Incidental Findings in TMJ Imaging
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For years, the conventional imaging modalities are generally used in dental practice routine. They provide precious information about dentomaxillofacial region; they also have some limitations such as superimpositions, magnifications, distortions, and low-image quality. Because of these limitations, some pathologies, calcifications, and anatomical variations may be missed. Any findings on a radiographic image which are not related to the research areas of interest could be a description of “incidental findings.” Calcifications are one of these incidental findings. They are asymptomatic and also common in TMJ images. Although most of the calcifications require no treatment, correct identification will reduce further diagnostic assessments. Incidental findings on TMJ images were rare, while examining images for temporomandibular disorders and associated issues, it is important not only to examine the TMJ structures but also to look at the nearby anatomical features to check for evidence of incidental findings pathologies that may have mimicked signs and symptoms of TMJ disorders.
KeywordsTMJ Incidental finding CBCT MRI
11.1 Incidental Findings in TMJ Imaging
MRI has been usually used to obtain information from the soft tissue of the TMJ especially for evaluating the articular disc and its pathology. Even if small field of view (FOV) and surface coils are used in TMJ MR imaging, it should be expected that some incidental findings may appear outside the primary area of interest. These incidental findings are common in TMJ MRIs. Orhan et al.  evaluated the incidental findings in TMJ MRI and found 117 incidental findings in 15% of the patients.
11.1.1 Epidermoid Cyst
Most commonly known as a sebaceous cyst but also known as epidermoid inclusion cyst or epidermal cyst is a slow-growing benign cyst which arises from the epidermis and develops out of the ectodermal tissue. Histologically the lining of the cyst contains only epithelium . Panoramic radiographs are not able to show these cysts even if they are located on the dentomaxillofacial region. Epidermoid cyst usually appears as a hypoattenuated well-circumscribed cystic mass with near cerebrospinal fluid (CSF) density on computed tomography (CT) images and does not enhance after the administration of the contrast material . This non-tender and dome-shaped cyst has low signal intensity on T1-W images while high signal intensity in T2-W images with focal low signal intensity debris . Cyst signal intensity in MRI is varying broadly according to the composition of the cyst, but most of the intracranial epidermoid cysts have low signal intensity on T1-W images and high or equal signal on T2-W images when compared the cerebral cortex . While DWI demonstrates high signal intensity, ADC maps have iso- or slightly hyperintense relative to the brain tissue . Seven percent of epidermoid cyst is seen in head and neck region, whereas in oral cavity they account for only 1.6% .
11.1.2 Arachnoid Cyst
11.1.3 Thornwald Cyst
11.1.4 Nasopharyngeal/Squamous Cell Carcinoma
Nasopharyngeal carcinoma (NPC) is a primary malignancy of the nasopharynx originating from the epithelial cells in lateral wall of the nasopharynx especially around the fossa of Rosenmuller and the Eustachian cushion [14, 15, 16]. Nasopharyngeal carcinoma is classified as squamous cell carcinoma and nonkeratinizing carcinoma (subdivided this type to differentiated nonkeratinizing carcinoma and undifferentiated carcinoma subtypes) by the World Health Organization (WHO) . Etiological factors of this malignancy include the Epstein-Barr virus (EBV) (especially in children), genetic susceptibility and environmental carcinogens, and consumption of food (in particular salted fish) containing carcinogenic volatile nitrosamines . MRI is sensitive than CT to perineural spread and for demonstrating early the bone marrow changes of infiltration (see normal bone marrow signal of the clivus), although not all bone marrow changes represent tumor extension. Signal characteristics on T1-W typically isointense to muscle, on T2-W image isointense to somewhat hyperintense to muscle fat saturation is helpful. Postcontrast sequences should be fat-saturated; prominent heterogeneous enhancement is typical. Perineural extension should be taken into consideration . On CT images it is difficult to differentiate the small nasopharyngeal carcinomas from the muscles in consequent of resemblance density of the tumor with muscles .
11.1.5 Pleomorphic Adenoma
11.1.6 Warthin’s Tumor
Warthin’s tumor, also known as a papillary cystadenoma lymphomatosum, adenolymphoma, lymphomatous adenoma, is the second most common benign neoplasm of the salivary gland and is arising from the ductal component of them . More males than females suffer from this tumor, and it is found bilaterally in 10% of patients . Warthin’s tumor has low signal intensity on T1-W images compared with the high signal intensity of the parotid gland. On T2-W images tumor is heterogeneous and variable in signal intensity . On CT scans, tumor density is equivalent to soft tissue or cystic density . This tumor has hypoechoic appearance on US like the pleomorphic adenoma comparing with the normal parenchyma. Characteristic sonographic features of the Warthin’s tumor include generally oval shape, well-defined borders, distal acoustic enhancement, heterogeneous echotexture, and grade 2 or 3 vascularity with central or mixed perfusion. Also more than half of the tumors have cystic areas . Klintworth et al.  reported that “half-half” sign (with a stiff area located in the superficial half of a lesion while the deeper part has a softer appearance) is specifically related with Warthin’s tumor.
Oncocytoma is a rare benign tumor arising from large, eosinophilic, granular, polygonal epithelial cells. This tumor usually occurs in the eighth decade of life with a slightly female predominance . This tumor has variable tissue signal on different MR sequences such as relatively hypointense on T1-W images, intermediate on fat-saturated T2-W, and postcontrast T1-W images . CT scans demonstrate the homogeneous well-circumscribed tumoral mass showing moderate enhancement after the contrast material administration . This tumor has no specific imaging features and represents well-defined, hypoechoic solid mass with posterior enhancement on US .
11.1.8 Adenoid Cystic Carcinoma
11.1.9 Lymphomas of Major Salivary Glands
Hemangiopericytoma is a kind of soft tissue sarcoma that arises from the Zimmerman’s pericytes surrounding the blood vessels in various parts of the body  but predominantly located in the lower extremities, pelvis, or retroperitoneum . Hemangiopericytomas have no gender predilection with a mean presentation in sixth and seventh decades of life . Only 15% and 25% of the hemangiomapericytomas occur in the head and neck region . On CT scans, hemangiopericytomas reveal round homogeneous masses with a sharp outline and often displace the adjacent structures [40, 41]. On T1-W images tumor shows intermediate signal intensity relative to the gray matter; after the gadolinium administration, it enhances heterogeneously . Contrast T1-W images shows intense enhancement and heterogeneous signal intensity. There may be a dural tail sign in Grade II tumors. On T2-W images the lesion is isointense to gray matter multiple flow voids on MRI. DWI shows an intermediate restricted diffusion.
It is still unclear if osteomas are benign tumors or hamartomas. Osteomas originate from the cartilage and embryonal periosteum that are usually nodular or pedunculated polyp attached to the bone with a narrow stalk. It occurs almost exclusively in the head and neck region, particularly in the posterior mandible and also in paranasal sinuses especially in frontal sinuses and are most often encountered after 40 years of age . Multiple osteomas which develop in the head and neck region and multiple epidermoid cysts of the skin and polyposis of the colon and rectum are the part of the Gardner syndrome [19, 20]. Histologically osteomas are divided into (1) ivory or compact osteoma, (2) spongious or trabecular osteoma, and (3) combination of the first two types [20, 42]. On MR T1-W and T2-W images, the ivory osteoma appears as a hypointense area and may be misinterpreted as air within the paranasal sinuses or nasal fossae . While the CT appearance of the compact osteoma is very dense calcified, well-defined lesions, trabecular osteoma’s density is changing depending on the fibrous component of the lesion [19, 42].
Cholesteatoma is a destructive and expanding well-demarcated noncancerous cystic lesion consisting of keratinizing squamous epithelium in the middle ear and/or mastoid process [43, 44]. Cyst cavity is filled with the keratinous debris . Cholesteatomas may be classified as either congenital or acquired types. Congenital cholesteatomas are derived from persistent embryologic remnants in the form of epithelial foci in the middle ear . The pathogenesis of acquired cholesteatoma is explained on the invagination of the tympanic membrane, basal cell hyperplasia, epithelial in-growth through perforation, and squamous metaplasia of middle ear epithelium . CT scans demonstrate the expansile nonenhancing hypodense lesions with neighboring smooth bony erosion . Generally cholesteatomas have nonspecific signal intensity on basic MRI sequences, but non-echo planar diffusion-weighted imaging is more specific to diagnose these lesions and differentiate the primer and recurrent cholesteatoma. Cholesteatomas have high signal relative to the brain tissue on DWI [46, 47].
Fibrosarcoma is a malignant spindle cell tumor of fibrous connective tissue with herringbone architecture or interlacing fascicular pattern without expression of other connective tissue cell markers [50, 51, 52, 53]. Fibrosarcomas have no sex predilection with a mean age in fourth decade . Clinical diagnosis of the soft tissue sarcoma in the head and neck region is often a challenging problem due to growing in a considerable size without any symptoms, and even if the symptoms is present, it may be frequently overlooked as dental problem such as TMD [51, 52, 54]. On CT scans fibrosarcomas are determined as an isodense attenuating soft tissue masses. On T1-W images fibrosarcomas have low signal intensity and on T2-W images show both low and high signal intensity (heterogeneous) together in comparison with adjacent muscle .
11.1.17 Empty Sella
11.1.18 Internal Carotid Artery (ICA) Dissection
11.1.19 Venous Angioma (VA)
11.2 Incidental Calcifications in Maxillofacial Area
Tonsillar calcifications also known as “tonsilloliths” are white-yellowish-calcified deposits which are commonly located in tonsillar crypts. Mesolella et al.  reported that calcified masses occur with a variation of sizes ranging from a few millimeters to several centimeters. Small-sized calculi are common and asymptomatic, but the giant-sized tonsilloliths are rare [73, 74]. Symptoms include dysphagia, recurrent sore throat, foreign body sensation, and halitosis . The mechanisms of formation are still controversial. It is believed that they occurred due to repeated inflammation of the tonsillar crypts with recurrent tonsillitis. Repeated episodes of inflammation may cause cryptal fibrosis. Bacterial and epithelial debris accumulates within these crypts, and calcification occurs subsequent to the deposition of inorganic salts which derived from saliva [73, 75]. In addition, it is reported that tonsilloliths are primarily consisted of calcium hydroxyapatite/calcium carbonate and some other minerals such as magnesium, sodium, silica, potassium, copper, aluminum, iron, and ammonia radicals [72, 74, 76]. The most common cause of tonsillitis is viral infection including enteroviruses, particularly coxsackie virus, respiratory viruses (e.g., adenovirus, rhinovirus, influenza virus, coronavirus, parainfluenza virus, and respiratory syncytial virus), and viruses of the Herpesviridae family like Epstein-Barr virus (EBV), cytomegalovirus (CMV), and herpes simplex virus (HSV), while the most common bacterial pathogen is Group A β-hemolytic streptococcus in pediatric population .
Salivary stones also known as “sialoliths” are described as an aggregation calcified deposits found within the salivary glands or ducts. It consists of hydroxyapatite, calcium and phosphate salts, desquamated epithelial cells, bacterial debris, and foreign bodies [20, 80]. They frequently occur in submandibular gland or its ducts (80–90%); calcified masses in the parotid and sublingual are, respectively, seen in 10–20% and 1–7% [20, 80]. They can also occur in minor salivary glands, primarily in the upper lip and buccal mucosa. According to literature, incidences of sialoliths are low, between 0.2% and 1.01% [81, 82].
11.2.4 Fungal Infection (Aspergilloma)
11.2.5 Osteoma Cutis
11.2.6 Calcified Lymph Nodes
Calcified lymph nodes are one of the calcification of structures in head and neck region. They are often an asymptomatic condition diagnosed accidentally during radiographic examination. Lymph node calcifications can occur in patients with chronic inflammatory diseases or tuberculosis. It can occur also in the patients who have been treated for lymphoma . They may be able to detect on palpation. Due to underlying serious conditions, diagnosing and monitoring is crucially important. The most commonly involved nodes are the submandibular and superficial and deep cervical nodes. Radiographically, they present below the inferior border of the mandible and near the mandibular angle, and they usually appear well-defined, lobulated irregularly shaped opacities. The irregular shape is described as “cauliflower-like” . Edwards et al.  reported that the incidence of calcified lymph nodes was rare, nearly 0.12%.
Cysticercosis is a parasitic tissue infection caused by larval cysts of Taenia solium. This affects the central nervous system, muscle, eyes, and skin. Infection is found most often in rural, developing countries where hygiene is poor. Cysticercosis can lead to neurologic and ocular complications, and rarely death. From the first located in the tissues, till death of the larvae, they are not visible in radiographic images. After their death calcification starts in subcutaneous and muscular sites . Muscular cysticercosis appears as “grains of rice” and dot-shaped or well-defined ellipsoidal calcifications and is found incidentally in radiographic images .
Phleboliths are the deposition of calcium salts which occurred in organized intravascular thrombi by the venous congestion or stagnation. It is always prediction of presence of hemangioma, because they are mostly found in veins, venulae, or the sinusoidal vessels of hemangiomas [20, 104]. Phleboliths are usually multiple; their shape is round or oval, up to 6 mm in diameter with a smooth periphery. Internal structure commonly has laminated appearance as a bull’s eye view . They may detect by conventional radiographic techniques as well as MR, USG, CT, and CBCT . Differential diagnosis should be made with sialoliths .
11.2.9 Myositis Ossificans
Myositis ossificans is a benign process characterized by arising bone or bone-like tissue within muscles, tendons, or ligaments. There are two types: localized traumatic or progressive. Localized myositis ossificans can occur as a result of acute or chronic trauma. It may occur not only when applied an intramuscular injection but only heavy muscular strain by sport [20, 104]. Progressive myositis ossificans is a rare hereditary disease which arises as a result of spontaneous mutation. The mostly involved muscles of maxillofacial area are masseter and sternocleidomastoideus. The less common muscles are medial and lateral pterygoid, buccinator, and temporalis .
Progressive form also known as fibrodysplasia ossificans progressiva is a rare autosomal dominant genetic disorder and most commonly affects male without any external injury of the muscle . Myositis ossificans circumscripta occur as a result of some traumatic injury to muscle such as acute and chronic trauma, heavy muscular strain, infections, burns, neuromuscular disorders, hemophilia, tetanus, and drug abuse . Masseter and sternocleidomastoid muscles are most commonly affected in the head and neck region . Also it may be localized in other masticatory muscles [106, 107]. MRI appearances depend on the age of the lesion. Early lesions can be misleading before the maturation, however soft tissue may appear with edema. T1-W images show ill-defined isointense images; T2-W shows mostly edema as high signal. The signal intensity may change depending on the localization of the lesion in which central part mostly seen with heterogeneous signal intensity T1-W contrast enhancement is often present. Standard radiographs do not disclose any anomaly in the early stages of myositis ossificans. Typically, they are distant from adjacent bony structures . CT scan examination is more sensitive than radiography for detecting ossification and may also show a central fatty metaplastic area .
11.2.10 Intracranial Calcifications
Intracranial calcifications can be physiological or pathological. They often occur due to mineral or metal deposition in the blood vessels, glands, cortices, or other structures within the brain . The most common sites include the pineal gland, habenula, choroid plexus, basal ganglia, falx, tentorium, petroclinoid ligaments, and sagittal sinus . According to the literature, intracranial calcifications were common and occur at any age and of any ethnicity . Kwak et al.  reported that intracranial calcifications have male predilection. According to literature reports, incidence rates of intracranial calcifications vary from 0.42%  to 71% . The great differences between incidence rates may be due to many reasons such as sample size, the size of the FOV, radiologic imaging devices, and their different abilities in detecting pathologies.
11.2.11 Stylohyoid Ligament Calcification
11.2.12 Arterial Calcifications
Arterial calcifications are a common consequence of aging and are more frequent in patients with some systemic diseases such as diabetes, dyslipidemia, genetic diseases, and diseases involving disturbances of calcium metabolism. They are a result of the deposition of calcium salts in the cells of the media or intima of the arterial wall . Arterial calcifications have two different forms which identified radiographically and histologically: Mönckeberg’s medial calcinosis and calcified atherosclerotic plaque . Calcifications of tunica media of medium-sized arteries are described as Mönckeberg’s medial calcinosis. The etiology is not well understood, but it is believed that glucose intolerance, age, sex, osteoporosis, or chronic renal failure is related. Medial calcinosis does not obstruct the lumina of the arteries, so tissue or organ ischemia is not seen . Hence, it is mostly asymptomatic. Only it is clearly seen in three-dimensional imaging modalities and plain radiography techniques as an incidental finding. Radiographically, calcified vessels of Mönckeberg appear as a parallel pair of thin, radiopaque lines which were described as “tram tracks,” “pipestem,” or “tramline.” In cross section, involved vessels can view a circular or ringlike pattern [20, 118].
One another arterial calcification is known as atherosclerosis. It is characterized by deposition of calcium in the atheromatous plaques within the intima of arteries. It causes narrowing the lumen of the affected vessels. The most common location for an atherosclerotic plaque lesion is at the carotid bifurcation; therefore, it may be visible in the panoramic radiograph. It may be seen as a heterogeneous radiopacity at the level of the intervertebral space between C3 and C4. They are usually multiple, unilateral or bilateral, irregular, and sharply defined from the surrounding soft tissues . These plaques have an aspect mostly circular when small and mostly linear or thin rectangular when enlarged. They are usually located posterior to the angle of mandible, approximately at the inferior margin of third cervical vertebra near the hyoid bone .
11.3 Paranasal Sinus Findings in TMJ Imaging
The paranasal sinuses are four-paired air-filled spaces which surround the nasal cavity. They consist of maxillary, frontal, and sphenoid sinuses and ethmoid cells. Their infectious diseases can mimic of an odontogenic infection, and practitioners should be careful diagnosing pathologies and familiar with variations .
Since the first invention of three-dimensional imaging modalities, paranasal sinus evaluations were made by using conventional imaging techniques. As everybody knows, two-dimensional plain radiographic techniques have some limitations such as superimpositions, magnifications, distortions, and low-image quality. Because of these limitations, some pathologies, calcifications, and anatomical variations may be missed . Especially in dentistry, panoramic radiography is routinely used by practitioners to evaluate dentomaxillofacial region. Panoramic radiography allows clinicians to evaluate not only normal anatomy, pathologies, and variations but also TMJ and paranasal sinuses. Evaluation of paranasal sinuses is quite difficult in panoramic images because of superimpositions. To overcome these limitations, CBCT use in dentistry spreads. Due to higher diagnostic sensitivity and specificity levels of CBCT, it became the first imaging tool option by clinicians. It became possible to make further analysis of the findings which could not have detected by conventional imaging modalities [81, 121].
As mentioned in previous chapters, “Any findings on a radiographic image which are not related to the research areas of interest” could be a description of “incidental findings.” Paranasal sinus findings are one of these incidental findings. And correct identification of pathologies or variations will reduce further diagnostic assessments .
11.3.1 Maxillary and Sphenoid Sinusitis
11.3.2 Sinus Opacifications
Sinus opacifications are common finding of paranasal sinuses. Radiopaque appearance of paranasal sinuses occurs in some cases such as mucosal thickening, air-fluid level, mucocele, mucous retention cyst, and total or partial opacifications.
Sinus mucosal thickening also known as mucositis is a common radiographic finding. All paranasal sinuses composed of respiratory epithelium and considered as normal if they are about 0.8–1 mm [20, 82]. Besides, some reports have considered mucosal thickening as ≥2 or ≥3 mm [82, 124, 125].
Lana et al.  reported that mucosal thickening was the most commonly detected lesion. They found mucosal thickening in 313 patients (62.6%). Avsever et al.  reported that incidence was 27.35%.
Mucocele is the lesion which shows certain characteristics such as complete filling of the maxillary sinus and bony expansion. It occurs as a result of blockade of sinus ostium for various reasons such as intra-antral or intranasal inflammation, polyp, or neoplasm. Because of its expansive and destructive character, it is easily diagnosed. Sinus cavity is uniformly radiopaque. Maxillary sinus is rarely affected. Ninety percent of the cases occur in the ethmoid air cells and frontal sinuses. Plain conventional imaging modalities are mostly visualized lesion. But comprehensive evaluation or exact diagnosis is easily made with CT or CBCT .
Total or partial opacifications occur in inflammatory conditions. Total opacification blockades the ostium and has some symptoms, but partial opacifications are sometimes asymptomatic. They both do not cause expansion or destruction. Sinus cavity is uniformly radiopaque.
11.3.3 Hypoplasia and Aplasia
The maxillary sinuses are the first developed among paranasal sinuses and lasts its growth at the end of puberty [126, 127]. Hypoplasia is underdevelopment or incomplete development of a tissue or organ. Aplasia is defined as absence of organ or tissue. Etiology of hypoplasia or aplasia includes both embryological and acquired causes like trauma or infection .
Hypoplasia of the paranasal sinuses is a rare condition . The frontal and sphenoid sinuses are most frequently involved . The incidence of maxillary sinus hypoplasia ranges between 1.5 and 10% . Hypoplasia of the maxillary sinuses occurs unilaterally in about 1.7% of patients and bilaterally in 7.2%. Hypoplasia of the frontal sinus is a common normal variant, and aplasia of the frontal sinuses is noted in approximately 4% of the population .
11.3.4 Haller Cell
Haller cell is an anatomical variation which is located lateral to the maxillo-ethmoidal suture along the inferomedial orbital floor. They are also known as infraorbital ethmoidal air cells . The prevalence of Haller cells is reported to be between 2% and 68% in the literature [133, 134]. Some reports have shown the Haller cells with larger than 3 mm in diameter, have a precious relationship with maxillary sinusitis [132, 135, 136].
With the increasing popularity of endoscopic sinus surgery, visualization of Haller cell or any anatomical variations became important. Infraorbital ethmoid cells may be visualized by a variety of imaging methods that show a view of the maxillary sinus. Plain radiography or three-dimensional imaging modalities which were performed to evaluate TMJ commonly visualize infraorbital ethmoid cells. CT and CBCT are commonly used for imaging infraorbital ethmoid cells, and they are frequently seen as incidental findings in CT/CBCT examination of paranasal sinuses. It is also possible to detect them by panoramic radiographs .
11.3.5 Onodi Air Cell
Sphenoethmoidal air cell, also known as the Onodi air cell, is an anatomic variant of the most posterior ethmoid cell that pneumatized superiorly and laterally to the sphenoid sinus and is in close relation to the optic nerve and internal carotid artery. Their importance is the close relationship with optic nerve and internal carotid artery. The damage of these structures causes some important complications during sinus surgery. They are usually asymptomatic, unless complicated by sinus disease. The incidence of Onodi cells on imaging studies is between 4.6% and 65.6% [121, 138]. The possible causes of the variety of prevalence are racial factors and differences of chosen imaging modalities. Using endoscopic dissection, Kainz and Stammberger  reported its prevalence was 42%.
11.3.6 Concha Bullosa
Concha bullosa is generally defined as the pneumatization of the middle turbinate in the nose. However, pneumatization may also be seen in the superior and inferior turbinate . In majority of cases, these variants are asymptomatic. Sometimes it is associated with deviation of the nasal septum. Although it was first described as a rare anatomical variation, Smith et al.  reported that incidence rate was 67.5%. In addition to radiologic reports, clinical data from Goldman , who encountered pneumatization in 80% of resected middle turbinates from patients undergoing ethmoidectomy for chronic sinusitis, suggested a higher prevalence.
11.3.7 Agger Nasi
Agger nasi cells are the most anterior ethmoid air cells and are located anterior, lateral, and inferior to the frontal recess. Presence of some anatomical variations such as Agger nasi may affect the functional drainage pathways and predispose for sinus disease. The recognition of sinonasal anatomic variations is important for the avoidance of surgical complications . The prevalence of Agger nasi cells has previously been reported as wide ranging. In a CBCT examination, Avsever et al.  found Agger nasi cells in 2.88% of specimens examined; however, Bolger et al.  noted this cell type in 98.5% of their sample. They should not be confused with Haller cells which located along the medial floor of the orbit.
11.3.8 Accessory Ostium
The maxillary sinuses communicate with the nasal cavity by the ostium, approximately 3–6 mm in diameter and positioned under the posterior aspect of the middle concha . Accessory maxillary ostium was another variation seen in nearly three-fourths of the cases which opened into membranous meatus inferior to the uncinate process . It is a rare variation of paranasal sinuses. The prevalence of accessory ostium has been reported between 0% and 43% [147, 148, 149, 150]. According to CT evaluations in a study, Earwalker  reported its prevalence as 14%. Jog and McGarry  used a flexible endoscope to investigate accessory ostium, and they reported the prevalence as 4%.
The authors would like to thank to Dr. Murat İçen for providing maxillofacial cysticercosis images.
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