| Objective: To measure the closing force of the upper and lower lips and to ascertain the relationship between the maximum closing force of the mentolabial muscles and types of malocclusion. Materials and Methods: Of those who showed the full eruption of a second molar and no permanent tooth loss, 99 subjects were chosen who showed a positive overbite and ANB and no skeletal asymmetry. By using the Y-meter, which can measure the lip force in the vertical direction using a load cell, the closing forces of the upper and lower lips were measured separately. A one-way analysis of variance (ANOVA) test and the Pearson's correlation test were used to evaluate the interrelationship between lip force and dentofacial morphology. Results: The lip closing force was greater in male and Class I subjects. Upper lip force was greater than that of the lower lip in all groups. The values of lip closing forces were related to the variables of upper incisor angulation. In Class II subjects, the values of lip closing forces were also related to the vertical skeletal pattern. Conclusions: The mentolabial muscle force was highly correlated with dentofacial structure and types of malocclusion.
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