Effect Of Forward Head Posture On The Electromyographic Activity Of Masticatory Muscles

Forward head posture (FHP) is characterized by a dorsal flexion of the head together with the upper cervical spine(C1-C3), and is accompanied by a flexion of the lower cervical spine(C4-C7)as well. Studies showed that FHP has the influence to the tooth occlusion as well as functional mandibular movement. Many scholars found that a forward protrusion of the head and neck position can be seen frequently in patients with temporomandibular joint disorders (temporomandibular disorders, TMDs). There is a forward cervical spine and an increased cranio-cervical angle in patients with TMDs when taking a lateral radiograph with head and the cervical spine. Currently some scholars tend to consider that abnormal head and neck posture can cause alterations in the stomatognathic system, such as TMDs, or it is a precipitating factor of TMDs.Other researchers believe that there is an association between some malocclusion and abnormal head and neck position. FHP accompanied by an increased craniocervical angle will restrict forward growth and development of maxilla and mandible.The effect of FHP on maxillofacial morphology and function may be due to corresponding changes in head and neck position. The soft tissue layers containing skin, muscle and fascia will be stretched with the change of head and neck position, which lead to the alteration of maxillofacial morphology and function. Many studies also showed that FHP has effect on electromyographic (EMG) activity of maxillofacial and neck muscle. However, most of reports focused on the impact on the EMG activity at mandibular postural position. Furthermore, the results are not entirely consistent.ObjectiveThe purpose of this study was to explore the effect of MFHP on EMG activity of masticatory muscles, and provide experimental basis for understanding the pathogenesis of TMDs as well as the etiology of some dental malocclusion. The Bio EMGⅡsurface electromyogram measuring equipment (produced by the U.S. Bio Research company) was used to record the EMG activity of bilateral anterior temporalis (TA), masseter muscles(MM), sternocleidomastoid muscles(SCM) and anterior digastric muscles(DA) during MPP, voluntary clenching in ICP and swallowing, maximum FHP and NHP position.Materials & Methods1. SubjectsThirty healthy young women with normal occlusion (age:18.67±1.90yrs) were selected from students in Guangzhou Medical College School of Nursing.The criteria for selection were based on:①Individual normal occlusion (normal anterior overbite and overjet, neutral relationship of molar and canine, the crowding of upper and lower arch was less than 2mm, the space of upper and lower dentition was less than lmm, good occlusal contact of full-mouth teeth, no obvious abrasion), complete dentition in addition to third molar;②No dental emergency, no dental traumatic or broken history;③No orthodontic and/or orthognathic surgery history, no restorative treatment history;④No history of bruxism or a tight biting habit, no unilateral chewing habit;⑤No TMDs, no relevant medical history, no otitis media, no temporomandibular joint trauma and/or surgery history, no chronic systemic diseases affecting on temporomandibular joint (such as rheumatoid joints disease);⑥No spontaneous pain or tenderness of neck muscles;⑦Symmetrical face, maxillofacial and neck without scars or history of trauma;⑧Good health, no mental disorders;⑧All participants understand the objective and process of this experiment, and signed an informed consent form. 2 research equipmentsEMG data were collected using a BioEMG II type surface electromyography measuring equipment (United States Bio Research Company's products, using Bio PAK system, Version5.0li,8 channel). Which include electric conductive lead lines, EMG amplifier,12bit signal converter, magnets (6×10×3mm), adhesive, surface electrodes, BioPAK system supporting analysis software; HP-branded desktop computers (HP, a 1237cl). Sterile dental equipment:mouth mirrors, probes, tweezers, ruler,75% alcohol cotton balls, sterile dry cotton ball and so on; Head posture auxiliary positioning devices:Mirrors (40 x25cm), chair (non-headrest, chair back to about the level of subjects'scapula),Two plumb lines(hanging on the simple rack), etc..3 History collection and clinical examination3.1 History collectionMainly includes general basic information of participants (name, gender, date of birth, race, native place, etc.), oral history, temporomandibular joint's function, history of maxillofacial and neck disease and trauma, systemic health status, mental and psychological conditions and systems history and so on.3.2 Clinical examination(1) Facial examination:facial symmetry, lateral profile and so on.(2) Oral examination:dental health, tooth abrasion, the arrangement of teeth, occlusal relationship and so on.(3) Examination of masticatory muscles and TMJ function, mainly include: Whether or not the masticatory muscles have spontaneous pain or tenderness under stationary or functional movement? Whether or not temporomandibular joint have clicking or pain under functional movement? Is there tenderness in joint region? Is the degree of mouth opening normal? Is there a limited ability to open mouth? Is the open-closed type normal?(4) Examination of stomatognathic system function:Whether ICP has premature contact or not? Is CR and CO consistent? Is there protrusive or lateral occlusal interference?(5) Examination of neck muscle function:Whether or not head and neck have spontaneous pain or tenderness under Stationary or functional movement?4 Experimental method and process4.1 PreparationTo ensure the head and neck posture auxiliary device is in place, mainly including:Put the chair in place; Put the mirror in front of the subjects with the distance of 2M; The simple rack hanging plumb lines was placed closely behind the back of the chair.Determination of the head and neck postural position:(1) NHP:in a quiet room, subjects were required to assume an upright position, whose back naturally leaned against the back of the chair, legs naturally separated, arms relaxed and naturally drooped, Stared at his or her own eyes in the mirror in front of their body. In the case of line of sight was parallel with the ground plane,let themself be in a subjective relaxed comfortable position of head and neck; (2) MFHP:subjects sat in a chair uprightly, and were in the NHP state, then were allowed to try to voluntary protrude to a limit position form NHP. In the whole process, subjects were required to look at their own eyes in front of their body, whose lines of sight were parallel with the ground plane, and back still leaned against the back of the chair. At the moment, the plumb line can be a reference with which we can determine whether the head is upright or not.4.2 EMG recording.In accordance with equipment instructions, we connected BioEMGⅡtype surface electromyography measuring instrument, and separately recorded the surface EMG activities of bilateral TA, MM, SCM and DA of maximum MFHP and NHP during mandibular postural position, voluntary clenching in ICP and swallowing in a quiet, static shielding room. (1) mandibular postural position:binocular smooth inspect ahead, no chewing, no swallowing, no speaking, the upper and lower dentition naturally separate. Record 10s EMG. (2) voluntary clenching in ICP: naturally close mouth to ICP, voluntary clench. Within 10s recording interface, voluntary clench Is every other 2s, repeat 3 times. (3) swallowing:in case of full moistening throat, subjects swallow their own saliva. Within 10s recording interface, swallow 1 times every other 2～3s, repeat 3 times.4.3 Data collection and statistical analysisSelect three 1s analytic interface in every experiment record, then computer can automatically give the average EMG amplitude (μV) of every muscle during mandibular postural position, voluntary clenching in ICP and swallowing in these three analytic interface. In this study, the average of left and right EMG amplitude was taken as a representative value of a particular muscle for statistical analysis. The mean and standard deviation ((?)±S) of EMG amplitude(μV) of NHP and FHP during MPP, ICP clenching and swallowing, the asymmetry index of bilateral TA and MM in ICP clenching and the number of cases of the ipsilaterally increased EMG amplitude were calculated.The statistical analysis was performed using SPSS version 13.0 software (SPSS, Chicago, IL, USA). Data were presented as means and standard deviations and compared by paired t-test, Wilcoxon's signed rank test and paired count data Chi-square test to determine differences between the EMG parameters of two head posture. In all cases, P-values were considered to be statistically significant when less than 0.05.Results1 Comparison of EMG amplitudes between two different head postures at rest positionThe EMG activities of TA, MM, SCM, and DA at NHP with the mandible at rest were small, while the significant increase (P<0.01) was identified at MFHP.2 Comparison of EMG amplitudes between two different head postures during voluntary clenching in ICPWhen subjects clenched their teeth in intercuspal position at NHP, the EMG activities of TA and MM were much higher than that at rest, the EMG activities of DA and SCM were also increased during ICP clenching. Compared with the EMG activities at NHP, the EMG activity of MM obviously increased at MFHP, the EMG activities of TA, SCM and DA also increased. There were significant differences in the EMG amplitudes of MM and DA during ICP clench between FHP and NHP (P<0.05), while the difference of EMG amplitudes of DA and SCM between MFHP and NHP were insignificant (P> 0.05).3 Comparison of EMG amplitudes between two different head postures during swallowingThe EMG activity of DA during swallowing at NHP was much higher than that during MPP, the EMG activities of TA and MM also obviously increased during swallowing. Compared with the EMG activities at NHP, the EMG activities of TA, MM, SCM and DA at MFHP increased, while the growth rate of DA was more apparent. There were significant differences in the EMG amplitudes of TA, MM, SCM and DA between MFHP and NHP (P<0.05)4 Asymmetry index of TM and MM during voluntary clenching in ICPWhen subjects clenched their teeth in intercuspal position at MFHP, the asymmetry index of bilateral and ipsilateral TA and MM was lower than that at NHP, but there was no significant difference(P>0.05).5 Comparison of the ipsilateral increase of TM and MM EMG amplitudes between two different head postures during voluntary clenching in ICPThe EMG amplitudes of TA and MM increased obviously during ICP clenching. At NHP, there were 14 cases whose EMG activities of TA and MM ipsilaterally increased, and 16 cases whose EMG activities of TA and MM un-ipsilaterally increased. At FHP, EMG activities of TA and MM ipsilaterally increased in 16 cases and un-ipsilaterally increased in 14 cases. However, there were no significant difference between MFHP and NHP (P>0.05).ConclusionsThe maximum forward head posture is able to influence the activity in masticatory muscles. The increased EMG activities of masticatory muscles at rest position, during swallowing, and MM and DA during ICP clenching are responded to MFHP obviously.