Abstract: BACKGROUND: The application of miniscrew in adolescents is increasing day by day, but at present, there are few studies on bone mass in the external oblique line of the mandible in adolescents at home and abroad, and there is no systematic study on bone mass in the external oblique line of the mandible in adolescents in different growth and development periods.
OBJECTIVE: To measure the bone mass in the external oblique line of the mandible in adolescents with different cervical vertebral bone ages using a cone-beam CT and to investigate the difference of bone mass in the external oblique line of the mandible in adolescents with different cervical vertebral bone ages and the correlation between bone mass in this area and the cervical vertebral bone age. 
METHODS: The cone-beam CT data of 105 adolescent patients before orthodontic treatment were collected and divided into CS3 group (n=24), CS4 group (n=26), CS5 group (n=29) and CS6 group (n=26) using the cervical vertebral maturation method. The adolescent mandibular buccal shelf was reconstructed by Mimics Medical 21.0 software. The width of buccal bone at 6 and 11 mm under the cemento-enamel junction and the bone height at 4 and 5 mm buccal to the cemento-enamel junction of right mandibular first and second molars were measured. The measured data were statistically analyzed. The measurement was made on four planes: plane 1 is the plane where the proximal mesial root of the mandibular right first molar is located; plane 2 is the plane where the distal mesial root of the mandibular right first molar is located; plane 3 is the plane where the proximal mesial root of the mandibular right second molar is located; and plane 4 is the plane where the distal mesial root of the mandibular right second molar is located.
RESULTS AND CONCLUSION: In each group, the bone width on the buccal side of the external oblique line increased gradually from the first molar proximally to the second molar distally in adolescents, and the width of buccal bone at 6 and 11 mm under the cemento-enamel junction showed significant difference among different layers (P < 0.05). The bone width of buccal bone at 11 mm under the cemento-enamel junction was greater than that at 6 mm. The bone height on the buccal side of the external oblique line increased gradually from the first molar proximally to the second molar distally in all four groups, and the bone height at 4 and 5 mm buccal to the cemento-enamel junction showed significant differences at different layers (P < 0.05). The bone height at 4 mm buccal to the cemento-enamel junction was greater than that at 5 mm. On the fourth plane, the bone width at 11 mm buccal to the cemento-enamel junction was smaller in the CS3, CS4, and CS5 groups than in the CS6 group (P < 0.05). On the third plane, the bone heights at 4 mm and 5 mm buccal to the cemento-enamel junction were smaller in the CS3 and CS4 groups than in the CS6 group (P < 0.05). On the fourth plane, the bone height at 5 mm buccal to the cemento-enamel junction was smaller in the CS3 and CS4 groups than in the CS6 group (P < 0.05). On the fourth plane, the bone height at 4 mm buccal to the cemento-enamel junction was smaller in the CS3 group than in the CS6 group (P < 0.05). Spearman correlation analysis showed that there was no correlation between bone mass and the cervical vertebral bone age, except that there was a weak correlation between bone mass at some measurement sites and cervical vertebral bone age. To conclude, the bone mass in the external oblique area of the mandible in adolescents does not change significantly with the increase of cervical vertebral bone age. The buccal side of the mesial root and distal root of the mandibular second molar in the external oblique area of CS3-CS6 adolescents meets the requirement of bone mass for miniscrew implantation, which is a site available for miniscrew implantation.

Key words: cervical vertebral bone age, external oblique line of the mandible, bone mass, miniscrew, cone-beam CT