Rapid Maxillary Expansion (RME) is one of the most powerful orthopedic tools available during growth. While most of us associate RME with “widening the palate” and correcting crossbites, its influence extends far beyond the transverse plane.
A landmark study by Farronato et al. evaluated 183 growing patients—Class I, II, and III—and revealed that RME also drives important sagittal and vertical changes. These effects vary significantly depending on the skeletal class, which is critical when planning early treatment.
This blog breaks down these findings into practical clinical insights you can apply from your very next patient.
🔍 Why RME Matters Beyond Transverse Correction
When the Hyrax appliance opens the midpalatal suture, it triggers a chain reaction:
- Circummaxillary sutures loosen
- Maxilla may reposition
- Mandible adapts to new occlusal contacts
- Vertical dimensions can shift
These effects can help or hinder skeletal correction—if you understand how they behave in each malocclusion.
🔹 CLASS I
Sagittal
- ANB ↓ slightly (–0.34°)
→ Mild improvement toward Class I
Maxilla & Mandible
- No significant sagittal movement
- Slight downward–backward rotation of palatal plane
Vertical
- No significant N–Me change
- Mandibular plane: unchanged
👉 Clinical Impact
- Improves transverse deficiency without disturbing sagittal or vertical balance.
Class I kids are like the straight-A students who also volunteer and play violin.
You expand them and—poof!—they get wider.
That’s it.
No tantrums. No drama. No sagittal plot twists.
Sagittally? Nothing much.
Vertically? Eh.
ANB changes by, what, –0.34°?
It’s like telling someone you changed your hairstyle and they say,
“Really? I… can’t see it.”
🔹 CLASS II
Sagittal
- SNB ↑ (+2.25°) → Mandible moves forward (statistically significant)
- ANB ↓ (–1.81°) → Skeletal Class II improves
- SNA unchanged (maxilla stable)
Mechanism
- Expansion “releases” the mandible → spontaneous forward posturing (McNamara effect)
Vertical
- No significant anterior facial height increase
- Palatal plane rotates down–back (slight)
👉 Clinical Impact
- RME can improve Class II sagittal pattern in early mixed dentition.
- Mandibular advancement occurs mainly during retention.
Ah, Class II.
The ones whose mandibles have been sitting back like they’re too cool to show up on time.
Enter RME.
Suddenly the mandible pops forward like:
“I’m here! I’m fabulous! I’m 2.25° more fabulous!”
SNB goes up.
ANB goes down.
Orthodontists everywhere go,
“Sweet mother of cephalometrics, it actually worked!”
It’s like giving someone better shoes and suddenly they walk straighter.
🔹 CLASS III
Sagittal
- SNA ↑ (+0.81°) → Maxilla moves forward
- SNB ↓ (–1.35°) → Mandible rotates down–back
- ANB ↑ (+2.16°) → Significant correction toward Class I
Vertical
- N–Me ↑ (+0.84 mm) → Increased anterior facial height
- Downward–backward rotation of mandible & palatal plane
👉 Clinical Impact
- RME improves early skeletal Class III by:
✓ Forward translation of maxilla
✓ Clockwise rotation of mandible
Now, Class III…
These kids don’t just enter the clinic.
They storm in with a plotline.
RME hits them and BAM—
the maxilla moves forward (+0.81°),
the mandible rotates down and back like it’s trying to avoid an awkward conversation,
and ANB shoots up like a Broadway finale (+2.16°).
Meanwhile, vertical height increases too.
Because of course it does.
Why do one thing when you can do five?
Class III kids after RME look like they’ve had a character arc.
Like they went to Paris and “found themselves.”
Summary of Cephalometric Changes After RME
| Parameter | Class I | Class II | Class III |
|---|
| Maxillary Position (SNA) | No significant change | No significant change | ↑ SNA (maxilla moves forward) |
| Mandibular Position (SNB) | No significant change | ↑ SNB (mandible moves forward) | ↓ SNB (mandible rotates down–back) |
| ANB Angle | ↓ slightly (minor Class I improvement) | ↓↓ significantly (Class II improves) | ↑↑ significantly (Class III improves) |
| Palatal Plane (SN–SNP.SNA) | ↑ (down–back rotation) | ↑ (down–back rotation) | ↑ more (significant rotation) |
| Anterior Facial Height (N–Me) | No significant change | No significant change | ↑ increased vertical height |
| Mandibular Plane (SN–GoGn) | No significant change | No significant change | Mild ↑ (not always significant) |
| Posterior Facial Height (S–Go) | No significant change | No significant change | No significant change |
Clinical Interpretation of RME Effects
| Clinical Aspect | Class I | Class II | Class III |
|---|---|---|---|
| Sagittal Effect | Minimal | Mandible moves forward → improves Class II | Maxilla advances + mandible rotates back → improves Class III |
| Vertical Effect | Stable | Stable | Vertical dimension increases (N–Me ↑) |
| Overall Skeletal Correction | Mild | Moderate | Strong |
| Most Active Phase | Active + Retention | Mainly retention | Active phase |
| Risk Areas | Few | Few | Vertical increase in hyperdivergent cases |
Mechanism Behind RME Changes
| Effect | How It Happens |
|---|---|
| Mandibular forward shift (Class II) | Removal of transverse “lock” → lower arch can posture forward (McNamara effect) |
| Maxillary forward movement (Class III) | Expansion affects circummaxillary sutures → allows slight anterior displacement |
| Palatal plane rotation | Down–back rotation from suture opening → common to all classes |
| Increase in N–Me (Class III) | Maxilla forward, mandible back/down |
Dentowesome 