Assessment of masticatory muscle activity during clear aligner orthodontic treatment

Orthodontic tooth movement occurs within a functional craniofacial complex that includes the masticatory muscles. Clear aligners cover the dental occlusion and may alter muscle function and patient perception. This study evaluated changes in masticatory muscle activity with aligner use and the associated impact on oral health-related quality of life.

Twenty-four adult patients (5 males and 19 females; mean age 38.2 ± 13.4 years) undergoing clear aligner treatment were assessed in a cross-sectional study. Surface electromyography (EMG) of the masseter and anterior temporalis muscles was recorded under 2 conditions: with and without the aligners in place. EMG indexes of overall muscle effort (impact) and intensity were analyzed, along with the distribution of bite forces among the 4 muscles. Each patient's EMG signals were normalized against a maximal clench on cotton rolls. Participants also completed the 14-item Oral Health Impact Profile questionnaire. Paired statistical comparisons, correlation analyses, and 1-sample tests for muscle balance were performed (α = 0.05).

Aligner use was associated with a significant increase in EMG activity. The impact index was 27.6% higher with aligners than without (P <0.001), and the EMG intensity was modestly but significantly elevated with aligners (4.1%, P = 0.007). Aligner wear produced a more balanced distribution of muscle activity: with aligners, no significant deviation from an equal 25% contribution per muscle was observed, whereas without aligners, the muscle contributions were uneven (P <0.001 for imbalance in each muscle). The 14-item Oral Health Impact Profile scores were low (median 4.5 on a 0-56 scale) and did not correlate with muscle activity (P = 0.806). No significant differences in muscle indexes were found between sexes, and longer duration of aligner use showed no significant correlation with muscle activity levels.

Aligners tended to distribute occlusal forces more evenly across muscles, potentially reducing localized overload. These findings suggest that clinicians should anticipate a period of adaptation in which masticatory muscles respond to the altered occlusion.