Journal of Clinical Medicine Article MRI-Based Assessment of Masticatory Muscle Changes in TMD Patients after Whiplash Injury Yeon-Hee Lee 1,* , Kyung Mi Lee 2 and Q-Schick Auh 1 1 Department of Orofacial Pain and Oral Medicine, Kyung Hee University Dental Hospital, #613 Hoegi-dong, Dongdaemun-gu, Seoul 02447, Korea; [email protected] 2 Department of Radiology, Kyung Hee University College of Medicine, Kyung Hee University Hospital, #26 Kyunghee-daero, Dongdaemun-gu, Seoul 02447, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-2-958-9409; Fax: +82-2-968-0588 Abstract: Objective: to investigate the change in volume and signal in the masticatory muscles and temporomandibular joint (TMJ) of patients with temporomandibular disorder (TMD) after whiplash injury, based on magnetic resonance imaging (MRI), and to correlate them with other clinical parameters. Methods: ninety patients (64 women, 26 men; mean age: 39.36 ± 15.40 years), including 45 patients with symptoms of TMD after whiplash injury (wTMD), and 45 age- and sex- matched controls with TMD due to idiopathic causes (iTMD) were included. TMD was diagnosed using the study diagnostic criteria for TMD Axis I, and MRI findings of the TMJ and masticatory muscles were investigated. To evaluate the severity of TMD pain and muscle tenderness, we used a visual analog scale (VAS), palpation index (PI), and neck PI. Results: TMD indexes, including VAS, PI, and neck PI were significantly higher in the wTMD group. In the wTMD group, muscle tenderness was highest in the masseter muscle (71.1%), and muscle tenderness in the temporalis (60.0%), lateral pterygoid muscle (LPM) (22.2%), and medial pterygoid muscle (15.6%) was significantly more frequent than that in the iTMD group (all p < 0.05). The most noticeable structural changes in the masticatory muscles occurred in the LPM with whiplash injury. Volume (57.8% vs. 17.8%) and signal changes (42.2% vs. 15.6%) of LPM were significantly more frequent in the wTMD group than in the iTMD group. The presence of signal changes in the LPM was positively correlated with the Citation: Lee, Y.-H.; Lee, K.M.; Auh, increased VAS scores only in the wTMD group (r = 0.346, p = 0.020). The prevalence of anterior disc Q.-S. MRI-Based Assessment of Masticatory Muscle Changes in TMD displacement without reduction (ADDWoR) (53.3% vs. 28.9%) and disc deformity (57.8% vs. 40.0%) Patients after Whiplash Injury. J. Clin. were significantly higher in the wTMD group (p < 0.05). The presence of headache, sleep problems, Med. 2021, 10, 1404. https://doi.org/ and psychological distress was significantly higher in the wTMD group than in the iTMD group. 10.3390/jcm10071404 Conclusion: abnormal MRI findings and their correlations with clinical characteristics of the wTMD group were different from those of the iTMD group. The underlying pathophysiology may differ Received: 2 March 2021 depending on the cause of TMD, raising the need for a treatment strategy accordingly. Accepted: 29 March 2021 Published: 1 April 2021 Keywords: whiplash injury; temporomandibular disorder; magnetic resonance imaging; masticatory muscle; lateral pterygoid muscle; masseter muscle Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. 1. Introduction Whiplash injury refers to a macrotrauma that occurs with a hyperextension of the head and neck vertebrae followed by hyperflexion when a sudden or excessive force is applied [1]. In road traffic accidents, injuries occur mainly due to side-impact or rear-end Copyright: © 2021 by the authors. collisions [2,3] (Figure1). Such cases present with a variety of clinical manifestations, Licensee MDPI, Basel, Switzerland. including neck stiffness, neck pain disability, psychological distress, headache, and tem- This article is an open access article poromandibular disorder (TMD) [4–6]. The incidence of TMD in patients with whiplash distributed under the terms and injury is low to moderate and ranges from 14–37.5% [4]. It is important to note that patients conditions of the Creative Commons suffering from chronic whiplash injury commonly have clinical pain in a wider range Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ of their bodies [7]. Approximately 40% of patients with whiplash injuries suffer from 4.0/). persistent pain and chronic disability [8]. J. Clin. Med. 2021, 10, 1404. https://doi.org/10.3390/jcm10071404 https://www.mdpi.com/journal/jcm J. Clin. Med. 2021, 10, x FOR PEER REVIEW 2 of 19 J. Clin. Med. 2021, 10, 1404 2 of 18 range of their bodies [7]. Approximately 40% of patients with whiplash injuries suffer from persistent pain and chronic disability [8]. FigureFigure 1. 1. DynamicDynamic mechanism mechanism of of whiplash whiplash injury injury with with hyperextension hyperextension–hyperflexion.–hyperflexion. Whiplash Whiplash injury injury refers refers to to a a macro- macro- traumatrauma that that occurs occurs with with a a sudden sudden hyperextension hyperextension followed followed by by hyperflexion hyperflexion,, which which can can initiate initiate or or aggravate aggravate the the temporo- temporo- mandibularmandibular disorder disorder ( (TMD)TMD) symptoms. symptoms. TMDTMD is is an an umbrella umbrella term term for for pain pain and and dysfunction dysfunction in in the the temporomandibular joint joint (TMJ),(TMJ), masticatory masticatory muscles, muscles, and and adjacent adjacent structures. Population Population-based-based studies studies showed thatthat 60 60–70%–70% of of the the general general population population has has at at least least one one sign sign of of TMD TMD,, 12.1% 12.1% have have TMD TMD-- relatedrelated pain, pain, and and only only 5% 5% seek seek treatment treatment [9,10] [9,10].. Plausible Plausible risk risk factors factors for for TMD, TMD, either either alonealone or or more more likely likely in in combination combination include include macrotrauma, macrotrauma, microtrauma, microtrauma, other other body body pain pain conditions,conditions, self self-reported-reported previous previous pain pain experience, experience, and and psychological psychological status. status. The The etiology etiology ofof TMD TMD moved moved from from a a mechanical mechanical-based-based p phenomenonhenomenon to to a a chronic chronic pain pain biopsychosocial biopsychosocial modelmodel [11] [11].. Typical Typical and frequentfrequent TMDTMD symptoms symptoms include include TMJ TMJ noise noise and and pain, pain, followed followed by byrestriction restriction of mandibular of mandibular movement, movement, headache, headache, neck pain, neck ear pain, pain, ear and pain, tinnitus. and Whiplashtinnitus. Winjuryhiplash is considered injury is considered an initiating an orinitiating aggravating or aggravating factor of TMD factor [5 of,12 TMD]. Thus, [5,12] patients. Thus, with pa- tientsTMD with suffering TMD from suffering whiplash from injurywhiplash have injury wider have and wider stronger and pain stronger than pain those than with those only withmicrotrauma only microtrauma [6]. [6]. TheThe pathophysiology pathophysiology ofof TMDTMD developmentdevelopment and and exacerbation exacerbation associated associated with with whiplash whip- lashinjury injury can can be distinguishedbe distinguished from from that that due due to to other other TMD TMD etiologies. etiologies. TMD caused by by functionalfunctional habits habits or or microtraumatic microtraumatic accumulations accumulations such such as as malocclusion, malocclusion, i.e., i.e., idiopathic idiopathic causes,causes, are are arise arise from from at at changes changes at at the the peripheral peripheral level. level. However, However, during during whiplash whiplash injury, injury, individualindividualss have have direct direct macrotrauma macrotrauma to to muscles, muscles, ligaments, ligaments, and and tendons tendons of of cervical cervical area, area, andand significant significant damage damage to to surrounding surrounding tissues tissues and and pain pain may may also also occur occur [13] [13].. In In addition, addition, suddensudden large large impact cancan bebe transmittedtransmitted to to the the central central nervous nervous system system (CNS) (CNS) because because of theof occurrence of diffuse axonal injury throughout the injury [14]. Sometimes referral patterns the occurrence of diffuse axonal injury throughout the injury [14]. Sometimes referral pat- may develop because of CNS involvement resulting from prolonged masticatory muscle terns may develop because of CNS involvement resulting from prolonged masticatory pain, and the individual’s pain pattern becomes more complex. Additionally, experiences muscle pain, and the individual’s pain pattern becomes more complex. Additionally, ex- of emotional trauma from the injury that pose a potential threat to life can negatively impact periences of emotional trauma from the injury that pose a potential threat to life can neg- personal future health and finances [15]. These physical and psychosocial risk factors create atively impact personal future health and finances [15]. These physical and psychosocial combined synergies and form complex neuropathologies of pain amplification in TMD. risk factors create combined synergies and form complex neuropathologies of pain am- Masticatory muscles are poorly understood in terms of posttraumatic sequelae. All plification in TMD. four main masticatory muscles, including the masseter, temporalis, medial pterygoid mus- Masticatory