The Illustrative Anatomy and the Histology of the Healthy Hyaline Cartilage

Hyaline articular cartilage is a 2–4 mm thick, avascular and aneural tissue, consisting of chondrocytes (only 1–2% of the total cartilage volume) embedded in an extracellular matrix [1, 2]. Its principal function is to provide a smooth, lubricated surface for articulation and to facilitate the transmission of loads with a low frictional coefficient [1]. The extracellular matrix contains mainly water (>70%) and two major organic components: type II collagen and the proteoglycan aggrecan, which provide tensile strength and compressive resilience to the tissue [2–4]. Histologically, the articular cartilage can be divided into the superficial, transitional, and deep (radial) zones based on the general orientation of the collagen fibrils, the morphology and arrangement of the chondrocytes, and the staining properties of the matrix [4–6]. Between the deep zone and the calcified cartilage layer, a radiologically denser, 5 μm thin discrete band of mineralized cartilage, called tidemark, can be found. Located below the tidemark, the calcified cartilage is a 20–250 μm thick transitional zone, which reduces the “stress riser” between the much stiffer bone and cartilage. Its physiological function is to form an interface between the cartilage and the bone for the transmitting forces, attaching cartilage to bone, and limiting diffusion from the bone to the deeper layers of cartilage [4, 7]. Under the calcified cartilage lies the subchondral bone which provides mechanical and metabolic support to the articular cartilage, absorbs shock, and maintains the joint shape [4, 5]. The subchondral bone consists of two parts with different macroscopic structures: the subchondral bone plate and the subarticular spongiosa [4]. The subchondral bone plate is a dense bony lamella, similar to the cortical bone of other bony structures, separating the calcified cartilage from the marrow cavity. The subarticular spongiosa is a more porous and metabolically active network of trabecular bone containing innervation and blood vessels [4]. This chapter describes the normal gross anatomy and histological characteristics of the hyaline articular cartilage that give the tissue its extraordinary load-bearing characteristics.