Cheat Sheet - Space Analysis

Purpose: compare the space available (between the permanent first molars) to the space required (erupted incisors measured directly + canines/premolars estimated from the Tanaka-Johnston table) to give an arch-length discrepancy.¹ Materials: mixed-dentition dental casts (all permanent first molars and incisors erupted), a profile sketch/image, a modified (pointed) Boley gauge, a space-analysis form, pencil.¹

1. Tanaka-Johnston Analysis — Step by Step

#	Step	What is measured (definition)	How / formula
1	Space available	Arch perimeter from the mesial of one 1st permanent molar to the mesial of the other, along the line of occlusion.	Measure in 4 straight segments (a, b, c, d) — two per side: a posterior segment (molar → canine) + an anterior segment (canine → midline). Straight lines approximate the curved arch. Sum a+b+c+d.
2	Incisor width	Greatest mesiodistal width of each erupted permanent incisor.	Measure each; sum the 4 mandibular incisors — this is the predictor variable for both arches.
3	Estimate unerupted C + P1 + P2 (Tanaka-Johnston)	Predicted mesiodistal width of the canine + two premolars per quadrant, at the 75% confidence level.2	½(Σ 4 mandibular incisors) + 10.5 mm (mandibular quadrant) or + 11.0 mm (maxillary quadrant); then ×2 per arch. The same lower-incisor sum predicts both arches.
4	Space required	Total mesiodistal width needed to align the arch.	Σ incisor widths + estimated (C + P1 + P2) ×2.
5	Arch-length discrepancy	The deficit/surplus of space.	Available − required. ==Negative = crowding; positive = spacing.==
6	Molar-shift adjustment	Expected mesial drift of the lower 1st molar (Class I, end-to-end molars only).	Measure each side, total, and subtract from the discrepancy.1
7	Refine for skeletal class / lip support	Projects whether incisor A-P position will add or remove arch space.	See §4 — read from the profile analysis.

Related measured terms

• Leeway space (Class I skeletal only) = (C + D + E) − (3 + 4 + 5) — the surplus of the primary canine + primary molars over their permanent successors. Confirm on the cast: drop a vertical at the MB cusp tip of the upper permanent 1st molar and another at the buccal groove of the lower permanent 1st molar; the gap is the distance to close for Class I occlusion (achieved by mesial shift of the lower 1st molar, greater mandibular than maxillary growth, or both).¹
• Molar relationship should correlate with skeletal class. In a skeletal Class I: a Class II molar → maxillary space loss; a Class III molar → mandibular space loss.¹

2. Assumptions (concise)

The Tanaka-Johnston prediction table is only valid if:³

1. A correlation exists between the sum of the erupted mandibular incisors and the size of the unerupted canines + premolars.
2. The patient fits the reference population — Caucasian / Northern European descent.
3. All succedaneous teeth are present and developing normally.
4. Arch dimensions do not change appreciably during growth.
5. Incisor position will not change in a way that increases or decreases arch circumference / available space.
6. The mesial shift of the first molars can be predicted accurately (holds mainly in a Class I skeletal pattern).

Why accuracy drops in non-Class I jaws

Assumptions 5 and 6 break down off Class I.⁴ Class II: lower incisors tip facially → increases lower-arch space; Class III: lower incisors tip lingually → reduces lower-arch space while upper incisors often procline. Molar mesial-shift also differs in Class II/III growth, so the prediction is less applicable.

3. Possible Modifications & Alternative Methods

Method	What it changes / uses	Notes
Recognise-limitations option	Run Tanaka-Johnston anyway on a non-matching patient	Accept reduced accuracy; the Workbook’s first fallback.3
Long-cone periapical radiographs	Measure unerupted tooth mesiodistal widths directly on film	Workbook’s second fallback; not routine (radiation exposure).3
Moyers probability tables	Same predictor (Σ 4 mandibular incisors) but reads C+P width from probability charts for both arches	Non-radiographic; Moyers recommended the 75% level (50% = optimistic, 80% = balanced over/under-estimations).5
Radiographic (Nance)	Measures unerupted teeth on a full periapical series	Seldom used — needs a complete radiographic set.5
Combination methods — Hixon-Oldfather (1958), Staley-Kerber (1980)	Combine regression + radiographic measurement	More accurate but still require radiographs.5
Population-specific regression / digital	Bespoke equations for other ethnic groups; may add predictor teeth (e.g. first molars); 3-D scanning / CBCT	Addresses Tanaka-Johnston’s ethnic & sex bias.6

Practical note

Tanaka-Johnston is popular precisely because it is simple, fast, and needs no radiographs — the trade-off is the population assumption (§2.2). Choose a modification when the patient is outside the Caucasian/N-European reference or when higher accuracy justifies radiographs.⁵⁶

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Sources / Footnotes

Footnotes

1. Workbook — space-analysis form, materials, segment measurement, Tanaka-Johnston procedure, leeway space, molar-shift & lip-support adjustments. Also Level II Unit B Summary—integrated (Space Analysis walkthrough) and 05 - Ackerman-Proffit Classification. ↩ ↩² ↩³ ↩⁴ ↩⁵

2. Level II Unit B Summary—integrated — Tanaka-Johnston estimate: ½(Σ 4 mandibular incisors) + 10.5 mm (mandibular) / + 11.0 mm (maxillary), ×2 per arch. ↩

3. Workbook and Level II Unit B Summary—integrated — assumptions of space analysis and the two fallback options for non-matching populations. ↩ ↩² ↩³

4. L4 Level II Unit B / Level II Unit B Summary—integrated — why accuracy decreases in non-Class I jaw relationships. ↩

5. Sarkar et al. Comparison of two non-radiographic techniques of mixed dentition space analysis — Moyers probability tables (75% level; 50% optimistic, 80% balanced), Nance radiographic method, Hixon-Oldfather & Staley-Kerber combination methods. PMC3514952 ↩ ↩² ↩³ ↩⁴

6. Mixed Dentition Space Analysis: A Review — ethnic/sex variation driving population-specific regression models, added predictor teeth, and digital/CBCT measurement. ResearchGate ↩ ↩²