Background/Objectives: This study aimed to evaluate the functional adaptation of the masseter muscle in growing individuals after one year of orthodontic treatment by assessing changes in its thickness.

Methods: Twenty children with a mean age of 10.4 ± 2.1 years undergoing orthodontic treatment were monitored over one year. Ultrasonographic measurements of masseter muscle thickness were taken before the commencement of orthodontic treatment and one year later. Eighteen orthodontically untreated children with a mean age of 9.9 ± 2.0 years served as the control group; their masseter muscle thickness was measured at baseline and after the same follow-up period. Comparisons were made between the two groups.

Results: At baseline, the mean masseter muscle thickness was 11.4 ± 1.3 mm in the control group and 11.7 ± 1.4 mm in the treatment group. After one year, children in the untreated control group showed an average increase of 0.5 ± 0.6 mm (p < 0.001) in the thickness of the masseter muscle, whereas those undergoing orthodontic treatment exhibited an average decrease of 0.6 ± 0.7 mm (p < 0.001). Multiple regression analysis accounting for age, gender, and initial masseter muscle thickness indicated that orthodontic treatment resulted in a reduction in masseter muscle thickness by 1.1 mm (p < 0.001) compared with the untreated control group.

Conclusions: Orthodontic treatment may influence the development of the masticatory muscles. In our sample, the masseter muscle showed an estimated atrophy of approximately 9% after one year of orthodontic treatment compared with the thickness it would likely have achieved in the absence of any orthodontic intervention.