Table 1.

Summary of the Characteristics of the Four Main Zones of Articular Cartilage.

Zone	Characteristics of Collagen	Characteristics of Chondrocytes	Main functional Characteristic
Superficial (tangential) Zone	Primarily type II and IX collagen packed tightly and aligned parallel to articular surface	Chondrocytes mainly flattened	Protects the deeper layers from shear stress and is in contact with the synovial fluid
Middle Zone	Thicker fibrils arranged obliquely surrounded by a large number of hydrated proteoglycans	Chondrocytes are spherical and have a very low density	Resists compressive forces and functions as a bridge between the superficial and deep zones
Deep (Basal) Zone	Large diameter fibrils arranged radially and perpendicular to the articular surface with a large number of dehydrated proteoglycans	Chondrocytes are columnar and run parallel to the collagen fibers and perpendicular to the joint line	Highly resistant to compressive forces and is the last zone before the tidal mark
Calcified Zone	Fibrils arranged perpendicular to the articular surface with a large number of proteoglycans	Very few chondrocytes and most are hypertrophic	Greatest resistance to compressive forces and functions mainly to anchor the cartilage to the bone