Ultrasonographic Images And Electromyographic Activity Of Masseter Muscle In Class Ⅱ~1 Malocclusions With Different Vertical Craniofacial Morphology

ObjectiveTo study the differences of ultrasound images and electromyography activities of masseter muscle in ClassⅡ~1 malocclusions with different vertical craniofacial morphology. To test whether the ultrasound images and electromyography activities of masseter muscle were related to the craniofacial morphology of individuals.MethodsThe subjects consisted of thirty-one female students with ClassⅡ~1 malocclusions(8 of them with high-angle facial skeletal type , 13 with normal-angle , 10 with low-angle) and five female students in normal occlusion with normal-angle. Ultrasound scanning was used to measure the perimeter, area, width, thickness, and length of masseter muscle. A computer-integrated KeypointⅣdiagnostic system was used to syn-chronously record the electromyography activities of masseter muscle All the items were measured under relaxing, maximal clenching and maximal protruding condition. Comparisons of masseter muscle images and electromyographic activity between ClassⅡ~1 malocclusions(normal-angle) and normal occlusion (normal-angle) were made using Student's t-test. Comparisons of masseter muscle images and electromyographic activity among ClassⅡ~1 malocclusions with different vertical craniofacial morphology were analyzed using ANOVA analysis . The relationship of masseter muscle images and electromyographic activity in ClassⅡ~1 malocclusions during maximal clenching to its vertical craniofacial morphology was estimated by Pearson's correlation coefficientResults(1) The average muscle thickness of the masseter muscle in normal occlusion (normal-angle) individuals was significantly greater than ClassⅡ~1 malocclusions(normal-angle) during maximal clenching. In the various mandibular positions,the masseter muscle size in ClassⅡ~1 malocclusions with low-angle facial skeletal type was larger than that of those with high-angle and normal-angle , significantly different between low-angle and high-angle;The average muscle thickness of the masseter muscle of various mandibular positions and maximal thickness during maximal clenching was significantly different among the three groups.(2) There was no significant difference in resting electromyographic activity. During maximal clenching, the masseter muscle electromyographic activity in ClassⅡ~1 malocclusions(normal-angle) individuals was significantly lower than that of normal occlusion (normal-angle), the masseter muscle electromyographic activity in ClassⅡ~1 malocclusions with high-angle was significantly lower than that of normal-angle and low-angle. During protrusive movement, activity in ClassⅡ~1 malocclusions(normal-angle) groups were beyond that of normal occlusion (normal-angle), the activity in ClassⅡ~1 malocclusions with high-angle was significantly lower than that of low-angle.(3) The result showed that the average muscle thickness of thirty-one female students with ClassⅡ~1 malocclusions during maximal clenching has a significantly negative relation to FH-MP , OP-FH , Ar-Go′-Me and positive relation to Ar- Go′,S - Go , N-ANS ; The maximal thickness has a significantly negative relation to FH-MP and positive relation to Ar- Go′,S - Go; The area of masseter muscle has a significantly negative relation to FH-MP and positive relation to S - Go, Co-Go; The length of masseter muscle has a significantly positive relation to Co-Go. The masseter muscle electromyographic activity in ClassⅡ~1 malocclusions during maximal clenching was significantly has a significantly negative relation to N-Me ,FH-MP , Ar-Go′-Me, SN-PP and positive relation to S - Go . The muscle area and thickness in ClassⅡ~1 malocclusions during maximal clenching has a significantly positive relation to the masseter muscle electromyographic activity.ConclusionThe present findings support the concept that subjects with different craniofacial morphology show the differences of form and function of the masseter muscle. When we treat the malocclusion with appliances, we should let the stomatognathic system adapt to rectification of malocclusion, then reach the harmony of morphology and function.