Assessment of Masticatory Muscle Activity and Mandibular Border Movement in Skeletal Class II Malocclusion Adolescents with Mandibular Retrognathia

Objective To investigate masticatory muscle activity and mandibular border movement in adolescents with skeletal class II malocclusion and mandibular retrognathia and explore the influence of mandibular retrognathia on masticatory system function. Methods Thirty adolescents aged 10–15 years diagnosed with skeletal class II malocclusion and mandibular retrognathia comprised the skeletal class II malocclusion group, while 30 adolescents with skeletal class I malocclusion constituted the control group. Functional assessments of the masticatory system were performed in both cohorts, encompassing Bio-EMG and JT-3D measurements during rest, clenching, chewing, swallowing, and border movements. Electromyographic (EMG) activity and mandibular kinematics were comprehensively measured and compared between the two groups. Results 1) EMG: During rest, there was no significant difference in the EMG amplitude of the muscles between the skeletal class II malocclusion and control groups. However, during clenching, the EMG amplitude of the anterior temporalis and masseter muscles was lower in the skeletal class II malocclusion group compared to the control group. Similarly, the EMG amplitude of the working-side temporalis and masseter muscles during chewing was lower in the skeletal class II malocclusion group. Furthermore, the mean EMG values of the anterior temporalis and both masseter muscles during swallowing were lower in the skeletal class II malocclusion group, while the mentalis muscle exhibited higher EMG activity. 2) Mandibular movement: The skeletal class II malocclusion group demonstrated increased vertical and coronal displacements in the mandibular resting position compared to the control group. However, during chewing, no significant disparity in mandibular displacement was observed between the skeletal class II malocclusion and control groups. In contrast, during swallowing, the skeletal class II malocclusion group exhibited greater vertical and coronal mandibular displacements, particularly with more pronounced forward sagittal displacement. Notably, there was no significant distinction in the three-dimensional mandibular displacement during maximum mouth opening between the two groups. Conclusion Functional impairments in the masticatory system of adolescents with skeletal class II malocclusion and mandibular retrognathia were evident across multiple activities, encompassing rest position, clenching, chewing, swallowing, and border movements. These results underscore the considerable influence of skeletal class II malocclusion on the functional integrity of the masticatory system.