Evaluation of Orthodontic Force Magnitude on Root Resorption: A Clinical and Radiographic Study

Abstract

Objectives: To evaluate the effect of orthodontic force magnitude on external apical root resorption (EARR) using clinical and radiographic outcomes. Materials and Methods: A prospective, parallel-group study was conducted at a university dental hospital in Riyadh, Saudi Arabia. Sixty-six patients were allocated to light (25–50 g), moderate (75–100 g), or heavy (150–200 g) continuous forces delivered by nickel–titanium closed-coil springs for three months. Imaging at baseline (T0) and follow-up (T1) used cone-beam computed tomography when ethically justified or standardized periapical radiographs. The primary endpoint was change in CEJ–apex root length (mm); secondary endpoints included Malmgren index grades, pain scores, and adverse events. Results: Groups were comparable at baseline. Mean EARR (Δ root length, mm ± SD) increased with force: light 0.29 ± 0.18, moderate 0.49 ± 0.24, heavy 0.83 ± 0.31. One-way ANOVA was significant (F(2,63)=26.43, p<0.001); Bonferroni-adjusted comparisons confirmed heavy > moderate > light (all p≤0.03). Clinically relevant shortening (≥2.0 mm) occurred in 0%, 4.5%, and 9.1% of participants, respectively. Malmgren grades shifted toward 2–3 with heavier forces. In mixed-effects models, force magnitude was the strongest predictor (βmoderate=+0.19 mm, 95% CI 0.09–0.30; βheavy=+0.52 mm, 95% CI 0.40–0.64), and pointed apical morphology independently increased risk (+0.18 mm, 95% CI 0.07–0.28). Conclusions: Heavier continuous forces substantially increase EARR, while light forces produce minimal, clinically insignificant changes. Treatment should prioritize light to moderate, well-calibrated forces and risk-stratified radiographic monitoring. Clinical Significance: Standardized periapicals suffice for routine surveillance; CBCT should be reserved for equivocal or high-risk cases where three-dimensional assessment will alter management.