Temporomandibular Joint (TMJ)

Type and Formation

Unique Features

1. Divided joint space - An intra-articular disc divides the joint into upper and lower cavities
2. Fibrocartilage, not hyaline - Articular surfaces are fibrous tissue, reflecting the joint's intramembranous development
3. Secondary condylar cartilage - Present in the condyle head until adolescence
4. Tooth influence - Movements are influenced by the occlusion of teeth

Articular Disc

• A biconcave fibrocartilaginous disc sits between the condyle and temporal fossa
• Divides the joint into:
  • Upper compartment (disc-temporal): permits gliding/translatory movement
  • Lower compartment (disc-condylar): permits hinge/rotatory movement
• Anteriorly attached to the superior head of the lateral pterygoid muscle (in ~60% of people, fibres insert directly into the medial aspect of the disc's anterior border)
• Posteriorly, the bilaminar zone connects the disc to the retrodiscal tissue: the upper lamina is fibro-elastic (aids disc return), the lower lamina is non-elastic

Joint Capsule

• Attached above to the mandibular fossa (extending to just anterior to the articular eminence crest and posteriorly to the squamotympanic and petrotympanic fissures)
• Attached below to the neck of the condyle
• Thin and lax capsule

Ligaments

Muscles of Mastication

Movements

• Mouth opening: combined rotation (lower compartment) + anterior translation of condyle down the articular eminence (upper compartment)
• Normal interincisal opening: 3-6 cm
• Protrusion / retrusion: gliding in upper compartment
• Lateral excursion: working side condyle rotates, balancing side condyle translates

Nerve and Blood Supply

• Nerve: Auriculotemporal nerve (branch of V3) - the main sensory supply; also masseteric and deep temporal branches
• Blood supply: Superficial temporal and maxillary arteries

Clinical Examination

• Palpate just anterior to the external auditory canal with jaw opening/closing and lateral excursion
• Assess for clicking, crepitus, tenderness, and range of motion
• Measure interincisal distance (normal 3-6 cm) - Firestein & Kelley's Rheumatology, p. 4608

TMJ Disorders

1. Internal Derangements

• Disc displacement (most common): anterior displacement with or without reduction
• With reduction: audible/palpable click on opening and closing
• Without reduction ("closed lock"): inability to fully open the mouth

2. TMJ Syndrome / Temporomandibular Disorder (TMD)

• Presents with: jaw pain, limited opening, clicking/crepitus, referred ear pain (close anatomical proximity to EAC)
• Thought to be related to bruxism and is considered a form of myofascial pain, similar to fibromyalgia
• Inflammatory cytokines (IL, TNF), matrix metalloproteinases, and VEGF may be elevated in synovial fluid

3. Arthritis

• Multiple arthritides affect the TMJ including juvenile and adult RA, osteoarthritis, psoriatic arthritis
• In children, RA of the TMJ can cause micrognathia (arrested mandibular bone growth)

4. Anterior Dislocation

• Condyle slips anteriorly over the articular eminence; muscle spasm prevents spontaneous reduction
• Unilateral: jaw deviates away from affected side
• Bilateral: jaw protrudes, mouth locked open, drooling, garbled speech, depression palpable over the TMJ
• Reduction technique: patient seated upright; thumbs/fingers in buccal sulcus pressing angle of jaw downward while rotating the chin upward and backward; gauze protection advised
• Rule out fracture-dislocation with Panorex/CT before blind reduction if trauma suspected - Rosen's Emergency Medicine, p. 194

Imaging

Key Clinical Pearl

TMJ Anatomy Diagrams

1. Sagittal Cross-Section - Articular Disc & Lateral Pterygoid (Scott-Brown's Otorhinolaryngology)

2. MRI - TMJ Disc Translation During Mouth Opening (Imaging Anatomy Atlas)

3. UTE MRI - Condylar Fibrocartilage & Bone (Imaging Anatomy Atlas)