Abstract
Two factors possibly break the lip seal at rest. The first is mouth breathing for some reason. The second is lip incompetence due to antero-posterior and vertical disharmony of the dentofacial complex. In this study, we investigated the relationship between lip sealing and mouth breathing, by measuring EMG activities of mentalis (MM), suprahyoid (SHM) and masseter muscles in nasal obstruction. Thirty-four subjects were divided into two groups of competent lip (CL : 11 subjects) and incompetent lip (IL : 23 subjects) groups on the basis of EMG activity of the mentalis muscle at rest. Mouth breathing was induced by obstructing the nostrils. EMG activities were recorded at rest and during chewing gum with the lips closed and apart. EMG activities of these muscles and the masticatory cycle time were estimated during chewing gum for 30 seconds. The Main findings were as follows : 1) EMG activity of MM was higher when obstructing the nostrils than when consciously opening the lips in the CL group at rest. 2) EMG activity of SHM was lowest when obstructing the nostrils in the IL group at rest. 3) The duration of the mastication cycle was shorter when chewing gum with open lips and by obstructing the nostrils than with closed lips in the IL group. This was ascribed to shortening the duration of the non-active masseter phase. 4) In both groups, coefficient of variation values for the total masticatory cycle increased when obstructing the nostrils, but in IL, this decreased when subjects consciously opened the lips and obstructed the nostrils. 5) In the active masseter phase, MM activity of the CL group was highest when subjects obstructed the nostrils and consciously opened the lips, while that of the IL group showed no significant change. 6) In the non-active masseter phase, MM activity of both groups was highest with closed the lips. Furthermore, that of the IL group was higher than the CL group. 7) In the active masseter phase, SHM activity of the two groups was highest when subjects obstructed the nostrils and consciously opened the lips. Furthermore, that of the CL group was higher than the IL group. 8) In the non-active masseter phase, SHM activity of the CL group was highest when subjects obstructed the nostrils and consciously, opened the lips, while, that of the IL group showed no significant change. The results suggest that subjects with incompetent lips tend to easily adopt the habit of mouth breathing. We conclude that it is neccessary to improve the lip sealing function by orthodontic or orthognathic surgical treatment to prevent damage by mouth btreathing.
