Superficial zone of articular cartilage

Figure 9.6. Diagram illustrating the pretension present in the superficial zone of articular cartilage. Normal articular cartilage shown at the top is loaded in tension across the surface like a drumhead that is pulled taut over a drum. When a piece of cartilage is cut from the surface, it curls as a result of release of this tension, as shown in the lower diagram. The presence of tension in the superficial zone makes articular cartilage behave like a drumhead, allowing compressive forces applied to the surface at specific points to be distributed across the surface to lower local stresses. The presence of tension on the chondrocytes in the superficial layer may be important to limit inflammation and support reparative processes by stimulating mechanochemical transduction.

Wise JK et al (2009) Chondrogenic differentiation of human mesenchymal stem cells on oriented nanofibrous scaffolds engineering the superficial zone of articular cartilage. Tissue Eng A 15(4) 913-921... 

Figure 11.9 Schematic of articular cartilage structure showing the superficial, middle and deep zones, the tidemark boundary between the noncalcified and calcified cartilage layer, and the subchondral bone that underlies the articular cartilage.

Figure 3.23. Diagram illustrating the zonal structure of articular cartilage. The superficial zone contains aligned collagen fibrils the intermediate zone contains unoriented collagen fibrils the deep zone contains collagen fibrils perperdicular to the subchondral bone.

The overall structure of articular cartilage is analogous to a jelly-filled balloon. The PG-rich middle zone is osmotically pressurized, with fluid restrained from exiting the tissue by the dense coUagen network of the superficial zone and the calcified structure of the deep bone. The interaction between the mechanical loading forces and osmotic forces yields the complex material properties of articular cartilage. 

Since hydrogen peroxide generated within the mitochondria of chondrocytes can freely permeate through the chondrocyte cell wall, one should admit the presence of in nil (deep, middle, and superficial) zones of the articular cartilage (Figure 4). The higher the tension, the greater is the content of H Oj and vice-versa. 

Khan, l.M. et al.. Expression of clusterin in the superficial zone of hovine articular cartilage. Arthritis Rheum, 2001. 44(8) 1795-9. 

Figure 10.1 Cross-sectional schematic of articular cartilage. The tissue consists of the superficial, middle/transitional, and deep zones with a distinct tidemark of calcified cartilage and the underlying subchondral bone (from top to bottom). Both cellular organization and collagen-fiber orientation are shown with respect to the zonal structure of cartilage.

Chondrocytes, the only cell type found in normal mature articular cartilage, are responsible for the maintenance and repair of this tissue, which happens to be the site of pathologic matrix remodeling and degradation in arthritis [51]. Indeed, chondrocyte apoptosis is a feature of osteoarthritic cartilage and is closely associated with extracellular matrix degradation. Fibrillated cartilage from osteoarthritic joints has been found to contain apoptotic cells in both the superficial and mid zones. In... 

Khalafi, A., Schmid, T. M., Neu, C., and Reddi, A. H. 2007. Increased accumulation of superficial zone protein (SZP) in articular cartilage in response to bone morphogenetic protein-7 and growth factors. 

In this study, a 5% confined compression test was carried out numerically. 5 % compression was chosen as the purpose of this study was to understand why articular cartilage chondrocytes may die when exposed to impact loading but survive when exposed to muscular loading of similar magnitude. Therefore, the strain magnitude was chosen very small so that it should not be injurious by itself. Previous experimental studies showed that 5% compression applied to cartilage tissue in 0.3s resulted in cell death in the superficial zone [7]. This made the 5% compression a reasonable choice for the current study. 

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