How Much BRT Do You Need to Expand Safely? What a Finite Element Study Tells Us

Expanding the upper arch with aligners tips premolars buccally. A new CBCT + FEA study quantifies how buccal bone thickness and buccal root torque interact, and finds a clear sweet spot.

Why this question matters

Aligners deliver force on the crown, and root control during transverse expansion is limited. The result is buccal tipping of posterior teeth instead of bodily movement, which concentrates stress in the cervical cortical bone and the PDL. In patients with a thin buccal plate, that is a direct path to dehiscence and fenestration. Buccal root torque (BRT) is the obvious tool to counter it, but how much, and does it depend on the bone?

What the authors did

Yu et al. (Am J Transl Res, 2026) combined two parts. First, a CBCT analysis of 100 adults (20–30 years, 800 teeth) measuring the buccal plate of PM1, PM2, M1 and M2 at 3, 6 and 8 mm from the CEJ and at the apex. Second, a 3D finite element analysis: three maxillary models with a thin (0.42 mm), medium (0.72 mm) or thick (1.02 mm) plate at PM1, each combined with four BRT prescriptions (0°, 0.5°, 1.0°, 1.5°) and a 0.2 mm buccal expansion step, 12 models in total.

Finding 1: PM1 is the weak point

At 3 mm below the CEJ, PM1 measured 0.72 mm, significantly thinner than every other site (PM2 1.07 mm, M1 mesiobuccal 0.89 mm, M2 mesiobuccal 1.59 mm). Bone thickens toward the apex at every tooth, so the cervical 3 mm is the real danger zone during expansion.

Finding 2: torque cuts the stress

In the thin-bone model without torque, PDL stress concentrated at the neck of PM1. Adding 1.0° of BRT reduced peak stress by 57.1% and moved the tooth more bodily. The lowest stress in each bone model came from 1.0°.

Finding 3: more torque is not better

Root control was measured as the R/C ratio (root movement ÷ crown movement). In thin bone at 0°, the ratio reached about −115%: the root swung further than the crown in the opposite direction, which is pure tipping. At 1.0° it improved to about −36% in all three models, so pure bodily expansion is still not fully achieved, but tipping is much better controlled. At 1.5°, stress increased again and crown tipping returned (around −50%). Overshooting torque costs control.

How I apply it in treatment planning

Limitations

The FEA model came from a single volunteer, used linear isotropic materials (the real PDL is non-linear and viscoelastic), included no attachments, simulated one 0.2 mm step and shows immediate stress only, with no remodelling. Treat these numbers as biomechanical guidance, not clinical proof, until clinical and CBCT follow-up studies confirm them.

Reference: Yu T, Yang Y, Liu Y, et al. Three-dimensional finite element analysis of bracketless clear aligner expansion with different buccal bone plate thicknesses. Am J Transl Res. 2026;18(5):3783–3793. doi:10.62347/EYJB3618