Cartilage Injuries and Healing

Courtesy: Prof Nabil Ebraheim, University of Toledo, Ohio, USA

Layers of the Articular Cartilage and Cartilage Healing

Introduction: Does Articular Cartilage Heal After Injury?

• Articular cartilage has a very limited capacity for healing.

• The ability of cartilage to heal depends on the depth of the injury and whether it reaches the underlying bone.

• Because articular cartilage is avascular, injuries confined to cartilage alone generally do not heal.

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Structural Anatomy of Articular Cartilage

When examining a joint surface in detail, the following structures are identified:

• Articular cartilage

• Subchondral bone

• Cancellous bone

Articular cartilage itself is composed of four distinct layers:

1. Superficial (Tangential) Layer

2. Middle (Transitional) Layer

3. Deep (Radial) Layer

4. Calcified Zone

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The Tidemark

• The tidemark represents the boundary between:

  • Uncalcified cartilage (deep layer)

  • Calcified cartilage

• It separates the deep zone from the calcified zone.

• This junction is critical in determining the healing potential of cartilage injuries.

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Collagen Composition of Cartilage

• Normal articular cartilage is composed predominantly of Type Two collagen.

• Fibrocartilage, which forms during repair after certain injuries, is composed mainly of Type One collagen.

• Fibrocartilage is mechanically inferior to native articular cartilage.

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Healing Response Based on Depth of Injury

1. Minimal Superficial Abrasion or Laceration

• These injuries involve only the superficial layer of cartilage.

• There is:

  • No hemorrhage

  • No inflammatory response

  • Minimal or no repair

• Limited chondrocyte proliferation may occur near the injury site.

• These lesions do not heal due to lack of blood supply.

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2. Partial Thickness Cartilage Injury (Above the Tidemark)

• Injuries involving up to fifty percent of cartilage thickness:

  • The remaining cartilage cannot adequately support mechanical stress.

  • Progressive degeneration occurs.

• The damaged cartilage may transform into fibrocartilage, which is inferior in quality.

• Chondrocytes may proliferate and synthesize matrix at the edges of the defect only.

• There is no true repair of the defect.

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3. Full Thickness Cartilage Injury Without Bone Involvement

• Lesions that do not cross the tidemark:

  • Do not reach the subchondral bone

  • Do not access circulation

• These injuries are unlikely to heal.

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4. Osteochondral Injury (Crossing the Tidemark)

• When the defect extends below the tidemark into the subchondral bone:

  • Blood supply is accessed

  • Mesenchymal stem cells are recruited

• Healing occurs through the formation of fibrocartilage.

• The repair tissue contains abundant Type One collagen.

• Although healing occurs, the resulting cartilage is biomechanically inferior to native articular cartilage.

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Key Characteristics of Deep Cartilage Lesions in Adults

• Lesions crossing the tidemark:

  • Heal by fibrocartilage formation

  • Contain predominantly Type One collagen

• The repair tissue lacks the durability and resilience of native articular cartilage.

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Summary

• Articular cartilage lesions that do not penetrate the subchondral bone do not heal due to the avascular nature of cartilage.

• Partial thickness cartilage injuries result in limited cellular response without true repair.

• Lesions that penetrate the subchondral bone can heal by recruiting mesenchymal stem cells.

• Healing in such cases occurs through fibrocartilage formation, which is structurally and mechanically inferior to normal articular cartilage.