1. Anatomy & Definition
- Infrazygomatic crest:
- Buccal process of maxilla connecting to zygoma
- Palpable pillar of cortical bone between:
- Zygomatic process
- Alveolar process of maxilla
- Intraorally:
- Crest of bone from buccal plate of alveolar process, lateral to roots of 1st and 2nd maxillary molars
2. Indications for IZC Screws
- Class II buccal segments with excessive overjet (avoiding orthognathic surgery)
- En‑masse retraction of maxillary arch
- Occlusal plane asymmetry / midline deviation correction
- Anchorage for cantilever in impacted canine traction
- Orthognathic surgery preparation in Class III cases
3. Placement Guidelines (Liou Lin et Al)
CBCT-Based Findings (Liou et al – AJODO 2007)
- Mean IZ crest thickness:
- ~5.2 mm (some sites)
- ~8.8 mm (other sites)
- Insertion angles:
- 40° to occlusal plane for thinner zones
- 75° for thicker zones
- If IZC thickness > lateral wall of maxillary sinus (~4.2 mm):
- Prefer 40° angle
- If thickness > 17 mm above occlusal plane:
- Prefer 75° angle
Liou’s Recommendations (IZC‑6)
- Height:
- 14–16 mm above maxillary occlusal plane and upper 1st molar
- Angle:
- 55°–70° to maxillary occlusal plane
4. Safe Zones by Facial Type (Almir Lima et al – AJODO 2022)
Study on 86 CBCTs: hyperdivergent, neutral, hypodivergent.
Safe Zones for IZC Miniscrew Insertion
| Facial Type | Between 1st & 2nd Molars | Mesial Root of 2nd Molar | Distal Root of 2nd Molar |
|---|---|---|---|
| Hyperdivergent | 11 mm from crest | 9 mm from crest | 11 mm from crest |
| Neutral | 11 mm from crest | 11 mm from crest | – |
| Hypodivergent | 11 mm from crest | 11 mm from crest | – |
General conclusion:
- Safe zones:
- 11 mm from alveolar crest between 1st & 2nd molars
- On mesial root of 2nd molar (for all facial types)
5. Sagittal Bone Availability (Furão et al – AJODO 2026)
- 100 CBCTs (40 males, 60 females)
- At 45° inclination:
- Sagittal dimension of IZC:
- Right: ~3.5 mm
- Left: ~3.6 mm
- No significant sex or side difference
- Older patients (>21 y): slightly greater sagittal bone availability than younger
- Sagittal dimension of IZC:
- Conclusion:
- Sufficient IZC bone volume at 45° for TAD insertion, with no sex/side variation (except slight age effect)
6. Primary Stability & Angular Insertion
- Angular insertion of 30° to bone surface showed:
- Greatest maximum insertion torque
- Use 30° angle when buccal bone thickness is sufficient
- Otherwise, follow Liou/Lin recommended angles (55°–70°)
7. Soft Tissue Guidelines (Lin & Roberts – IZC‑7)
- Attached gingiva: ~1.5 mm clearance from soft tissue to TAD platform
- Screw composition example:
- ~1.5 mm cortical bone
- ~7.5 mm non‑cortical (for 12 mm screw)
- or ~1.5 mm cortical + ~3.5 mm non‑cortical (for 8 mm screw)
- Placement:
- In attached gingiva with ~1.5 mm clearance from mucogingival junction to base of TAD platform
8. IZC‑6 vs IZC‑7 (Liou vs Lin)
| Feature | Liou IZC‑6 | Lin IZC‑7 |
|---|---|---|
| Position | Lateral to MB root of 6 | Lateral to MB root of 2nd molar |
| Buccal bone | Thin | Thick |
| Inter‑radicular risk | Often inter‑radicular | Mostly extra‑alveolar |
| En‑masse distalization | Some limitation | Facilitates |
| Root damage risk | Higher | Lower |
| Angle | 55°–70° | 55°–70° |
9. Biomechanics of En‑Masse Maxillary Distalization
Main Effects
- Distalization of posteriors
- Extrusion of posteriors
- Intrusion of anteriors
- Clockwise rotation of maxillary occlusal plane
Force Vector & Rotation
- Line of action passes below maxillary center of resistance (CR)
→ Clockwise rotation of occlusal plane
→ Posterior open bite tendency + anterior deep bite reduction
→ Favorable for:- Anterior open bite
- Class II correction
Transverse Considerations
- Force from buccally placed screw → rolling in of molars possible
- Countermeasures:
- Expanded arch form
- Torquing of archwire
10. Power Arm (Hook) Height & Anterior Tooth Response (Schwertner et al – FEA)
Three PA heights: 4 mm, 7 mm, 10 mm
| PA Height | Incisor Response | Canine Response |
|---|---|---|
| 4 mm (short) | More extrusion + lingual tipping | – |
| 7 mm (middle) | Preservation of anterior torque, no occlusal plane change | – |
| 10 mm (long) | Buccal tipping + intrusion of lateral incisors; no extrusion of centrals | Increased lingual tipping + extrusion |
Key point:
- Increasing PA height → shift from lingual to buccal tipping of incisors, less extrusion; canines show more lingual tipping + extrusion.
11. Clinical Outcomes (Wu et al – Implant Dent 2017)
- 20 patients, 8 months average
- Effects:
- Incisor retraction: 4.3 mm, crown extrusion: 3.8 mm
- Canine distalization: 3.7 mm, width increase: 3.1 mm
- 1st MB cusp distalization: 3.5 mm, intrusion: 2.1 mm, width: 5.0 mm
- 1st DB cusp distalization: 2.8 mm, intrusion: 3.7 mm, width: 6.2 mm
Conclusion:
- IZC miniscrews are efficient for maxillary dentition distalization.
12. FEA Comparison of TAD Positions (Sanap et al)
Models:
- Model‑1: Miniscrews between 1st–2nd premolar and 2nd premolar–1st molar
- Model‑2: IZC screws between 1st & 2nd molars
- Model‑3: IZC on MB root of 1st molar
Results:
- Maximum distalization: Model‑2 (IZC between 1st & 2nd molars)
- Maximum intrusion + less distalization: Model‑1 (buccal miniscrews anteriorly)
- No bucco‑palatal rotation in any model
Conclusion:
- IZC screws in buccal inter‑molar region are most effective for maxillary arch distalization.
13. Prospective Clinical Study (Rosa et al – Angle Orthod 2022)
- 25 adolescents, mean 7.7 months
- Effects:
- 4 mm total arch distalization
- 1.2 mm intrusion of 1st molar with 11.2° distal tipping
- Incisor retraction: 4.7 mm, lingual tipping: 13.4°
- Overjet reduction: 3.6 mm, overbite: 2.4 mm
- Occlusal plane clockwise rotation: 2.8°
- Upper lip retraction: 1 mm, nasolabial angle increase: 5.1°
Conclusion:
- Total arch distalization with IZC miniscrews is effective for Class II.
14. Gummy Smile Correction (Shaikh et al – JCDP 2021)
- 10 Class II gummy smile patients
- IZCs (14 mm) between 1st & 2nd molars + anterior mini‑implants
- Results:
- Maxillary arch distalization: 4.6 mm
- Anterior intrusion: 3.8 mm (min)
- Gummy smile reduction: 3.4 mm
- Overbite correction: 4 mm
Conclusion:
- IZC + anterior implants effective for full‑arch distalization + intrusion, improving smile esthetics.
15. Asymmetric Distalization
Biomechanical Consideration
- If no cant in occlusal plane:
- Hook height should be same as screw height (force line through CR)
Advantages
- Single‑step retraction of buccal teeth
- Midline correction simultaneously
- No separate premolar distalization step
16. Failure of IZC Screws
Reported Failure Rates
- Chang et al (Angle Orthod 2019): ~7%
- Uribe et al (Prog Orthod 2015): ~21.8%
Causes of Failure
- Poor bone quality
- Immediate loading
- Sinus floor penetration
- Placement in movable mucosa
Factors for Success
- Placement in attached mucosa
- No/mild sinus pneumatization
- High placement for distalization (to control vertical effects)
17. Maxillary Sinus Penetration (Jia et al – AJODO 2018)
- 32 patients, IZC miniscrews
- Success rate: 96.7%
- Penetration into sinus: 78.3%
- Outcomes:
- Penetration >1 mm:
- Membrane thickening incidence: 88.2%
- Mean thickening: 1.0 mm
- Penetration ≤1 mm:
- Thickening incidence: 37.5%
- Mean thickening: 0.2 mm
- Penetration >1 mm:
Conclusion:
- High penetration incidence is common, but:
- Penetration through double cortical plates with depth ≤1 mm is safe and recommended.
Quick Viva Summary
- IZC = cortical pillar lateral to 1st–2nd molar roots, connecting maxilla–zygoma
- Indications: Class II en‑masse distalization, asymmetry, cantilever, surgery prep
- Safe zone: ~11 mm from crest between 1st–2nd molars; 55°–70° to occlusal plane
- Biomechanics: distalization + posterior extrusion + anterior intrusion + clockwise rotation
- Power arm height controls anterior tipping/extrusion vs intrusion
- Failure: due to bone quality, immediate loading, sinus penetration, mucosa type
- Sinus penetration is common but acceptable if ≤1 mm.
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