Online space analysis: arch length discrepancy from an occlusal photo

The principle: space available versus space required

Space analysis (Nance) compares two lengths mesial to the first molars: the room the arch offers the teeth and the room the teeth need. It is done first on the mandibular arch: the mandible is treated first, then the maxilla is adapted to it.

Taking a usable occlusal photo

1. Preferably photograph the cast, lying flat, with the camera perpendicular to the occlusal plane: a straight top view, without tilt.
2. Place a millimeter ruler at the height of the occlusal plane, close to the teeth: a ruler lying lower, and therefore further from the lens, does not have the same scale as the teeth.
3. Frame the whole arch up to the first molars, moving the camera back and zooming in rather than coming closer: perspective distorts less.
4. Intraoral photos are taken with a mirror: the image is reversed and rarely perpendicular. Flip it, or simply tell the tool on which side of the image the patient’s right side is: the teeth are numbered accordingly.
5. For tooth widths, the caliper remains the reference: enter its values directly; on the photo, you then only set the scale, the three landmarks and the ideal arch.

Measuring the space required and the space available

The scale. A ruler photographed at the height of the occlusal plane remains the safest: two clicks on its marks and their actual distance. Without a ruler, a single tooth measured with a caliper is enough: its two contact points on the photo give the scale. With neither, the tool estimates the scale from the tooth proportions and gives the discrepancy with a margin; a scale error only acts in proportion on the discrepancy, since both lengths are divided by the same scale. In every case, a scale that makes the teeth clearly smaller or larger than average is flagged.

Three landmarks, then the teeth from first molar to first molar. Point the mesial surface of the right first molar, the inter-incisor point, then the mesial surface of the left first molar: an average arch template then proposes each contact point as a dashed circle. For each tooth, from the right first molar to the left one, click its distal then its mesial point (the ends of its greatest mesiodistal diameter), or keep the proposal with Enter. When two teeth overlap, the hidden contact is hard to locate on a photo: this is where caliper entry is most useful.

Missing, impacted or barely visible teeth. A tooth that cannot be measured is declared instead of pointed: its width comes from the contralateral tooth (or the mean if that one is missing too), or is 0 if the space will be closed. The detailed calculation lists them.

Space available, the wire method. A soft brass wire is laid over the incisal edges, the canine cusp tips and the premolar contact points up to the mesial surfaces of the first molars, following an ideal arch form that ignores isolated malpositions, then straightened and measured. The tool proposes this ideal arch as five points, its ends on the mesial surfaces of the first molars; you adjust the three middle points, and it measures the smooth curve through all five.

The segmental method. It measures the arch in four straight segments with dividers, which removes the variability of the wire in the incisor–canine region. The tool also gives the sum of the four segments between the five points, and the discrepancy per side, worth checking when the arch is asymmetric.

Teeth off the ideal arch. Once the ideal arch is in place, the tool flags the teeth whose middle lies at least 1.5 mm from it (the tool’s threshold), buccally or lingually, and calls them ectopic beyond 3 mm: a pointer for the examination, not a diagnosis.

Reading the result

• Negative: there is not enough room to align the ten teeth as they are. Positive: room is left over, usually as spacing. Some textbooks write it the other way round (required − available): check the convention before comparing two figures.
• Scale from tooth proportions: the result comes with a margin (± mm). The larger the discrepancy, the larger the margin; a ruler or one tooth measured with a caliper removes it.
• Only one line of the space analysis: at the very least, the curve of Spee and the incisor repositioning are added to it. And no crowding does not mean no space deficiency: proclined incisors or a deep curve of Spee can mask it.
• No cut-off on crowding alone: the decision guidelines taught (for example 0 to 4 mm, 4 to 6 mm, 6 mm or more) apply to the complete analysis; they vary between courses and with the origin of the crowding.
• Lower incisor crowding with aligned or spaced upper incisors: consider a tooth-size discrepancy and check the Bolton ratio. The tool passes the measured widths on to it, first molars included: with both arches measured, the overall ratio is complete.

Where crowding fits in the space analysis

In the Steiner box, crowding is a fixed line of the analysis: it is added to the incisor repositioning, the curve of Spee, molar uprighting, any leeway space and the cost of Class II elastics, and only the incisor repositioning is open to discussion. Tweed–Merrifield’s total space analysis splits it by zone (anterior, midarch, posterior), with difficulty coefficients. The two methods do not give the same figure for the same case: always state which one you use.

Limits of the tool

• Photo: perspective, camera angle and the mirror distort lengths, and a ruler away from the occlusal plane skews the scale. Values are approximate.
• Contact points: a contact hidden by overlap or rotation is hard to locate on an image.
• Proposals: they come from an average arch template, not from your patient. A very narrow, very wide or asymmetric arch departs from it: correct each point.
• Mixed dentition: the width of the unerupted canines and premolars has to be predicted (radiographic method, Tanaka-Johnston). The tool does not do this: it assumes the permanent teeth are in place.
• Rest of the analysis: curve of Spee, incisor repositioning, leeway space and molar uprighting are not calculated here.

Frequently asked questions

Learning and decision-support tool: it applies Nance’s method (space available − space required) and does not replace clinical examination, measurement on the cast or the practitioner’s judgment. The example is a fictitious arch.