Articular cartilage refers to the cartilaginous connective tissue covering the bony surfaces of diarthrodial joints.
On this page:
Gross anatomy
Articular cartilage is hyaline cartilage and is composed of a low density of chondrocytes surrounded by an abundant extracellular matrix. The extracellular matrix consists of water, collagen, an amorphous ground substance of proteoglycan aggregates, rich in glycosaminoglycans and a small amount of non-collagenous proteins and glycoproteins 1,2. Like other types of hyaline cartilage, it is characterized by a simple anatomical structure and a firm consistency. It does not contain any blood vessels, lymphatics or nerves 1,2, and it has a translucent, glassy pearl-white to bluish appearance 2.
Function
The main function of hyaline cartilage is the absorption and distribution of loading forces and, to a lesser degree, tensile forces while providing low friction on its surface to facilitate smooth movements 2,3. The water that is bound within the extracellular matrix has an important role in buffering compressive loading forces. When applied, these cause an immediate rise in interstitial fluid pressure and squeeze the water slowly out of the extracellular matrix creating a large frictional resistance due to its low permeability; once the compressive loading forces subside, the interstitial fluid flows back in 1.
Relations and/or boundaries
Articular cartilage is surrounded by joint fluid produced by the adjacent synovial membrane, or it is bordering the subchondral bone plate. It is in close contact with other cartilaginous structures in some joints as menisci or articular disks, which consist of fibrocartilage.
Histology
The articular cartilage is characterized by the following zonal anatomy 1,3:
- superficial or tangential zone (~10-20% of the articular cartilage volume):
- dense alignment of collagen fibers parallel to the surface
- a relatively high number of flattened ovoid chondrocytes
- highest water content
- protection of deeper layers from tensile forces as shear stress
- intermediate or transitional zone (~ 40-60% of articular cartilage volume):
- oblique or haphazard orientation of collagen fibers
- abundant proteoglycan content
- a low density of round or spherical chondrocytes
- resistance of compressive loading forces
- deep or basal zone (~30% of the articular cartilage volume)
- perpendicular arrangement of collagen fibers to the articular surface and subchondral bone
- highest proteoglycan content, lowest water concentration
- columnar alignment of the chondrocytes parallel to the collagen fibers and perpendicular to the surface
- resistance of compressive loading forces
- tidemark: separates the deep zone from the calcified zone
- calcified zone
- scarce and hypertrophic chondrocytes
- absence of proteoglycans
- fixation of the perpendicularly arranged collagen of the deep zone to the subchondral bone plate
Besides, the extracellular matrix of the articular cartilage is characterized by different regions based on chondrocyte proximity, which show distinctive features concerning content and fiber structure 1:
- pericellular matrix (adjacent to the chondrocytes)
- mainly comprises proteoglycans glycoproteins and non-collagenous proteins
- considered to play a role in signal transmission
- territorial matrix (surrounding the pericellular matrix)
- composed of a basket-like network of collagen fibrils around the chondrocytes
- thought to protect the chondrocytes against mechanical stress
- interterritorial region
- mainly characterized by the respective zonal anatomy
- the main contributing factor to the biomechanical properties of articular cartilage
Radiographic features
Plain radiograph
Hyaline cartilage cannot be differentiated from joint fluid or adjacent soft tissues on plain radiographs unless calcified.
Ultrasound
Articular cartilage is characterized by low echogenicity.
CT
On CT, articular cartilage features a slightly higher density than normal soft tissue.
MRI
Hyaline cartilage displays a homogeneous appearance on conventional MRI sequences with signal intensity similar to or slightly hyperintense to muscle due to high water content.
Signal characteristics
- T1: intermediate signal intensity
- T2: low signal intensity
- PD: intermediate to mildly hyperintense signal intensity
- PDFS/STIR: mildly hyperintense signal intensity
There are several functional approaches for the evaluation of articular cartilage available which target the following extracellular matrix components 4:
- collagen: T2 mapping, DWI/DTI
- proteoglycans: T1 rho, dGEMRIC
- glycosaminoglycans: sodium imaging, gagCEST, dGEMRIC
However, those techniques are still more subject to research than to clinical application. This is due to demanding technical requirements (e.g. specific RF pulse, 7T magnet), contrast media-related issues and low reproducibility between different vendors 4,5.
Related pathology
Pathologies related to articular cartilage include the following 2: