Articular cartilage (AC) functions as a load-bearing, low friction, and wear resistant material in diarthrodial joints. The distribution of AC matrix composition is highly depth-dependent. The fluid fraction in AC is 80% and decreases from surface to the depth of the tissue [1]. Collagen constitutes 70% of the tissue dry weight, and is highest in the superficial and deep zones and lowest in the middle zone [2]. Proteoglycans (PG’s) constitute 20–30% of the tissue dry weight. PG’s are lowest in the superficial zone, and highest in the middle zone. Although the PG content is lower in the deep zone than in the middle zone, the fixed charge density (FCD) is highest in the deep zone [3]. Apart from AC composition, its structure is also depth-dependent. In the superficial zone collagen fibers are densely packed, and are arranged parallel to the articular surface. In the middle zone collagen fibers are more randomly arranged. In the deep zone, the collagen fibers have their largest diameters and are arranged perpendicular to the subchondral bone (Fig. 1) [4].

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