Electromyography (EMG) is a method for studying the motor system, based on recording the biopotentials of skeletal muscles. EMG is often used in surgical and orthopedic dental practice as a functional and diagnostic method for studying the functions of the peripheral neuromotor apparatus and for assessing the coordination of the muscles of the maxillofacial area in time and intensity, in normal and pathological conditions.
EMG is based on recording action potentials of muscle fibers functioning as part of motor (motor, or neuromotor) units. A motor unit (MU) consists of a motor neuron and a group of muscle fibers innervated by that motor neuron. The number of muscle fibers innervated by one motor neuron varies in different muscles. In the masticatory muscles there are about 100 muscle fibers per motor neuron, in the temporal muscle there are up to 200, in the facial muscles the MEs are smaller, they include up to 20 muscle fibers. In small facial muscles this ratio is even lower, which ensures a high level of differentiation of contractions of facial muscles, which determine a wide range of facial expressions.
At rest, the muscle does not generate action potentials, so the EMG of a relaxed muscle has the form of an isoelectric line. The action potential of an individual ME when recorded with a needle electrode usually has the form of a 2-3 phase oscillation with an amplitude of 100-3000 μV and a duration of 2-10 ms. On the EMG, an increase in the number of working MEs is reflected in an increase in the frequency and amplitude of oscillations as a result of the temporal and spatial summation of action potentials. EMG reflects the degree of motor innervation, indirectly indicates the intensity of contraction of an individual muscle and gives an accurate idea of the time characteristics of this process.
Fluctuations in potentials detected in a muscle during any form of motor reaction are one of the most subtle indicators of the functional state of a muscle. Oscillations are recorded using a special device – an electromyograph. There are two ways to discharge biocurrents: with cutaneous electrodes with large discharge areas, and with needle electrodes, which are inserted intramuscularly.
The functional state of the masticatory muscles is examined during the period of functional rest of the lower jaw, during the closure of teeth in the anterior, lateral and central occlusions, during swallowing and during chewing. Analysis of the obtained EMG consists of changing the amplitude of biopotentials, their frequency, studying the shape of the curve, the ratio of the period of rhythm activity to the rest period. The magnitude of the oscillation amplitude allows one to judge the strength of muscle contractions.
The electromyogram during chewing in people with normal dentition has a characteristic shape. There is a clear change between active rhythm and rest, and volleys of biopotentials have spindle-shaped outlines. There is coordination between the contraction of the muscles of the working and balancing sides, which is expressed in the fact that on the working side the EMG amplitude is high, and on the balancing side it is approximately 2.5 times less.
In therapeutic dentistry, MG is carried out in periodontal disease and periodontal disease to record changes in the strength of contractions of the masticatory muscles, since these diseases cause functional and dynamic disorders of the masticatory apparatus. EMG is carried out in combination with gnathodynamometric tests, which allow one to compare the intensity of muscle excitation with their strength effect.
In surgical dentistry, surface EMG is used for jaw fractures, inflammatory processes in the maxillofacial area (phlegmon, abscesses, periostitis, osteomyelitis), and for myoplastic operations for persistent paralysis of the facial muscles and tongue. In case of jaw injuries, EMG serves to objectively assess the degree of dysfunction of the masticatory muscles, as well as to monitor the timing of rehabilitation of patients. Jaw fractures lead to a significant decrease in the bioelectrical activity of the masticatory muscles and the appearance of tonic activity at rest in the temporal muscles, which persists for a long time.
In inflammatory processes of the maxillofacial region, there is a significant decrease in bioelectrical activity on the affected side. The reasons for this are reflex (pain) limitation of muscle contraction and disruption of the conduction of nerve impulses due to tissue swelling.
During myoplastic operations for persistent paralysis of the facial muscles and tongue, EMG is used to determine the usefulness of the innervation of the transplanted muscle before the operation, and after the operation - the restoration of its function.
In dental neurology, for traumatic and infectious injuries to the nerves of the maxillofacial region containing motor fibers, local EMG is used to objectively identify signs of muscle denervation and early signs of muscle and nerve regeneration.
In orthopedic dentistry, EMG is used to study the bioelectrical activity of the masticatory muscles in the complete absence of teeth and in the process of adaptation to removable dentures. Orthopedic treatment with complete removable dentures leads to an increase in the bioelectrical activity of the masticatory muscles during chewing and a decrease in the bioelectrical activity after their removal. In the process of adaptation to complete removable dentures, the time of the entire chewing period is shortened by reducing the number of chewing movements and the time of one chewing movement.
In pediatric dentistry, interference EMG is used to monitor the progress of the restructuring of the coordination relationships of the functions of the temporal and masticatory muscles in the treatment of malocclusions; the participation of muscles in certain natural acts (for example, swallowing) is revealed. Local EMG is performed to study the bioelectrical activity of the soft palate muscles in children under normal conditions and with congenital developmental anomalies. After surgical removal of clefts of the soft palate, EMG is used to determine the prognosis of the possibility of speech restoration and to monitor the process of muscle training using a special set of myogymnastic exercises. question number 6
Physiological rationale for local anesthesia (infiltration or conduction) in dental practice. The significance of the laws of conduction of excitation along the nerve. The phenomenon of parabiosis.
Infiltration anesthesia (anesthesia) - anesthesia, in which an anesthetic is injected under the mucous membrane/skin, acting on a small area.
In dentistry, using this method, you can anesthetize the mucous membrane, periosteum, teeth, including chewing ones in the lower jaw (intraligamentary anesthesia).
Conductor o.- a method that allows you to anesthetize a large area with small doses of anesthetic (reversible blockade of nerve impulse transmission along a large nerve)
Electromyography is a method for studying the motor system, based on recording the biopotentials of skeletal muscles. Electromyography is used in surgical and orthopedic dentistry, orthodontics, dental neurology as a functional and diagnostic method for studying the functions of the peripheral neuromotor apparatus, assessing the coordination of the muscles of the maxillofacial area in time and intensity, normally and in pathology - for injuries and inflammatory diseases of the maxillofacial area , malocclusions, myoplastic surgeries, dystrophies and hypertrophies of the masticatory muscles, clefts of the soft palate and other diseases.
PHYSICAL AND PHYSIOLOGICAL BASES OF ELECTROMYOGRAPHY
Contraction of muscle tissue is caused by a flow of impulses arising in various parts of the central nervous system and spreading through motor nerves into the muscles. Excitation of the motor unit of the neuromotor apparatus is manifested by the generation of action potentials with integral expression of individual muscle fibers. Excitation of muscle tissue is a complex set of phenomena consisting of increased metabolic processes, increased heat production, specific activity (contraction of muscle fibers), and changes in the electrical potential in the excited area of the muscles. For the purposes of electromyography, changes in the electrical potential of the muscle fiber are of immediate practical interest.
In the occurrence of electrical (membrane) potentials, a decisive role is played by changes in the ionic permeability of cell membranes, the regulatory mechanisms of this process, sodium and potassium ions, as well as chlorine and calcium. Using the example of the function of the so-called sodium-potassium pump, we can consider the mechanism of the emergence of resting potentials and action potentials of a muscle cell.
The resting potential is due to the function of the cell pump, i.e., the movement of sodium ions from the cell into the intercellular fluid, and potassium ions from it into the cell through the cell membrane. The consequence of this transition is a change in the concentration of ions in the cell and the occurrence of EMF. The scheme for the occurrence of an action potential in a muscle cell is as follows: under the influence of a stimulus (nerve impulse), the permeability of the muscle cell membrane for sodium ions sharply increases (about 20 times more than for potassium ions). Due to the significant difference in the concentrations of sodium and potassium ions during this depolarization phase, the muscle cell membrane becomes negatively charged (depolarization phase). The second phase (repolarization phase) is caused by inactivation of the sodium-potassium pump: the movement of sodium ions from the intercellular fluid into the cell stops. When exposed to subsequent nerve impulses, the cycle of de- and repolarization phases is repeated. Thus, the difference in the concentrations of sodium and potassium ions in a muscle cell causes the occurrence of EMF - resting and action potentials, which can be recorded graphically with the help of electrodes, electronic amplifiers and recorders.
Using electromyography, changes in the potential difference inside or on the surface of the muscle that arise as a result of the propagation of electrical energy are recorded.
Judgments based on muscle fibers. The recorded changes in the potential difference (or bioelectrical activity) of the muscles are called an electromyogram (EMG).
Electromyography is based on recording action potentials of muscle fibers functioning as part of motor units (MU). MU is a functional unit of voluntary and reflex muscle activity. It consists of a motor neuron and a group of muscle fibers innervated by this motor neuron (Fig. 43).
Muscle fibers included in one motor unit are excited and contracted simultaneously as a result of excitation of the motor neuron. The number of muscle fibers innervated by one motor neuron, i.e., included in one motor unit, is not the same in different muscles. In the masticatory muscles proper there are 100 muscle fibers per motor neuron, in the temporal muscle there are 200; in facial muscles, the MUs are smaller, they include up to 20 muscle fibers. In small facial muscles this ratio is even smaller; Thus, a high level of differentiation of contractions of facial muscles is ensured, which determines a wide range of facial expressions.
At rest, the muscle does not generate action potentials, so the EMG of a relaxed muscle has the form of an isoelectric line. As a result of the passage of impulses from motor neurons along the nerve through the neuromuscular endings, motor units are excited, which can be recorded with a needle electrode in the form of a motor unit action potential, which is an algebraic sum of the action potentials of individual muscle fibers. The action potential of an individual motor unit usually has the form of a 2-3-phase oscillation with an amplitude of 100-3000 μV and a duration of 2-10 ms (Fig. 44).
An increase in the force of muscle contraction occurs due to an increase in the number of working motor units and the frequency of their discharges. On the EMG, this process is expressed in an increase in the frequency and amplitude of oscillations, as a result of the temporal and spatial summation of action potentials of motor units (Fig. 45). This kind of EMG is called interference. Interference EMG is usually recorded with cutaneous electrodes, i.e., the activity of a large number of motor units of a muscle area located near the electrodes, summarized in time and space. The conditions for the spatial summation of action potentials of motor units (i.e., the spatial arrangement of muscle fibers), the varying distances of the “generators” of biopotentials from the recording electrodes are one of the factors that determine the parameters of the recorded EMG. EMG reflects the degree of motor innervation, indirectly indicates the intensity of contraction of an individual muscle and gives an accurate idea of the time characteristics of these processes.
There are three main types of electromyography:
- 1) interference electromyography (synonyms: superficial, total, global), it is carried out by deducing muscle biopotentials, applying electrodes to the skin, the deduction area is large;
- 2) local electromyography - registration of the activity of individual motor units is carried out using needle electrodes;
- 3) stimulation electromyography - recording the electrical response of a muscle to stimulation of the nerve innervating this muscle.
Since EMG recording is the result of the combined activity of the muscle as a source of biopotentials and the equipment with which these biopotentials are removed and recorded, the influence of methodological conditions on the process of EMG recording should be taken into account.
Electromyography began with preliminary preparation of the patient for the study, and the essence of the study was explained to him. To relieve excess muscle tension,
Rice. 74. Computer tomograms of two patients with TMJ arthrosis in two projections (sagittal, frontal).
which can arise as a result of excitement, fear, etc., the patient was explained about the painlessness and harmlessness of all manipulations.
We used a six-channel electromyograph from the Medicor company, which does not require a special tyrannized chamber (Fig. 76). Reducing the interference created by the electric field of the alternating current network was achieved by grounding the patient through the body of the electromyograph, which is grounded with a common contour grounding. Biopotentials were removed using cutaneous bipolar electrodes. The distance between the electrodes was always constant and equal to 15 mm, since they were fixed with plastic. The electrodes were fixed in the center of the motor points of the temporal (anterior abdomen) and the masticatory muscles themselves.
Until now, researchers have identified the motor point by palpation and fixed the electrodes using a rubber cuff and adhesive medical tape. For identical recording of electromyograms at different periods of the study, a very important point is the fixation of bipolar electrodes in the same area of the motor point of the temporal and masticatory muscles. In order to identify recordings of electromyograms at different periods of the study during the treatment of patients with TMJ pathology, we, together with A.I. Dovbenko and N.Yu. Seferyan proposed an apparatus for electromyography of the temporal and masticatory muscles (Fig. 76). It consists of a crosspiece, a head clamp, clamps with ear olives, a nose bridge clamp, and a nape clamp. In the auricular olive clamp, a horizontal plate with a scale and a flat spring is movably installed above the auricle in the area of the projection of the temporal muscles, and a sector with a scale is attached under the auricle on the same lever, equipped with a spring arrow with a longitudinal groove and divisions. First, the approximate localization of the motor points of the temporal and masticatory muscles is determined by palpation. The skin surface in these areas is thoroughly treated with alcohol and ether. To achieve better “electrode-skin” contact and reduce interelectrode resistance, the electrodes are moistened with a 0.9% sodium chloride solution. The electrodes are installed under a flat spring with divisions in the area of the temporal muscles and the floor with a springy arrow of the buccal semi-oval sector. Then visually using a control device located on the front The electromyograph panels, moving the electrode along the groove of the spring clamps, find the exact localization of the center of the motor point and control the quality of contact with the skin.The location of the electrodes is accurately recorded using a scale with divisions and entered into the study protocol.
With the correct application of electrodes in a state of relative physiological rest of the lower jaw, the electromyogram has an isoelectric line. With maximum jaw compression, the appearance of bioelectrical activity is checked and the equipment is adjusted before recording functional tests. The amplifier switch is set to the 50 mm/sec position, and the subject is asked to compress and relax the jaws several times. By adjusting the switch, we make sure that the maximum amplitude of oscillations does not exceed the screen frame or is excessively small. The amplitude is measured using a scale ruler. After preliminary setup of the equipment, they begin to study the functional activity of the masticatory muscles.
When analyzing the data obtained, a qualitative and quantitative assessment of electromyograms was carried out:
a) the transition of the bioelectrical activity phase (BEA) to the bioelectrical resting phase (BRE) is abrupt or with continued excitation in the resting phase (myological delay);
b) the degree of amplitude of oscillations during the act of chewing and with maximum compression of the jaws in the position of central occlusion;
c) duration of the act of chewing and the act of swallowing in seconds;
d) rhythmicity, synchronicity of contraction of the masticatory muscles, the presence of oscillations both in the state of relative physiological rest of the masticatory muscles and in the BEP phase during the act of chewing. Amplitude indicators of electromyograms during the act of chewing and clenching the jaws were subjected to quantitative calculations (Fig. 77).
Each closure of the dentition is reflected by the appearance of biopotentials with varying degrees of vibration amplitude. The magnitude of the amplitude of biopotentials depends on the degree of contraction of the masticatory muscles. When recording a voluntary act of chewing with a stimulus (1 cm3 of black bread), a clear transition from the bioelectrical activity (BEA) phase to the bioelectrical rest phase (BER) of all muscle groups studied was noted in the control group. In order to obtain initial and compare electromyographic data obtained on patients with joint pathology with normal indicators, an additional examination of the temporal and masticatory muscles was carried out in 10 practically healthy individuals aged 16 to 36 years with intact teeth and orthognastic occlusion (control group) .
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Electromyography (EMG)- an objective method for studying the neuromuscular system by recording the electrical potentials of the masticatory muscles, which allows one to assess the functional state of the dental system.
There are three main EMG methods:
1) interference (superficial, total, global), in which electrodes are applied to the skin;
2) local, in which the study is carried out using needle electrodes;
3) stimulation, in which the speed of propagation of an electrical impulse from the place of its application to another part of the stimulated nerve or muscle innervated is measured.
To judge the state of the masticatory muscles, it is sufficient to conduct interference EMG using surface electrodes.
EMG research technique. Many works have been devoted to EMG studies of masticatory muscles in dental diseases [Persii L.S., Khvatova V.A., Erokhina I.G., 1982; Petrosov Yu.A., 1982; Khvatova V.A., 1985; Malevich O.E., Zhitny N.I., 1991; Grechko V.E. et al., 1994; Onopa E.N. et al., 2003; Bessette R. et al., 1971; Freesmey-erW., 1993].
The electrical activity of the masticatory muscles is recorded simultaneously from both sides. To remove biopotentials, surface cup electrodes are used. The electrodes are fixed in the area of motor points (areas of greatest muscle tension, which are determined by palpation).
Functional tests are used to record EMG. EMG is recorded in the physiological rest of the lower jaw, when the jaws are compressed in the usual occlusion, voluntary and specified chewing (Fig. 3.57).
In addition, the mandibular reflex is studied (when tapping the chin in the midline with a neurological hammer) when clenching the jaws in a position of central occlusion. Mandibular reflex - the time of reflex inhibition of the activity of the masticatory muscles, has diagnostic significance (Fig. 3.58).
When analyzing EMG, the following indicators are determined: the average amplitude of biopotentials, the number of chewing movements in one chewing cycle, the duration of one chewing cycle, the time of bioelectric activity (BEA) and bioelectric rest (BER) of the masticatory muscles in the phase of one chewing movement. The obtained data are compared with indicators of normal EMG activity of the masticatory muscles.
Electromyography of the external pterygoid muscles uses concentric needle electrodes. Each electrode is a thin hollow needle with a diameter of 0.45 mm, into which a wire is inserted, insulated from the outer sheath throughout its entire length except for the tip. Before insertion, the needle electrodes are kept for 30 minutes in a special sterilizer.
The literature describes two methods of introducing electrodes - intraoral and extraoral. The intraoral method is technically difficult to perform, it is not accurate and does not provide the opportunity to study muscle activity during chewing. The extraoral method of introducing needle electrodes through the semilunar notch of the mandible does not allow recording of EMG during the chewing function, since the needle electrode passes through the tendon of the masticatory muscle.
Rice. 3.57. The EMG activity of the masticatory (1), temporal (2), lateral pterygoid (3) and suprahyoid muscles (4) during clenching of the jaws (A) and specified chewing (B) is normal.
a - right, b - left.
A method has been developed for introducing a needle electrode directly into the muscle near the neck of the articular process of the lower jaw (V.A. Khvatova, A.A. Nikitin A.A., etc.)
After treating the facial skin with alcohol, the electrode is inserted into the soft tissue of the neck of the articular process of the lower jaw, slightly pulled back so that its working part is in the muscle. This position of the electrode allows you to freely and painlessly make all movements of the jaw (Fig. 3.59). Complications in the form of short-term restriction of mouth opening were rarely observed.
Normally, there is a coordinated function of synergist and antagonist muscles, a clear rhythmic change in the phases of BEA and BEP. In the phase of one chewing movement, the time of EMG activity of the masticatory, temporal and external pterygoid muscles is less, and the suprahyoid muscles are equal to the EMG “rest” time.
During the rest period there is no spontaneous muscle activity. The average EMG amplitude of all muscles studied during clenching of the jaws is less than during chewing. During voluntary chewing, a periodic change in the functional center occurs, and alternating muscle activity on the right and left is observed.
Rice. 3.58. The time of reflex inhibition of the activity of the right (a) and left (b) masticatory muscles is normal.
At the same time, the masticatory and external pterygoid muscles react more clearly to a change in the functional center than the temporal and suprahyoid muscles. With a given chewing, the average EMG amplitude of the masticatory, temporal and suprahyoid muscles increases on the working side, and on the opposite side - the external pterygoid muscle.
The masticatory and temporal muscles exhibit synchronous activity during chewing, and volleys of EMG activity of the external pterygoid and suprahyoid muscles are located between the volleys of activity of the masticatory and temporal muscles.
Normally, with physiological rest of the masticatory muscles, there is no EMG activity, while with muscular-articular dysfunction such activity reaches 170 μV, and with bruxism, higher amplitudes can be observed. The duration of the latent period of the mandibular reflex increases more than 2 times.
In the phase of one chewing movement, the BEP time decreases, and the BEA time increases.
The EMG activity of the levator muscles with muscular-articular dysfunction decreases, and the muscles of the floor of the mouth increases [Khvatova V.A., 1986].
The degree of disturbances in muscle EMG activity corresponds to the severity of the pain syndrome. In patients with complete regression of the clinical manifestations of dysfunction after treatment, the parameters of the EMG study and the latent time of the mental reflex are close to normal. At the same time, in the group of people with residual symptoms of the disease at the end of the course of treatment, changes in the EMG pattern persist: a decrease in muscle BEA and an increase in the latent time of the reflex [Semenov I.Yu., 1997].
Rice. 3.59. Moment of EMG recording of the external pterygoid muscles. Needle electrodes are inserted directly into the muscle near the neck of the articular process (our own technique).
J.Travell, D.Simons (1989) discovered trigger points (TPs) in the masticatory muscles in the pain syndrome of TMJ dysfunction - areas of increased irritability of muscle tissue, painful when squeezed, from which irradiation of pain occurs in certain areas.
All TTs are characterized by common features:
Hyperirritability;
increased metabolism;
decreased blood flow;
the presence of a palpable cord.
Studies have shown that muscle damage is observed in cases of occlusion (35%), bruxism (24%), emotional stress (15%), lack of teeth (20%) and other pathology of the dentofacial system (6%).
The reasons why occlusion in some people leads to the formation of TT in the masticatory muscles, and in others not, are still unclear.
Experimental studies with induced occlusal disturbances showed that only one of the five subjects with artificially created occlusal disharmony developed muscle discomfort by the end of the second week of the experiment. It is likely that occlusal disorders can maintain TT in the masticatory muscles, but not form and activate them.
The formation of TT in muscles, according to biochemical studies, is facilitated by disturbances in the metabolism of hormones, minerals, and vitamins in common diseases (liver, thyroid, gastrointestinal disorders).
Interpretation of the obtained EMG data is possible with a comprehensive study of the dentofacial system, since the same changes in the EMG pattern occur in various pathological conditions (loss of teeth, malocclusion, decreased occlusal height).
V.A. Khvatova
Clinical gnathology
Graphic registration of movements of the lower jaw, on the basis of which articulators were built - the first mechanical models of the musculoskeletal system of the masticatory system, played a positive role. The design of dentures adapted to the simplest movements of the lower jaw, which immeasurably increased the quality of prosthetics, simultaneously opened up new perspectives for the theory and practice of orthopedic dentistry. Solving these problems required the use of modern functional methods for studying the musculoskeletal system in the orthopedic dentistry clinic.
The most fundamental studies of the biomechanics of the masticatory system have been carried out using masticationography and electromyography.
Masticationography. The chewing stereotype depends on many conditions: the nature of articulation, bite, extent and topography of dentition defects, the presence or absence of a fixed bite height and, finally, on the constitutional and psycho-sthenic characteristics of the patient, formed under the influence of these conditions. Masticography, which allows you to graphically record the dynamics of chewing and non-chewing movements of the lower jaw, is a method for objectively studying this stereotype. With the help of mastication, it is possible to study changes in the biomechanics of the masticatory system in case of anomalies of its development and loss of teeth, as well as the effectiveness of orthopedic and prosthetic measures.
By the nature of masticationograms, one can judge not only the most subtle changes in the masticatory system (intactness of individual teeth, dentition, malocclusion), but also the type of higher nervous activity of the person being studied.
The first attempt to record the movements of the lower jaw using a kymograph was made by N. I. Krasnogorsky (1906). Then this technique underwent many modifications, and currently it looks relatively simple. To obtain a masticatiogram, you need a mechanical or electrical kymograph with a time recorder, as well as a rubber balloon enclosed in a plastic case shaped like the lower jaw (Fig. 34). Using the case, the balloon is pressed to the chin and secured to the head with a special bandage. The balloon is connected through a rubber tube to the Marey capsule, on which the scribe is attached.
Regardless of individual characteristics, several phases are distinguished on the kymogram.
The first phase is the resting phase, recorded before the introduction of a food stimulus into the oral cavity, and is characterized by an isoline.
The second phase is due to the opening of the mouth to accept the food stimulus. It corresponds to the first rise of the kymogram, the height of which depends on the degree of opening of the mouth, and the steepness depends on the duration of the introduction of food into the oral cavity.
The third phase is the adaptation phase. It is characterized by a descending curve, the most extended in time, the lower knee of which lies at the level of the resting phase. The degree of its fracture and the total length after a certain “plateau” at the apex indicate the complexity of the adaptive process to the initial grinding of food, which, on the one hand, is determined by the consistency of the food, and on the other, by the usefulness of the masticatory apparatus.
The fourth phase is characterized by relatively similar, regularly alternating waves, the amplitude, frequency and uniformity of which depend, on the one hand, on the consistency of the food, and on the other, on the usefulness of the masticatory apparatus. This phase is called the main phase. In each oscillation of this phase, an ascending and descending knee is distinguished, of which the first is caused by the lowering of the lower jaw, and the second by bringing it to its original position, i.e., to a state of central occlusion. The top of each wave corresponds to the limit of lowering of the lower jaw, and the magnitude of the angle corresponds to the speed of transition to lifting of the lower jaw.
In this phase, when chewing soft food, frequent, uniform rises and falls of chewing waves are observed. When chewing solid food, at the beginning of the phase of the main chewing function, more rare descents of the chewing wave are noted. The harder the food is and provides more resistance, slowing down the moment of raising the lower jaw, the more flat the descending knee.
The fifth phase is the phase of formation of a lump followed by swallowing it. Graphically, this phase is marked by a wave-like curve with a slight decrease in wave amplitude. The act of forming a bolus and preparing it for swallowing depends on the properties of the food. After swallowing a bolus of food, a new state of rest of the masticatory apparatus is established. Graphically, this state of rest is represented as a horizontal line. It serves as the first phase of the next chewing period.
When using the mastication method, the appropriate recording apparatus should be used correctly.
Electromyography. Over the past 10-15 years, electromyography as a method of functional research of the neuromuscular system has been increasingly used not only in the clinic of nervous diseases, surgery and anesthesiology, but also in dental practice. It is used in surgical and orthopedic, dentistry, dental neurology as a functional and diagnostic method for studying the function of the peripheral neuromotor apparatus and for assessing the coordination of the muscles of the maxillofacial area in time and intensity, normally and pathologically in case of injuries and inflammatory diseases of the maxillofacial area; malocclusions, myoplastic surgeries, dystrophies and hypertrophies of masticatory muscles, clefts of the soft palate, etc.
This method is based on recording action potentials of muscle fibers functioning as part of motor units, consisting of a motor neuron and a group of muscle fibers innervated by this motor neuron. An electromyogram is a graphic expression of bioelectrical activity, which accompanies all basic life processes and is a universal and most accurate indicator of the course of any physiological functions.
In the occurrence of muscle bioelectrical activity, a decisive role is played by changes in the ionic permeability of muscle fiber membranes for K+ and Na+ ions, as well as CL- and Ca 2 - ions due to the different content of K+ and Na+ ions inside muscle fibers and in the intercellular fluid at rest there is a potential difference between the inner and outer surfaces of the muscle fiber membrane (resting potential). As a result of the passage of a nerve impulse along the motor nerve from the motor neuron and the neuromuscular ending, acetylcholine is released from the neuromuscular endings and, as a result, the permeability of the membranes of the corresponding muscle fibers for K + and Na + ions sharply changes, i.e., action potentials are generated.
Any modern electromyographic installation (regardless of its technical design) includes three sequentially located links: output electrodes, or sensors, amplifiers and oscilloscopes (Fig. 35).
The discharge electrodes can be contact electrodes, that is, they directly discharge muscle potentials to the amplifying and recording parts of the installation. There are two types. The first type (type) of electrodes has a discharge surface of up to 10 mm or more, an interelectrode distance of up to 30 mm or more. Such electrodes make it possible to capture the total difference in voltages that develop during the excitation of numerous myoneural endings and muscle fibers located under each electrode of a given pair. The electromyograms obtained using this method characterize “globally” electrical oscillations in the muscle, regardless of whether both electrodes are placed on the skin or immersed intramuscularly.
The second type (type) of electrodes has a small outlet surface (0.65 mm or less) and a small interelectrode distance (0.1 mm or less). With any variants of the technical design of the electrodes, they divert “locally” potential fluctuations from relatively limited areas of the muscle, from their individual fibers, or motor units.
There are three main types of electromyography: global, or superficial, total, interference - removal of biopotentials using cutaneous electrodes; local - recording the activity of individual motor units using needle electrodes; stimulation - registration of muscle biopotentials in response to stimulation of the nerve innervating this muscle.
The choice of program is determined by the specific research objective. Thus, in cases where electromyograms should only confirm the normalization of muscle function, its stability and an increase in the force of contraction, it is sufficient to limit ourselves to recording the activity during maximum voluntary contraction of the muscles of interest to the researcher. And, conversely, in cases where electromyography should help clarify the point of the lesion and identify changes in muscle potentials typical for a particular syndrome, the research program is expanded. Thanks to this expansion, it has been established that often pathological changes in muscle electrogenesis can be detected at rest or during weak tonic tension, whereas with maximum active contraction of the same muscle they are masked by the electrical activity of intact motor units and are not reflected in the electromyogram.
With all the diversity and multiplicity of human motor reactions, they can be schematically classified into three main categories: muscle relaxation reactions; - various reflex-induced tonic tensions; - voluntary or involuntary phasic contractions, providing all types of normal or pathological movements. Since each of these three types of motor reactions, which determine the functional state of the neuromotor apparatus, is based on different physiological and pathophysiological mechanisms, then for a more complete electromyographic characterization of each muscle under study, it is necessary to record electromyograms during at least three functional states: at rest (at active relaxation of the muscle), with its tonic tension and with various (in tempo, strength, target setting) voluntary contractions.
Clinicians widely use advanced techniques that have already been developed and tested in the clinic and experiment. The variety of such methodological techniques both in general medicine and in dentistry is increasing. In the vast majority of cases, the authors record electromyograms of the maxillofacial area during the following functional tests:
- 1) in a state of relative physiological rest of the lower jaw (active relaxation of the masticatory muscles);
- 2) with various non-chewing movements of the lower jaw;
- 3) when performing the main function of the masticatory apparatus (chewing, swallowing);
- 4) with maximum tension of the masticatory muscles in a state of central occlusion;
- 5) with the friendly movement of facial muscles;
- 6) when tapping the chin with a hammer (a special test for studying reflex reactions of the masticatory muscles, used for diseases of the temporomandibular joint). Tapping on the chin with the jaws tightly closed causes a reflex inhibition of the activity of the muscles that lift the mandible - a “silent period”, the duration of which has diagnostic significance. The same test with the lower jaw freely lowered causes a reflex excitation of the masticatory muscles (myostatic reflex), the cause of which is the excitation of muscle stretch receptors (muscle spindles).
Electromyographic research in dentistry has developed in two main directions. The first of these includes works in which electromyographic analysis of the normal activity of the masticatory muscles was carried out. The conducted studies confirmed the existing, based on anatomical data, understanding of the function of the masticatory muscles. The study of the dynamic activity of the masticatory muscles made it possible to determine the average values of quantitative indicators of the bioelectrical activity of these muscles in normal people.
In works related to the second direction, an attempt was made to study functional disorders of the masticatory muscles in various pathological conditions of the dentofacial apparatus. The first studies in this direction were devoted to identifying functional changes in the masticatory muscles with various malocclusions. The works of many domestic and foreign authors are devoted to the study of the EMG characteristics of the masticatory muscles with various partial defects of the dentition. However, most of them came to the conclusion that the absence of even one chewing tooth leads to a decrease in the contractility of the masticatory muscles, an increase in the duration of the bioelectrical activity phase and a decrease in the time of bioelectrical rest.
By conducting electromyographic studies of the masticatory muscles, it was possible to determine the optimal permissible limits for increasing the height of the bite for clinical purposes. Thus, an increase in the height of the bite within acceptable limits causes the appearance of bioelectrical activity in the anterior belly of the temporal muscle in a state of relative physiological rest of the lower jaw. The appearance of such activity in the chewing muscles themselves is a symptom of an excessive increase in the height of the bite. This fact opens up certain methodological possibilities for a true functional determination of the permissible limits for increasing the height of the bite for clinical purposes.
Global electromyography is also used to study functional changes in the masticatory muscles in edentulous patients, both before and at various periods after prosthetics. Conducted studies indicate that prosthetics with complete removable dentures leads to an increase in the bioelectrical activity of the masticatory muscles during chewing in dentures and after their removal. In the process of adaptation to complete removable dentures, there is a reduction in the time of the entire chewing period due to a decrease in the number of chewing movements and the time of one dynamic cycle. According to EMG data, adaptation to total prostheses occurs, as a rule, during the first 6 months. using them.
Analyzing the data from EMG studies conducted in orthopedic dentistry, we can conclude that this method allows us to objectively assess the effectiveness of various types of prosthetic interventions, monitor the consistency (coordination) of the work of symmetrical muscles and the restructuring of the coordination relationships of the functions of the masticatory muscles in the treatment of malocclusions, and identify the pathological participation of facial muscles in some natural acts of the masticatory apparatus.