The electromyographic activity of masseter and anterior temporalis during orofacial symptoms induced by experimental occlusal highspot.

The aim of the present study was to investigate the short-term impact of an occlusal highspot on the occurrence of orofacial symptoms by collecting self-evaluation and using electromyography (EMG) evaluation. A rigid unilateral intercuspal occlusal highspot (A cast onlay of 0.5 mm) was placed on the right lower first molar of six adult volunteers (three males, three females), and remained for 6 days. Continuously all the induced orofacial symptoms were collected and the subjects scored the orofacial pain on a 10-cm visual analogue scale (VAS) during the placement of onlay. The surface EMG was recorded before the placement of onlay, during (on the 3rd and 6th day) and after the onlay was removed. Then the contractile symmetry of bilateral masseter (MAL, MAR) and anterior temporalis (TAL, TAR) was measured by using an asymmetry index. On the 3rd day of the placement of the occlusal highspot, all subjects complained of headache in right temporal region (mean VAS +/- s.d.=3.7+/-0.5); the activity of TAR at rest position of mandible increased significantly (P=0.027). In addition, on the 3rd and 6th day with the highspot the EMG activity of the tested muscles during maximal voluntary contraction (MVC) was significantly reduced; the asymmetry index of bilateral anterior temporalis during MVC was increased significantly (P(3rd)=0.028; P(6th)=0.046). A unilateral occlusal highspot may make the ipsilateral anterior temporalis become tenser at rest position. Furthermore, the activity of bilateral anterior temporalis becomes more unsymmetrical during MVC although there are inter-individual differences between subjects. The changes in muscular activity may have some relationship with the occurrence of tension-type headache in temporal region.