To evaluate the efficacy of the modified lingual arch in treating the ectopic eruption of the permanent mandibular first molar.

The subjects consisted of 15 children who visited the department of orthodontics of Jianli Stomatological Hospital of Dongguan between January 2018 and June 2021, aged 7 to 8 with ectopic eruption of the first mandibular permanent molar, 5 of which were bilateral and 10 of which were unilateral. By welding the distal extension hook via lingual arch and bonding the lingual button on the first permanent molar, the first permanent molar was moved distally to the normal position using chain rubber band. The inclination angle and the ratio of mesio crown height to distal before and after treatment were analyzed respectively with software (SPSS 20.0) with the second deciduous molar as direct anchorage and the ectopic first mandibular permanent molar. The paired t-test was used for comparison, and the significant level was set at 0.05.

The inclination angle of the second primary molars were 64°±3° before treatment and 63°±4° after treatment respectively, where there was no significant difference (t = 1.399, P = 0.178) . Similarly, there was no significantly difference comparing the ratio of the mesio crown height to distal of the second primary molars before treatment 85%±4%, the ratio after treatment 84%±4% (t = 0.608, P = 0.550) . Significant difference (t = -10.423, P<0.001) was found in the inclination angle of the ectopic mandibular first permanent molars before treatment 52° ± 6° and after treatment 69° ± 4°. The ratio of the mesio crown height to distal of the ectopic mandibular first permanent molars after treatment was 90% ± 3%, which was significantly higher than that before treatment (80% ± 5%; t = -8.231, P<0.001) .