Objectives: To examine the effects of orthodontic treatment with fixed appliances on masseter muscle thickness in growing children.

Subjects and method: Forty children with a mean pre-treatment age of 12.9 ± 1.19 years with mild to moderate malocclusions were treated with fixed appliances. Forty untreated children served as the control group, with a mean age of 9.14 ± 0.91 years at the start of the study. Masseter muscle thickness was measured by ultrasonography at treatment start and 1 year later in the patient group, and at equivalent times in the control group. Differences within and between the groups were evaluated using paired and independent sample t-tests, respectively. Multiple regression analysis was conducted to examine associations between different variables, including age, gender, and treatment.

Results: Masseter thickness at the beginning of the study was smaller (P = .004) in the control group (10.57 ± 1.04 mm) than in the treatment group (11.26 ± 1.03 mm), an observation that can be partly explained by the initial age difference between the groups. One year later, masseter muscle thickness increased in the control group (0.38 ± 0.50 mm, P < .001) while no statistically significant changes were found in the treatment group (-0.08 ± 0.55 mm, P = .37). The observed change in masseter muscle thickness in the two groups presented statistically significant differences (0.46 ± 0.11 mm, P < .001).

Conclusions: Masseter muscles in growing children normally increase in thickness annually. Fixed appliance treatment seems to hinder this growth during a 1-year treatment period. This information should be considered when assessing the progress and stability of the orthodontic treatment of certain malocclusions.