Retinoic acid signalling regulates branchiomeric neck muscle development at the head/trunk interface.

Skeletal muscles of the head and trunk originate in distinct lineages with divergent regulatory programmes converging on activation of myogenic determination factors. Branchiomeric head and neck muscles share a common origin with cardiac progenitor cells in cardiopharyngeal mesoderm (CPM). The retinoic acid (RA) signalling pathway is required during a defined early time window for normal deployment of cells from posterior CPM to the heart. Here, we show that blocking RA signalling in the early mouse embryo also results in selective loss of the trapezius neck muscle, without affecting other skeletal muscles. RA signalling is required for robust expression of myogenic determination factors in posterior CPM and subsequent expansion of the trapezius primordium. Lineage-specific activation of a dominant-negative RA receptor reveals that trapezius development is not regulated by direct RA signalling to myogenic progenitor cells in CPM, or through neural crest cells, but indirectly through the somitic lineage, closely apposed with posterior CPM in the early embryo. These findings suggest that trapezius development is dependent on precise spatiotemporal interactions between cranial and somitic mesoderm at the head/trunk interface.

The deflection of fatigued neck.

The human neck is a unique mechanical structure, highly flexible but fatigue prone. The rising prevalence of neck pain and chronic injuries has been attributed to increasing exposure to fatigue loading in activities such as prolonged sedentary work and overuse of electronic devices. However, a causal relationship between fatigue and musculoskeletal mechanical changes remains elusive. This work aimed to establish this relationship through a unique experiment design, inspired by a cantilever beam mechanical model of the neck, and an orchestrated deployment of advanced motion-force measurement technologies including dynamic stereo-radiographic imaging. As a group of 24 subjects performed sustained-till-exhaustion neck exertions in varied positions-neutral, extended, and flexed, their cervical spine musculoskeletal responses were measured. Data verified the occurrence of fatigue and revealed fatigue-induced neck deflection which increased cervical lordosis or kyphosis by 4-5° to 11°, depending on the neck position. This finding and its interpretations render a renewed understanding of muscle fatigue from a more unified motor control perspective as well as profound implications on neck pain and injury prevention.

Vestibular Contributions to Primate Neck Postural Muscle Activity during Natural Motion.

To maintain stable posture of the head and body during our everyday activities, the brain integrates information across multiple sensory systems. Here, we examined how the primate vestibular system, independently and in combination with visual sensory input, contributes to the sensorimotor control of head posture across the range of dynamic motion experienced during daily life. We recorded activity of single motor units in the splenius capitis and sternocleidomastoid muscles in rhesus monkeys during yaw rotations spanning the physiological range of self-motion (up to 20 Hz) in darkness. Splenius capitis motor unit responses continued to increase with frequency up to 16 Hz in normal animals, and were strikingly absent following bilateral peripheral vestibular loss. To determine whether visual information modulated these vestibular-driven neck muscle responses, we experimentally controlled the correspondence between visual and vestibular cues of self-motion. Surprisingly, visual information did not influence motor unit responses in normal animals, nor did it substitute for absent vestibular feedback following bilateral peripheral vestibular loss. A comparison of muscle activity evoked by broadband versus sinusoidal head motion further revealed that low-frequency responses were attenuated when low- and high-frequency self-motion were experienced concurrently. Finally, we found that vestibular-evoked responses were enhanced by increased autonomic arousal, quantified via pupil size. Together, our findings directly establish the vestibular system's contribution to the sensorimotor control of head posture across the dynamic motion range experienced during everyday activities, as well as how vestibular, visual, and autonomic inputs are integrated for postural control.SIGNIFICANCE STATEMENT Our sensory systems enable us to maintain control of our posture and balance as we move through the world. Notably, the vestibular system senses motion of the head and sends motor commands, via vestibulospinal pathways, to axial and limb muscles to stabilize posture. By recording the activity of single motor units, here we show, for the first time, that the vestibular system contributes to the sensorimotor control of head posture across the dynamic motion range experienced during everyday activities. Our results further establish how vestibular, autonomic, and visual inputs are integrated for postural control. This information is essential for understanding both the mechanisms underlying the control of posture and balance, and the impact of the loss of sensory function.

Functional involvement of the sternohyoid muscle during breathing and swallowing in rats.

The sternohyoid muscle depresses the hyoid bone, but it is unclear whether the muscle contributes to respiratory and swallowing mechanisms. This study aimed to clarify whether the sternohyoid muscle participates in the respiration and swallowing reflex and how the activity is modulated in two conditions: with airway stenosis and with a fixed sternohyoid muscle length. Electromyographic activity in the sternohyoid, digastric, thyrohyoid, and diaphragm muscles was recorded in anesthetized rats. The sternohyoid muscle activity was observed in the inspiratory phase and during swallowing, and was well coordinated with digastric and thyrohyoid muscle activity. With airway stenosis, the respiratory activity per respiratory cycle was facilitated in all assessed muscles but the facilitation of activity per second occurred only in the digastric, thyrohyoid, and sternohyoid muscles. With airway stenosis, the swallowing activity was facilitated only in the digastric muscle but not in the thyrohyoid and sternohyoid muscles. Swallowing activity was not observed in the sternohyoid muscle in the condition with the sternohyoid muscle length fixed, although increased inspiratory activity remained. The current results suggest that 1) the sternohyoid muscle is slightly activated in the inspiratory phase, 2) the effect of airway stenosis on respiratory function may differ between the upper airway muscles and diaphragm, and 3) swallowing activity in the sternohyoid muscle is not dominantly controlled by the swallowing central pattern generator but instead occurs as a myotatic reflex.NEW & NOTEWORTHY We found that the sternohyoid muscle was activated in the inspiratory phase. However, increased airway resistance had different effects on the extrathoracic muscles than on the diaphragm. The swallowing activity of the sternohyoid disappeared when the muscle length was fixed. These findings suggest that the sternohyoid muscle may be activated not by the swallowing central pattern generator but as a myotatic reflex.

Sex differences in cervical disc height and neck muscle activation during manipulation of external load from helmets.

Neck pain associated with helmet-wear is an occupational health problem often observed in helicopter pilots and aircrew. Whether aircrew helmet wearing is associated with physiological and biomechanical differences between sexes is currently unknown. This study investigated neuromuscular activation patterns during different helmet-wearing conditions. The helmet load was manipulated through a novel Helmet Balancing System (HBS) in healthy, non-pilot male and female participants (n = 10 each, age 19-45 years) in two phases. Phase A assessed the acute effects of helmet-wear on neck muscles activation during head movements. Phase B examined changes in muscle activity and cervical disc height after wearing a helmet for 45 min. In Phase A, muscle activity was similar between sexes in many movements, but it was higher in female participants when wearing a helmet than in males. The HBS reduced muscle activity in both sexes. In Phase B, female participants exhibited a greater level of muscular fatigue, and male participants' cervical disc height was significantly decreased [5.7 (1.4) vs. 4.4 (1.5) mm, P < 0.001] after continuous wearing. Both sexes showed no significant change in muscle fatigue and disc height [male: 5.0 (1.3) vs. 5.2 (1.4) mm, P = 0.604] after applying HBS. These findings demonstrate sex-specific physiological and biomechanical responses to wearing a helmet. They may indicate different postural and motor control strategies, associated with different neck pain aetiologies in male and female aircrew, the knowledge of which is important to reduce or prevent musculoskeletal injuries associated with helmet wearing.

Botulinum toxin and conservative treatment strategies in people with cervical dystonia: an online survey.

Isolated cervical dystonia is a focal, idiopathic dystonia affecting the neck muscles. Treatment usually consists of botulinum neurotoxin (BoNT) injections into the dystonic muscles. Our aim is to investigate the use of BoNT treatment and conservative treatments by people living with cervical dystonia. An online survey in English was conducted between June and August 2022. Participants were eligible to participate if they were living with cervical dystonia, were over 18 years old and could read and understand English. The survey consisted of demographic questions, characteristics of dystonia, questions relating to BoNT use and the perceived utility of conservative treatments. The data were analysed descriptively, and open-ended questions were grouped into similar topics represented by direct quotes. We received 128 responses from people with cervical dystonia, with an average age of 59 years and 77% women. Most participants (52%) described their cervical dystonia as mild to moderate with an average pain score of 5/10. Eighty-two (64%) participants were having regular BoNT injections, with overall positive perceived effects. Common activities reported to improve the symptoms were the use of heat packs, massage, relaxation, physiotherapy and participation in general exercise. Common coping strategies reported were getting sufficient rest, having the support of friends and family, and remaining engaged in enjoyable hobbies. We found that most participants received regular BoNT injections and that heat packs, exercise, massage, physiotherapy and relaxation were mostly perceived as effective in reducing the symptoms of cervical dystonia.

The functional role of the rabbit digastric muscle during mastication.

Muscle spindle abundance is highly variable in vertebrates, but the functional determinants of this variation are unclear. Recent work has shown that human leg muscles with the lowest abundance of muscle spindles primarily function to lengthen and absorb energy, while muscles with a greater spindle abundance perform active-stretch-shorten cycles with no net work, suggesting that muscle spindle abundance may be underpinned by muscle function. Compared with other mammalian muscles, the digastric muscle contains the lowest abundance of muscle spindles and, therefore, might be expected to generate substantial negative work. However, it is widely hypothesised that as a jaw-opener (anatomically) the digastric muscle would primarily function to depress the jaw, and consequently do positive work. Through a combination of X-ray reconstruction of moving morphology (XROMM), electromyography and fluoromicrometry, we characterised the 3D kinematics of the jaw and digastric muscle during feeding in rabbits. Subsequently, the work loop technique was used to simulate in vivo muscle behaviour in situ, enabling muscle force to be quantified in relation to muscle strain and hence determine the muscle's function during mastication. When functioning on either the working or balancing side, the digastric muscle generates a large amount of positive work during jaw opening, and a large amount of negative work during jaw closing, on average producing a relatively small amount of net negative work. Our data therefore further support the hypothesis that muscle spindle abundance is linked to muscle function; specifically, muscles that absorb a relatively large amount of negative work have a low spindle abundance.

Longitudinal visibility of MRI findings in living victims of strangulation.

Initial experiences with magnetic resonance imaging (MRI) of living strangulation victims demonstrated additional findings of internal injuries compared to the standard clinical forensic examination. However, existing studies on the use of MRI for this purpose mostly focused on the first 48 h after the incident. The aims of this study were (a) to evaluate the longitudinal visibility of MRI findings after violence against the neck by performing two MRI examinations within 12 days and a minimum of four days between both MRI scans and (b) to assess which MRI sequences were most helpful for the detection of injuries. Twenty strangulation victims participated in this study and underwent one (n = 8) or two (n = 12) MRI scans. The first MRI examination was conducted during the first five days, the second five to 12 days after the incident. Two blinded radiologists assessed the MRI data and looked for lesions in the structures of the neck. In total, 140 findings were reported in the 32 MRI examinations. Most of the findings were detected in the thyroid and the muscles of the neck. T2-weighted SPACE with fat suppression, T1-weighted TSE and T1-weighted MPRAGE were rated as the most helpful MRI sequences. Subjects who showed findings in the initial scan also demonstrated comparable results in the second scan, which was performed on average 8.4 days after the incident. Our results show that even up to 12 days after the incident, the criminal proceeding of strangulation cases may greatly profit from the information provided by an MRI examination of the neck in addition to the standard clinical forensic examination.

Advanced US of the Skin, Nerves, and Muscles of the Neck: Pearls and Pitfalls with Use of High-Frequency Transducers.

High-frequency US provides excellent visualization of superficial structures and lesions, is a preferred diagnostic modality for anatomic characterization of neck abnormalities, and has a central role in clinical decision making. Recent technological advancements have led to the development of transducers that surpass 20 MHz, elevating high-frequency US to a highly valuable diagnostic tool with broader clinical use and enabling greater spatial resolution in the assessment of skin and superficial nerves and muscles. The authors focus on evolving applications of high-frequency US in neck imaging, emphasizing practical insights and strategies in skin and neuromuscular applications. ©RSNA, 2024 Supplemental material and the slide presentation from the RSNA Annual Meeting are available for this article.

Surgical approach through the anterior scalene muscle resection for invasive tumours in the supraclavicular fossa (with video).

BACKGROUND: The supraclavicular fossa contains many vital organs to be preserved, such as the brachial plexus, subclavian artery and vertebral artery. Various surgical methods have been reported. However, no well-standardized surgical procedure has been established. METHODS: Between 2010 and 2020, we performed 28 surgical treatments of the superior mediastinum and supraclavicular fossa. Of these, we retrospectively reviewed seven cases of supraclavicular invasion using a unified surgical technique in which the anterior scalene muscle was resected, and the inter-scalene triangle was approached. RESULTS: We performed claviculectomy in four cases and a transmanubrial approach in three cases. In all cases, by resecting the anterior scalene muscle, the brachial plexus, subclavian artery and vertebral artery were preserved. There were no critical postoperative complications other than tracheostomy and lymphatic leakage. Median bleeding amount and operative time were 438 (range; 76-1144) ml and 328 (range; 246-615) minutes, respectively. CONCLUSIONS: The anterior scalene muscle resection method might be a safe and standardized method for preserving the brachial plexus, subclavian artery and vertebral artery.

Vibro-tactile stimulation of the neck reduces pain in people with cervical dystonia: a proof-of-concept study.

BACKGROUND: Pain is a common non-motor symptom in patients with cervical dystonia (CD), severely impacting their quality of life. The pathophysiology of CD is incompletely understood but it involves altered processing of proprioceptive and pain signals. OBJECTIVES: The purpose of this proof-of-concept study was to determine if vibro-tactile stimulation (VTS)-a non-invasive form of neuromodulation targeting the somatosensory system-can modulate neck pain in people with CD. METHODS: In a multi-center study, 44 CD patients received VTS to sternocleidomastoid and/or trapezius muscles for up to 45 min under 9 different stimulation conditions that either targeted a single or a pair of muscles. The primary outcome measure was a perceived pain score (PPS) rated by participants on a 100-point analogue scale. RESULTS: During VTS, 29/44 (66%) of participants experienced a reduction in PPS of at least 10% with 17/44 (39%) reporting a reduction in pain of 50% or higher. After VTS cessation, 57% of participants still reported a 10% or higher reduction in PPS. Effects were significant at the group level and persisted for up to 20 min post-treatment. No distinct optimal stimulation profiles were identified for specific CD phenotypes. Clinical markers of disease severity or duration did not predict the degree of VTS-induced pain reduction. CONCLUSION: This proof-of-concept study demonstrates the potential of VTS as a new non-invasive therapeutic option for treating neck pain associated with CD. Further research needs to delineate optimal dosage and long-term effects.

Intermittent neck flexion induces greater sternocleidomastoid deoxygenation than inspiratory threshold loading.

PURPOSE: To compare deoxygenation of the sternocleidomastoid, scalenes, and diaphragm/intercostals (Dia/IC) during submaximal intermittent neck flexion (INF) versus submaximal inspiratory threshold loading (ITL) in healthy adults. METHODS: Fourteen participants performed a randomized, cross-over, repeated measures design. After evaluation of maximal inspiratory pressures (MIP) and maximum voluntary contraction (MVC) for isometric neck flexion, participants were randomly assigned to submaximal ITL or INF until task failure. At least 2 days later, they performed the submaximal exercises in the opposite order. ITL or INF targeted 50 ± 5% of the MIP or MVC, respectively, until task failure. Near-infrared spectroscopy (NIRS) was applied to evaluate changes of deoxy-hemoglobin (ΔHHb), oxy-hemoglobin (ΔO2Hb), total hemoglobin (ΔtHb), and tissue saturation of oxygen (StO2) of the sternocleidomastoid, scalenes, and Dia/IC. Breathlessness and perceived exertion were evaluated using Borg scales. RESULTS: Initially during INF, sternocleidomastoid HHb slope was greatest compared to the scalenes and Dia/IC. At isotime (6.5-7 min), ΔtHb (a marker of blood volume) and ΔO2Hb of the sternocleidomastoid were higher during INF than ITL. Sternocleidomastoid HHb, O2Hb, and tHb during INF also increased at quartile and task failure timepoints. In contrast, scalene ΔO2Hb was higher during ITL than INF at isotime. Further, Dia/IC O2Hb and tHb increased during ITL at the third quartile and at task failure. Borg scores were lower at task failure during INF compared to ITL. CONCLUSION: Intermittent INF induces significant metabolic activity of the sternocleidomastoid and a lower perception of effort, which may provide an alternative inspiratory muscle training approach for mechanically ventilated patients.

The effect of suboccipital muscle dysfunction on the biomechanics of the upper cervical spine: a study based on finite element analysis.

OBJECTIVE: Muscle dysfunction caused by repetitive work or strain in the neck region can interfere muscle responses. Muscle dysfunction can be an important factor in causing cervical spondylosis. However, there has been no research on how the biomechanical properties of the upper cervical spine change when the suboccipital muscle group experiences dysfunction. The objective of this study was to investigate the biomechanical evidence for cervical spondylosis by utilizing the finite element (FE) approach, thus and to provide guidance for clinicians performing acupoint therapy. METHODS: By varying the elastic modulus of the suboccipital muscle, the four FE models of C0-C3 motion segments were reconstructed under the conditions of normal muscle function and muscle dysfunction. For the two normal condition FE models, the elastic modulus for suboccipital muscles on both sides of the C0-C3 motion segments was equal and within the normal range In one muscle dysfunction FE model, the elastic modulus on both sides was equal and greater than 37 kPa, which represented muscle hypertonia; in the other, the elastic modulus of the left and right suboccipital muscles was different, indicating muscle imbalance. The biomechanical behavior of the lateral atlantoaxial joint (LAAJ), atlanto-odontoid joint (ADJ), and intervertebral disc (IVD) was analyzed by simulations, which were carried out under the six loadings of flexion, extension, left and right lateral bending, left and right axial rotation. RESULTS: Under flexion, the maximum stress in LAAJ with muscle imbalance was higher than that with normal muscle and hypertonia, while the maximum stress in IVD in the hypertonic model was higher than that in the normal and imbalance models. The maximum stress in ADJ was the largest under extension among all loadings for all models. Muscle imbalance and hypertonia did not cause overstress and stress distribution abnormalities in ADJ. CONCLUSION: Muscle dysfunction increases the stress in LAAJ and in IVD, but it does not affect ADJ.

How I do it: Sternocleidomastoid sparing approach for the right vertebral artery to common carotid artery transposition.

BACKGROUND: Direct surgery is an important option to treat vertebral artery (VA) stenosis. METHOD: A patient with symptomatic stenosis at the origin of the right VA underwent transposition of the right VA to the common carotid artery (CCA). Using the sternocleidomastoid sparing approach, the VA was anastomosed to the posterior wall of the CCA by twisting the CCA to expose its posterior wall to face the operative field. CONCLUSION: This approach, consisting of securing the proximal VA and then following it to its distal end, not only preserves the sternocleidomastoid muscle but also protects the sympathetic chains and thoracic duct.

A Novel Exercise to Improve Suprahyoid Muscle Area and Intensity as Evaluated by Ultrasonography.

Insufficient suprahyoid muscle strength with poor opening of the upper esophageal sphincter can cause dysphagia. This study investigated whether an exercise of the suprahyoid muscle, named forehead exercise for suprahyoid muscles (FESM, "Enge-Odeko-Taiso" in Japanese), improves the geniohyoid muscle area and intensity using ultrasonography. Sixty-four participants (15 men and 49 women, 82.8 ± 6.0 years) living independently with no symptoms of swallowing difficulties were enrolled. The participants were divided into the FESM and the control group. The FESM is an isometric exercise involving repetitions of looking into the navel as if the chin is pulled back with little neck motion using a hand pushed against the forehead for resistance. This exercise is performed five times in 10 courses a day (total 50 times) for 8 weeks. Participants in the control group did not conduct any exercises. Body mass index, hand grip strength, gait speed, calf circumference, Mini Nutritional Assessment short-form, eating assessment tool, repetitive saliva swallowing test (RSST), and Food Intake LEVEL Scale scores were examined. The ultrasonographic geniohyoid muscle area, intensity, and RSST were investigated before and after the program. In the FESM group, the geniohyoid muscle area increased from 2.24 to 2.52 cm2 (P < 0.05), intensity decreased from 34.6 to 32.0 (P < 0.05), and the median RSST increased from 5 to 6 (P < 0.05) significantly. Conversely, no significant differences were observed in the control group. The FESM was effective to increase the area and decrease the intensity of the geniohyoid muscle and may improve swallowing function.

Reliability and usability of a novel inertial sensor-based system to test craniocervical flexion movement control.

BACKGROUND: Neck pain has a significant global impact, ranking as the fourth leading cause of disability. Recurrent neck pain often leads to impaired sensorimotor control, particularly in craniocervical flexion (CFF). The Craniocervical Flexion Test (CCFT) has been widely investigated to assess the performance of deep cervical flexor muscles. However, its use requires skilled assessors who need to subjectively detect compensations, progressive increases in range of motion (ROM) or excessive superficial flexor activation during the test. The aim of this study was to design and develop a novel Craniocervical Flexion Movement Control Test (CFMCT) based on inertial sensor technology and real-time computer feedback and to evaluate its safety and usability, as well as inter and intra-rater reliability in both healthy individuals and patients with neck pain. METHODS: We used inertial sensor technology associated with new software that provides real-time computer feedback to assess CCF movement control through two independent test protocols, the progressive consecutive stages protocol (including progressive incremental stages of ROM) and the random stages protocol (providing dynamic and less predictable movement patterns). We determined intra and inter-rater reliability and standard error of the measurement for both protocols. The participants rated their usability was analysed through the System Usability Scale (SUS) and possible secondary effects associated with the tests were registered. RESULTS: The progressive consecutive stages protocol and the random stages protocol were safe and easy to use (SUS scores of 82.00 ± 11.55 in the pain group and 79.56 ± 13.36 in the asymptomatic group). The progressive consecutive stages protocol demonstrated good inter-rater reliability (intraclass correlation coefficient [ICC] ≥ 0.75) and moderate to good intra-rater reliability (ICC 0.62-0.80). However, the random stages protocol exhibited lower intra-rater reliability, especially in the neck pain group, where the reliability values were poor in some cases (ICC 0.48-0.72). CONCLUSION: The CFMCT (progressive consecutive stages protocol) is a promising instrument to evaluate CCF motor control in patients with chronic neck pain. It has potential for telehealth assessment and easy adherence for exercise prescription and seems to be a safe and usable tool for patients with pain and asymptomatic individuals. Its use as a test or for exercise needs to be further investigated to facilitate its transfer to clinical practice.

Variations and thicknesses of anterior belly of the digastric muscle: an ultrasonographic study.

PURPOSE: This study aims to evaluate the anatomy and anatomical variations of the anterior belly of the digastric muscle. METHODS: Hundred and fifty one ultrasonographic images of the digastric muscle pairs were evaluated in Near East University Faculty of Dentistry Department of Dentomaxillofacial Radiology. Morphological variations were recorded using the classification of the digastric muscle into 12 types by Kim et al. For the analysis, the Mann-Whitney U test and Chi-square test were used, and for the correlational analysis, Spearman's rho test was applied. P < 0.05 was considered statistically significant in all tests. RESULTS: Seventy female and 81 male patients aged 19-60 years were evaluated. Type 1 digastric muscle was observed in 145 of 151 patients, Type 2 in 3 patients, and Type 7 in 3 patients. The thicknesses of the right and left digastric muscles were measured, and a statistically significant difference was observed between the groups when the genders were compared. It was observed that both right and left digastric muscle thicknesses were higher in males than females. There was a statistically significant positive high correlation between right and left digastric muscle thicknesses (p = 0.000; r = 0.736). No statistically significant difference was found between genders (p = 0.596) in terms of anatomical variations. CONCLUSION: Considering that the variations of the digastric muscle may have a clinical significance role, the normal anatomy and variations of this muscle should be well known by maxillofacial surgeons and radiologists.

Navigating the void: outcomes and adaptations during parotid surgery in the absence of posterior belly of digastric muscle.

It is well known that the digastric posterior belly is one of the essential landmarks for facial nerve identification during parotid surgery. While there were multiple reports about variations of the digastric anterior belly, only a few anatomical variations of the posterior belly of the digastric muscle have been described.In this article, we describe an anatomical variation of the posterior belly of digastric muscle found during superficial parotidectomy of a patient with pleomorphic adenoma. This anatomical variation also led to an anatomical variation in the position of the facial nerve.To our knowledge, this is the first report of an absent posterior belly of digastric muscle found during live parotid surgery. The knowledge of current anatomical variation may help to avoid facial nerve injury during parotid surgery and preserve the function of muscles of facial expression.

MRI and frozen section evaluation of mylohyoid muscle in determining surgical approach for T2-T3 floor of the mouth cancer.

PURPOSE: The choice of surgical approach for floor of the mouth (FOM) cancer, particularly for intermediate-stage tumors (cT2-cT3), remains controversial. This study aims to evaluate a method considering mylohyoid muscle (MM) invasion as a determinant for surgical approach selection, utilizing magnetic resonance imaging (MRI) preoperatively and frozen section (FS) analysis intraoperatively. METHODS: This observational retrospective cohort study analyzed patients undergoing surgical resection of cT2 and cT3 FOM squamous cell carcinoma (SCC) between January 2013 and June 2023. MM infiltration assessed by preoperative MRI determined the surgical approach: clear infiltration led to compartmental surgery (CS), while doubtful or absent infiltration led to transoral surgery (TOS). Conversion from TOS to CS occurred intraoperatively based on macroscopic evidence or positive FS. Data collected included demographic, clinical, surgical, and pathological variables. Survival analysis was conducted using Kaplan-Meier method. RESULTS: Among 44 patients included, majority had cT2 tumors (59.1%). MM resection was necessary in 22.7% of cases. Overall survival (OS) and progression-free survival (PFS) did not significantly differ between TOS and CS groups. Radiological depth of invasion (rDOI) < 10 mm is correlated with MM preservation in 89% of cases, while rDOI > 10 mm is correlated with MM resection only in 23.8% of cases. Pathological depth of invasion (pDOI) discrepancies were observed in the two groups: in CS group is shown a higher pDOI (> 10 mm) confirmation (90%). Surgical complications and functional outcomes differed between TOS and CS groups. CONCLUSION: Considering MM invasion for surgical approach selection in cT2-cT3 FOM tumors appears oncologically safe, with better functional outcomes in muscle preservation. Preoperative MRI for MM assessment combined with intraoperative FS analysis provides reliable guidance for surgical decision-making.

Mylohyoid Muscle: Current Understanding for Clinical Management-Part I: Anatomy and Embryology.

The mylohyoid is one of the suprahyoid muscles, along with the geniohyoid, digastric, and stylohyoid muscles. It lies between the anterior belly of the digastric muscle inferiorly and the geniohyoid superiorly. In Part I, the anatomy and embryology of the mylohyoid muscle will be reviewed in preparation for the clinical discussion in Part II.