Ng J, Major PW, Heo G, Flores-Mir C.
American Journal of Orthodontics and Dentofacial Orthopedics. 2005;128:212–219.
🎯 STUDY AIM
To determine how much true incisor intrusion is actually achieved during orthodontic treatment.
Key question:
When incisors appear to intrude clinically, how much is genuine intrusion versus apparent intrusion caused by growth or other tooth movements?
🔑 WHAT IS TRUE INTRUSION?
True intrusion
Apical movement of the tooth’s center of resistance/root relative to a stable skeletal reference.
Important distinction
Clinical crown appears to move apically
≠
True intrusion of the entire tooth
Apparent intrusion may result from:
- Labial tipping of incisors
- Posterior extrusion
- Growth changes
- Reference-plane changes
- Crown movement without bodily intrusion
📊 SYSTEMATIC REVIEW — KEY FINDINGS
True incisor intrusion achieved:
| Incisor group | Mean true intrusion |
|---|---|
| Maxillary incisors | ~1.5 mm |
| Mandibular incisors | ~1.9 mm |
Overall conclusion
True incisor intrusion is possible, but the amount achieved is relatively small.
⚙️ FACTORS AFFECTING TRUE INTRUSION
1. FORCE SYSTEM
Forces should be directed through or near the center of resistance.
Correct force vector
→ Intrusive movement
Incorrect force vector
→ Intrusion + tipping
2. ANCHORAGE
Loss of posterior anchorage may produce:
Posterior extrusion
↓
Mandibular clockwise rotation
↓
Apparent bite opening
This may be mistaken for true incisor intrusion.
3. GROWTH
In growing patients:
- Vertical facial growth
- Eruption of adjacent teeth
- Changes in skeletal reference points
may influence measured intrusion.
4. INCISOR INCLINATION
Labial tipping of incisors
can produce an apparent reduction in overbite without equivalent bodily intrusion.
🏆 GOLD MEDAL ANSWER
Ng et al. (2005), in a systematic review and meta-analysis, evaluated the amount of true incisor intrusion achieved during orthodontic treatment. The study demonstrated that genuine incisor intrusion is achievable, but the magnitude is relatively limited, approximately 1.5 mm for maxillary incisors and 1.9 mm for mandibular incisors. Therefore, the reduction in overbite observed clinically should not be attributed entirely to true incisor intrusion, as it may also result from incisor inclination changes, posterior tooth extrusion, mandibular rotation, and growth-related changes. This finding emphasizes the importance of distinguishing true intrusion from apparent or pseudo-intrusion when evaluating deep-bite correction.
🔥 ONE-LINE MEMORY HOOK
“Deep bite correction ≠ pure intrusion; true intrusion is modest—~1.5 mm upper, ~1.9 mm lower.”
Dentowesome 