Temporomandibular Joint (TMJ)

Overview

Unique Anatomical Features

1. Divided joint space - an intra-articular disc divides the cavity into upper and lower compartments
2. Fibrocartilage articular surface - unlike most synovial joints, the articular surfaces are covered by fibrous tissue (not hyaline cartilage), reflecting the joint's intramembranous development
3. Secondary condylar cartilage - present beneath the fibrous articular surface until puberty (~16 years); disappears with skeletal maturity
4. Tooth-influenced movement - occlusal contacts directly modify joint kinematics

Bony Components

• Mandibular condyle: The oval-shaped articular head of the mandible. Its articular surfaces are the anterior and superior aspects. The neck of the condyle is a frequent fracture site. The pterygoid fovea on the anterior neck marks the attachment of the lateral pterygoid.
• Mandibular (glenoid) fossa + articular eminence: The concave fossa and its anterior convex eminence form the temporal component. The shape mismatch between condyle and fossa is part of why the intra-articular disc is needed.

The Articular Disc (Meniscus)

• The disc is thickest at its posterior band, thinnest centrally (intermediate zone), and thicker again anteriorly
• Anterior attachment: fibrous bands to the articular eminence above and condyle below; the superior head of the lateral pterygoid inserts here in ~60% of cases
• Posterior attachment: the retrodiscal tissue (bilaminar zone)
  • Superior lamina: loose, highly vascular, elastic fibres - attaches to squamotympanic fissure; provides elastic recoil to return disc on closing
  • Inferior lamina: relatively avascular, inelastic - attaches to posterior condyle
• Medial/lateral attachments: to the joint capsule and poles of the condyle via triangular connective tissue zones
• Retrodiscal tissue volume increases 4-5x through venous engorgement as the jaw opens

Joint Capsule

• Attached above to the mandibular fossa, anteriorly to just in front of the articular eminence crest, posteriorly to the squamotympanic and petrotympanic fissures
• Below, attached to the neck of the condyle
• The capsule is thin and relatively weak - particularly at the anteromedial portion, where loose synovial tissue allows translation; this anatomical weakness is one reason anterior dislocations occur

Ligaments

Innervation

• Auriculotemporal nerve (primary)
• Masseteric nerve
• Deep temporal nerves

Blood Supply

• Superficial temporal artery (primary)
• Maxillary artery - anterior tympanic and deep auricular branches
• Contributions from deep temporal and transverse facial arteries

Movements

• Normal maximal mouth opening (measured between upper and lower incisors): 3-6 cm
• The condyle undergoes rotation then anterior translation during opening (the meniscus translates with it)

Clinical Examination

• Palpate by placing a finger just anterior to the external auditory canal while the patient opens/closes and moves the jaw side to side
• Assess vertical movement: measure distance between upper and lower incisors at maximal opening (normal 3-6 cm)
• Assess lateral movement using incisors as landmarks
• Note: audible or palpable crepitus or clicking may be present with or without severe arthritis

Clinical Conditions

TMJ Disorders (TMD)

• Inflammatory or degenerative damage to articular surfaces increases friction on the disc
• Disc gradually shifts anteromedially from its normal position
• The posterior retrodiscal tissue - now between bony surfaces - undergoes abnormal loading, loses structure, becomes vascularised, and can eventually perforate
• Degenerative changes can expose bone (osteoarthritic change)
• Synovial fluid in TMD shows elevated pro-inflammatory cytokines (IL, TNF), matrix metalloproteinases, and VEGF

Inflammatory Arthritis

• Both juvenile and adult rheumatoid arthritis can involve the TMJ
• Micrognathia may develop in children if the TMJ is involved, due to arrested mandibular bone growth
• Other arthritides (psoriatic, ankylosing spondylitis, reactive arthritis) may also affect the joint

Anterior Dislocation

• Occurs after any wide mouth opening (yawning, dental procedures, vomiting)
• Condyle translates anterior to the articular eminence and becomes trapped there by masseter/temporalis spasm
• Unilateral: jaw deviates away from affected side
• Bilateral: significant jaw protrusion, open-mouth drooling, garbled speech
• A depression is palpable just anterior to the ear
• Reduction: patient seated upright, operator places thumbs/index fingers in buccal sulcus, applies downward pressure on posterior mandible while rotating chin upward and backward; protect fingers from bite when jaws snap shut
• Rule out fracture before reduction with plain film or Panorex

Trauma/Internal Derangement

• Tearing of the meniscus or collateral ligaments causes abnormal translation - clicking, popping, or incomplete opening
• Paediatric patients with post-traumatic TMJ derangement are prone to asymmetric facial growth and retrognathia
• Management: soft diet, avoid wide opening, refer to oral surgeon with TMJ expertise

Imaging

Treatment Approaches

• Conservative: soft diet, NSAIDs, muscle relaxants, occlusal splints, physiotherapy
• Arthrocentesis: lavage and aspiration of the joint - reduces inflammatory mediators, improves mobility; effective as initial treatment for TMJ arthropathy
• Arthroscopy: for internal derangements refractory to conservative treatment
• Open surgery: reserved for failures of arthroscopy; includes disc repositioning, discectomy, or total joint replacement