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      Anatomical Study of the Innervation of the Masseter Muscle and Its Correlation with Myofascial Trigger Points

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          Abstract

          Background and Purpose

          Myofascial pain syndrome (MPS) is widely prevalent in the general population; some reports estimate its prevalence ranges from 9 to 85%. Among the different locations where MPS may arise, pain related to the masseter muscle is referred as masticatory myofascial pain. MPS is characterized by myofascial trigger points (MTPs), which represent tender anatomical areas of a muscle where painful symptoms are elicited whenever stimulated. Previous publications have found MTPs to coincide with neuromuscular junctions at the motor end plate, at the innervation zone (IZ). Our study aimed to describe the innervation of the masseter muscle and relate it to clinically described myofascial trigger points (MTPs).

          Materials and Methods

          We mapped the nerve fiber distribution into the masseter muscles from 16 cadavers by anatomical dissection. We divided the muscle into six regions, three superior (I–III) and three inferior (IV–VI), and classified the nerve’s branches distribution according to these predetermined areas. Statistical analyses was made by Poisson distribution and logarithm link function followed by Bonferroni multiple comparisons ( P<0.05).

          Results

          All six areas received branches from the masseteric nerve. Areas I and II (upper posterior and upper intermediate, respectively) had a significant higher number of nerve entries as compared to the remaining areas.

          Conclusion

          The penetration areas of the masseteric nerve have been established and MTPs are found in the innervation zones, clinicians should focus initially on the regions of the penetration points, for diagnostics and therapeutic measures, such as injections, dry needling and soft tissue interventions. Anatomical study of nerve supply to the masseter muscle can provide useful additional knowledge to further understanding masticatory myofascial pain and to direct therapeutic interventions and diagnostic studies of temporomandibular junction dysfunction.

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          Most cited references 42

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          An expansion of Simons' integrated hypothesis of trigger point formation.

          Simons' integrated hypothesis proposed a model of trigger point (TrP) activation to explain known TrP phenomena, particularly endplate noise. We propose an expansion of this hypothesis to account for new experimental data and established muscle pathophysiology.
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            Pathophysiologic and electrophysiologic mechanisms of myofascial trigger points.

            To review recent clinical and basic science studies on myofascial trigger points (MTrPs) to facilitate a better understanding of the mechanism of an MTrP. English literature in the last 15 years regarding scientific investigations on MTrPs in either humans or animals. Research works, especially electrophysiologic studies, related to the pathophysiology of MTrP. (1) Studies on an animal model have found that a myofascial trigger spot (MTrS) in a taut band of rabbit skeletal muscle fibers is similar to a human MTrP in many aspects. (2) An MTrP or an MTrS contains multiple minute loci that are closely related to nerve fibers and motor endplates. (3) Both referred pain and local twitch response (characteristics of MTrPs) are related to the spinal cord mechanism. (4) The taut band of skeletal muscle fibers (which contains an MTrP or an MTrS in the endplate zone) is probably related to excessive release of acetylcholine in abnormal endplates. The pathogenesis of an MTrP appears to be related to integrative mechanisms in the spinal cord in response to sensitized nerve fibers associated with abnormal endplates.
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              Classification, epidemiology, and natural history of myofascial pain syndrome.

               R Gerwin (2001)
              Myofascial pain syndrome is a disease of muscle that produces local and referred pain. It is characterized by a motor abnormality (a taut or hard band within the muscle) and by sensory abnormalities (tenderness and referred pain). It is classified as a musculoskeletal pain syndrome that can be acute or chronic, regional or generalized. It can be a primary disorder causing local or regional pain syndromes, or a secondary disorder that occurs as a consequence of some other condition. When it becomes chronic, it tends to generalize, but it does not change to fibromyalgia. It is a treatable condition that can respond well to manual and injection techniques, but requires attention to postural, ergonomic, and structural factors, and toxic or metabolic factors that impair muscle function.
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                Author and article information

                Journal
                J Pain Res
                J Pain Res
                jpr
                jpainres
                Journal of Pain Research
                Dove
                1178-7090
                02 December 2020
                2020
                : 13
                : 3217-3226
                Affiliations
                [1 ]Department of Surgery, Laboratory of Medical Research—Division of Human Structural Topography, Faculty of Medicine of the University of São Paulo (FMUSP) , São Paulo, SP, 01246903, Brazil
                Author notes
                Correspondence: Flávia Emi Akamatsu Tel +55 11 971515516Fax +55 11 30618277 Email flaea@usp.br
                Article
                265717
                10.2147/JPR.S265717
                7719441
                © 2020 Procópio Pinheiro et al.

                This work is published and licensed by Dove Medical Press Limited. The full terms of this license are available at https://www.dovepress.com/terms.php and incorporate the Creative Commons Attribution – Non Commercial (unported, v3.0) License ( http://creativecommons.org/licenses/by-nc/3.0/). By accessing the work you hereby accept the Terms. Non-commercial uses of the work are permitted without any further permission from Dove Medical Press Limited, provided the work is properly attributed. For permission for commercial use of this work, please see paragraphs 4.2 and 5 of our Terms ( https://www.dovepress.com/terms.php).

                Page count
                Figures: 3, Tables: 12, References: 44, Pages: 10
                Categories
                Original Research

                Anesthesiology & Pain management

                pain, masseteric nerve, innervation zone, anatomy

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