Muscle response to the oral-screen activator. An EMG study of the masseter, buccinator, and mentalis muscles.

The myofunctional changes that occur during orthodontic treatment with functional appliances constructed with vestibular screen elements, such as buccal shields and lip pads, have not been sufficiently investigated. The aim of this study was to examine electromyographically the response of the masseter, buccinator and mentalis muscles to the oral-screen activator (OSAC), which is a conventional activator constructed with buccal shields and lip pads. The material consisted of 10 children, with Angle Class II, division 1 malocclusion and narrow dental arches (mean age 10.7 years), treated at the Department of Orthodontics, School of Dentistry, Malmö, Sweden. The group had a mean overjet of 8.5 mm, and a mean ANB angle of 6 degrees. Six out of the 10 cases had incompetent lips at relaxed lip position. The construction bite of the OSAC was taken with the anterior teeth in edge-to-edge position. The recording procedure included the following positions: (a) postural position of the mandible with lips relaxed and lips closed; (b) clenching; (c) swallowing saliva; and (d) spatula exercise. EMG recordings were made after 1, 2, and 3 months of appliance treatment. Differences between means were tested with a two-way analysis of variance (ANOVA). The results showed that during postural position with lips closed there was a statistically significant increase in mentalis EMG with the oral-screen activator in place. The buccinator and masseter muscles showed no EMG activity during rest with or without the appliance in the mouth. During clenching we found no statistically significant difference in masseter EMG with or without the appliance in place. During swallowing saliva a statistically significant decrease was recorded in mentalis EMG with the oral-screen activator in the mouth, while buccinator activities remained the same with and without the appliance. The masseter activity, on the other hand, increased significantly with the activator in place. During spatula exercise mentalis and buccinator were highly activated. The conclusions drawn from this study are that the oral screen activator (OSAC) increased masseter activity during swallowing, but it remained unchanged during clenching. Lip pads increase mentalis activity during lip closure, but reduce mentalis hyperactivity during swallowing. The buccinator activity was insignificant and buccal shields did not change that activity. Spatula exercise stimulates both mentalis and buccinator activity.