent contact points on a study model or digital scan and summing the results. It produces a single score that lets crowding be graded as mild, moderate or severe, and lets a change in crowding be tracked objectively over the course of treatment rather than judged by eye. It focuses specifically on the lower incisors because this segment is a common site of relapse and is often used as a reference point in research and case records.

Moyer's prediction method for mixed dentition

In children who still have some baby teeth present alongside their permanent teeth — the mixed dentition stage — the width of the not-yet-erupted permanent canines and premolars cannot be measured directly. Moyer's prediction method estimates their combined width using tables based on the size of the already-erupted lower incisors, which are proportionally related to the unerupted teeth with reasonable accuracy across a population. This lets an orthodontist estimate how much space will be needed once all the permanent teeth arrive, and plan interceptive treatment — such as space maintenance — before that space is lost.

Bolton analysis for upper and lower tooth size matching

Bolton analysis compares the combined width of the upper teeth to the combined width of the lower teeth, to check whether the two arches are proportionally matched. A significant Bolton discrepancy means that even with perfectly aligned arches individually, the upper and lower teeth will not fit together correctly at the front or back, which can affect the final bite. Where a discrepancy is identified, it may be addressed by selectively reducing the width of specific teeth (interproximal reduction) or by adjusting the extraction pattern to compensate.

Digital methods versus manual measurement

Space analysis has traditionally been done by hand on plaster study models using callipers or a specialised gauge, tooth by tooth. Digital scanning software now automates much of this — measuring tooth widths and arch length directly from an intraoral scan and calculating Little's Irregularity Index, Bolton ratios and arch length discrepancy automatically. Digital methods reduce measurement error and speed up the process considerably, though the underlying analyses being calculated are the same established methods used in manual assessment.

How space analysis informs extraction decisions

The size of the arch length discrepancy is one of the key factors — alongside facial profile, the position of the lips relative to the teeth, and the amount of crowding — that determines whether an orthodontist recommends extractions to create room, or whether crowding can instead be resolved by expanding the arch, reducing tooth width (interproximal reduction), or a combination of these. As a general guide, mild crowding is often manageable without extractions, while more severe discrepancies are more likely to need extractions or a combination approach, though this is always assessed on an individual basis rather than by a fixed threshold.

How results are explained to patients

Most UK practices will show patients their arch length discrepancy or crowding score as part of explaining a proposed treatment plan, often alongside the study models or digital scan the measurements were taken from. This is a useful point to ask why a particular approach — expansion, interproximal reduction, or extraction — has been chosen for your specific measurements, since the same visual level of crowding can sometimes be managed in more than one way depending on other factors such as your bite and facial profile.

Useful related pages

This guide is for general information only. It is not a diagnosis, treatment plan, or substitute for advice from a registered dentist or orthodontist.