Temporomandibular Joint (TMJ). Anatomical Structures. Vascular Supply. Innervation.

The temporomandibular joint (TMJ) is a uniquely complex structure, both in its anatomy and biomechanics. It comprises the mandibular condyle, the glenoid fossa of the temporal bone, and an interposed fibrocartilaginous articular disc. This disc separates the joint into upper and lower compartments, each filled with synovial fluid. Enclosed by a fibrous capsule lined with synovial membrane, the disc is supported externally by attachments to various muscles and tendons. The capsule not only houses the lateral ligament but also connects with other supporting ligaments such as the sphenomandibular and stylomandibular ligaments. Its primary roles include shielding the joint and limiting excessive movement. Key muscles involved in TMJ function include:

• The masticatory muscles, notably the lateral pterygoid, which anchors to the pterygoid fossa and the articular disc.
• Muscles of facial expression.
• Anterior neck muscles.

This joint exhibits significant mobility, with independent actions occurring in the upper and lower compartments. When the jaw is closed (central occlusion), the condylar heads rest within the glenoid fossae. During mouth opening, the condylar heads rotate on a central axis, moving predominantly within the lower compartment. As the mouth opens further, these heads glide forward alongside the articular discs, reaching the crests of the articular tubercles and engaging the upper compartments.

When the mouth closes, the condylar heads and discs retract. Forward and backward movements are generally confined to the upper compartment. Lateral jaw movements, limited to roughly 15 degrees, involve one condylar head and disc advancing forward and downward along the tubercle, while the opposite condylar head stays in place, rotating within the fossa.

The temporomandibular joint plays a crucial role in cases of orofacial pain. However, these conditions are often challenging to diagnose and treat due to their intricate nature and the limited understanding of the underlying mechanisms contributing to their etiology and pathogenesis. For those seeking a comprehensive understanding of TMJ dysfunction, facial pain, and occlusion, we recommend the course "Occlusion, TMJ dysfunctions, and orofacial pain from A to Z" by Dr. Jeffrey P. Okeson.

Osseous structures of the TMJ

The temporomandibular joint's osseous framework comprises three formations:

• mandibular condyle
• temporal bone's glenoid fossa
• articular eminence (tubercle)

Condyle

The condyle, located on the mandibular ramus, has an oval shape, with its long axis oriented at a slight medial angle of about 140° relative to the horizontal plane. Its articular surface is covered by fibrocartilage, beneath which lies cortical and trabecular bone layers. In the frontal view, the lateral pole of the condyle is positioned lower than the medial pole.

Glenoid fossa

The glenoid fossa, a shallow oval depression in the temporal bone's infratemporal region, forms a key component of the TMJ. It is bordered anteriorly by the zygomatic process and posteriorly by the external auditory meatus. The articular eminence projects from its anterior boundary, while the posterior aspect is defined by the tympanic portion. In the medial-lateral direction, the squamotympanic fissure traverses the area, branching medially into the petrosquamous and petrotympanic fissures. The petrotympanic fissure is particularly significant as it transmits the chorda tympani nerve—a branch of the facial nerve responsible for taste sensation in the anterior two-thirds of the tongue and parasympathetic innervation to the salivary glands.

This close anatomical relationship between the TMJ and the chorda tympani suggests that TMJ dysfunction could potentially affect the nerve, leading to symptoms such as altered taste or salivary gland dysfunction. The glenoid fossa’s bony structure is notably thin, sometimes comprising only a single compact bone layer that separates it from the cranial cavity. 

Articular eminence

The articular eminence, located anterior to the glenoid fossa at the base of the zygomatic process, plays a pivotal role in condylar motion during mouth opening. This cylindrical projection is oriented mediolaterally and features a sigmoid curve in the sagittal plane. From a frontal perspective, the eminence displays a concave curvature, giving it a saddle-like shape when observed inferiorly. The eminence has two distinct slopes: the anterior slope, positioned in front of the apex, and the posterior slope, located just before the articular fossa. Both are covered with fibrocartilage and work alongside the articular disc to facilitate smooth articulation and load distribution during mandibular movement.

Articular disc

The articular disc separates the temporomandibular joint (TMJ) into upper and lower chambers and functions to facilitate smooth movement. It is shaped like a laterally flattened oval and includes three key regions: the thick posterior band, the thinner anterior band, and the slender intermediate zone, which bears the most functional load. The posterior band transitions into the bilaminar zone, composed of elastic and collagen fibers, anchoring the disc to the articular fossa and condylar neck. The intermediate zone, devoid of blood vessels, receives nutrients through fluid perfusion and is susceptible to damage under excessive stress or joint dysfunction. The anterior disc is connected differently on each side: medially, it blends with the lateral pterygoid muscle's tendon and loosely attaches to nearby structures, while laterally, it is securely fixed by a ligament.

Ligaments supporting the TMJ

Ligaments provide structural support and restrict excessive movement in the TMJ. They are composed of durable collagen fibers but may stretch under pathological stress. There are three functional and two accessory ligaments associated with the TMJ:

1. Lateral ligaments: These connect the disc's medial and lateral edges to the condyle, enabling rotational movements while preventing lateral displacement.
2. Capsular ligament: Encircling the TMJ, this ligament connects to the temporal bone and condyle, ensuring stability, resisting separation of joint components, and retaining synovial fluid for lubrication.
3. Lateral (temporomandibular) ligament: Strengthening the joint laterally, this ligament includes external oblique and internal horizontal components that prevent excessive mouth opening, backward movement of the disc and condyle, and overextension of the lateral pterygoid muscle.
4. Sphenomandibular ligament: Originating from the sphenoid bone, it extends to the lingula on the mandibular ramus, supporting joint alignment while allowing passage of the maxillary artery and auriculotemporal nerve.
5. Stylomandibular ligament: Extending from the styloid process to the mandibular ramus, it restricts excessive forward movement of the mandible during protrusion.

Muscles

Temporal muscle

The temporal muscle, the strongest of the masticatory muscles, originates from the temporal fossa on the skull's lateral surface. It consists of three groups of fibers:

• Anterior fibers run vertically downward from the front of the temporal fossa.
• Middle fibers descend slightly forward from the central region of the temporal fossa.
• Posterior fibers extend forward from the rear portion of the fossa.

All fibers converge and insert near the coronoid process of the mandible. As the muscle passes through the infratemporal region, it slips beneath the zygomatic arch, intermingling with fibers of the masseter muscle. It may also attach near the mandibular notch and retromolar area. This muscle plays a vital role in jaw closure.

Innervation is provided by the deep temporal nerves from the mandibular branch of the trigeminal nerve, while the deep temporal arteries from the maxillary artery supply blood.

Masseter muscle

The masseter muscle, quadrilateral in shape, originates along the inferior edge of the zygomatic arch and attaches to most of the mandibular ramus's lateral surface. It has two parts:

• Superficial portion: Fibers descend posteriorly and attach around the zygomatic-maxillary suture.
• Deep portion: Fibers descend anteriorly and interweave with the temporal muscle.

Some deep fibers originate independently from the zygomatic bone and arch, anchoring near the mandibular notch. The masseter elevates the mandible vertically for optimal intercuspal contact. When contracted unilaterally, it shifts the mandible laterally.

The muscle is innervated by the mandibular branch of the trigeminal nerve and receives blood from the masseteric branch of the maxillary artery, the facial artery, and direct branches of the external carotid artery.

Medial pterygoid muscle

The medial pterygoid is a rectangular muscle that originates from the pterygoid fossa and pterygoid process of the sphenoid bone, inserting into the pterygoid tuberosity at the mandible's angle. It has two parts:

• Deep part: Arises primarily from the medial surface of the lateral pterygoid plate and the pyramidal process of the palatine bone.
• Superficial part: Fibers interlace with the masseter muscle along the mandible's lower border and with the lateral pterygoid muscle anteriorly.

Bilateral contraction aids in mandibular elevation, while unilateral contraction causes lateral mandibular movement. The muscle is innervated by the medial pterygoid branch of the mandibular nerve, with blood supplied by branches of the maxillary artery.

Lateral pterygoid muscle

The lateral pterygoid has two distinct heads:

• Superior head: Originates from the infratemporal surface of the greater wing of the sphenoid.
• Inferior head: Arises from the lateral surface of the lateral pterygoid plate and partially from the maxillary bone.

Fibers run posterolaterally, inserting into the pterygoid fossa of the mandibular neck and the articular disc and capsule of the TMJ. The superior head primarily attaches to the joint capsule and disc, while the inferior head anchors to the condyle.

The lateral pterygoid shifts the condylar head forward during mouth opening, moving the disc with it. Unilateral contraction leads to lateral mandibular displacement. Innervation is by the lateral pterygoid branches of the mandibular nerve, with blood supplied by pterygoid branches of the maxillary artery. A notable racial variation exists in the position of the maxillary artery relative to the muscle.

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Vascular supply to the TMJ

The TMJ's blood supply comes from:

1. Superficial temporal artery branches: Transverse facial, middle temporal, and zygomatico-orbital arteries.
2. Maxillary artery branches: Deep auricular, anterior tympanic, middle meningeal, posterior deep temporal, and masseteric arteries.

These vessels form an arterial network around the joint capsule. The intermediate portion of the articular disc is avascular, with peripheral regions supplied anteriorly and posteriorly, especially posteriorly.

Innervation Overview

• TMJ and masticatory muscles: Mandibular branch of the trigeminal nerve.
• Facial muscles: Facial nerve.
• Anterior neck muscles: Mandibular nerve, facial nerve, and cervical plexus.
• Lateral neck and scalene muscles: Accessory nerve and cervical plexus branches.

Innervation of the TMJ

The temporomandibular joint (TMJ) is primarily innervated by branches of the auriculotemporal nerve, a branch of the mandibular nerve. The auriculotemporal nerve divides into five branches:

1. Skin of the external auditory canal
2. Parotid gland
3. Connecting branch with facial nerve
4. Anterior auricular nerve
5. Terminal branch of the superficial temporal nerve

From these, additional smaller branches innervate the TMJ. Also, masseteric and posterior deep temporal nerves, which primarily serve the masticatory muscles, contribute to the joint's innervation. The role of facial and lateral pterygoid nerve branches in TMJ innervation is not fully understood. The nerves ascend from both anterior and posterior sides to supply the condylar head.

Innervation of the joint capsule

The joint capsule receives innervation from different directions:

• Medially: Direct joint branches of the auriculotemporal nerve.
• Laterally: Joint branches from the facial nerve's communicating branch, with some contribution from the superficial temporal nerve.
• Anteriorly: Innervated by joint branches from the masseteric and posterior deep temporal nerves.
• Posteriorly: Joint branches from the auriculotemporal nerve, external auditory canal nerve, anterior auricular nerve, and superficial temporal nerve.

The density of nerve fibers varies across the capsule. The posterior part, with looser connective tissue, is more richly supplied with nerves and blood vessels than the anterior part. Nerve fiber density decreases medially, particularly in the medial third of the capsule, where most fibers end as free nerve endings.

Distribution of nerves in the articular disc

The articular disc receives nerve fibers from the surrounding joint capsule. Nerves enter primarily at the anterior and posterior regions, with a particularly high concentration at the posterior margin. The middle and lateral portions of the disc have fewer nerve fibers, concentrated along the periphery. Notably, the intermediate zone is devoid of nerve fibers, indicating it undergoes significant loading during mastication.

Nerve distribution in the condylar head

Understanding the condyle's movements and its role in mandibular dynamics is crucial for diagnosing and treating TMD (temporomandibular disorders), muscle dysfunctions, and occlusal misalignments. Join the advanced course "Condylography: Data Interpretation for Occlusal Rehabilitation" to master the use of condylography—a specialized tool that reveals the movements of the condyle during mastication, speech, and other functional activities!

The condylar head is innervated by branches of the nerves that supply the joint capsule. These nerves enter the lower compartment, passing through the fibrous layer and ascending to innervate the synovial membrane, connective tissue lining, and articular cartilage. Some fibers also descend to innervate the neck of the condylar process. There are more nerve fibers in the anterior portion of the condylar head compared to the posterior part.

This extensive innervation of the TMJ and its components supports essential functions such as proprioception, movement, and overall joint function, enabling coordinated jaw movements during activities like mastication.