Corpus: Mandibular joint

The mandibular joint is the joint between the temporal bone and the lower jaw. It is responsible for the chewing movement.

2.1. Joint Socket

The joint socket is the mandibular fossa of the temporal bone (pars squamosa ossis temporalis). Only the anterior part, covered by fibrocartilage and extending to the petrotympanic fissure, forms the articular surface. The posterior part is extracapsular. A small bony elevation covered with fibrocartilage – the articular tuberosity of the temporal bone – is situated in front of the articular surface.

2.2. Joint Head

The articular head, known as the caput mandibulae, lies on the articular process of the mandible (processus condylaris). Its radius of curvature is significantly smaller than that of the mandibular fossa. The joint head is covered with fibrocartilage from the anterior to the posterior joint edge and is separated from the joint socket by an articular disc.

2.3. Articular disc

The articular disc comprises several layers. The thin intermediate zone in the center consists of tight connective tissue, while the thickened peripheral zones are made of fibrocartilage. These thickenings, known as the "anterior" and "posterior ligaments", attach to the joint capsule all around and divide the joint into two chambers. The disc accompanies the mandibular head during its movements and forms a socket for the articular tubercle with its upper surface.

The disc is fused medially and laterally with the joint capsule and dissolves posteriorly into a connective tissue bar (bilaminar zone) that delimits the joint space dorsally. Thus, there is no fibrous posterior joint capsule wall. The bilaminar zone contains many elastic fibers, fat cells, and a venous plexus. Fibers of the lateral pterygoid muscle radiate into the disc. The middle and lateral sections are also reached by fibers from the masseter and temporalis muscles.

2.4. Joint Capsule

The joint capsule originates from the petrotympanic fissure between the petrous bone and the tympanic part of the temporal bone. It includes the joint tuberosity at the front. The capsule is very wide, consists of many elastic fibers, and is reinforced on the sides by connective tissue, a "medial ligament," and a "lateral ligament." Only the lateral ligament, which runs from the zygomatic arch to the condylar process, has any mechanical significance.

The bilaminar zone is located in the dorsal part of the joint capsule.

2.5. Ligaments

In addition to the ligaments of the joint capsule, the mandibular joint is stabilized by other fibrous ligaments:

• Stylomandibular ligament: Runs from the styloid process to the angle of the jaw and to the fascia of the medial pterygoid muscle
• Sphenomandibular ligament: Runs from the sphenoid bone to the lingula mandibulae, located at the inner opening of the mandibular canal

2.6. Innervation

The mandibular joint capsule is supplied by the articular branches of the auriculotemporal nerve, the profundus temporalis nerve, and the masseteric nerve. The proprioception of the temporomandibular joint includes several types of receptors, such as Ruffini corpuscles, Vater-Pacini corpuscles, Golgi tendon organs, and free nerve endings.

2.7. Blood Supply

The arterial supply to the mandibular joint is largely provided by the superficial temporal artery from the external carotid artery. Other branches of the external carotid artery, such as the profunda auricular artery, the anterior tympanic artery, the ascending pharyngeal artery, or the maxillary artery, can also contribute to the blood supply.

2.8. Topography

Immediately behind the mandibular joint is the external auditory canal (Meatus acusticus externus) and directly above it the cranial fossa.

The mandibular joint enables the following movements, where both joints must always work together as the lower jaw is involved in the left and right mandibular joint:

• Abduction: Lowering of the lower jaw (mouth opening)
• Adduction: Lifting of the lower jaw (mouth closure)
• Protrusion: Translation forwards (pushing the lower jaw forwards)
• Retrusion: Translation backwards (pushing the lower jaw back)
• Laterotrusion: External rotation of a condyle (ipsilateral outward grinding movement)
• Mediotrusion: Translation of the contralateral condyle forwards inwards (contralateral movement inwards)

Chewing is always a compound movement. In the resting position, the rows of teeth are not completely closed. The mandibular head is not located in the depth of the socket but on the posterior slope of the articular tubercle. The thick posterior part of the disc is located in the mandibular fossa.

The degree of lateral mandibular tilt is measured in degrees and is referred to as Bennett's angle.

3.1. Hinge Movements

When the mouth is opened, the hinge heads move forwards and downwards. When closing, they move backwards. The lower jaw's movement is restricted by the sphenomandibular ligament. During the opening movement, there is sliding above the discus and rotation below it. Both mandibular joints together can be regarded as a hinge joint with a movable socket. The rotational movement takes place around a transverse axis that passes through the joint heads, but which shifts during opening.

3.2. Slide Movement

A carriage movement is the forward and backward movement of the lower jaw with a range of approximately 1.5 cm. The mouth can also remain closed during this movement.

3.3. Grinding Movement

When grinding movements are performed in the mandibular joint, one joint head moves ventrally and laterally, while the other rotates around its vertical axis and moves slightly laterally.

When the mouth is very wide open, the mandibular head may move to the front of the articular tubercle and get caught, preventing the mouth from closing. This condition is known as lockjaw. With lockjaw, the mouth cannot be opened. The mandibular joint can also be affected by osteoarthritis or cause headaches as part of Costen's syndrome.

The first step in the examination of the mandibular joint is clinical inspection and palpation. Functional diagnostics provide additional information. Special questions can be clarified using imaging procedures, usually an MRI of the mandibular joint.