My first Magazine | Page 121

STANDARDIZED ELECTROMYOGRAPHIC INDEXES ALLOW A RELIABLE MEASUREMENT OF MASTICATORY MUSCLES FUNCTION

Table 1 . Comparison of standardized sEMG indexes obtained in two different data collection sessions ( all values are percentages ). No statistically significant differences were found

Masticatory sEMG standardized indexes repeatability

Session 1

Session 2

Difference

T Test 0.166 0.408 0.691 0.166 0.821

chloride bipolar surface electrodes ( rectangular shape , 21x41 mm , 20 mm inter-electrode distance ) ( F3010 , Fiab , Firenze , Italy ) were positioned . The electrodes were placed on the muscular bellies parallel to muscular fibres as follows ( Fig . 1 ):

• MM : the operator , standing in front of the seated subject , palpated the muscular belly while the subject clenched his / her teeth . The electrodes were fixed parallel to the exocanthion-gonion line and with the upper pole of the electrode under the tragus-labial commissura line .

• TA : the muscular belly was palpated during tooth clenching and the electrodes were fixed vertically along the anterior margin of the muscle ( corresponding to the fronto-parietal suture ) 24 .

A disposable reference electrode was applied to the forehead . To reduce skin impedance , the skin was carefully cleaned prior to the electrode placement , and recordings were performed 5 min later , allowing the conductive paste to adequately moisten the skin . 2.4 . sEMG recordings and measurements Instrumentation The surface EMG activity was recorded using a computerized instrument ( Easymyo , 3 Technology S . r . l ., Udine , Italy ). The analogic sEMG signal was amplified ( gain 100 , bandwidth 0 – 1000 Hz , peakto-peak input range from 0 to 3600 µ V ) using a differential amplifier with a high common mode rejection ratio ( CMRR = 115 dB in the range 0 – 60 Hz , input impedance 100 GΩ ), digitized ( 24-bit resolution , 4000 Hz A / D sampling frequency ), and digitally filtered ( Butterworth type , high-pass filter set at 30 Hz , low-pass filter set at 400 Hz , band-stop for common 50 – 60 Hz noise ). The signals were averaged over 25 ms , with muscle activity assessed as the root mean square ( RMS ) of the amplitude (µ V ). sEMG signals were recorded for further analysis . Before the acquisition session the subjects were properly trained to elicit true teeth maximal voluntary contraction using an on-time sEMG signal visualization .

3 . Results 3.1 . Standardization procedure To normalize the sEMG signals two 10-mm thick cotton rolls were positioned on the mandibular second premolars / first molars of each subject and a 5-s maximum voluntary contraction ( MVC ) was recorded . The mean sEMG potential of each muscle obtained in that first acquisition was set at 100 %, and all further sEMG potentials were expressed as a percentage of this value (µ V / µ Vx100 ). 3.2 . Analyzed task The sEMG activity was recorded during a 5-s MVC test in intercuspal position ( IP ): the subject was invited to clench as hard as possible and to maintain the same level of contraction for all the test . For each patient , the central 3 s of the MVC test were analysed , and the sEMG potential was standardized as detailed before . 3.3 . sEMG data analysis Separately for each acquisition session ( T1 and T2 ), the sEMG waves were compared by computing a series of standardized indexes using the instrument software tools : 1 . the percentage overlapping coefficient ( POC , unit %), an index of symmetric muscular contraction . The index ranges between 0 % and 100 %: when two paired muscles contract with perfect symmetry , a POC of 100 % is obtained . Masseter and Temporalis Anterior POCs were obtained for each patient 24 , 25 .

2 . the torque coefficient ( TORQUE , unit %) was assessed to evaluate if an unbalanced contractile activity of the contralateral Masseter and Temporalis Anterior muscles , such as that of the right Temporalis Anterior and the left Masseter , might give rise to a potential lateral displacing component . TORQUE ranges between 100 % ( complete prevalence of the right Temporalis Anterior and left Masseter ) and -100 % ( complete prevalence of the left Temporalis Anterior and the right Masseter ) 24 , 25 .