The structure of the extracellular matrix of articular cartilage varies considerably with age. These changes are attributable to variations in molecular abundance and structure, and they can affect all the matrix components, but none more so than the proteoglycans. Some of these changes are attributable to variations in synthesis whereas others are attributable to variations in degradation, some of the changes occur during juvenile development whereas others occur throughout life, and some of the changes are beneficial to cartilage function whereas others are detrimental. These variations result in a cartilage that not only changes in its phenotype with age, but also in one whose functional properties are changing continuously throughout life. In a similar manner, the cartilage formed during repair also may show considerable variation in structure and function, depending on whether tissue is being replaced or regenerated and whether mature or immature cells are being used. Because all cartilage is not created equal, different repair techniques may not be equally efficacious.

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