TMJ Anatomy and Function

Temporomandibular Joint (TMJ) Anatomy and Function

• The TMJ is a synovial joint formed by the temporal bone and the mandible
• It facilitates jaw movement (hinging and sliding)
• The joint is bilateral, meaning the two sides work together and depend on each other

Temporal Bone: Lateral View

• The temporal bone features the zygomatic process, articular tubercle, mandibular fossa, styloid process, mastoid process, and external acoustic meatus
• The articular tubercle and mandibular fossa are crucial for the TMJ's function

Temporal Bone: Inferior View

• The temporal bone includes the stylomastoid foramen, styloid process, and other features relevant to the TMJ
• The stylomastoid foramen and carotid canal are anatomical features of the temporal bone

Mandibular Condyles

• The mandible has condyles that articulate with the temporal bone
• There are heads, necks, and condylar processes related to the mandibular condyles

Muscles of Mastication

• Temporalis: Originates in the temporal fossa, inserts into the coronoid process of the mandible, responsible for closing the jaw, retruding the jaw
• Masseter: Originates from the zygomatic arch, inserts into the angle and ramus of the mandible; responsible for closing the jaw
• Lateral Pterygoid: Originates on the sphenoid bone, inserts into the articular disc and neck of mandible; responsible for opening, protracting, and lateral movement of the jaw
• Medial Pterygoid: Originates on the sphenoid bone and maxillary bone; inserts into the ramus and angle of mandible, responsible for closing the jaw, protracting the jaw

Design of the Masticatory System

• The design involves a fixed socket in the temporal bone, a lever arm with a fulcrum (mandibular fossa), a movable fulcrum (articular disc), and ligaments to limit movement
• The spinal column's forward movement, when upright posture developed, necessitated a movable fulcrum
• Ligaments and a fibrous capsule are needed to limit the range of motion and maintain stability
• A variety of muscles allow extensive movement while consuming food, talking, and expressing emotions

Joint Capsule

• The joint capsule is a fibrous membrane surrounding the joint, attaching to the articular eminence, neck of the mandible, and the articular disc
• It restricts excessive movement and ensures stability during jaw function, while the inner layer produces synovial fluid
• Two layers comprise the capsule: (1) the outer fibrous layer which holds the ligaments, and (2) the inner synovial membrane which creates synovial fluid

Articular Disc

• The articular disc is a biconcave structure within the TMJ, extending from the joint capsule
• It's the thinnest in the middle, with an anterior band (2mm) and posterior band (3mm)
• The central part of the disc (denser) is avascular and non-innervated, being nourished by the synovial fluid; this central portion is designed to withstand pressure from the condyle
• The peripheral parts have nerve and blood supply, positioning the condyles within the TMJ

Synovial Fluid

• The synovial membrane produces synovial fluid, essential for nourishing avascular surfaces within the joint, lubricating the joint surfaces, removing waste
• It's composed of a polysaccaride-protein complex (hyaluronate)

Ligaments

• Lateral (temporomandibular) ligament: Thickened section of the capsule, connecting the zygomatic arch to the mandible's neck; limits excessive lateral movement during jaw motion
• Stylomandibular ligament: A ligament situated between the styloid process and mandible; aids in maintaining jaw stability
• Sphenomandibular ligament: Positioned between the sphenoid bone and mandible; assisting in maintaining jaw stability

Blood Supply and Innervation

• External carotid arteries (superficial temporal and maxillary arteries) supply blood to the TMJ
• The auriculotemporal nerve carries sensory information (V3)

Function of TMJs

• Chewing, swallowing, speaking
• Facial expressions

Complex Movements of the TMJ

• Opening of the jaw involves a rotation of the condyles in the initial stage
• Translation (forward gliding) occurs in the upper joint compartment as the jaw opens wider
• The two condyles and discs move simultaneously during all phases of jaw motion

Lower and Upper Joint Compartments

• The lower compartment involves the condyle and disc and allows for rotational movement during the beginning phase of opening the mouth
• The upper compartment includes the disc and fossa, contributing to gliding/translational movement during wider opening movements

MRI Images

• MRI (magnetic resonance imaging) scans of the TMJ in both closed and opened mouth positions aid in diagnosis of jaw disorders and assessing joint movement