8
EMG and Ultrasonography of Masticatory Muscles
Stavros Kiliaridis
Introduction
During the postnatal growth of bones, a continuous remodeling process takes place to maintain a form that is appropriate for its biomechanical environment. Masticatory muscle function has been considered to be a local environmental factor that plays an important role in influencing craniofacial growth, as has been shown in animal experiments and clinical studies. These investigations have shown that the elevator muscles of the mandible influence both the transverse and the vertical facial dimensions. It is possible that the loading of the jaws by masticatory muscles stimulates sutural growth, increases bone apposition, and results in greater transverse growth of the maxilla with broader bone bases for the dental arches. Furthermore, increased demands in masticatory muscle function are often associated with an anterior growth pattern and well‐developed angular, coronoid, and condylar processes in the mandible (Kiliaridis 2006). Thus, certain facial asymmetries have been considered to be related to an asymmetric muscular balance, induced after an environmentally established unilateral malocclusion, as is the functional lateral crossbite (Pirttiniemi 1998).
Understanding the Functional Profile of the Muscle
The investigations which explored the interrelation between masticatory muscles and dentofacial morphology have relied on various characteristics of muscles that could be measured in order to define the level of the muscle capacity. In animal experimental studies, electrophysiologic methods and muscle biopsies could identify the characteristics of the functional profile of the muscle cells composing the muscle, i.e. the muscle fibers, that through their contraction create force and movement. Nevertheless, most of the clinical studies do not permit extensive use of such procedures, so other methods were implemented to identify the functional profile of the muscles of different individuals. In order to better understand the implementation of these methods, an effort will be done to present an overview of the basic function of the muscles.
The contraction of the muscle fiber is based on the size changes of sarcomers which are the basic functional elements of the striated fibers. Sarcomeres are organized so that their contractile proteins are situated in striated form; they function as a unit during development of active tension. Under physiologic and not experimental conditions, the contraction of a muscle fiber is initiated after a signal from the nervous system that reaches the motor endplate, situated on the fiber’s sarcolemma, where an amount of acetylcholine is released from the nerve ending after the nerve stimulation. Due to this shower of acetylcholine at the motor endplate, the ion channels in the sarcolemma of the muscle fiber open and allow sodium ions to pass through. This induces a depolarization of the sarcolemma in a waveform that departs from the end plate and travels rapidly along the entire fiber and the internal of the fiber via the T‐tubules, which are an organized network of channels. The initiation of this wave, called the action potential, releases calcium ions that then trigger almost simultaneous contraction of all the sarcomeres in the fiber (Franzini‐Armstrong 2018).
In the center of the sarcomere thick filaments composed of myosin molecules are arranged in a hexagon with adjacent thin filaments. The thin filaments, actin, are attached to the Z‐plate, which forms the borders between the sarcomeres (Luther 2009). The procedure of the muscle contraction is based on the sliding of the actin filaments along the thick myosin filaments, to bring the Z‐plates closer to each other. The sliding process is achieved thanks to the “head” of the myosin molecules attached to an active site on an actin molecule, forming a cross‐bridge between the thick and thin filaments. The transformation of the ATP to ADP releases energy that bends the myosin heads toward the center of the myosin filament, pulling also the attached actin filament to this direction and reducing the length of the sarcomere (Canepari et al. 2010).
Electrophysiologic methods of isolated muscle fibers after electric stimulation of the single fiber could distinguish them to slow‐twitch and fast twitch and after repetition of the stimulation, the fibers were distinguished if they were reaching fatigue quickly, fast fatigue, or not, i.e. fatigue resistant. The electrophysiologic contractive profile of the fibers was related to an oxidative contraction for fibers that are slow twitch and resistant to fatigue, in contrast to glycolytic contraction of fibers which are fast twitch and fast fatigue. Thus, another way to characterize the muscle fibers is through the application of histochemical or immunohistochemical methods to differentiate them in Type I fibers which are those with oxidative contraction, slow twitch, and resistant to fatigue, and the Type II fibers which are those with glycolytic contraction and fast twitch. Two subcategories of the Type II fibers presented differentiation in fatigue, i.e. the Type IIa characterized by fast twitch and more fatigue resistant than the Type IIb fibers characterized by fast twitch and fast fatigue (Schiaffino and Reggiani 2011). The muscle fibers characteristics are decided by the motoneuron, and all the fibers innervated by the same motoneuron, i.e. belong to the same motor unit and present the same functional profile. Analysis of the composition and size of the fibers provides a good insight in the functional profile of the muscle. Nevertheless, the analysis of biopsy samples of masticatory muscles in humans is a rare method used in the clinic.
The muscle strength of each of the elevator muscles cannot be measured independently under clinical conditions. Nevertheless, the result of their synergy is evaluated as bite force. The level of bite force is a complicated function, as it depends on the number of the activated motor units and their frequency of activation.
Thus, as it is explained beautifully by Astrand and Rodahl (1986) “in activities with low force demands, slowly contracting small motor units are recruited first, with relatively low frequency of contraction. With increasing demand for force, the ‘old’ motor units increase their discharge rate, and in addition, new motor units are recruited. The fast‐contracting motor units gradually start their activity and then at a relatively high frequency. The gradation of a muscle contraction is brought about by varying the number of active motor units (recruitment) and their frequency of excitation (rate coding).” The number and size of the muscle fibers determine the muscle strength. Thus, an evaluation of the strength of a muscle can be based on the amount of the contractive elements producing the muscle force by measuring the cross‐section or the thickness of a muscle.
Clinical Methods to Record Masticatory Muscles Functional Capacity
Bite Force
Bite force is the measured effect of the contraction of the jaw elevator muscles, mainly the masseter, temporalis, and medial pterygoid. Various levels of measurements of the bite force were recorded, the one mostly used, was the maximal bite force, while others as the chewing bite force, and the maximal bite force endurance have been also recorded but in less extent. Maximal bite force was used as an indirect measure of the functional capacity of all the elevator muscles of the mandible, relatively simple to perform (Kiliaridis et al.1993). The suggested site for recording the maximal bite force was the molar region where the highest recordings can be performed (Koc et al. 2010), while keeping the intermolar distance approximately 11 mm apart by adapting the thickness of the bite fork to this size. This distance was suggested as the optimal one, as the bite force levels increase when clenching is performed with gradual augmentation of the jaw opening until about 11 mm of intermolar distance, while there is a decrease of the level of maximal bite force with further opening of the mandible, probably reflecting the average result of the optimum sarcomere length of the jaw elevator muscles (Bakke 2006).
The recording of maximal molar bite force as a measure of the functional capacity of the masticatory muscles provides big advantages as it is relatively simple, inexpensive, and quick to perform, serving large epidemiologic purposes, and being well accepted by the subjects. Nevertheless, this method has a drawback as the maximum bite force is generated bilaterally from the synergy of all the elevator masticatory muscles, transferred on the mandible. The contribution of the contraction forces of both sides is the reason why the method is less sensitive to identify minor or moderate asymmetric functional differences between the elevator muscles of the two sides. Another disadvantage of the maximum bite force recordings is associated with a large method error as reported in different studies due to the motivation level of the subject to bite as hard as possible, the intellectual level of the subjects to understand this demand, the triggering of dental, periodontal, muscle or joint pain, as well as the fear of breaking a tooth or restoration, or the result of fatigue (Carlsson 1974; Hellsing and Hagberg 1990; Bakke 2006).
Electromyography
Electromyography (EMG) is a method available for imaging muscle function and efficiency, by identifying their electrical potentials. The EMG is recording the action potentials generated by muscle fibers when they are recruited by the motoneuron and trigger almost simultaneous contraction of all the sarcomeres in the fiber. This method can be implemented to assess the electrical activity on different levels: from a muscle fiber, a motor unit, a single muscle, or a group of muscles. Nevertheless, the recording of the activity of a single muscle fiber or a motor unit is performed by an intramuscular electromyography with bipolar needle electrodes inserted through the skin into the muscle tissue, most of the time being used to detect single motor unit action potential. Though the intramuscular EMG is a very useful diagnostic method in the neurology, its invasive approach is not practical for routine use in dental clinical settings and demands a thorough knowledge in neurophysiology and pathology for correct interpretation of the findings.
Surface electromyography (sEMG) has been a widely used method in the fields of Oral Physiology as well as in Orthodontics using surface electrodes located on the surface of the skin, and it detects superimposed motor unit action potentials from many muscle fibers (Dahlstrom 1989). The recordings depend on the location of the electrodes, where the voltage between two electrodes (with a standardized interelectrode distance) is measured. Nowadays, surface bipolar, self‐adhesive, and pre‐gelled electrodes are often used with an inter‐electrode distance of 20 mm, decreasing a source of recording error.
Masseter and anterior temporalis muscles are the muscles most frequently assessed by sEMG. The EMG activity of these muscles can be evaluated during static tests (in rest position, maximum, or sub‐maximum voluntary contraction during clenching) or during active tests, such as chewing, swallowing, opening, or closing the mouth, protrusion, retrusion, and lateral deviation of the mandible. The clinical rest position of the mandible, determined by freeway space, is an active muscle position because of the tone of the muscles involved in it (Suvinen et al. 2003). Maximum voluntary contraction (MVC) is another static test frequently analyzed. The recordings of the EMG activity during isometric contraction are usually performed while maximum clenching of the teeth for 3–5 seconds in an intercuspal position or biting on a control substance, i.e. with cotton rolls placed on the lower second premolar and molars (Ferrario et al. 2000; Tartaglia et al. 2008).
The recordings of sEMG were not found to be very reproducible, presenting a larger error in the method (Cecere et al. 1996). This may be because of different factors as the inaccuracy of electrode placement with respect to underlying structures, variation in impedance of the skin, the subcutaneous fat layer, and the depth of the muscle under study (Mohl et al. 1990; Ferrario et al. 1991; Lund et al. 1995; Nordander et al. 2003).
The impact of the large methodologic error may influence longitudinal recordings where the exact placement of the electrodes is crucial. This is more serious in growing individuals when recordings are performed some years later, and changes have occurred due to growth of the face, creating difficulties to localize exactly the site where the electrodes should be placed. Similar problems may appear when bilateral recordings are compared, as the bilateral position of the electrodes may not correspond perfectly to the position of the underneath muscles, thus creating problems when evaluating a symmetric or asymmetric function.
Cross‐section Surface and Thickness of Masticatory Muscles: Imaging Techniques – Ultrasonography of the Masticatory Muscles
Stay updated, free dental videos. Join our Telegram channel
VIDEdental - Online dental courses
