Short-term changes in muscle activity and jaw movement patterns after orthognathic surgery in skeletal Class III patients with facial asymmetry

Abstract

Objective: To evaluate the short-term changes in masticatory muscle activity and mandibular movement patterns after orthognathic surgery in skeletal Class III patients with facial asymmetry.

Methods: Twenty-seven skeletal Class III adult patients were divided into two groups based on the degree of facial asymmetry: the experimental group (n = 17 [11 male and 6 female]; menton deviation ≥ 4 mm) and control group (n = 10 [4 male and 6 female]; menton deviation < 1.6 mm). Cephalography, electromyography (EMG) for the anterior temporalis (TA) and masseter muscles (MM), and mandibular movement (range of motion [ROM] and average chewing pattern [ACP]) were evaluated before (T0) and 7 to 8 months (T1) after the surgery.

Results: There were no significant postoperative changes in the EMG potentials of the TA and MM in both groups, except in the anterior cotton roll biting test, in which the masticatory muscle activity had changed into an MM-dominant pattern postoperatively in both groups. In the experimental group, the amount of maximum opening, protrusion, and lateral excursion to the non-deviated side were significantly decreased. The turning point tended to be shorter and significantly moved medially during chewing in the non-deviated side in the experimental group.

Conclusions: In skeletal Class III patients with facial asymmetry, the EMG activity characteristics recovered to presurgical levels within 7 to 8 months after the surgery. Correction of the asymmetry caused limitation in jaw movement in terms of both ROM and ACP on the non-deviated side.

Keywords: Class III orthognathic surgery; Electromyography; Facial asymmetry; Jaw movement.

Figure 1. Cephalometric measurements.

1, Sella-Nasion to A Point angle (SNA); 2, sella-nasion to B point angle (SNB); 3, A point-Nasion to B point angle (ANB); 4, upper incisor to Frankfort plane angle (U1-FH); 5, lower incisor to mandibular plane angle (IMPA); 6, menton deviation-the distance from the perpendicular bisector line of both frontozygomatic points (FZPs) to the menton.

Figure 2. A, Electromyographic recording device (Bio-EMG III™; BioResearch, Inc., Milwaukee, WI, USA) and the data for clenching obtained from one subject. B, Jaw movement tracking device (JT-3D™; BioResearch, Inc.) and the sequence for right-sided gum chewing obtained from one subject.

Figure 3. Schematic drawing of the jaw movement. A, Two cycles of the three-dimensional mandibular incisor-point chewing sequences. The three graphs indicate the tracing in the vertical (upper), anteroposterior (middle), and lateral (lower) axes. The dotted vertical line indicates the “turning point” between opening and closing of the mouth. B, Average chewing pattern in the frontal and horizontal view. Opening and closing angles in the frontal or horizontal view were defined as the angles between each plane and the tangent line of the average chewing trajectory.

Figure 4. The mean paths of the average chewing pattern during unilateral gum chewing at T0 (blue) and T1 (red) in the frontal and horizontal views.

T0, Initial; T1, 7–8 months after orthognathic surgery.