Mandible Movement and Jaw Closure

Temporomandibular Joint

• The temporomandibular joint is a synovial joint that articulates between the articular tubercle and the anterior portion of the mandibular fossa of the temporal bone above and the head (condyloid process) of the mandible below.
• The articular surfaces are covered with fibrocartilage.
• The joint is divided into upper and lower cavities by the articular disc.

Capsule and Ligaments

• The capsule surrounds the joint and is attached above to the articular tubercle and the margins of the mandibular fossa and below to the neck of the mandible.
• The lateral temporomandibular ligament strengthens the lateral aspect of the capsule and limits the movement of the mandible in a posterior direction, protecting the external auditory meatus.
• The sphenomandibular ligament lies on the medial side of the joint and represents the remains of the first pharyngeal arch in this region.
• The stylomandibular ligament lies behind and medial to the joint and is a band of thickened deep cervical fascia that extends from the apex of the styloid process to the angle of the mandible.

Articular Disc

• The articular disc is an oval plate of fibrocartilage that is attached circumferentially to the capsule.
• The disc is also attached in front to the tendon of the lateral pterygoid muscle and by fibrous bands to the head of the mandible.
• These bands ensure that the disc moves forward and backward with the head of the mandible during protraction and retraction of the mandible.

Retrodiscal Tissue

• The retrodiscal tissue is vascular and highly innervated, making it a major contributor to the pain of Temporomandibular Disorder (TMD) when there is inflammation or compression within the joint.
• The retrodiscal zone (bilaminar zone) of the TMJ is located between the posterior band of the articular disc and the posterior part of the articular capsule.
• The superior retrodiscal layer of the TMJ joint is a lamina composed of connective tissue and elastic fibers that continues the posterior part of the articular disk and connects the disc to the tympanic plate of the temporal bone.
• The inferior retrodiscal layer of the TMJ joint is a lamina composed of collagen fibers that continues the posterior part of the articular disk and inserts into the condylar neck.

Nerve and Vascular Supply

• The TMJ is supplied by the auriculotemporal and masseteric branches of the mandibular nerve.
• The TMJ is supplied mainly by three arteries: the deep auricular artery (from the maxillary artery), the superficial temporal artery (a terminal branch of the external carotid artery), and the anterior tympanic artery (a branch of the maxillary artery).
• The venous drainage of the TMJ is via the superficial temporal vein and the maxillary vein.

Movements

• The mandible can be depressed or elevated, protruded or retracted, and rotated.
• Depression of the mandible is brought about by contraction of the digastrics, geniohyoids, and mylohyoids, with the lateral pterygoids playing an important role in pulling the mandible forward.
• Elevation of the mandible is brought about by contraction of the temporalis, masseter, and medial pterygoids.
• Protrusion of the mandible is brought about by contraction of the lateral pterygoid muscles of both sides, assisted by both medial pterygoids.
• Retraction of the mandible is brought about by contraction of the posterior fibers of the temporalis.

Muscles

• Masseter: originates from the zygomatic arch, inserts into the lateral surface of the ramus of the mandible, and elevates the mandible to occlude teeth.
• Temporalis: originates from the floor of the temporal fossa, inserts into the coronoid process of the mandible, and elevates the mandible while posterior fibers retract the mandible.
• Digastric: has two bellies, posterior and anterior, which depress the mandible or elevate the hyoid bone.
• Mylohyoid: originates from the mylohyoid line of the mandible, inserts into the body of the hyoid bone, and depresses the mandible or elevates the floor of the mouth and hyoid bone.
• Geniohyoid: originates from the inferior mental spine of the mandible, inserts into the body of the hyoid bone, and depresses the mandible or elevates the hyoid bone.
• Lateral pterygoid: originates from the greater wing of the sphenoid and lateral pterygoid plate, inserts into the neck of the mandible and articular disc, and pulls the neck of the mandible forward.
• Medial pterygoid: originates from the tuberosity of the maxilla and lateral pterygoid plate, inserts into the oblique line on the lamina of the thyroid cartilage, and elevates the mandible.

Clinical Notes

• The temporomandibular joint lies immediately in front of the external auditory meatus.
• The great strength of the lateral temporomandibular ligament prevents the head of the mandible from passing backward and fracturing the tympanic plate when a severe blow falls on the chin.
• Dislocation of the temporomandibular joint can occur when the mandible is depressed, and the joint is unstable.
• Reduction of the dislocation can be achieved by pressing the gloved thumbs downward on the lower molar teeth and pushing the jaw backward.