orthodontics

External root resorption and rapid maxillary expansion: TB vs BB comparison at post-retention

Anisha ValliLeave a comment

So, we’ve all sat through those ortho lectures where the professor keeps throwing around terms like “external root resorption” and “tooth-borne versus bone-borne expanders,” and honestly, at first, it feels like way too much. But here’s the simple breakdown of what’s actually happening.

Rapid Maxillary Expansion (RME) is used to fix transverse maxillary deficiency. The problem? The forces aren’t exactly gentle—they’re around 0.9 to 4.5 kg—and sometimes your roots pay the price. That’s where ERR (External Root Resorption) comes in.

Now, there are two main types of expanders:

  • Tooth-borne (Hyrax type): All the force is on the teeth.
  • Bone-borne (MARPE type): Screws in the palate take the load instead.

Now, how do we actually see ERR? That’s where CBCT comes in. It’s almost as accurate as micro-CT (which is super precise but can only be used on extracted teeth). Studies using CBCT showed that first molars (M1) and first premolars (P1) lose root volume after expansion, and even second premolars (P2) — the ones not holding the appliance — can get affected too. Forces spread everywhere!

Here’s the important point: most studies only looked at ERR right after expansion. But remember, cementum can repair itself a bit over time. So if you only check right away, you might overestimate the “permanent” damage. That’s why this study looked at ERR after 6 months of retention — to see what happens once the dust settles.

Here’s what the research shows:

  • Tooth-borne RME → more ERR. First molars lose the most root volume (around 17 mm³), followed by premolars. Even second premolars, which aren’t directly attached, still show resorption.
  • Bone-borne RME → less ERR. Molars only lose about 3 mm³. There’s still some resorption, but it’s way less compared to tooth-borne.

📊 Findings (6-month post-retention, CBCT-based)

ToothERR Volume Loss (mm³)TB GroupBB Group
M1 (1st molar)Highest17.033.11
P1 (1st premolar)Moderate6.421.04
P2 (2nd premolar)Least5.261.24
  • All teeth showed ERR (anchored + unanchored).
  • M1 palatal root most affected in length shortening.
  • ERR localized to apical, bucco-apical & bucco-medial areas.
  • Greater in TB vs. BB, but differences clinically questionable.
  • Mechanism of ERR: The buccal forces from the RME appliance compress the periodontal ligament, leading to tissue hyalinization. ERR occurs during the subsequent removal of this necrotic tissue on the compressed (buccal) side of the root. The root apex is also a sensitive area due to high force concentration and denser bone.

⚠️ Clinical Insights

  • ERR occurs in both abutment and non-abutment teeth.
  • Amount of root shortening (~0.3 mm) unlikely to affect longevity.
  • Bone-borne expanders ↓ ERR risk but do not eliminate it.
  • Cementum repair may occur post-retention.

📖 Citation

Leonardi R, Ronsivalle V, Barbato E, Lagravère M, Flores-Mir C, Lo Giudice A.
External root resorption and rapid maxillary expansion: TB vs BB comparison at post-retention.
Progress in Orthodontics. 2022; 23:45.

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