Cheat Sheet - Key Statistics and Numbers
The memorizable numbers flagged across the learning objectives, in one place. The ceph / facial / space sections are compact — full detail lives in the dedicated cheat sheets they link to.1
Unit A — Occlusion, Need & Etiology
1. Ideal occlusion norms
| Statistic | Value | Meaning |
|---|---|---|
| Line of occlusion contacts | ≈ 138 | Occlusal contact points engaging the arches |
| Overjet (normal) | 2–3 mm | Horizontal incisor overlap |
| Overbite (normal) | 1–2 mm | Vertical incisor overlap |
2. Treatment need vs demand
| Statistic | Value |
|---|---|
| Population with malocclusion | ~65% |
| Dentists feel need treatment | ~55% |
| Lay people feel need treatment | ~35% |
| Would accept if free | ~50% |
| Refuse even if free | ~5% |
| Actually receive treatment (utilization) | 5–40% |
| Have normal occlusion | ~1/3 (≈35%) → two-thirds have some malocclusion |
3. Etiology / prevalence
| Statistic | Value | Meaning |
|---|---|---|
| Specific known cause | only ~5% | ~95% are multifactorial (inherited × environment) |
| Major etiologic factors | 4 | Heredity · interference with development · trauma · disturbed function |
4. Heredity
| Statistic | Value | Note |
|---|---|---|
| Max hereditary contribution | ~50% | Environmental contribution likely ≥ genetic |
| Mandibular prognathism heritability | ≈0.32 | Most strongly inherited pattern; autosomal dominant, incomplete penetrance (long-face = 2nd highest) |
| Twin study — spacing / tooth position | 69–89% | High hereditary component |
| Twin study — overbite | ~53% | |
| Twin study — overjet | ~28% | Largest environmental component |
5. Trauma & growth
| Statistic | Value | Meaning |
|---|---|---|
| Condylar process fracture outcome | ~75% normal growth / ~25% asymmetry | Asymmetry driven by soft-tissue scarring (functional-matrix theory), not the bony injury |
6. Force & duration (tooth movement / habits)
| Statistic | Value | Meaning |
|---|---|---|
| Force magnitude | ~3 g | Eruption experiments; threshold is very low |
| Duration threshold | ~4–8 h/day (≈6 h) | Sustained force needed to move a tooth — duration > magnitude |
| Intermittent force — 50% of time | ≈ continuous-force effect | |
| Intermittent force — 25% of time | intermediate effect | |
| Intermittent force — 10% of time | little / no effect | |
| 12 h/day (50% of time) | ≈ 24 h effect | Why the night-long thumb is what matters |
7. Swallowing, tongue & nasal
| Statistic | Value | Meaning |
|---|---|---|
| Tongue-thrust pressure per swallow | ~1 s | Far too brief to move teeth |
| Mature swallow appears | as early as age 3 | |
| Mature swallow in majority | by ~age 6 | |
| Never achieve mature swallow | ~10–15% | |
| Tongue thrust vs anterior open bite (age >6) | ~10× more common | Tongue thrust is an adaptation, not the cause |
| Long-face airway | minority <40% nasal breathing; none of normals below that | Mouth-breathing/long-face link is weak |
Unit B — Diagnostic Numbers
8. Facial analysis — full detail in Cheat Sheet - Facial Analysis
| Statistic | Value |
|---|---|
| Chin / mandibular deviation noticeable | ≥4 mm (≤3 mm not noticeable) |
| Dental midline shift noticeable | >3 mm |
| Maxillary incisor display at rest | 2–4 mm |
| Maxillary incisor display on smile | 75–100% of crown |
| Buccal corridor | males 15–24 mm / females 10–17 mm |
| Lip incompetence (separation at rest) | >3–4 mm |
| Nasolabial angle | ~90–110° |
9. Cephalometric norms — full table in Cheat Sheet - Cephalometric Measurements and Norms
| Statistic | Value | Note |
|---|---|---|
| Cephalometrics introduced | 1931, Broadbent (US) & Hofrath (Germany) | ⚠️ Unit B summary states 1934 — both dates appear in the notes |
| SNA / SNB / ANB (rounded) | ~82° / ~80° / ~2° | ⚠️ UNC/UWA table gives 81° / 78.2° / 2.8° (Black 84.7 / 79.2 / 5.5) — use the table for the exam |
| Significant skeletal change | ≥4° | |
| Lower incisor to NB | proclined if >25°, protruded if >4 mm | |
| IMPA (lower incisor to mandibular plane) | ~90° (Tweed) | UNC/UWA norm 97.3° |
| Interincisal angle | ~131° (lecture) | ⚠️ Workbook table = 126.8° (Black 113.8°) |
| Lower incisor ahead of B-point | ~2 mm (range 1–5 mm) |
10. Space analysis — full detail in Cheat Sheet - Space Analysis
| Statistic | Value | Note |
|---|---|---|
| Tanaka-Johnston (per quadrant) | ½ Σ lower incisors + 10.5 mm (lower) / + 11.0 mm (upper) | At 75% confidence |
| Prediction range | ~±1.5 mm | |
| Moyers confidence level used | 75% | |
| Bolton ratio — anterior / overall | ~77.2% / ~91.3% | Tooth-size discrepancy affects ~5% of population |
| Boley gauge precision | 0.1 mm |
Watch the conflicting figures
A few values differ between the lecture/LO sheet and the Workbook/UNC-UWA table — date 1931 vs 1934, ANB ~2° vs 2.8°, interincisal ~131° vs 126.8°. For the practical/written exam, default to the UNC/UWA Workbook table values (see Cheat Sheet - Cephalometric Measurements and Norms).
Sources: Ortho Learning Objectives · Level II Unit A Summary—integrated · Level II Unit B Summary—integrated · Workbook
Footnotes
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Numbers compiled from the High-yield lines of Ortho Learning Objectives, cross-checked against Level II Unit A Summary—integrated and Level II Unit B Summary—integrated. ↩