Skeletal sagittal and vertical facial types and electromyographic activity of the masticatory muscle

To investigate the electromyographic activities of the anterior temporal (T) and masseter (M) muscles in different facial skeletal types.

The samples consisted of 105 subjects (38 males and 67 females; mean age 22.0 +/- 6.7 years) and were classified into six groups according to the values of ANB and SN-GoMe: group 1 for Class I malocclusion and normodivergent type (n = 27), group 2 for Class I and hyperdivergent type (n = 20), group 3 for Class II and normodivergent type (n = 10), group 4 for Class II and hyperdivergent type (n = 23), group 5 for Class III and normodivergent type (n = 12), and group 6 for Class III and hyperdivergent type (n = 13). Temporal muscle activity (TMA), masseter muscle activity (MMA), and T/M ratio were evaluated at resting and clenching status.

Although there was no significant difference in resting MMA among all groups, group 6 showed a higher resting TMA than did other groups and a significant difference in resting T/M ratio compared with groups 1 and 3. There were no significant differences in clenching TMA and MMA among all groups. Although all groups showed a significant increase of TMA and MMA from resting to clenching status, group 6 showed a significant decrease of clenching T/M ratio compared with resting T/M ratio.

The results suggest that the more Class III and the more hyperdivergent type, the higher resting TMA and the lesser increase of clenching MMA than expressed by other groups. Significant differences existed in TMA and MMA according to sagittal and vertical facial skeletal types.