Banks P, Carmichael G.
JCO. 1999;33(11):620–623.

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🎯 KEY CONCEPT

Modified Twin-Block = Controlled, Stepwise Mandibular Advancement

Instead of advancing the mandible immediately to an edge-to-edge incisor position, advancement is performed gradually using chairside-activated advancement screws + acetal resin spacers.

⭐ Main advantage

Large overjet + limited mandibular protrusion → gradual advancement without forcing the mandible forward.

---

🧠 WHY STEPWISE ADVANCEMENT?

Conventional approach

Edge-to-edge bite registration

⬇️

May cause:

• Excessive muscular tension
• Patient discomfort
• Speech difficulties
• Reduced compliance
• Difficulty maintaining appliance position during sleep
• Greater incisor tipping

Stepwise advancement

Small initial advancement
⬇️
Adaptation
⬇️
Progressive advancement
⬇️
Reduced overjet

Potential benefits

✅ Greater orthopedic effect
✅ Less incisor tilting
✅ Better patient comfort
✅ Better speech
✅ Improved compliance
✅ Better appliance positioning during sleep

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⚙️ MODIFIED TWIN-BLOCK DESIGN

Components

Maxillary appliance blocks
⬇️

Contain advancement screws

⬇️

Activated by inserting:

Cylindrical acetal resin spacers

Different spacer thicknesses allow controlled incremental advancement.

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📏 ADVANCEMENT CAPACITY

Screw	Maximum/indicated advancement
Standard screw	Up to 7 mm
16 mm screw	For greater activation
20 mm screw	For very large advancement

Reported clinical use

Up to 12 mm stepwise advancement

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🩺 CASE SNAPSHOT

Patient

12-year-old female

Clinical findings

• Severe overjet: 12 mm
• Excessive but incomplete overbite
• Mandibular retrusion
• Dolichofacial pattern
• Incompetent lips
• Limited mandibular protrusion
• Slight bilateral Class II molar relationship
• Severe maxillary crowding
• Buccally positioned canines
• Palatally inclined lateral incisors
• Minimal mandibular crowding
• Previous loss of mandibular first premolars

Cephalometry

• Marked Class II dental relationship
• Mandibular retrusion
• Excessive maxillomandibular plane angle

---

🦷 TREATMENT PLAN

Phase 1

Modified Twin-Block

→ Stepwise mandibular advancement

→ Correct overjet

Phase 2

Extraction of maxillary first premolars

Phase 3

Fixed appliances

→ Align and level both arches

Retention

• Upper Hawley retainer
• Lower fixed multistranded retainer

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🔥 STEPWISE ACTIVATION PROTOCOL

Initial condition

Overjet = 12 mm

Maximum comfortable protrusion = only 3 mm

⬇️

Initial bite advancement

3 mm

⬇️ 6–8 weeks

First reactivation

+3 mm spacer

⬇️ 6–8 weeks

Second reactivation

+3 mm spacer

⬇️ 6–8 weeks

Third reactivation

+2 mm spacer

⬇️

Total advancement

3 + 3 + 3 + 2 = 11 mm

⬇️

Final result

Overjet reduced to 0 mm

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⏱️ TREATMENT TIMELINE

Stage	Duration
Active Twin-Block phase	7 months
Continued full-time wear	2 additional months
Maxillary first premolar extraction	During continued Twin-Block wear
Fixed appliance therapy	17 months
Total active treatment	24 months

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🧩 CLINICAL PROBLEM → SOLUTION

Problem:

12 mm overjet

but

Mandibular advancement possible:

Only 3 mm initially

⬇️

Conventional Twin-Block

❌ Immediate large advancement difficult

⬇️

Modified Twin-Block

✅ Start with 3 mm advancement

⬇️

Gradually add spacers

⬇️

Progressive mandibular advancement

⬇️

Final overjet correction

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💡 CHAIR-SIDE PROTOCOL

When using a modified Twin-Block:

1. Assess maximum comfortable mandibular protrusion

↓

2. Start with a small advancement

↓

3. Allow neuromuscular adaptation

↓

4. Review at approximately 6–8 weeks

↓

5. Add bilateral spacers

↓

6. Reassess overjet and patient comfort

↓

7. Continue progressive advancement until correction

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🚨 KEY INDICATION

Particularly useful in:

⭐ Severe overjet

⭐ Limited mandibular protrusive movement

⭐ Patients unable to posture edge-to-edge

⭐ Patients who may not tolerate large initial advancement

⭐ Cases requiring large total mandibular advancement

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⚠️ IMPORTANT CLINICAL POINT

Do NOT confuse:

Initial mandibular advancement

with

Total required overjet correction

Example:

Initial advancement: 3 mm

Overjet: 12 mm

The patient does not need to achieve 12 mm advancement immediately.

Instead:

Small advancement → adaptation → reactivation → further advancement

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📌 EXAM PEARLS

Banks & Carmichael, 1999

🔹 Modified Twin-Block allows chairside progressive mandibular advancement

🔹 Advancement achieved using screws + cylindrical acetal resin spacers

🔹 Initial advancement can be small when mandibular protrusion is restricted

🔹 3 + 3 + 3 + 2 mm incremental advancement in the reported case

🔹 12 mm overjet → 0 mm overjet

🔹 Active Twin-Block phase: 7 months

🔹 Total active treatment: 24 months

🔹 16 mm and 20 mm screws may be required for greater activation

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🏆 GOLD MEDAL ANSWER

Banks and Carmichael described a modified Twin-Block appliance that permits controlled, progressive mandibular advancement using advancement screws incorporated into the maxillary blocks and cylindrical acetal resin spacers. This approach is particularly advantageous in patients with severe overjet who cannot initially posture the mandible to an edge-to-edge position. By using small incremental advancements at 6–8-week intervals, muscular adaptation and patient comfort may be improved while potentially reducing unwanted incisor tipping. In the reported 12-year-old patient, an initial 3 mm advancement was progressively increased by 3 mm, 3 mm and 2 mm spacers, reducing a 12 mm overjet to zero over a 7-month Twin-Block phase.

🔥 ONE-LINE MEMORY HOOK

“Can’t advance 12 mm? Don’t force it—start at 3 mm, adapt, add spacers, and advance step-by-step.”

Ng J, Major PW, Heo G, Flores-Mir C.
American Journal of Orthodontics and Dentofacial Orthopedics. 2005;128:212–219.

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🎯 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?

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🔑 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

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📊 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.

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⚙️ 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.”

Danz JC et al. European Journal of Orthodontics. 2014;36:522–530

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🔑 STUDY AT A GLANCE

Parameter	Finding
Study design	Retrospective long-term follow-up
Original sample	855 former orthodontic patients
Eligible deep bite cases	185
Final analyzed complete-treatment sample	43
Relapse cases	4/43
Median follow-up	11.9 years
Relapse definition	Incisor overlap <50% after treatment → ≥50% at follow-up
Relapse prevalence	10.3%
Median increase in overlap in relapse group	6.7%
Range of increase	3.2–19.8%
Stable cases	~90%
Major conclusion	Long-term vertical relapse was low and clinically small

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📌 DEEP BITE — QUICK BASICS

Definition

Increased vertical overlap of maxillary and mandibular incisors

Measured as:

• Absolute: mm
• Relative: % incisor overlap
• Qualitative: Dental/gingival/palatal contact

Etiology

1. Dentoalveolar

• Incisor overeruption
• Altered incisor inclination

2. Skeletal

• Reduced lower facial height
• Low mandibular plane angle
• Hypodivergent growth pattern

---

⚠️ WHY DOES DEEP BITE RELAPSE?

Potential contributors

Growth
→ Mandibular growth rotation

Function
→ Muscular and functional influences

Dental changes
→ Incisor overeruption
→ Loss of arch form
→ Lower incisor crowding
→ Upper anterior spacing

Periodontal remodeling
→ Reorganization of periodontal tissues

Retention
→ Inadequate or absent retention

Treatment outcome
→ Incomplete leveling
→ Persistent deep bite
→ Poor incisor contact

Other factors
→ Lower lip pressure
→ Sagittal relapse
→ Bolton discrepancy
→ Interincisal angle

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🦷 THREE WAYS TO CORRECT DEEP BITE

1️⃣ INCISOR INTRUSION

↓
Intrude upper and/or lower incisors

Best when: Excessive incisor eruption contributes to deep bite

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2️⃣ INCISOR LABIAL INCLINATION

↓
Pseudo-intrusion

Incisor proclination increases effective vertical clearance.

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3️⃣ POSTERIOR EXTRUSION

↓
Posterior tooth eruption
↓
Possible clockwise mandibular rotation
↓
↑ Lower anterior facial height
↓
Open bite / reduce deep bite

⚠️ Clockwise rotation does not necessarily occur in every patient.

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🔴 THE MOST IMPORTANT CLINICAL FINDING

Successfully corrected deep bite

More stable than expected

90% remained without vertical relapse

Relapse

Only 10.3% demonstrated recurrence to ≥50% incisor overlap.

And even in these cases:

The amount of relapse was small

Median increase = 6.7%

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🚨 PARTIAL TREATMENT ≠ TRUE RELAPSE

This is the most clinically important message from the study.

True relapse

T1: Deep bite
↓
T2: Successfully corrected
↓
T3: Deep bite returns

Partial correction / noncompliance

T1: Deep bite
↓
T2: Still deep bite
↓
T3: Deep bite persists or worsens

Study finding

Deep bite at long-term follow-up was more likely due to PARTIAL CORRECTION than TRUE RELAPSE.

Group	Deep bite at T3
True relapse	4 patients
Partial treatment	7 patients

⭐ Clinical takeaway

Finish the correction!

Incomplete leveling and incomplete establishment of anterior contact may be more problematic than relapse after successful correction.

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🦷 CONTACT AT LONG-TERM FOLLOW-UP

Completely treated group

• Dental incisor contact: 88%
• No incisor contact: 12%
• Gingival contact: 0%
• Palatal impingement: 0%

Partial treatment group

• Dental incisor contact: 29%
• Gingival contact: 57%
• Palatal impingement: 14%

🚨 Clinical implication

Persistent deep bite + incomplete treatment
→ ↑ Gingival contact
→ ↑ Palatal impingement
→ ↑ Risk of traumatic contact

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🔒 RETENTION PEARLS

In the relapse group:

• No patient had an upper fixed retainer at T3
• Only 2/4 had a lower fixed retainer
• Only 1/4 received removable retention

Possible clinical implication

Absence of retention may facilitate:

• Upper anterior spacing
• Loss of incisor contact
• Arch form changes
• Deepening of overbite

⚠️ However: The study was not large enough to prove retention as a relapse risk factor.

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🧠 WHY WAS RELAPSE LOW?

Possible explanations:

1. Older age at debonding

Median age at T2 ≈ 17 years

↓ Less remaining growth

2. Long treatment duration

More periodontal remodeling may have occurred before debonding.

3. Retention

Majority received:

• Lower fixed retainer
• Temporary upper removable bite plate

4. Successful correction

Most patients achieved:

• Proper incisor contact
• Adequate leveling

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❌ RISK FACTORS COULD NOT BE CONFIRMED

The study could not identify reliable predictors of deep bite relapse because:

• Relapse prevalence was low
• Relapse magnitude was small
• Only 4 relapse cases
• Sample size was limited
• Retention protocols were not standardized

Therefore:

No clinically reliable prediction model for individual deep bite relapse could be established.

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🩺 CHAIR-SIDE CLINICAL CHECKLIST

Before debonding

☐ Deep bite fully corrected
☐ Curve of Spee adequately leveled
☐ Stable anterior incisor contact
☐ No gingival contact
☐ No palatal impingement
☐ Evaluate incisor inclination
☐ Assess interincisal angle
☐ Check arch form
☐ Check anterior spacing
☐ Evaluate sagittal relationship
☐ Plan long-term retention

After treatment

Monitor for:

🔍 Increased overbite
🔍 Loss of anterior contact
🔍 Upper anterior spacing
🔍 Lower anterior crowding
🔍 Loss of arch length
🔍 Sagittal relapse
🔍 Growth-related mandibular rotation

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⭐ GOLD MEDAL EXAM TAKE-HOME

Danz et al. 2014

Long-term relapse of successfully treated moderate deep bite was relatively uncommon and small. Approximately 90% remained stable after a median 11.9-year follow-up, while 10.3% showed relapse, with a median increase in incisor overlap of only 6.7%. Importantly, persistent deep bite at long-term follow-up was more commonly associated with incomplete treatment than true relapse.

🔥 ONE-LINE MEMORY HOOK

“Correct it completely → Establish anterior contact → Retain it → Relapse is usually small.”