Temporomandibular joint

The temporomandibular joint (TMJ, jaw joint; Latin: articulatio temporomandibularis) is a paired articulation between the temporal bone of the skull and the lower jaw (mandible). It is located on the lateral side of the face anteriorly to the external ear. Overall, the temporomandibular joints are the most frequently used articulations in the human body. They provide complex movements and allow a person to open and close the mouth, as well as chew and move the jaw from side to side.

TMJ classification

Functionally (depending on the provided movement), the TMJ is classified as a diarthrosis. Diarthroses can move freely, and they are joints with the widest range of motions. Histologically (depending on the involved tissue), the TMJs are synovial complex-type joints.

Synovial joints can be further subdivided according to the type of movements they provide. The TMJs combine two joint types - a hinge and plane joints. They are also referred to as ginglymoarthrodial synovial joints as the Latin word ginglymus means hinge joint, but Latin arthrodia stands for plane joint.

The TMJ is composed of two compartments (floors), and both belong to different joint types. The superior floor of the TMJ has characteristics of a plane-type synovial joint. Plane-type joints allow translatory or gliding movements. The superior compartment of the TMJ permits translation of the mandibular condyle, and it occurs when the mandibular condyle moves forward and backward along the articular tubercle of the temporal bone.

The inferior floor of the TMJ is a hinge-type joint. A hinge joint is a uniaxial joint, meaning it allows movement only in one plane. The articulating surfaces of this joint type are between a convex end of one bone and a concave end of another bone. It allows rotatory or hinge movements. The inferior compartment of the TMJ permits rotation of the condyle, and it happens between the articular disc and the head of the mandibular condyle.

Note: To read more in detail about joint types and their functions, please visit our article dedicated to the classification of joints.

Articulating structures of TMJ

The articulating structures of the TMJ include the following:

• Mandibular fossa (also called the glenoid fossa) of the temporal bone
• Articular tubercle of the temporal bone
• Head of the mandibular condyle
• Articular disc

The mentioned structures actually do not directly articulate with each other. Instead, they are separated by a fibrocartilaginous articular disc that is located within the joint cavity and divides it into two smaller cavities, also known as floors or compartments - superior and inferior.

The superior floor of the TMJ is limited by the temporal bone and the articular disc (disco-temporal compartment). The inferior floor is found between the articular disc and the mandible (disco-mandibular compartment).

Articular disc

The articular disc of the TMJ is a round or oval biconcave fibrocartilaginous plate located within the joint cavity between the articular surfaces of the mandible and temporal bone. It is an avascular structure made of dense fibrous connective tissue. Its main function is to absorb stress and distribute the compressive load on the joint. Besides that, the articular disc also reduces the wear and tear of the articulating structures of the temporal bone and mandible by separating the translational and rotational movements.

As mentioned before, the articular disc divides the joint cavity into two smaller cavities (floors or compartments) - superior (disco-temporal) and inferior (disco-mandibular). The superior compartment is slightly larger than the inferior. Both cavities are lined by the synovial membrane and filled with synovial fluid. The upper floor is a plain joint responsible for translational movements, while the lower is a hinge joint providing rotational movements.

The articular disc of the TMJ contains three functional zones - anterior and posterior bands and intermediate zone. The intermediate zone is the central part of the disc. It is thin and allows flexibility and smooth articulations, as well as protects articulating structures. Both bands are located in the periphery of the disc, and they are thicker than the center. They fill the space created by the condyle of the mandible, and they also stabilize the disc and provide structural integrity.

The anterior and posterior aspects of the disc are fused with the fibrous joint capsule of the TMJ. Besides the joint capsule, the anterior band of the disc is attached by collagenous fibers to the articular tubercle of the temporal bone superiorly and the mandibular condyle inferiorly. Moreover, between both attachments is the upper head of the lateral pterygoid muscle. This attachment may help during translational movements when the jaw is moved forward.

The posterior band is in contact with retrodiscal tissue - loose connective tissue located behind the articular disc. The retrodiscal tissue is composed of three portions. The upper part is known as the superior retrodiscal lamina - a connective tissue plate mostly made of elastic fibers. It connects the articular disc with the tympanic plate of the temporal bone. The lower portion of the retrodiscal tissue is the inferior retrodiscal lamina, and it mainly contains collagenous fibers. The inferior retrodiscal lamina goes from the articular disc to the mandibular condyle. Between both laminae is an extensive venous plexus.

Joint cavity and joint capsule

As described before, the TMJ is a unique articulation in several ways. Another specific feature of the TMJ is that its articular surfaces are lined by fibrocartilage, unlike most synovial joints that are covered by hyaline cartilage. Fibrocartilage is a dense cartilaginous tissue with a fibrous texture. It is very strong and durable, and it resists long-term degenerative effects such as pressure and friction. Fibrocartilage also has a great repair potential.

Like other diarthrodial-type joints, the temporomandibular joint also contains a synovial membrane. A synovial membrane is a vascular connective tissue layer that lines the inner surface of the joint capsule and produces synovial fluid. The synovial fluid is a thick liquid that lubricates and nourishes structures within the joint and allows them to move smoothly. Each compartment of the TMJ has its own synovial membrane that attaches to the lateral aspects of the articular disc.

The whole TMJ complex is enclosed by a very loose fibrous joint capsule that is attached along the articular surfaces. The capsule originates from the margins of the mandibular fossa. It goes along the sides of the fossa and in front of the articular tubercle, enclosing it. Posterior to the joint, the capsule attaches in front of the petrotympanic fissure. It inserts at the neck of the mandible, usually above the pterygoid fovea. The capsule is so loose that the mandibular ramus can easily dislocate, and usually, it does not cause any damage to the capsular fibers.

The upper fibrous fibers of the capsule attach the articular disc to the temporal bone of the skull, and the lower capsular fibers connect the disc to the mandibular condyle. Therefore, the articular disc divides a single joint capsule of the TMJ into two smaller fibrous joint capsules. These capsules provide additional stabilization of the disc, at the same time allowing rotational and transitional movements.

TMJ ligaments

The temporomandibular joint is strengthened by several ligaments that increase its stability. All TMJ ligaments can be classified into two groups - intracapsular and extracapsular ligaments. The intracapsular ligaments are situated within the joint capsule, while the extracapsular ligaments are positioned outside the capsule. The intracapsular ligaments include the lateral and medial collateral ligaments. In contrast, the extracapsular ligaments associated with the TMJ are the following:

• Temporomandibular (lateral) ligament
• Stylomandibular ligament
• Sphenomandibular ligament

The lateral and medial collateral ligaments are two tight bands that are located within the lower floor of the TMJ and laterally and medially are attached to the condyle of the mandible. They are minor ligaments of the TMJ, and because of their attachment to the articular disc, they are also called the discal ligaments.

The collateral ligaments originate at the level of the intermediate fascia of the articular disc and stretch from the peripheral aspects of the disc to the lateral and medial sides of the mandibular condyle. It is why they are named the lateral and medial collateral (discal) ligaments. Sometimes they are simply referred to as parts of the fibrous joint capsule of the TMJ.

The temporomandibular ligament (also called the lateral temporomandibular ligament or simply the lateral ligament) is a thickened lateral aspect of the fibrous joint capsule. It runs from the articular tubercle of the temporal bone to the lateral surface and posterior margin of the mandibular neck. The temporomandibular ligament prevents posterior dislocation of the joint.

The stylomandibular ligament is considered a thickening of the parotid fascia that is formed by the investing (most superficial) layer of the deep cervical fascia. It extends from the styloid process of the temporal bone to the mandibular angle and posterior margin of the mandible. The stylomandibular ligament limits excessive protrusive movements of the jaw. However, it is a minor extracapsular ligament and is described as an accessory ligament, as it does not provide significant support to the TMJ.

The sphenomandibular ligament is located medially to the fibrous capsule of the TMJ. It extends from the spine of the sphenoid bone to the medial wall of the TMJ capsule and lingula of the mandible located on the internal surface of the mandibular ramus. The sphenomandibular ligament limits excessive translational movements of the condyle when the mouth is opened more than 10 degrees. Like the stylomandibular ligament, it is also considered an accessory ligament of the TMJ.

The sphenomandibular ligament is relatively long, and various neurovascular structures are related to it. The upper aspect of the ligament laterally borders with the auriculotemporal nerve, maxillary artery and vein and lateral pterygoid muscle, but medially with the chorda tympani and medial pterygoid muscle. The inferior alveolar nerve and vessels laterally cross its lower parts. At the level of the lingula, the ligament is pierced by the nerve to the mylohyoid. The inferior portion of the sphenomandibular ligament also relates to the blood vessels headed to the mylohyoid muscle.

Movements and muscles of TMJ

The temporomandibular joints provide a wide variety of movements that allow such functions as chewing, sucking, phonation, swallowing, breathing, facial expressions, opening and closing the mouth and many more. Both TMJs should move in a coordinated manner to perform any of these functions.

Since the rotational movements of the jaw are not enough to crush the food, the translational movements are extremely important due to additional force for mastication. The rotational movements include elevation and depression of the mandible, while translational movements correspond to protrusion (protraction), retrusion (retraction) and lateral deviation of the lower jaw.

The unique anatomy of the TMJ allows it to perform both movement types at the same time. Sliding motions can happen because of the loose joint capsule, the loose connective tissue around the superior floor of the TMJ and tighter extracapsular ligaments around the inferior compartment.

The principal movements performed at the temporomandibular joint include the following:

• Depression of the mandible - jaw opening; occurs when the head of the mandible moves forward onto the articular tubercle; apart from gravity, this movement is mainly provided by the lateral pterygoid muscles, as well as by the geniohyoid, mylohyoid and digastric muscles;
• Elevation of the lower jaw - jaw closing; happens when the head of the mandible moves backward into the mandibular fossa; the main muscles that close the mouth are the temporalis, masseter and medial pterygoid muscles; also, lateral pterygoids show a small activity during the elevation of the mandible;

• Protrusion of the lower jaw - anterior movement of the mandible that is primarily generated by the lateral pterygoid and, to a lesser degree, the medial pterygoid muscle; however, there is evidence of activity coming from the middle part of the temporalis, digastric, masseter and geniohyoid muscles;
• Retrusion - posterior movement of the lower jaw that mainly occurs with the involvement of the posterior fibers of the temporalis muscle; the digastric and mylohyoid muscles also show some muscular activity;

• Lateral deviation - side-to-side movement of the mandible; on the opposite (contralateral) side, it is mainly provided by the lateral and medial pterygoids and, to a lesser degree, by the middle part of the temporalis; on the same (ipsilateral) side, the posterior fibers of the temporalis, mylohyoid, digastric and geniohyoid muscles show activity.

Relations of TMJ

Close to the temporomandibular joint pass various important neurovascular structures. The joint is also related to structures associated with the ear, different areas of the skull and the parotid gland.

Medially to the joint capsule runs the maxillary artery with its proximal branches, such as the middle meningeal and anterior tympanic arteries. Laterally to the joint capsule lies the upper part of the parotid capsule (capsule of the parotid gland, formed by the investing layer of the deep cervical fascia) and branches of the facial nerve (CN VII).

Anterior to the mandibular condyle, the infratemporal fossa is located, containing both pterygoid muscles (lateral and medial). In cases of TMJ dislocation, the mandibular condyle usually moves into this fossa. Posterior to the TMJ lies the tympanic plate of the temporal bone and the external acoustic meatus of the external ear. Superior to the joint is the temporal fossa.

Neurovascular supply of TMJ

The arterial blood supply to the TMJ laterally comes from the superficial temporal artery, while medially, it is provided by the maxillary artery. Branches of the maxillary artery that supply the TMJ include the deep auricular, anterior tympanic and masseteric arteries.

Venous drainage of the TMJ is provided by the superficial temporal vein and maxillary vein. Both veins merge and form the retromandibular vein that carries venous blood further to the internal and external jugular veins.

Lymphatics collecting lymph from the TMJ mainly drain into the upper deep cervical lymph nodes around the internal jugular vein. However, lymph drained from the TMJ is also carried to the submandibular and parotid lymph nodes.

The temporomandibular joint receives nerve supply from the branches of the mandibular nerve (the third division of the trigeminal nerve (CN V)) - the auriculotemporal nerve, masseteric nerve and deep temporal nerves.

As mentioned before, the articular disc is an avascular structure, meaning it is not supplied by any blood vessel. Also, it does not receive nerve supply. Instead, the articular disc is nourished by the synovial fluid. In contrast, the retrodiscal tissue behind the disc is highly vascularized and innervated.

TMJ disorders

Temporomandibular joint disorders may affect around 15 to 25 % of adults at some point in their lives, with a greater risk for women to suffer from various diseases associated with the TMJs. The TMJ disorders are typically unilateral, meaning that they affect only one side of the jaw.

The main symptoms signifying TMJ dysfunction include headaches, pain in the face around the ears and ear pain, ringing and pressure in the ears, pain behind or below the eyes and in cheeks, unexplained teeth pain, limited motions of the jaw, jaw muscle stiffness and clicking or popping temporomandibular joint sounds.

The most common cause of temporomandibular joint dysfunction (in around 85% of the cases) is bruxism. Bruxism is a term used to describe teeth grinding or clenching, especially during sleep (sleep bruxism). It is usually caused by stress and may also be associated with certain personality types (aggressive and hyperactive persons) and anxiety.

Bruxism may result in pain within the jaw, tense facial and jaw muscles, flattened, fractured and loose teeth, jaw or facial pain, as well as headaches. Other possible causes of TMJ dysfunction include such conditions as displacement or chronic dislocation of the articular disc, degenerative joint disease (osteoarthritis), rheumatoid arthritis and various traumas.

References:

1. Cleland, J., Koppenhaver, S., & SU, J. (2021). Netter’s Orthopaedic Clinical Examination Elsevier eBook on VitalSource (Retail Access Card): An Evidence-Based Approach (Netter Clinical Science) (4th ed.). Elsevier.
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3. Gray, H., & Carter, H. (2021). Gray’s Anatomy (Leatherbound Classics) (Leatherbound Classic Collection) by F.R.S. Henry Gray (2011) Leather Bound (2010th Edition). Barnes & Noble.
4. Insalaco, L. F. (2022). Temporomandibular Joint Syndrome. Clinical Overview. Elsevier.Magee, D., & Manske, R. (2020). Orthopedic Physical Assessment, 7e (7th ed.). Saunders.