A Practical Review of the Muscles of Facial Mimicry With Special Emphasis on the Superficial Musculoaponeurotic System

Transcription

1 Neuroradiology/Head and Neck Imaging Review Hutto and Vattoth Muscles of Facial Mimicry Neuroradiology/Head and Neck Imaging Review FOCUS ON: Justin R. Hutto 1 Surjith Vattoth 2 Hutto JR, Vattoth S Keywords: facial muscles, mimic muscles, superficial facial anatomy, superficial musculoaponeurotic system (SMS) DOI: /JR Received March 16, 2014; accepted after revision pril 30, Department of Radiology, University of labama at irmingham, JT N338, th St S, irmingham, L ddress correspondence to J. R. Hutto (jrhutto@uabmc.edu). 2 Neuroradiology Section, Hamad Medical Corporation, Doha, Qatar. This article is available for credit. WE This is a web exclusive article. JR 2015; 204:W19 W X/15/2041 W19 merican Roentgen Ray Society Practical Review of the Muscles of Facial Mimicry With Special Emphasis on the Superficial Musculoaponeurotic System OJECTIVE. In this article, we elaborate a practical approach to superficial facial anatomy enabling easy identification of the facial mimic muscles by classifying them according to their shared common insertion sites. CONCLUSION. The facial mimic muscles are often difficult to identify on imaging. y tracing them from their common group insertion sites back to their individual origins as well as understanding key anatomic relationships, radiologists can more accurately identify these muscles. S uperficial facial anatomy is terra incognita on cross-sectional imaging and unfortunately not a routinely discussed topic during residency training. This is partly due to the complexity of facial anatomy and that these structures are only infrequently mentioned in imaging reports. The superficial facial muscles can also be difficult to accurately identify on routine imaging because they are usually closely intertwined structures. The entangled nature of the facial muscles is principally due to their common embryologic origin from the mesoderm of the second brachial arch [1]. These muscles initially form as a large muscular sheet early in development and begin to differentiate rapidly into individual muscles as the embryo reaches between 26 and 37 mm in length [2]. The primary function of the facial muscles is facial expression, and the term mimic muscles has subsequently been coined. These muscles of facial mimicry are typically very thin, usually acting as sphincters and dilators of facial orifices as well as elevators and depressors of facial structures. Most of the mimic muscles are unique in that they have their origin on bone but insert directly into the dermis through an intricate structure called the superficial musculoaponeurotic system (SMS) [3]. In the remainder of the body, the skin and subcutaneous tissue are physically separated from the musculature by an investing layer of fascia. This unusual attachment of the facial muscles directly into the dermis is what enables them to accomplish facial expression. detailed overview of the imaging appearance of the facial muscles had not been previously discussed in the literature until March 2012 when a complete atlas of facial muscle anatomy was published. This was accomplished by performing MRI on a cadaver both before and after dissection [1]. This method enabled accurate identification of these small and often insubstantial muscles. The purpose of this article is to elaborate a more practical and simplified approach to this potentially complex anatomy and to provide the radiologist with multiple tools for quick and accurate identification of the facial muscles on routine cross-sectional imaging. Materials and Methods We performed a detailed search of the literature to identify articles containing both gross and radiologic descriptions of the anatomy of the superficial facial muscles, SMS, and superficial fat compartments of the face. CT was chosen as the primary imaging modality for this article because it is a routine imaging technique that is frequently encountered by all readers. lso, multiple anatomic relationships between the facial muscles and surrounding bony structures are stressed, and these relationships are best shown on CT. lthough only CT images are shown in this article, these anatomic principles can be applied interchangeably between all cross-sectional imaging modalities, including CT and MRI. Images used to show facial anatomy were acquired using a routine JR:204, January 2015 W19

2 Hutto and Vattoth protocol for CT angiography (CT) of the head and neck with a slice thickness of 1.4 mm. The images showing examples of abnormalities involving the facial muscles were also acquired using routine protocols for both unenhanced head CT with 5.0 mm slice thickness and CT of the head and neck with 1.4 mm slice thickness. Superficial Facial natomy: n Introduction The face is typically divided into six layers. Moving from superficial to deep, these include the skin, superficial fat compartments, SMS, superficial facial muscles, deep fascia and deep fat compartments, and the retaining ligaments that anchor the overlying structures to the periosteum of the facial bones [3]. This article will primarily focus on the superficial facial muscles and the SMS as well as a brief discussion of the superficial and deep fat compartments. The mimic muscles receive innervation via the facial nerve. The facial nerve exits the brainstem at the ventral surface of the pons near the junction with the medulla and quickly enters the internal auditory canal. Following a tortuous course through the temporal bone, the facial nerve exits the skull via the stylomastoid foramen. Soon after exiting the skull, it pierces into the parotid gland where it gives off its five main branches. This branch pattern is highly variable but is usually composed of a temporal, zygomatic, buccal, mandibular, and cervical branch [4]. fter exiting the parotid gland, the facial nerve branches travel just deep in relation to the SMS before innervating their respective muscles. Trigeminal sensory nerve branches supplying the face travel just superficial to the SMS. This relationship becomes extremely important for enabling facial surgeons to avoid facial nerve injury during procedures in this region [5]. lood supply to the superficial face is primarily accomplished via the facial artery, which is a branch of the external carotid artery. fter its origin from the external carotid, the facial artery courses anteriorly lying inferior to the mandible and anterior to the masseter [4]. The artery then turns cranially until it terminates near the medial canthus of the eye. Throughout its course, it gives off multiple branches including the inferior and superior labial arteries, lateral nasal artery, and terminates as the angular artery. dditional arterial supply to the superficial face is also provided to a lesser extent by the superficial temporal and internal maxillary arteries [4]. Venous drainage is accomplished predominantly by the facial vein. The anterior facial vein and the anterior branch of the retromandibular vein merge to form a common facial vein, which then courses inferiorly and posteriorly to empty into the internal jugular vein. dditional venous drainage is provided by the posterior branch of the retromandibular vein, which joins the posterior auricular vein prior to empyting into the external jugular vein [4]. s mentioned previously, identifying the facial muscles on cross-sectional imaging presents many challenges. We have already discussed the intertwined nature of the mimic muscles because of their common embryologic origin. These muscles are also flat and insubstantial in many patients, making identification even more taxing. dditionally, some of the muscles are oriented in an oblique plane in the face, making them difficult to follow when imaged in the traditional sagittal, axial, and coronal planes [1]. Overcoming these challenges can be difficult; however, there are multiple tools that can be used to correctly identify these structures. The primary concept to keep in mind when trying to locate these muscles is that they share common insertion sites. Thus, correctly identifying the mimic muscles is most easily accomplished by tracing them from the insertion site of the group back to their individual origin, which is often much less crowded. To show this concept, we will review facial muscle anatomy by organizing the muscles into six groups on the basis of their common insertion sites (Fig. 1). Muscles Inserting at the Lower Lip The most caudal insertion site of the mimic muscles in the face is the lower lip. There are three muscles within this group: the mentalis, depressor labii inferioris, and platysma (Fig. 2). The mentalis and depressor labii inferioris are both easily identifiable. The mentalis courses inferiorly from the lower lip to its origin at the incisive fossa of the mandible. The depressor labii inferioris is located superficial and lateral to the mentalis with its origin lying on the lateral mandibular surface between the symphysis menti and the mental foramen [1] (Fig. 2). The platysma is a very thin sheetlike muscle stretching from its origin in the upper pectoral and deltoid fascia up the neck to its insertion along the lower lip [1]. The platysma actually becomes continuous within the SMS in the lower face and can be easily visualized as a hyperdense line in the lateral face at this level [3]. s seen in Figure 2, there is an additional prominent muscle at the lower mandibular level that lies lateral to the depressor labii inferioris. This muscle is the depressor anguli oris. Note that this muscle does not insert at the lower lip but can be traced more superiorly to its insertion site at the angle of the mouth (modiolus). detailed description of the depressor anguli oris is provided in the next section, along with the additional muscles inserting at the angle of the mouth. Muscles Inserting at the ngle of the Mouth (Modiolus) Moving cranially, the next group of muscles insert at an anatomic structure, the modiolus, which is a conglomerate of closely intertwined muscle and fibrous tissue located just lateral and superior to the angle of the mouth [6] (Figs. 3 and 3). This is the largest muscle group and contains six muscles: the orbicularis oris, buccinator, risorius, levator anguli oris, depressor anguli oris, and zygomaticus major. The orbicularis oris is a sphincter muscle surrounding the mouth, which is closely interdigitated with numerous adjacent muscles and contains multiple origins around the mouth as well [1, 7]. The buccinator muscles are not actually mimic muscles because their primary function is mastication; however, these muscles are included in this article to show the association with the parotid duct. The parotid duct pierces the buccinator at the level of the second maxillary molar tooth [8]. This relationship is important because the parotid duct is easily confused with another muscle in this group, the zygomaticus major. The zygomaticus major is located anterior to the parotid duct where it pierces the buccinator (Fig. 3C). It then courses laterally and obliquely in the cheek region to its origin on the zygomatic portion of the zygomatic arch. The risorius muscle is very thin and cannot be identified in most patients. This muscle is unique in that it has no bony origin. oth its insertion at the modiolus and its origin in the soft tissues superficial to the masseter are within the dermis [1]. The other mimic muscle without a bony origin is the malaris. The malaris also inserts near the angle of the mouth, with its origin in the soft tissues adjacent to the lower margin of the orbicularis oculi. However, like the risorius, it almost always cannot be consistently distinguished on imaging [1]. The levator anguli oris courses superiorly from its insertion to W20 JR:204, January 2015

3 Muscles of Facial Mimicry its origin in the canine fossa of the maxilla. This course makes it easy to identify because it is the deepest muscle in the face overlying the inferior portion of the maxillary sinus (Fig. 3D). Finally in this group of muscles, there is the depressor anguli oris, which as we have already discussed, tracks inferiorly from the modiolus to its origin at the oblique line of the mandible lying lateral to the depressor labii inferioris [1]. Muscles Inserting at the Upper Lip The levator labii superioris, levator labii superioris alaeque nasi, and zygomaticus minor are the three muscles that insert at the upper lip (Fig. 4). The levator labii superioris extends superiorly from its insertion at the upper lip to its origin at the inferior margin of the orbit just cranial to the infraorbital foramen [1]. t this origin, the levator labii superioris lies deep in relation to the orbicularis oculi muscle. Knowledge that the levator anguli oris is the deepest muscle overlying the inferior maxillary sinus enables easy identification of the levator labii superioris because it is the muscle just superficial to the levator anguli oris (Fig. 4). The levator labii superioris alaeque nasi is also easily located. This muscle also tracks superiorly from the upper lip; however, it is located more medially than the levator labii superioris, with its origin on the frontal process of the maxilla [1]. ecause the frontal process of the maxilla is easy to locate, the levator labii superioris alaeque nasi is simple to find because of its close association with this bony structure (Fig. 4C). This also makes it an excellent starting point to identify the remainder of the mimic muscles. The final muscle in this group is the zygomaticus minor, which like the zygomaticus major courses obliquely in the cheek region to form its origin on the zygomatic portion of the zygomatic arch. However, it can be distinguished from the zygomaticus major by its more anterior origin site as well as its more cranial orientation in the cheek [1] (Fig. 4D). Muscles Inserting at the Nose More cranially, there are three muscles that insert around the nose: the nasalis, depressor septi nasi, and dilator naris (Fig. 5). The dilator naris is also known as the alar portion of the nasalis muscle but is too small to identify on routine imaging and is only mentioned here for completeness [1, 9]. The nasalis muscle is easily located overlying the bridge of the nose with its origin on the canine eminence of the maxilla (Fig. 5). It has the primary function of compressing the nasal aperture and therefore may also be referred to as the compressor naris [9]. The depressor septi nasi, like the dilator naris, is small in size; however, it can usually be located at its origin on the nasal ala and within the incisive fossa of the maxilla, which is just medial to the canine eminence (Fig. 5C). Its primary function is to pull the tip of the nose inferiorly [1, 9]. Muscles Inserting at the Orbit There are four muscles that have an insertion around the orbit (Fig. 6). The first is the orbicularis oculi, which is the sphincter muscle surrounding the eye. It contains three distinct portions, each containing its own origin. The orbital portion origin is located along the medial orbital rim, and the palpebral and lacrimal portions contain origins at the palpebral ligament and lacrimal bone, respectively [1]. The procerus muscle is seen on the midline between the orbits because it has its origin in the fascia overlying the medial and lateral nasal cartilages where it depresses the lower forehead [9] (Fig. 6). The depressor and corrugator supercilii muscles are both located cranial to the orbit and are often interdigitated. The depressor supercilii is also closely intertwined with the orbicularis oculi as well. The corrugator supercilii is located more cranial and lateral to the depressor supercilii, having its origin on the frontal bone at the medial supraorbital margin. This enables it to achieve its function of depressing the brow and pulling it medially. It is infrequently identified on imaging because it is usually covered by the frontal belly of the occipitofrontalis but if seen is most frequently identified on coronal images [1, 9] (Fig. 6C). Muscle Inserting at the Scalp The frontal belly of the occipitofrontalis is easily identified on imaging in the forehead region because it is the only muscle located in this area. It is located deep in relation to the galea aponeurotica and has its origin in the epicranial aponeurosis near the coronal suture. It functions as the main elevator of the brow [1] (Fig. 7). The Superficial Musculoaponeurotic System s mentioned, the SMS is a continuous organized fibrous network connecting the facial muscles to the overlying dermis. It was first described by Mitz and Peyronie [10] in 1976 as being composed of a 3D scaffold of collagen and elastic fibers with interspersed fat cells. Since its initial description, the SMS has been found to consist of three distinct layers: a fascial layer superficial to the musculature, a layer intimately associated with the mimic muscles, and a deep layer extensively attached to the periosteum of the bones of the face [3]. Two distinct types of SMS have been described. Type 1 SMS is the more common architecture and composes the entire SMS lateral to the nasolabial fold [3]. Histologically, type 1 SMS is composed of a meshwork of fibrous septa that envelop large lobules of fat cells (Fig. 8). These fat lobules provide important viscoelastic properties, and the interconnecting fibrous septa provide the connection between the facial muscles and the periosteum [11]. Clinically, type 1 SMS provides a looser connection enabling more mobility of the mimic muscles; however, this results in more susceptibility to the aging process [3]. Type 2 SMS is located medial to the nasolabial fold and differs in that it contains a much denser network of connective tissue with only interspersed fat cells as opposed to distinct lobules of fat (Fig. 8). The superficial facial muscle fibers in type 2 SMS extend up into the dermis. These differences enable a much stronger more rigid connection. The junction between the rigid type 2 SMS and the loose type 1 SMS in the nasolabial fold is clinically significant in the realm of facial surgery because this area becomes very challenging during facial rejuvenation procedures [12]. On imaging, the SMS appears as a hyperattenuating tortuous line on CT separating the superficial and deep fibroadipose tissue (Fig. 4C). On MRI, this will appear as a T1 and T2 hypointense line in the same region [13]. The SMS becomes continuous with the parotid fascia as it courses laterally in the face (Fig. 3C). Medially, the SMS becomes less distinct as it transitions between type 1 and type 2 in the nasolabial fold (Fig. 5). Caudally, it blends with the upper platysma [3] (Fig. 2). The Superficial and Deep Fibroadipose Tissue discussion on superficial facial anatomy would not be complete without mentioning the superficial and deep fibroadipose tissue. These fat planes provide volume and shape to the face as well as provide a gliding plane for the mimic muscles [9, 14]. s already mentioned, facial fat is divided into super- JR:204, January 2015 W21

4 Hutto and Vattoth ficial and deep compartments by the SMS and facial muscles. The superficial fat is divided into numerous subcompartments by delicate fascial tissue that cannot be visualized on imaging and is beyond the scope of this article [15]. The superficial fibroadipose tissue is well represented in the parotid and cheek regions, whereas the deep fat compartment is thicker medially in the region of the nasolabial folds. The deep fibroadipose tissue is also composed of the large buccal fat pads. Clinically, this anatomy is important in the realm of cosmetic surgery because atrophy of these fat pads plays a significant role in the aging process [9]. Clinical Significance of Superficial Facial natomy We have already discussed the significance of the SMS and fibroadipose tissue in the realm of cosmetic surgery. Rhytidectomy procedures are actually classified according to whether the SMS is dissected and mobilized during the procedure [16]. Facial muscle anatomy surrounding the mouth is also crucial for surgical treatment of cleft palate for both aesthetic and functional outcomes [17, 18]. Previous articles have shown that presurgical MRI to evaluate facial anatomy in cleft palate patients is useful for surgical planning and postoperative follow-up [18, 19]. The facial muscles can also be involved with multiple pathologic processes. Denervation and subsequent atrophy of the mimic muscles can occur from a number of causes affecting the facial nerve including ell palsy; myotonic dystrophy; myasthenia gravis; parotidectomy; and neoplasms, such as perineural tumor spread and schwannoma. MRI imaging techniques have been developed for patients with myasthenia gravis that can accurately assess the degree of atrophy of multiple superficial facial muscles on the basis of quantitative measurements of muscle volume as well as muscle signal intensity characteristics. This has enabled clinicians to more accurately predict which specific type of antibody a patient possesses [20]. Various tumors can also directly involve the facial muscles and SMS as well, including but not limited to lymphoma, squamous cell cancers, adenoid cystic carcinomas, and dermoid cysts (Figs. 9 and 10). Finally, the most common abnormality encountered on imaging of the superficial face is involvement of the facial muscles in the setting of trauma and infection (Fig. 9). Conclusion This systematic review of the superficial facial anatomy provides a practical approach to identifying the facial mimic muscles by stressing that they are more accurately identified by tracing them from their common group insertion sites back to their individual origins. Emphasis should also given to certain anatomic relationships between the mimic muscles and surrounding facial structures that provide critical pieces of information to the interpreting radiologist. Hopefully, this article will shed light on this terra incognita on imaging and provide practical tools to enable quick and accurate identification of the superficial facial structures when necessary. References 1. Som PM, Ng S, Stuchen C, Tang CY, Lawson W, Jeffrey T. The MR imaging identification of the facial muscles and the subcutaneous musculoaponeurotic system. Neurographics 2012; 2: Gasser R. The development of the facial muscles in man. m J nat 1967; 120: Ghassemi, Prescher, Riediger D, xer H. natomy of the SMS revisited. esthetic Plast Surg 2003; 27: Moore KL, Dalley F. Clinically oriented anatomy. Toronto, ON, Canada: Lippincott Williams & Wilkins, 2006: Campiglio GL, Candiani P. natomical study on the temporal fascial layers and their relationship with the facial nerve. esthetic Plast Surg 1997; 21: Pessa JE, Zadoo VP, Garza P, drian EK Jr, Dewitt I, Garza JR. Double or bifid zygomaticus major muscle: anatomy, incidence, and clinical correlation. Clin nat 1998; 11: Farrugia ME, ydder GM, Francis JM, Robson MD. Magnetic resonance imaging of facial muscles. Clin Radiol 2007; 62: Kang HC, Kwak HH, Hu KS, et al. n anatomical study of the buccinator muscle fibers that extend to the terminal portion of the parotid duct, and their functional roles in salivary secretion. J nat 2006; 208: Prendergast PM. Cosmetic surgery. erlin, Germany: Springer-Verlag, 2012: Mitz V, Peyronie M. The superficial musculoaponeurotic system (SMS) in the parotid and cheek area. Plast Reconstr Surg 1976; 58: Har-Shai Y, odner SR, Egozy-Golan D, et al. Viscoelastic properties of the superficial musculoaponeurotic system. Plast Reconstr Surg 1996; 98: Mendelson C. Correction of the nasolabial fold: extended SMS dissection with periosteal fixation. Plast Reconstr Surg 1992; 89: Macchi V, Tiengo C, Porzionato, et al. natomo-radiological study of the superficial musculoaponeurotic system of the face. Ital J nat Embryol 2007; 112: Mendelson C, Muzaffar R, dams WP. Surgical anatomy of the midcheek and malar mounds. Plast Reconstr Surg 2002; 110: Rohrich RJ, Pessa JE. The fat compartments of the face: anatomy and clinical implications for cosmetic surgery. Plast Reconstr Surg 2007; 119: Hönig JF. Concepts of face-lifting: state of the art [in German]. Mund Kiefer Gesichtschir 1997; 112(suppl 1):S21 S Freilinger G, Gruber H, Happak W, Pechmann U. Surgical anatomy of the mimic muscle system and the facial nerve: importance for reconstructive and aesthetic surgery. Plast Reconstr Surg 1987; 80: Kleinheinz J, Joos U. Imaging of cartilage and mimic muscles with MRI: anatomic study in healthy volunteers and patients with unilateral cleft lip and palate. Cleft Palate Craniofac J 2001; 38: Gosain K, marante MT, Hyde JS, Yousif NJ. dynamic analysis of changes in the nasolabial fold using magnetic resonance imaging: implications for facial rejuvenation and facial animation surgery. Plast Reconstr Surg 1996; 98: Fischbein NJ, Kaplan MJ, Jackler RK, Dillon WP. MR imaging in two cases of subacute denervation change in the muscles of facial expression. JNR 2001; 22: (Figures start on next page) W22 JR:204, January 2015

5 Muscles of Facial Mimicry Fig. 1 rtist rendering shows superficial facial muscles that have been color coded into six groups on basis of their common insertion sites. Maroon = lower lip, orange = angle of mouth (modiolus), yellow = upper lip, purple = nose, blue = orbit, green = scalp. Fig. 2 Muscles of lower lip., rtist rendering shows muscles inserting at lower lip highlighted in maroon., xial CT image through level of mandibular body in 36-year-old man with trauma shows muscles that insert at lower lip. Note, depressor anguli oris is prominent muscle at this level but can be traced superiorly to its insertion at modiolus. SMS = superficial musculoaponeurotic system. Fig. 3 Muscles inserting at modiolus., rtist rendering shows muscles inserting at modiolus highlighted in orange. (Fig. 3 continues on next page) JR:204, January 2015 W23

6 Hutto and Vattoth C Fig. 3 (continued) Muscles inserting at modiolus. D, xial CT image () in 36-year-old man with trauma shows muscles through level of modiolus. xial CT image more cranially (C) shows muscles begin to fan out from their insertion, becoming more easily identified. Note parotid duct piercing buccinator and more anteriorly located zygomaticus major muscle at this level. xial CT image further cranial from insertion site (D) shows muscles inserting at modiolus have become even more distinct. Note levator anguli oris is easily identified as deepest muscle overlying inferior maxillary sinus. SMS = superficial musculoaponeurotic system. C D Fig. 4 Muscles inserting at upper lip. SMS = superficial musculoaponeurotic system., rtist rendering shows muscles inserting at upper lip highlighted in yellow., xial CT image through inferior maxillary sinus in 36-year-old man with trauma shows levator anguli oris as deepest muscle seen anterior to maxilla at this level. This deep location is important identification tool to separate this muscle (inserting into modiolus) from more superficial levator labii superioris and levator labii superioris alaeque nasi muscles (which can be traced toward their insertions in upper lip). C, Coronal CT image through cheek in 36-year-old man with trauma shows zygomaticus minor located superior to zygomaticus major. D, xial CT image through superior maxillary sinus in 36-year-old man with trauma shows intimate relationship between frontal process of maxilla and levator labii superioris alaeque nasi muscle. This relationship makes this muscle easy to identify. D W24 JR:204, January 2015

7 Muscles of Facial Mimicry C C Fig. 5 Muscles inserting at nose., rtist rendering shows muscles inserting at nose highlighted in purple., xial CT image in 36-year-old man with trauma nicely shows nasalis muscle overlying bridge of nose. SMS = superficial musculoaponeurotic system. C, xial CT image at more caudal level in 36-year-old man with trauma shows depressor septi nasi within incisive fossa of maxilla. Fig. 6 Facial muscles inserting at orbit., rtist rendering shows facial muscles inserting at orbit highlighted in blue. and C, xial () and coronal (C) CT images identify mimic muscles surrounding orbit. Corrugator supercilii is infrequently identified on imaging but can occasionally be seen in coronal plane before being covered by frontal belly of occipitofrontalis as shown. Fig. 7 xial CT image through frontal sinus in 36-year-old man with trauma. Frontal belly of occipitofrontalis is easily identified because it is only superficial facial muscle that inserts in scalp. JR:204, January 2015 W25

8 Hutto and Vattoth Fig. 8 Type 1 and type 2 superficial musculoaponeurotic system (SMS). and, rtist renderings of type 1 () versus type 2 () SMS. Note discrete fat lobules in type 1 SMS as opposed to interspersed fat cells of type 2 SMS. Dense collagen fiber network of type 2 SMS also provides much stronger connection between underlying structures and overlying dermis. Fig. 9 Superficial musculoaponeurotic system (SMS)., xial CT image through level of mastoids in 80-year-old woman who initially presented for evaluation of dizziness shows incidentally discovered hyperattenuating nodule (arrow) within type 1 SMS. This finding likely indicates dermoid cyst., xial CT image in 27-year-old man with cut injury shows severing of right parotid duct (red arrow) and overlying SMS (yellow arrow), with adjacent fat stranding. gain, zygomaticus major muscle should not be mistaken for parotid duct. Facial nerve branches lying deep in relation to and trigeminal sensory nerves lying superficial to SMS are likely injured. Fig year-old man who presented for evaluation of parotid mass., xial CT image shows heterogeneously enhancing mass lesion in right parotid gland (biopsy-proven adenoid cystic carcinoma with facial nerve perineural spread) with direct extension into right masseter muscle (white arrows). Right parotid duct is normal (yellow arrow). Right zygomaticus major muscle shows denervation atrophy at this level where it is seen coursing toward its insertion at angle of mouth (red arrow); normal left zygomaticus major muscle is shown for comparison (arrowhead)., xial CT image at more cranial level shows thickening of right superficial musculoaponeurotic system (SMS) due to contiguous involvement from perineural spread of tumor along facial nerve, which lies just deep in relation to it (white arrows). Note that normal left SMS is seen as thin hyperdense line (yellow arrows). Right levator anguli oris muscle shows denervation atrophy (red arrow); normal left levator anguli oris muscle is shown for comparison (arrowhead). FOR YOUR INFORMTION This article is available for CME and Self-ssessment (S-CME) credit that satisfies Part II requirements for maintenance of certification (MOC). To access the examination for this article, follow the prompts. W26 JR:204, January 2015