To observe the morphological feature of calcified cartilage zone (CCZ) in mild to moderate degeneration of cartilages from patients with knee osteoarthritis (OA), revealing the pattern of CCZ during OA progression and its correlation to the surrounding structures.

Osteochondral specimens were collected from the center of the lateral tibial plateau of 42 OA patients undergoing total knee replacement. Sections were stained with hematoxylin-eosin and Safranin-O/Fast green. Morphological parameters (thickness of CCZ, hyaline cartilage, and subchondral bone, roughness of tidemark and cement line, number of tidemarks and chondrocytes in CCZ, area and number of vascular channels in CCZ) of OARSI grades 0∼4 cartilages were measured.

The thickness of CCZ increased with grading except in grade 2. This changing trend of CCZ was in accordance with chondrocyte number and area of vascular channel. The roughness of cement lines increased with the grading, and was correlated with the thickness of subchondral bone. The roughness of tidemarks was associated with thickness of hyaline cartilage in grade 0 to grade 3.

In mild OA, the thickness of CCZ was increased at first and then decreased, the roughness of tidemark and cement line was nearly unchanged, which suggests that the pathological change of CCZ is reversible. However, in moderate OA, the thickness of CCZ, the roughness of tidemark and cement line were progressively increased, which suggests that the pathological change of CCZ is irreversible.