1. Why examiners love space analysis
Space analysis appears often in orthodontic OSCEs and written exams because it links measurement to a clear treatment decision. Examiners want you to:
- State the arch length discrepancy (crowding or spacing).
- Choose the right method for the clinical situation.
- Explain how the result affects extraction or non-extraction planning.
You do not need every detail from the textbook — you need a clean explanation of each method.
2. Bolton analysis in three exam steps
Use Bolton when you suspect mismatch between upper and lower tooth sizes (e.g. excess lower anterior width).
Bolton analysis — OSCE flow
1. Measure mesiodistal widths of teeth 6–6 (overall) or 3–3
(anterior) in each arch.
2. Calculate the ratio = (mandibular sum / maxillary sum) ×
100.
3. Compare with Bolton’s ideal ratios: overall ≈ 91.3%, anterior
≈ 77.2%.
Excess in one arch → add or remove space by IPR, build-ups,
or extractions.
In viva, always finish with a sentence linking the ratio to treatment choice, e.g. “A lower anterior excess suggests need for stripping or build-ups.”
3. Moyers mixed dentition analysis — chart based
Moyers is mainly tested in mixed-dentition cases (eruption of incisors, canines and premolars unerupted).
- Measure the sum of mandibular permanent incisors.
- Use Moyers probability chart (commonly 75% level) to read predicted width of canine + premolars per quadrant.
- Measure space available from mesial of first molar to midline.
- Compare predicted space required vs space available to decide if crowding or spacing is present.
Exam phrase: “In Moyers analysis, we use lower incisors plus a probability chart to estimate unerupted canine and premolar size.”
4. Tanaka–Johnston formulas you can say without a chart
Tanaka–Johnston is an easy mental shortcut derived from Moyers, used widely in exams.
Tanaka–Johnston — remember these numbers
1. Add the mesiodistal widths of the four lower incisors.
2. For mandibular canine + premolars in one
quadrant:
Predicted width = (sum of lower incisors / 2) + 10.5
mm.
3. For maxillary canine + premolars in one
quadrant:
Predicted width = (sum of lower incisors / 2) + 11.0
mm.
4. Compare with space available in each quadrant to decide if
there is crowding or spacing.
Because there is no chart, this method is perfect when an examiner asks you to “estimate space analysis at the chair.”
5. How to present space analysis in OSCE or viva
Keep your answer predictable and structured:
- Introduce the method (Bolton, Moyers, or Tanaka–Johnston) and when you use it.
- Explain the measurements you take (which teeth, which arch, chart or formula used).
- State the result as crowding or spacing in millimetres.
- Link to treatment — non-extraction vs extraction, stripping, build-ups, or space maintenance.
This is exactly the structure examiners score: method → measurement → result → decision.
6. Example examiner-style answer
Sample viva line
“For this case I would start with a space analysis. Using Tanaka–Johnston, I add the four lower incisors, divide by two and add 11 millimetres for the upper canine and premolars in each quadrant. When I compare this to the space available, I find approximately 4 mm crowding per side, which supports an extraction-based treatment plan.”
7. How DentAIstudy helps
DentAIstudy can instantly convert any orthodontic case description into exam-ready notes:
- Step-by-step Bolton, Moyers, and Tanaka–Johnston flows.
- Ready-made viva lines explaining each method.
- Problem lists linking space analysis to extraction decisions.
- Flashcards for key ratios and formulas.
References
- Proffit WR, Fields HW, Larson BE. Contemporary Orthodontics. 6th ed. Elsevier; 2018.
- Moyers RE. Handbook of Orthodontics. 4th ed. Year Book Medical Publishers; 1988.
- Tanaka MM, Johnston LE. The prediction of the size of unerupted canines and premolars in a contemporary orthodontic population. J Am Dent Assoc. 1974;88(4):798-801.