Anatomy

The Temporomandibular Joint

Specifically the temporomandibular joint is between the condylar head of the mandible and the glenoid (mandibular) fossa and articular tubercle of the squamous part of the temporal bone. The anatomy of the TMJ is unusual. Normal synovial joints consist of 2 bones covered by a hyaline cartilage and surrounded by a capsule which keeps in the synovial fluid. However the TMJ has two separate compartments within the capsule, one above the other. These are separated by an oval disc of fibrocartilage known as the articulate disc. This is a very unique structure found in only a few joints in the body; it is very flexible and highly moveable. It separates the joint into two parts (inferior and superior). The inferior compartment is made up of the mandibular condyle and the superior compartment is made from the glenoid fossa and articular tubercle. The capsule encloses the entire of the joint and attaches from the articular eminence (articular tubercle is the lateral part of the articular eminence) to the condylar head.

Ligaments

The TMJ has 3 ligaments associated with it; the temporomandibular ligament, stylomandibular ligament and sphenomandibular ligament. The temporomandibular is the most important one and makes up the lateral part of the capsule. It’s made up of 2 fasciculi; the outer oblique portion (OOP) and an inner horizontal portion (IHP). They connect from the lateral surface of the zygomatic arch to the lateral surface and posterior border of the neck of the mandible. The other 2 ligaments are secondary ligaments and do not connect directly to any part of the joint itself. The sphenomandibular ligament attaches from the spine of the sphenoid bone to the lingula of the mandible. The stylomandibular ligament attaches from the styloid process to the angle of the mandible. All the ligaments are used for limiting the movement of the mandible, meaning they don’t allow movement beyond the extents. If this were to occur it would result in painful stimuli.

Movement

The TMJ is a ginglymoarthrodial joint meaning it allows for 2 types of movement; rotation and translation. Translational movement is the sliding of the joint allowing for protrusion, retrusion and side to side movement.

Rotation is the hinge movement of the joint and is an action mainly involving the inferior compartment. Translation is the sliding movement and an action of the superior compartment. To open the mouth you require both types of movement as rotational movement alone can only open the mouth to around 20mm, further movement cannot be achieved without translational movement of the joint. This is shown in the short clip at the bottom of the page.

Muscles

There are 4 muscles involved with movement of the mandible; the masseter, the pterygoids (lateral and medial) and the temporalis.

·         Masseter – Has 2 parts (superficial and deep portions). They arise from the medial surface of the zygomatic arch (deep) and zygomatic process of the maxilla bone (superficial). They insert into the ramus of the mandible (deep) and the angle of the mandible (superficial). The masseter acts to elevate the mandible (close the mouth) when contracted.

·         Lateral pterygoid – Has 2 heads (superior and inferior). Originate from the infratemporal surface of the greater wing of the sphenoid bone (superior) and from the lateral surface of the lateral pterygoid plate (inferior). They insert into the articular disc of the TMJ (superior) and the condyloid process of the mandible (inferior). It acts to depress the mandible (open the mouth) and also translates the mandible from side to side.

·         Medial Pterygoid – Also has 2 heads (deep and superficial). Originate from the medial surface of the lateral pterygoid plate (deep) and the maxillary tuberosity of the palatine bone (superficial). They both insert into the lower part of the ramus and angle of the mandible. Acts to elevate the mandible (close the mouth) and also translates the mandible from side to side with the lateral pterygoid and protrudes it.

·         Temporalis – Arises from the temporal fossa and inserts into the coronoid process of the mandible. Its action is to elevate the mandible (close the mouth).

Innervation, Blood Supply and Proprioception

All of the muscles of mastication are innervated by the mandibular branch (V3) of the trigeminal nerve (the 5^th cranial nerve). The blood supply is from branches off the external carotid artery. Predominantly from the superficial temporal branch, but some other branches are also involved; deep auricular artery, anterior tympanic artery, ascending pharyngeal artery and maxilliary artery.

Proprioception of the TMJ is regulated by 4 receptors as shown in the table below.