• Articular cartilage is made up of hyaline cartilage that lines the bone of synovial joints. It is attached directly to the subchondral surface of the bone. (11)
  
  
  Microscopic picture of hyaline articular cartilage
  Permission granted by Mr. Pimpalnerkar of Orion Clinic
  
  
• Healthy articular cartilage is capable of withstanding high compressive and shear loads during strenuous activities. The cartilage allows for the easy movement of joint surfaces due to its frictionless properties. If a healthy knee joint is deprived of motion, synovial fluid builds up and is stagnant, thus depleting the cartilage of nutrients. This is why the rehabilitation schedule is important to follow. (6, 11)
• Traumatic knee articular cartilage defects are common. In one study of 25, 124 knee arthroscopies, 67% showed localized focal osteochondral or chondral lesions. (19)
• Articular cartilage injuries can occur from impact and excessive repetitive usage of the area. Injuries can also occur due to obesity, and immobilization. However, sports activity, without trama, does not increase ones risk of developing osteoarthritis or another defect. (5, 6)
• Symptoms of articular chondral lesions: (1, 5)
  • Pain
  • Joint dysfunction and effusions
  • Swelling
  • Weakened or giving out
  • Locking
  • Limping
  • MRI confirmation
• Possible locations of chondral lesions: (23)
• Trochlear Groove
• Medial Femoral Condyle
• Lateral Femoral Condyle
• Lateral Tibial Plateau
• Medial Tibial Plateau
• Patella
  
  
  
  Diagram of the knee
  A reproduction from Gray's Anatomy, provided by the public domain of Wikimedia Commons
  

• Severity of Chondral lesions: (1)
  • Grade 1- Articular cartilage appears normal, it is difficult to locate the borders of lesion and healthy cartilage
  • Grade 2- Mild fibrillation, discoloring, and the cartilage is softer than normal
  • Grade 3- Deep fissures or cobblestone surface but no exposed bone
  • Grade 4- Full-thickness cartilage loss with exposed subchondral bone
• Full-thickness cartilage lesions of the distal femur extend into the subchondral bone causing pain, loss of function, and osteoarthritis. These defects occur due to the detachment of osteochondritic dissecting flaps, fractured osteeochondral fragments, or from chronic wear of degenerative articular cartilage. (9)These injuries are common in football, basketball, and rugby players and do not heal without surgical intervention. (1, 9, 17, 19)
• Articular cartilage has a limited capacity for healing and regeneration. Animal studies have shown that articular cartilage lesions that do not penetrate the subchondral bone do not heal and typically progress to degeneration of the articular surface. However, there is an early reparative response by the chondrocytes when the lesion does not penetrate the bone. The response is insufficient to provide cells and matrix to repair even small lesions. After 1-2 weeks this natural response stops and surgical intervention is needed. (1, 9, 24)
• Lesions that do penetrate the subchondral bone do undergo some repair through the formation of fibrous, fibrocartilaginous, or hyaline-like cartilaginous. This depends on the age of the patient, the size, and location of the lesion. These reparative tissue experience extensive degeneration in about half of the full-thickness defects. Thus surgery is needed as the body's response is insufficient. (12, 24)
• Although the success of microfracture does not depend on the size of the lesion, the status of the lesion and surrounding cartilage depends on the size of the defect. Smaller lesions do not progressively worsen and do not appear to lead to further cartilage damage. Larger lesions do progress and usually result in osteoarthritis. The size of the lesion is a main determinant in deciding if microfracture is necessary. (1)
  
  Table 1: Chondral and osteochondral injuries
  

  Injury	Clinical Presentation	Repair Response	Potential for Healing
  Damage to chondral matrix and/or cells and/or subchondral bone without visible disruption of the articular surface	No known symptoms, although subchondral bone injury may cause pain; Inspection of the articular surface and current clinical imaging methods for articular cartilage cannot detect this type of injury; Imaging of subchondral bone by scintigraphy or magnetic resonance imaging may show abnormalities	Synthesis of new matrix macromolecules; Cell proliferation	If the basic matrix structure remains intact and enough viable cells remain, the cells can restore the normal tissue composition; If the matrix and/or cell population sustains significant damage or if the tissue sustains further damage, the lesion may progress to cartilage degeneration
  Cartilage disruption (chondral fractures or ruptures)	May cause mechanical symptoms, synovitis, pain, and joint effusions	No fibrin clot formation or inflammation; Synthesis of new matrix macromolecules and cell proliferation, but new tissue does not fill the cartilage defect	Depending on the location and size of the lesion and the structural integrity, stability, and alignment of the joint, the lesion may or may not progress to cartilage degeneration
  Cartilage and bone disruption (osteochondral fractures)	May cause mechanical symptoms, synovitis, pain, and joint effusions	Formation of fibrin clot, inflammation, invasion of new cells, and production of new chondral and osseous tissue	Depending on the location and size of the lesion and the structural integrity, stability, and alignment of the joint, the lesion may or may not progress

  Adapted from Joseph Buckwalter, (5)
  

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