(Fig 5-A.01) Articular Cartilage: Structure, Composition & Tidemark: Part 1

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📌 Articular Cartilage: Structure, Composition & Tidemark

 

✅ Hyaline Cartilage Composition

• Healthy articular cartilage is primarily composed of an extracellular matrix that includes water (65%–85%), collagen (10%–20% by wet weight), and proteoglycans (5%–10% by wet weight). 
• The chondrocytes, which comprise 1%–5% by volume, are sparsely distributed within this matrix.

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✅ Four Layers of Articular Cartilage

1️⃣ Superficial Zone:

• This zone consists of a thin mat of densely packed, interwoven collagen fibrils aligned roughly parallel to the articular surface, resisting shear forces.

 

2️⃣ Middle and Deep Zones:

• The middle and deep zones feature densely packed collagen fibrils in a columnar organization that handles compressive loads. In the deep zone, these fibrils radiate from the bone like blades of grass and curve through 90° arcs in the middle zone.

 

3️⃣ Deepest Calcified Cartilage Layer:

• Located at the interface with the subchondral bone plate, this layer of compact cortical bone overlies the cancellous marrow-containing trabecular bone. 
• In imaging, the calcified cartilage and subchondral bone appear as a single low-signal-intensity band, often referred to as the subchondral bone plate.

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✅ Subchondral Bone

1️⃣ Subchondral Bone Plate

• The subchondral plate comprises compact subchondral bone and calcified cartilage, collectively termed the subchondral plate. 
• This plate is not uniform in thickness; it is thicker in weight-bearing areas and thinner in the peripheral regions of the condyle.

 

2️⃣ Subchondral bone

• The subchondral bone consists of the subchondral bone plate (or articular bone plate) and the subarticular spongiosa. 
• In radiology, "subchondral bone" or "subchondral region" generally refers to the subchondral plate and nearby cancellous bone. 
• Osteochondral lesions encompass diseases affecting both the articular cartilage and subchondral bone. 
• The thickness of the bone plate varies both within and between joints, with CT being the best assessment method.

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✅ Tidemark

• The tidemark is the interface between the calcified cartilage and the overlying noncalcified cartilage. 
• It is vulnerable to damage from shear forces, potentially resulting in cartilage delamination. 
• Collagen fibrils pass only into this calcified layer of cartilage, not into the bone plate.

 

References

1. RadioGraphics 2022; 42:1457–1473.
2. RadioGraphics 2018; 38:1478–1495.
3. Jpn J Radiol. 2022 May;40(5):443-457.

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#ArticularCartilage, #OsteochondralUnit, #HyalineCartilage, #Chondrocytes, #ExtracellularMatrix, #SubchondralBone, #Tidemark, #Radiology, #CartilageHealth, #JointFunction