Masticatory muscle function three years after surgical correction of class III dentofacial deformity


Individuals with dentofacial deformities have masticatory muscle changes. The objective of the present study was to determine the effect of interdisciplinary treatment in patients with dentofacial deformities regarding electromyographic activity (EMG) of masticatory muscles three years after surgical correction. Thirteen patients with class III dentofacial deformities were studied, considered as group P1 (before surgery) and group P3 (3 years to 3 years and 8 months after surgery). Fifteen individuals with no changes in facial morphology or dental occlusion were studied as controls. The participants underwent EMG examination of the temporal and masseter muscles during mastication and biting. Evaluation of the amplitude interval of EMG activity revealed a difference between P1 and P3 and no difference between P3 and the control group. In contrast, evaluation of root mean square revealed that, in general, P3 values were higher only when compared with P1 and differed from the control group. There was an improvement in the EMG activity of the masticatory muscles, mainly observed in the masseter muscle, with values close to those of the control group in one of the analyses.

Individuals with dentofacial deformities benefit from orthognathic surgery to correct skeletal morphology. Orthognathic surgery, in combination with orthodontic treatment, corrects the dentofacial deformity, and improves occlusal contacts , masticatory efficiency , the electromyographic (EMG) activity of the masticatory muscles , the bite force and the thickness of the masseter muscle .

Other studies detected no changes in masticatory efficiency, in the EMG activity of masticatory muscles or in bite force 12–18 months after surgery , 2 years after surgery regarding bite force and occlusal contacts , or in muscle activity per unit of bite force 3 years after surgery . No improvement in bite force was detected 5 years after surgery .

In an investigation of functional changes after combined treatment with orthodontics and orthognathic surgery in individuals with dentofacial deformities, Y ang et al . detected a worsening of bite force and masticatory efficiency 3 months after surgery, with the values being close to presurgical levels 6 months after surgery in patients undergoing bilateral sagittal split osteotomy for mandibular retrocession. U eki et al . found no changes in the pattern of the masticatory curve in class III patients who underwent sagittal and intraoral osteotomy of the mandibular ramus about 1 year after surgery.

Orthognathic surgery can also impair orofacial function due to the sensorineural sequelae following sagittal osteotomy of the mandibular ramus and after the period of maxillomandibular fixation . Some errors in speech articulation may persist after surgical correction , justifying functional rehabilitation with speech and language therapies. According to several investigators, changes in masticatory function or in its components after the correction of dentofacial deformities by orthognathic surgery are evident. The period of time for this to occur is a matter of controversy and may be related to differences in the methods of evaluation and treatment. Most studies describe ortho-surgical treatment without mentioning orofacial myofunctional therapy.

The objective of the present study was to investigate the effect of interdisciplinary treatment by orthodontics, surgery and orofacial myofunctional therapy in patients with class III dentofacial deformity on the EMG activity of masticatory muscles 3 years after surgical correction, compared with a control group.

Materials and methods

The study was approved by the Research Ethics Committee of the University Hospital, Faculty of Medicine of Ribeirão Preto, University of São Paulo (HCFMRP-USP) and the patients gave written informed consent to participate in the study.

The study was conducted on 13 patients with class III corrected dentofacial deformity characterized by mandibular prognathism and/or maxillary deficiency. The patients were considered as two groups: P1 (before surgery) and P3 (3 years to 3 years and 8 months after surgery). Three patients were male aged 21–25 years (mean 22 years) and 10 were female aged 22–42 years (mean 27 years). The surgical procedures and the gender distribution are presented in Table 1 .

Table 1
Patient distribution according to the surgical procedure.
Surgery Patients
Males Females Total
Combined (Le Fort I and bilateral sagittal mandíbular) 1 3 4
Advancement of the maxilla (Le Fort I) 0 4 4
Mandibular retrocession (vertical bilateral of the ramus) & MMF * 2 3 5
Total 3 10 13

* MMF = maxillomandibular fixation.

The control group (CG) consisted of 15 volunteers with natural dentition, with no alterations of facial morphology or dental occlusion and no signs or symptoms of temporomandibular joint dysfunction. Eleven were female, aged 21–29 years (mean 24 years) and 4 were male aged 19–25 years (mean 21 years).

All patients underwent orthodontic treatment before and after surgery and orofacial myofunctional therapy follow-up according to the routine of the service of Head and Neck Surgery of the Integrated Center of Studies of Facial Deformities of HCFMRP-USP .

Presurgical orthodontic treatment was performed in order to align, level and decompensate the teeth, often increasing skeletal discrepancy. The teeth were positioned in an ideal manner on their osseous bases without taking into consideration the inter-arch relation. After orthognathic surgery, occlusal adjustments were made to obtain the largest possible number of dental contacts during a bite. Presurgical orthodontic treatment lasted on average 12.4 months and postsurgical treatment lasted 14 months.

Orofacial myofunctional evaluation was carried out before surgery, with investigation of the predominant mode of respiratory function (oral, nasal or oronasal). Alterations were observed in the oral phase of deglutition, in mastication and speech, and in the tonic and postural aspects of lips and tongue. The authors worked only with tongue muscles in cases in which important muscle changes were observed in addition to the use of nasal respiration. After evaluation, the patients received instructions and clarification regarding the surgery, the proposed treatment and the care needed during the postoperative period. After surgery, the patients initially had a smooth diet with a gradual return to a solid diet. Patients who reported pain or discomfort in the temporomandibular joint after surgery were instructed to maintain a soft diet and simultaneous bilateral mastication. All patients were instructed to perform mandibular movements with emphasis on the gradual recovery of these movements. Exercises were used to strengthen and improve the posture of lips and tongue. Corrective work was performed on deglutition, mastication and speech when alterations were observed in these aspects. The patients were instructed to apply lukewarm compresses and massage starting 2 weeks after surgery to reduce facial edema and to relax the mandibular levator muscles and improve mandibular movements. Postoperative oromyofunctional therapy lasted on average 11 months.

The period of maxillomandibular fixation was 3 weeks for all patients who had undergone vertical osteotomy of the mandibular ramus.

The participants were evaluated for bilateral EMG activity of the temporal (T) and masseter (M) muscles during right mastication (RMa) and left mastication (LMa) of mint-flavored Trident ® chewing gum (Warner-Lambert, Adams Division, Bauru, SP, Brazil) and for bite force using a Neuropack 8, MEM 4200K model computerized electroneuromyograph (Nihon Kohden Corporation, Shinjuku-ku, Tokyo, Japan). The motor unit action potentials of the muscles under study were recorded with silver-chloride surface electrodes immersed in conductive gel and attached to the skin with micropore tape. Fig. 1 illustrates the device and the positioning of the electrodes. The procedures were carried out in the Laboratory of Neurophysiology of the Department of Neurology, Psychiatry and Medical Psychology of FMRP-USP. In the masticatory situation, the sides of work were considered (ζ) and the side of the mouth where the chewing gum was held, and the balance side (B) was considered to be the one contralateral to ζ.

Fig. 1
Equipment for EMG evaluation and positioning of the surface electrodes.

The authors analyzed in each muscle, in the three situations, the values (in μV) of the amplitude interval (AI) of EMG activity and of the root mean square (RMS), which also represents the amplitude of the EMG signal. The methodology used has been described in a previous study .

The data were tabulated and analyzed statistically using the Wilcoxon test to determine differences between P1 and P3 (dependent samples) and by the Mann Whitney test to determine differences between P3 and CG (independent samples) regarding the EMG activity of the M and T muscles in the mastication and biting situations. For the purpose of analysis, the mean EMG activity values were calculated using the median. All statistical tests were carried out using the statistical software Graph Pad InStat version 3.0 for Windows 95, with the level of significance set at P < 0.05.


The EMG activity of the T and M muscles differed significantly between the P1 and P3 groups in both situations (mastication and biting), with higher values in P3, whereas no significant difference ( P > 0.05) was detected between P3 and CG ( Table 2 ).

Feb 8, 2018 | Posted by in Oral and Maxillofacial Surgery | Comments Off on Masticatory muscle function three years after surgical correction of class III dentofacial deformity
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