Fibrillar wear of the articular cartilage. The yellowish bone is showing through on the top of the tibia.
Articular cartilage damage and arthritis
Articular cartilage is also called joint cartilage or hyaline cartilage. It is damage to the articular cartilage which is called 'arthritis' and in the knee all efforts are geared towards preventing any damage or breakdown of this all-essential joint surface, as it has a poor blood supply and does not heal well if damaged.
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Imaging of articular cartilage. Paunipagar BK and Rasalkar DD. Indian J Radiol Imaging. 2014 Jul-Sep; 24(3): 237–248.