Anatomy of the Temperomandibular Joint

Original Editor - Jess Bell based on the course by Victoria Reboredo
Top Contributors - Jess Bell, Kim Jackson, Tarina van der Stockt, Rucha Gadgil, Lucinda hampton and Olajumoke Ogunleye

Introduction

Figure 1. TMJ Anatomy.

The temporomandibular joint (TMJ) is involved in mastication (chewing) and speech and it is one of the most frequently moved joints in humans. This synovial joint must be able to respond to significant biomechanical load. It is made up of the articulating surface of the temporal bone and the head of the mandible. The whole joint is enclosed in a fibrous capsule. The TMJ is defined as a ginglymoarthrodial joint (i.e. both a hinge and a gliding joint). It allows a rotational movement in the sagittal plane and a translation movement on its own axis. The translation motion generates more movement.

Unique Features

The TMJ is unique from other synovial joints for the following reasons:

1. Intra-articular disc
   • This disc divides the joint into two compartments (i.e. superior and inferior), which allows different actions to occur simultaneously in different parts of the joint in a coordinated manner.
2. Bilaterality
   • The mandible articulates on both sides of the skull - from a mechanical point of view, any movement on one side will always affect the other side.
3. Fibrous cartilage
   • While hyaline cartilage typically covers the articular surfaces in synovial joints, the main articular surfaces of the TMJ (i.e. the mandibular condyle and the temporal bone) are covered by fibrous cartilage. Fibrocartilage contains type I and II collagens whereas articular hyaline cartilage only contains type II collagen. Fibrocartilage is more resistant to sheer forces, so it is better able to absorb load and compressive forces. It is, however, also more sensitive to friction, which is an important factor when considering degenerative pathology of the TMJ.
4. Occlusion
   • Occlusion refers to the relationship between the upper and lower teeth when the jaw closes. The TMJ is the only human joint that has a rigid end point (caused by the teeth coming into contact). Occlusion via the teeth maintains the vertical dimension in a sagittal plane and ensures that pressure is distributed along the whole joint.
   • The role of occlusion in the TMJ is controversial. Differences in occlusion are not necessarily associated with TMJ dysfunction. However, a loss of height and separation in the mandibular bone and the jaw at the end of the closing movement (i.e. in individuals who lose teeth) can cause changes in the joint's intra-articular pressure and lead to earlier degeneration.

Intra-Articular Disc

Figure 2. TMJ disc location.

The intra-articular disc is the central anatomic structure of the TMJ. It is made of fibrocartilage. Its extracellular matrix is largely made up of collagen I and elastin. Collagen I helps the disc to resist tensile forces while elastin helps it to maintain its shape after deformation.

It is located between the mandible condyle and the temporal fossa (see Figure 1 and 2). It is not vascularised and has little innervation.

Functions of the intra-articular disc include:

• Creating articular congruence
• Absorbing and distributing load over larger areas in order to prevent damage to the articular surfaces

The disc divides the TMJ fossa into two compartments, thus creating two new articular surfaces:

1. Upper or temporodisc chamber
2. Lower or condylar disc chamber

These surfaces have the ability to move between each other in an isolated way or in a synchronised way. Essentially, the disc acts as a largely passive, but movable articular surface which can accommodate translational movements of the condyle.

When increased load is applied to the TMJ (i.e. during mastication), the fibrocartilage is able to modulate its water content and ensure that the load is shared.

Unlike the disc, the retro-discal area is highly vascularised and richly innervated. In cases of articular dysfunction, this is a key area that can cause symptoms.

Please watch the following video for an overview of the anatomy of the TMJ.

Articular Capsule

Like any synovial joint, the TMJ has a joint capsule that surrounds all the articular elements. This capsule has an external and internal layer. The external layer is a fibrous membrane, which is thickened laterally by the temporomandibular ligament. The anterior and posterior part of the capsule are thinner and more elastic, thus allowing for the glide of the condyle disc complex in the sagittal plane.

The internal or synovial layer generates the intra-articular fluid that lubricates the jaw and decreases friction between the surfaces. It is also involved in the nourishment of the intra-articular tissues and helps to remove metabolic residues.

The capsule is highly innervated by the trigeminal nerve (i.e. cranial nerve V):

• The anterolateral portion is innervated by the masseteric nerve, which is a motor branch of the anterior division of the mandibular division of the trigeminal nerve
• The lateral area is innervated by the auriculotemporal nerve, which is a sensory branch of the posterior division of the mandibular division of the trigeminal nerve

The trigeminal nerve also innervates the tympanic membrane, the anterior cochlear surface, the external auditory canal, the temporal region, the parotid gland and, alongside the facial nerve (i.e. cranial nerve VII), the skin and posterior part of the cheek. This is significant because an articular problem at this level can cause painful sensations around the auricle and on the skin in the area.

The articular capsule is supplied by the maxillary, temporal and masseteric arteries.

Ligaments

The ligaments of the TMJ can be divided into intrinsic and extrinsic ligaments.

• Intrinsic ligaments:
  • collateral disc ligaments
  • temporomandibular ligament
• Extrinsic ligaments:
  • affect the position of the bony levers in the TMJ

Collateral Disc Ligaments

The collateral ligaments (also known as the discal ligaments) are intra-capsular. They connect the articular disc to the joint capsule and are made of two fascicles: one lateral and one medial.

Their primary function is to help to prevent movement of the intra-articular disc away from the condyle. Movement of the condyle-disc complex depends on these ligaments - the disc modifies its position depending on the position of the condyle during movement in order to improve pressure distribution. As these ligaments are richly innervated, they also provide proprioceptive and pain information.

Temporomandibular Ligament

As mentioned above, the temporomandibular ligament is considered a reinforcement of the capsule. It originates at the lower articular tubercle of the zygomatic arch and inserts into the lateral and posterior border of the neck of the mandible. It restricts jaw closure and mandible retraction, thus protecting the retro-discal tissues.

Extrinsic Ligaments

• Stylomandibular - acts to limit excessive protrusion of the jaw
• Sphenomandibular - primarily acts to protect the TMJ from excessive translation of the condyle after the mouth is opened more than 10 degrees
• Pterigomandibular - limits excessive jaw movements

Together these ligaments maintain the position of the jaw against gravity (i.e. jaw suspension).

Please watch the video below for further discussion of the anatomy of the TMJ.

Innervation and Vascularisation

The TMJ is innervated by the mandibular nerve, which is the third root of the trigeminal nerve (cranial nerve V).

The superficial temporal artery and maxillary artery run along the lateral and medial sides of the condylar neck. The temporal superficial and internal maxillary arteries supply the TMJ through the auricular root. Both of these arteries arise from the external carotid.

The key veins are the pterygoid plexus and superficial temporal veins.

Musculature

• 

  Figure 3. Temporalis, masseter, platysma

• 

  Figure 4. Lateral pterygoid

• 

  Figure 5. Mylohyoid, sternohyoid, digastric

Movements at the TMJ are:

• Opening and closing (descending and ascending movements)
• Lateral movements (lateral glide, left and right)
• Anterior posterior movements in a horizontal plane (mandibular protraction and retraction)

The muscles involved in elevating the mandible (i.e. closing the jaw) are:

• Temporalis
• Masseter
• Medial pterygoid

See figure 3.

The muscles involved in descension of the mandible (i.e. opening the jaw) are:

• Lateral pterygoid (see figure 4)
• Suprahyoid
• Infrahyoid

Table 1. Muscles acting on the TMJ.

Muscle	Origin	Insertion	Innervation	Actions
Temporalis	Temporal bone	Coronoid process of the mandible	Central part: deep temporal nerves of the mandibular nerve Anterior part: branches of the buccal nerve Posterior part: branches of the masseteric nerve	Elevates and retracts the mandible
Masseter	Zygomatic arch	Angle of the mandible	Mandibular branch (V3) of the trigeminal nerve.	Elevates the mandible
Lateral pterygoid (superior and inferior heads)	Superior head: sphenoid bone, In ferior head: sphenoid bone	Superior head: capsule and articular disc of the TMJ, Inferior head: pterygoid process of the mandible	Mandibular branch (V3) of the trigeminal nerve	Unilateral action: lateral mandible deviation to the opposite side. Bilateral action: mandible protraction. Depresses the chin.
Medial pterygoid	Sphenoid, maxilla, and palatine bones	Angle of the mandible	Mandibular branch (V3) of the trigeminal nerve	Works with masseter to elevate the mandible and aids in protraction. Also assists in lateral deviation.
Digastric: Anterior belly (DAB) Posterior belly (DPB)	DAB: digastric fossa of mandible DPB: mastoid notch of the temporal bone	DAB/DPB: Body of the hyoid bone	DAB: Mylohyoid nerve of the inferior alveolar nerve (Mandibular division of the trigeminal nerve (CN V) DPB: digastric branch of the facial nerve (CN VII)	Depresses the mandible. Assists with chewing and swallowing as it elevates the hyoid bone.
Stylohyoid	Styloid process of temporal bone	Body of the hyoid bone	Stylohyoid branch of facial nerve (CN VII)	Depress the mandible against resistance when the infrahyoid muscles stabilise or depress the hyoid bone
Mylohyoid	Mandible near the molars	Hyoid bone	Mylohyoid nerve of the inferior alveolar nerve (Mandibular division of the trigeminal nerve (Cranial nerve V, CN V)	Elevates the hyoid bone and floor of the mouth. Depresses the mandible.
Geniohyoid	Inferior mental spine: the posterior surface of the mandible near the lower part of the mandibular symphysis	Superior border of the body of the hyoid bone	C1 via the hypoglossal nerve	Depress the mandible against resistance when the infrahyoid muscles stabilise or depress the hyoid bone
Platysma	Skin and fascia of the shoulder Superior thoracic regions	Mandible, lower lip, modiolus	Cervical branch of the facial nerve (CN VII) Transverse cervical nerve	Depresses the mandible against resistance. Pulls down the corner of the mouth and hee lower lip.