Anatomy of the Temperomandibular Joint
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Introduction[edit | edit source]
The temporomandibular joint (TMJ), which is involved in mastication (chewing) and speech, 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[edit | edit source]
- 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.
- 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.
- 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.
- 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[edit | edit source]
The intra-articular disc 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.
- Creating articular congruence
- Absorbing and distributing load over larger areas in order to prevent damage to the articular surfaces
- Upper or temporodisc chamber
- 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.
Articular Capsule[edit | edit source]
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 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.
Ligaments[edit | edit source]
- Intrinsic ligaments:
- Collateral disc ligaments
- Temporomandibular ligament
- Extrinsic ligaments:
- Affect the position of the bony levers in the TMJ
Collateral Disc Ligaments[edit | edit source]
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[edit | edit source]
As mentioned above, the temporomandibular ligament is considered a reinforcement of the capsule. It restricts jaw closure and mandible retraction, thus protecting the retro-discal tissues.
Extrinsic Ligaments[edit | edit source]
- 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).
Innervation and Vascularisation[edit | edit source]
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[edit | edit source]
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:
- Medial pterygoid
See figure 3.
- Lateral pterygoid (see figure 4)
|Temporalis||Elevates the mandible|
|Masseter||Elevates the mandible|
|Lateral pterygoid||Protracts the mandible, depresses the chin, and lateral deviation of mandible|
|Medial pterygoid||Works with masseter to elevate the mandible and aids in protrusion|
Geniohyoid (see figure 5)
|Depress the mandible against resistance when the infrahyoid muscles stabilise or depress the hyoid bone|
|Platysma||Depresses the mandible against resistance|
References[edit | edit source]
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