Imaging of the temporomandibular joint: An update



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Name of the journal: World Journal of Radiology

ESPS Manuscript NO: 8525

Columns: REVIEW
Imaging of the temporomandibular joint: An update
Bag AK et al. Imaging of the temporomandibular joint
Asim K Bag, Santhosh Gaddikeri, Aparna Singhal, Simms Hardin, Benson D Tran, Josue A Medina, Joel K Curé
Asim K Bag, Aparna Singhal, Simms Hardin, Benson D Tran, Josue A Medina, Joel K Curé, Section of Neuroradiology, The Department of Radiology, School of Medicine, The University of Alabama at Birmingham, Birmingham, AL 35249-6830, United States
Santhosh Gaddikeri, Department of Radiology, University of Washington, SS-202 Seattle, WA 98195-7117, United States
Author contributions: Bag AK contributed to the arthritis, images and overall integrity, manuscript revision and final approval the manuscript; Gaddikeri S contributed to the internal derangement and manuscript revision; Singhal A contributed to the normal joint anatomy, anatomic variation and manuscript revision; Hardin S contributed to the anatomic variation and manuscript revision; Tran BD contributed to the deposition diseases and tumors, manuscript revision; Medina JA contributed to the imaging techniques, manuscript revision; Curé JKC contributed to the images, overall integrity, manuscript revision and final approval.
Correspondence to: Asim K Bag, Assistant Professor, Section of Neuroradiology, The Department of Radiology, School of Medicine, The University of Alabama at Birmingham, JT N432, 619 19th Street South, Birmingham, AL 35249-6830, United States. abag@uabmc.edu
Telephone: +1-205-9349952 Fax: +1-205-9759262
Received: December 28, 2014 Revised: February 27, 2014

Accepted: June 20, 2014



Published online:
Abstract

Imaging of the temporomandibular joint (TMJ) is continuously evolving with advancement of imaging technologies. Many different imaging modalities are currently used to evaluate the TMJ. Magnetic resonance imaging (MRI) is commonly used for evaluation of the TMJ due to its superior contrast resolution and its ability to acquire dynamic imaging for demonstration of the functionality of the joint. Computed tomography and ultrasound imaging have specific indication in imaging of the TMJ. This article focuses state of the art imaging of the temporomandibular joint. Relevant normal anatomy and biomechanics of movement of the TMJ are discussed for better understanding of many TMJ pathologies. Imaging of internal derangements is discussed in detail. Different arthropathies and common tumors are also discussed in this article.


© 2014 Baishideng Publishing Group Inc. All rights reserved.
Key words: Temporomandibular joint; Magnetic resonance imaging; Imaging; computed tomography; Anatomy; Pathologies

Core tip: “Imaging of the temporomandibular Joint” is a thorough review of the imaging techniques and imaging appearances of normal anatomy, anatomic variation and pathologies of the temporomandibular joint (TMJ). Numerous images are appropriately used for illustration of the key concepts of TMJ imaging. Nice blend of exquisite details and beautiful illustrative images is the main feature of this article. The purpose of this article is easy understanding of many difficult aspects of imaging of the TMJ.
K Bag AK, Gaddikeri S, Singhal A, Hardin S, Tran BD, Medina JA, Curé JK. Imaging of the temporomandibular joint: An update. World J Radiol 2014; In press
INTRODUCTION

Pain related to the temporomandibular joint (TMJ) is common in the general population. Only about 3%-7% of the patients with pain related to TMJ seek medical attention[1,2]. Although TMJ disorders or dysfunctions are the most common clinical conditions for imaging referrals, pathologies specific to the bone and the joints are also common. Different imaging modalities are available to image the TMJ, each with inherent strengths and weaknesses. Magnetic Resonance Imaging (MRI) is the most widely used and is diagnostic technique of choice. In this article, we review the imaging techniques, anatomy pathology involving the TMJ with special emphasis on MRI findings.


Embryology and development of TMJ

The temporomandibular joint (TMJ) is one of the last diarthrodial joints to appear in utero and does not emerge in the craniofacial region until the 8th week of gestation. The maxilla, mandible, muscles of mastication, and biconcave disc develop embryologically from the first branchial arch through the 14th wk of gestation. The TMJ is considerably underdeveloped at birth in comparison to other diarthrodial joints making it susceptible to perinatal and postnatal insults. The joint continues to developing in the early childhood years as the jaw is utilized for sucking motions and eventually chewing.


ANATOMY OF TEMPOROMANDIBULAR JOINT

The TMJ is a ginglymoarthrodial synovial joint (latin: ginglymus = hinge joint) that allows both backward and forward translation as well as a gliding motion[3]. Similar to the other synovial joints in the body, the TMJ has a disk, articular surfaces, fibrous capsule, synovial fluid, synovial membrane, and ligaments. What makes this joint unique is the articular surfaces are covered by fibrocartilage instead of hyaline cartilage. The articular surfaces of the TMJ are formed inferiorly by the mandibular condyle and superiorly by the glenoid fossa (a.k.a. mandibular fossa) and articular eminence of the temporal bone.


Articular surfaces

The mandibular component consists of the ovoid condylar process that is 15-20 mm wide in the transverse dimension an 8-10 mm wide in the antero-posterior dimension[3]. The appearance of the mandibular condyle is extremely variable between patients and different age groups.

The cranial component of the TMJ lies below the squamous portion of the temporal bone anterior to tympanic plate. The articular fossa is formed entirely by the squamous portion of the temporal bone. The posterior part of the articular fossa is elevated form the posterior articular ridge. In most individuals the posterior articular ridge becomes thicker on the lateral aspect and forms a cone shaped projection known as postglenoid process (PGP). The tympanosquamosal fissure lies at the posterior and lateral part of the glenoid fossa, between the squamous and tympanic portion of the petrous bone and separates the articular surface form the nonarticular surface of the glenoid fossa. Along the medial aspect of the glenoid fossa is the petrotympanic fissure anteriorly and the petrosquamous fissure posteriorly. The articular eminence (AE) forms the anterior boundary of the glenoid fossa. The AE is a transverse bony bar anterior to the glenoid fossa and medial to the posterior margin of the zygomatic process. The anterior slope of the AE is known as the preglenoid plane (PEP) and rises gently from the infratemporal surface of the squamous bone. The mandibular condyle and the articular disk travel anteriorly to the summit of the AE and onto PEP during wide mouth opening. The gentle anterior slope facilitates smooth backward movement of the condyle and disk from the anterior position back to neutral position. The articular tubercle is a small bony knob at the lateral aspect of the AE where the lateral collateral ligament attaches. The lateral border of glenoid fossa is slightly raised from the fossa joining the anterior tubercle with the PGP (Figure 1).
Articular disk

The articular disk is round or oval, biconcave, avascular fibrocartilage between the condyle and glenoid fossa. The disk is considerably thinner centrally in the intermediate zone. The triangular anterior band is approximately 2 mm in thickness and blends with the joint capsule. The posterior band is approximately 3 mm in thickness and continues as bilaminar zone (a.k.a. retrodiskal region and posterior attachment), which consists of superior fibroelastic layer (a.k.a. temporal lamina) that attaches to PGP and an inferior fibrous layer (a.k.a. the inferior lamina) that attaches to the posterior condylar neck. The superior layer prevents slipping of the disk during wide mouth opening and the inferior layer prevents excessive rotation of the disk over the condyle. Both the lamina are separated by loose elastic fibers with blood vessels and nerves. These fibers attach to the posterior joint capsule and augments disk retraction during moth closing. The bands are longer in the mediolateral dimension than in the antero-posterior dimension[4]. The smaller anterior band attaches anteriorly to the joint capsule, condylar head, and AE. Some patients have an additional antero-medial attachment to the superior belly of the lateral pterygoid muscle. Unlike its anterior and posterior attachments, the disk is not attached to the joint capsule medially and laterally. Instead, the disk is firmly attached to the medial and lateral poles of the mandibular condyle. This allows simultaneous movements of the disk and the condyle (Figure 2).


Muscles

The muscles of mastication (medial and lateral pterygoids, masseter, and temporalis) in addition to other accessory muscles help opening and closing of the jaw[4-6]. The lateral pterygoid in conjunction to the stylohyoid, mylohyoid and geniohyoid muscles is used to open the jaw. The temporalis, medial pterygoid, and masseter muscles close the jaw. The lateral pterygoid, part of the masseter muscle and the medial pterygoid assist in the anterior translation of the mandible. The protrusive muscles (helping forward movement) are used alternately to move the jaw laterally from side to side. Individual muscle origins and attachments are listed below[4,6].


Jaw-closing muscles/adductors

The masseter is the strongest muscle of mastication and has two parts that blend together anteriorly. The superficial part originates from the anterior two-thirds of the zygomatic arch and inserts on the lower one-third of the lateral surface of the mandibular ramus. The deep part originates from the whole zygomatic arch and inserts on the upper two-thirds of the ramus.

The medial pterygoid courses parallel to the masseter along the medial aspect of the mandible. The anterior part arises from the lateral surface of the palatine pyramidal process and the maxillary tuberosity. The posterior part originates from the pterygoid fossa and the medial surface of the lateral pterygoid plate. The medial pterygoid inserts on the inferomedial surface of the mandibular ramus.

The temporalis muscle originates from the temporalis fossa and inserts on the coronoid process and inner side of the mandibular ramus. The fibers also attach directly to the medial side of the coronoid process and ramus.


Jaw-opening muscles/abductors

The lateral pterygoid muscle has two bellies. The superior belly originates from the infratemporal surface of the greater wing of sphenoid. The inferior belly originates from the lateral surface of the lateral pterygoid plate. There is a wide gap between the two heads of the lateral pterygoid muscle that come together anterior to the TMJ. The fibers from the superior head primarily attach to the anteromedial surface of the mandibular neck at the pterygoid fovea. Additionally, in some patients part of the superior head directly attaches to the superomedial aspect of the joint capsule and extends to the anteromedial aspect of the articular surface. All of the fibers of the inferior head attach to the pterygoid fovea. Variability in the attachment of the lateral pterygoid muscle is reported with insertions of the muscle described only to the condyle or to the condyle, capsule, and the disk[7-9].

The superior belly helps maintain the physiologic position of the disk in the open mouth position. This is accomplished by pulling the disk forward with a combined translation and rotation while exerting forward pressure on both the condyle and the disk thus stabilizing their relationship to each other. The inferior belly pulls the condyle forward out of the fossa. When the inferior belly alternately contracts, this produces lateral movement of the jaw.

The digastric muscle has a posterior and an anterior belly united by an conjoined tendon. The posterior belly is attached to the mastoid process of the temporal bone and extends to the hyoid bone becoming continuous with the intermediate tendon. A fibrous loop attached to the hyoid holds the tendon in place. The anterior belly extends from the tendon to the digastric fossa on the lower aspect of mandible near the symphysis. Contraction of the digastric muscles pulls the symphysis menti backwards producing the retrusive and opening movements of the mandible.

The geniohyoid, myolohyoid, stylohyoid and infrahyoid muscles also have supportive role in mandibular movements that are beyond the scope of this review.
Biomechanics of TMJ movements

Jaw movement involves a high level of interaction and coordination between bilateral mandibular condyles, disk, muscles, and ligaments of the joints. The functional interactions within the TMJ are complex and incompletely understood[10,11]. A simplistic view of the complex interactions in open and closed mouth positions is described below.

In a normal joint the thin intermediate zone of the disk is always interposed between the condyle and the temporal bone in both the closed-mouth and open-mouth positions. This is for the prevention of articular damage.

In the closed mouth position the condyle is centered in the glenoid fossa. The disk is interposed between the condyle inferiorly and the glenoid fossa superiorly. The articular eminence is anterior to the disk (Figure 2). The normal disk is positioned such that the anterior band is in front of the condyle and the junction of the posterior band and bilaminar zone lie immediately above the condylar head near the 12 o’clock position[1,3,4,9,12-14]. However, some controversy exists over the range of normal position of the disk[1,3,4,14-18]. Drace and Enzmann suggest that the junction of the posterior band and bilaminar zone should fall within 10 degree of vertical to be within 95 percentile of normal[15]. There is significant variation in relationship of the posterior band and bilaminar zone in normal population, resulting in inappropriate classification of anterior disk displacement[16,18]. Rammelsberg et al[17] suggest that disk positions of up to + 30 degrees from the vertical be considered normal. Many other authors have proposed that the intermediate zone be the point of reference so that in a normal joint it is interposed between the condyle and the temporal bone in all joint positions[4,19,20]. Comparing to the different disk positions of 12, 11 and 10 o’clock, Orsini et al[19] found the intermediate zone criterion for disk displacement to be more stringent. Recently Provenzano and colleagues draw similar conclusions[20] (Figure 2).


IMAGING TECHNIQUES

A variety of modalities can be used to image the TMJ. This includes non-invasive imaging modalities such as conventional radiographs, ultrasound, Computed tomography (CT) and MRI to more invasive imaging such as arthrography. Each imaging modality has its uses.

Conventional radiographs have a limited role in evaluation of the TMJ. They can be used to evaluate only the bony elements of the TMJ. They do not give useful information when it comes to the non-bony elements such as cartilage or adjacent soft tissues. They also do not give useful information concerning joint effusions, which are commonly associated with pain and disc displacements. Another disadvantage concerning conventional radiographs is the problem of superimposition of adjacent structures. Many different views such as the submentovertex, transmaxillary, and the transcranial are used to reduce superimposition.

Ultrasound is a less expensive and easily performed imaging modality that can be used to evaluate the TMJ. This is simple way to look for the presence of a joint effusion[21]. Ultrasound is used to evaluate cartilage as well as disk displacement with both open and closed mouth imaging[21]. It is used for image-guided injections for both diagnostic and therapeutic purposes[21]. Typically, a linear transducer of 8 MHz or higher is ideal. The patient should be lying supine with the transducer placed parallel to a line extending from the tragus of the ear to the lateral surface of the nose over the TMJ.

CT is useful to evaluate the bony elements of the TMJ as well as the adjacent soft tissues. CT is ideal for the evaluation of fractures, degenerative changes, erosions, infection, invasion by tumor, as well as congenital anomalies[21]. A typical imaging protocol is: 120 kV, 100 mA, 1 mm collimation, 1mm/rotation (pitch), and imaged with a closed mouth. CT also allows 3D reconstructions, which can be used for evaluating congenital anomalies and fractures[21]. CT is predominantly done when there is suspicion of bony involvement from the MRI and if primary bony pathologies are suspected clinically. Relative advantages of CT over MRI include, exquisite bone details and 3D assessment of congenital, traumatic and postsurgical conditions.

Clinical evaluation of the TMJ can be nonspecific due to overlap of symptoms between internal derangement and myofacial pain dysfunction[1]. MRI should be part of the standard evaluation when an internal structural joint abnormality is suspected because MRI provides high resolution and great tissue contrast. This allows for a detailed evaluation of the anatomy was well as biomechanics of the joint through open and closed mouth imaging[1].

For optimal imaging of the TMJ, small bilateral surface coils with small field of view are used to achieve higher signal to noise ratio and simultaneous bilateral acquisition. Closed mouth coronal and axial T1 sequences are needed to evaluate the overall anatomy and bone morrow as well as the adjacent soft tissues to exclude other adjacent pathology. In our institution, axial T1 is obtained as a localizer[14]. Bilateral closed mouth and open mouth T2, proton density (PD) and dynamic sequences are obtained in a oblique sagittal plane. In the our institution, dynamic images are obtained as rapid acquisition of static images using a single shot fast spin echo (SSFSE) proton density sequence during progressive opening and closing of the mouth. These images are displayed the sequential images as a cine loop. Mouth opening devices such as Burnett opening devices may be used for incremental opening of the mouth controlled by the patient. It can be argued that passive mouth opening with a Burnet device might not reproduce the physiologic conditions occurring during mouth opening given the possible role of the lateral pterygoid muscle in disc stabilization during mouth opening. Oblique imaging entails 30 degrees medial from the true sagittal plane[1]. Please see the table for specific MRI protocol[1]. A total of 8 sequences will need to be performed (Table 1).

Arthrography is an invasive imaging technique to evaluate the TMJ. This imaging modality requires injection of radiopaque contrast into the TMJ under fluoroscopic guidance. Once the contrast is injected, the joint can be evaluated for adhesions, disk dysfunction, as well as disk perforation based on how contrast flows in the joint. This modality is rarely used today because MRI can be used to evaluate the TMJ without being invasive, exposing the patient to a possibility of allergic reaction from the contrast, possibility of infection, or using radiation.


IMAGING APPEARANCE OF NORMAL TMJ

MRI

On MR imaging marrow fat in the condyle has a high T1 signal intensity. The cortical bone and the disk have low signal intensity on both T1 and T2 weighted images because of low proton density and short T2[12]. Sometimes high T2 and PD signal intensity can be seen in the central portion of the disk similar to a centrally hydrated vertebral disk[1,4]. The disk is otherwise homogeneous, hypointense and biconcave in shape. The center of the posterior band may be slightly hyperintense (Figure 2).

The disk’s posterior attachment has higher signal intensity than muscle on proton density and T1 weighted images secondary to fatty tissue. The bilaminar zone is visible as intermediate signal intensity structures.

In closed mouth position the junction of the posterior band and posterior attachment normally lies above the condylar head near the 12 o’clock position. The posterior band and retrodiskal tissue are best depicted in the open mouth position. In open mouth position the intermediate zone lies between the condyle and the articular eminence and the posterior band is against the posterior surface of the condyle[1,9] (Figure 2).

The superior belly of lateral pterygoid attaches to the anterior band of the disk. The inferior belly of the lateral pterygoid attaches to the anterior surface of the condylar neck with a thin linear hypointense fibrous band. This band is seen just inferior to the position of the disk, and can sometimes be mistaken for the disk, particularly when the disk is medially or laterally displaced[22].

In the coronal plane the disk is crescent shaped and its medial and lateral borders are attached to the respective aspects of the condylar head and joint capsule. The lateral and medial capsules do not demonstrate any outward bulges beyond the borders in normal condition[1,22].


PATHOLOGIES RELATED TO ANATOMIC VARIATIONS

Anatomic variations in the TMJ can be symptomatic and/or have implications during arthroscopy and surgery. There can also be several variations in the appearance of the mandibular condyles including intra-individual variations between the two sides. The disease processes can be developmental, due to remodeling related to malocclusion, trauma or other secondary developmental abnormalities[3].



Bifid condyle

A bi-lobed or duplicated mandibular head is an infrequently encountered incidental imaging finding. While the etiology is unknown, theories include reminiscence of congenital fibrous septum and peripartum or early childhood trauma. The duplicated heads may lie in either an antero-posterior or transverse orientation. Dennison et al[23] have suggested that the term “bifid condyle” should be reserved for describing multiple condyles in the sagittal plane only. No treatment is required for asymptomatic patients. However surgery may be performed if there is displacement of the disc or ankylosis of the joint space (Figure 3A).


Foramen of huschke

In some individuals there may be persistence of a developmental defect in the tympanic plate. The tympanic plate is present as an incomplete U-shaped cartilaginous ring at birth. Over time the ossification proceeds laterally and posteriorly leaving a defect in the floor of the external meatus, called the foramen tympanicum (foramen of Huschke). With growth of the mastoid process, this defect changes in position from inferior to anterior and usually closes by the 5th year of life. Rarely a 3-4 mm defect persists and is found to be located at the antero-inferior aspect of the external auditory canal and posteromedial to the TMJ. These patients can present with a defect or polyp on the anterior wall of the external auditory canal (EAC) or with salivary otorrhea during mastication, TMJ tissue may also herniate into the EAC during mastication[24,25]. During arthroscopy, there can be inadvertent passage into the EAC resulting in otologic complications. This foramen also can act as a path of communication between the EAC and TMJ or infratemporal fossa allowing the spread of infection, inflammation or tumor[24,25] (Figure 3B).


Condylar hypoplasia

Aplasia and hypoplasia of the mandibular condyle is secondary to non-development or underdevelopment of the condyle and can be congenital or acquired. Congenital aplasia or hypoplasia of the mandibular condyles is a rare anomaly and usually occurs as a part of more widespread 1st and 2nd branchial arch anomalies (e.g., Treaher-Collins syndrome). Acquired condylar hypoplasia may be secondary to local factors (trauma, infection, radiation) or systemic factors (toxic agents, rheumatoid arthritis, mucopolysaccharoidosis)[26]. Traumatic vaginal delivery has been implicated as a cause of hypoplasia[27]. Hypoplasia may involve one or both of the condyles. Unilateral disease produces mandibular rotation or tilt and associated facial asymmetry. The diagnosis of bilateral condylar hypoplasia may be delayed secondary to facial symmetry. Hypoplastic condyles are frequently complicated with ankylosis[28].


Idiopathic condylar resorption

Idiopathic condylar resorption (a.k.a. condylysis or ‘cheerleader syndrome’) is primarily a disease of TMJ affecting teenage girls. There is rapidly progressive condylar erosion resulting in widening of the joint space with the chin becoming less prominent from retrognathia[29]. Many causes have been hypothesized including estrogen influence on osteogenesis, avascular necrosis, and TMJ internal derangement. Orthognathic surgery has been implicated as a cause of the disease but also is one of the corrective approaches for idiopathic condylar resorption (Figure 3C).


Condylar hyperplasia

Condylar hyperplasia is a rare disorder characterized by increased volume of the mandibular condyle, and is frequently associated with increased volume of the ramus and mandibular body[30]. Condylar hyperplasia is usually a unilateral process. This disease presents in the second and third decades of life during brisk periods of osteogenesis suggesting a hormonal influence upon the growth disturbance. Trauma has also been implicated in asymmetric condylar hyperplasia due to hypervascularity during healing producing inducing excessive osteogenesis. The hyperplasia produces facial asymmetry with the chin rotating away from the affected side[30]. Resection of the hyperplastic condyle causes the abnormal growth to cease and restores facial symmetry (Figure 3D).


Extensive pneumatization

Extensive pneumatization of the mastoid bone can involve the glenoid fossa and articular eminence. Knowledge of extensive neumatization is necessary prior to surgery to prevent perforations. Complications can occur during TMJ surgery due to forceful flap retraction, dissection or with placement of screws in cases where fossa- eminence prostheses are required[31,32]. Pneumatization can also provide a path of minimal resistance and facilitate the spread of pathological tumors, inflammation, infection or fracture into the joint. For these reasons, a CT must be performed prior to TMJ surgery when extensive pneumatization is detected in the panoramic radiographs[31,32] (Figure 3E).

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