Articular Cartilage

Structure and Composition:

• Composed of chondrocytes embedded in an extracellular matrix (ECM) of collagen (mainly type II), proteoglycans, and water (up to 80%).

• Organised into four histological zones:
  Superficial (Tangential) Zone – flat chondrocytes, high collagen, resist shear
  Middle (Transitional) Zone – rounder chondrocytes, high proteoglycans
  Deep Zone – vertical collagen fibres, resist compressive forces
  Calcified Zone – anchors cartilage to subchondral bone

Functions:

• Provides smooth articulation and shock absorption

• Distributes mechanical loads across the joint

• Lacks blood vessels, lymphatics, and nerves → limited intrinsic repair capacity

Biomechanical Properties:

• Viscoelastic behaviour due to proteoglycan-water interaction

• Maintains joint congruity and lubrication

• Damage leads to increased friction and joint degeneration

Injury and Degeneration:

• Can occur due to trauma, overuse, or inflammatory conditions

• Partial-thickness defects often do not heal

• Full-thickness defects may undergo limited fibrocartilage repair

Assessment Modalities:

• MRI with cartilage-sensitive sequences (e.g., dGEMRIC, T2 mapping)

• Arthroscopy provides direct visualisation

• Scoring systems: Outerbridge classification, ICRS grading

Repair Techniques:

• Microfracture: promotes fibrocartilage repair

• Osteochondral autograft/allograft transplantation

• Autologous chondrocyte implantation (ACI)

• Matrix-assisted ACI (MACI)

• Biologic adjuncts (e.g., PRP, stem cells) under research

Orthopaedic Relevance:

• Central to sports injuries, osteoarthritis, and joint preservation surgery

• Success of cartilage procedures depends on patient age, defect size, and mechanical alignment