Joints are the enabler of motion between two bones. For example, during running, the knee and ankle joints are in coordinated motion to reduce the impact sustained during landing. But, what is the element in the joint that reduces and sustains the vertical and lateral forces typical of running? It is the articular cartilage.
Cartilage comprises a meshwork of proteins, collagen and other biomolecules clustered around cells known as chondrocytes which, on the evolutionary timescale, organizes into sheet like structures suitable for absorbing the lateral forces associated with joint movement. In the case of the articular cartilage, another layer of chondrocytes lie below the first layer that helps absorb lateral forces. Specifically, the second layer of chondrocytes are vertically aligned and is capable of sustaining vertical loads better. Thus, comprising two layers of cells capable of absorbing different types of forces, both vertical and lateral, articular cartilage is the key component in knee joints that help maintain running gait and absorb impact.
Despite the presence of articular cartilage for sustaining the impact that inevitably comes from running, inappropriate running style and landing can lead to disproportionate amount of stress on specific part of the knee which, in turn, could lead to hairline fractures in the articular cartilage surface. Typically, having a smooth surface in aid of movement between the two bones comprising a joint, articular cartilage are avascular (i.e., without blood vessels), and thus, any injuries to the cartilage would not heal in a manner enabling total regeneration of the original smooth cartilage. What would more likely happen is the generation of fibrocartilage, which can be construed as a poorer form of cartilage. But what is fibrocartilage and how different it is from articular cartilage?
Fibrocartilage comprises damaged cartilage with layers of fibroblasts, and has much poorer lubricating properties compared to articular cartilage, which delivers best in class performance in running and normal walking. Generated in response to injuries to the articular cartilage, but whose regeneration is impeded by the avascular nature of cartilage, fibrocartilage is not endowed with the structural features useful for reducing impact stress.
Being architecturally smooth and capable of enabling movement between two bones such as those in the knee joint, articular cartilage plays critical roles in absorbing forces during high impact sports, for example, running. Although evolutionary selection forces have generated a design architecture useful for sustaining both lateral and vertical forces through a two layer chondrocytes cell sheet arrangement, articular cartilage is not immune to impact associated injuries such as hairline fractures typical of localized forces on some part of the smooth lubricating surface. Having sustained an injury, what are the prospect of full recovery typified by the regeneration of the entire impact absorbing surface? Avascular in nature, lack of blood vessels meant that delivery of nutrients to the injured sites would be hampered; thus, severely constraining the regeneration of the articular cartilage. What typically transpire is the formation of cell sheets of random shapes and arrangement around the injured sites after months of recovery. Known as fibrocartilage, this poorer form of cartilage is unable to absorb significant amount of impact and could easily lead to another more serious injury to the cartilage and joint. Overall, being able to absorb the stress and forces associated with high impact sports such as running, articular cartilage serves as the first line of defense against serious knee injuries due primarily to its shock absorbing properties that prevents the transmission of vertical and lateral forces to other parts of the joint.
Category: sport science, sport medicine,
Tags: articular cartilage, fibrocartilage, vertical and lateral forces, avascular, chondrocytes,
Acknowledgement: Ng Wenfa thank Seah Kwi Shan for co-authoring this blog post.