Articular Cartilage Definition
Articular cartilage is found only in diarthroidal joints (synovial joints) and is comprised of hyaline cartilage – a particularly smooth type of cartilage that allows for easy articulation, increased weight distribution, and shock absorption
What is Articular cartilage?
Articular cartilage is a type of cartilage that is found only in synovial joints, also known as diarthroidal joints. This type of cartilage is composed of hyaline cartilage, which is a very smooth and elastic type of cartilage that allows for easy movement and articulation of joints. It also helps distribute weight and absorb shock, especially in the lower limbs.
As a person ages or engages in intensive sports activities, articular cartilage may degenerate, leading to inflammation and friction between the surfaces of two bones in the joint. This can cause pain and limited mobility.
However, it’s important to note that any pain associated with degenerated articular cartilage is not caused by damage to the cartilage itself, as cartilage does not have a nerve network. Instead, the pain is caused by the nerves surrounding the bony layers which are no longer protected by the cushioning effect of cartilage.
Articular cartilage is usually found in layers between 2 and 4 mm thick. Like all types of cartilage, it lacks blood vessels and lymph vessels, which creates a slow metabolic environment.
Chondrocyte proliferation and apoptosis (cell death) occur at much lower rates in articular cartilage compared to other types of cartilage.
Additionally, the low levels of oxygen in the joint mean that chondrocytes primarily depend on anaerobic metabolism. Nutrients are provided directly from the synovial fluid, which bathes the joint, rather than from the perichondrium, which is absent in articular cartilage.
Articular Cartilage Function
Articular cartilage is a specialized type of cartilage that covers the ends of bones in diarthrodial joints, which are joints that allow for a wide range of movement.
Its function is based on its composition of hyaline cartilage, which has a glass-like surface and can self-lubricate via glycoproteins in the extracellular matrix.
This makes it practically frictionless and resistant to wear, much like oil added to a squeaky door hinge to prevent erosion of the touching surfaces.
The structure of articular cartilage is organized into three different zones with varying characteristics. This enables it to effectively distribute compressive forces and withstand the stresses generated during joint loading and motion.
However, if there is an abnormal movement or excessive force applied to the load-bearing cartilage, such as at the knee joint between the femur and tibia, it can cause injury to the articular cartilage. Excessive rotation, as often seen in football injuries, can lead to damage such as meniscus tears.
Another important function of articular cartilage is its ability to allow for movement on one or more planes, depending on the specific type of diarthrodial joint. This allows for smooth and efficient joint motion, which is necessary for daily activities and physical function.
Where Is Articular Cartilage Found?
Articular cartilage is found in the diarthroidal joints throughout the body. These joints are the ones that allow for movement between two bones in at least one axis. Articular cartilage is a type of hyaline cartilage that covers the surfaces of the bones within these joints, providing a smooth, low-friction surface that allows the bones to glide against each other during movement.
The term “articular cartilage” does not refer to a specific type of cartilage structure, but rather to its location within these joints. Other types of cartilage, such as fibrocartilage and elastic cartilage, can be found in other parts of the body.
Diarthroidal joints are also characterized by the presence of a synovial membrane and fluid, which helps to lubricate and nourish the articular cartilage. Synovial fluid is a thick, clear fluid that contains a variety of substances, including hyaluronic acid and lubricin, that help to reduce friction and wear on the articular cartilage.
Articular cartilage is only found in the presence of a synovial membrane and fluid, meaning that it is not present in other types of joints, such as cartilaginous and fibrous joints. These joints have very little movement and lack synovial membranes and fluid.
Planar (Gliding) Joints
Planar or gliding joints are a type of synovial joint where the articular surfaces of the bones involved are relatively flat or slightly curved and can slide or glide over one another in a variety of directions. Unlike hinge joints or ball-and-socket joints, planar joints do not permit rotation, but rather allow bones to move back and forth or side to side in a sliding motion.
Examples of planar joints in the human body include the small bones in the wrist and hand, called the carpals, which allow for fine, intricate movements of the hand and fingers. The tarsals, which are the small bones in the foot, also have planar joints that allow for gliding movement and contribute to the foot’s ability to adapt to uneven surfaces.
Another example of a planar joint is the temporomandibular joint, which is located in the jaw and connects the mandible to the temporal bone of the skull. This joint allows for the movement of the jaw in various directions, such as side-to-side and forward-and-backward movements, necessary for actions like chewing and speaking.
Hinge joints are a type of synovial joint, which means they have a lubricating fluid-filled cavity between the bones. In hinge joints, the two bones that meet have a unique shape. One of the bones has a rounded surface, while the other has a hollowed surface. This creates a type of joint where one bone remains stationary while the other moves.
Hinge joints have a very limited range of motion, allowing only for flexion and extension movements. They are primarily found in the limbs, such as the elbow and knee. The elbow joint is a simple hinge joint, with only one plane of motion, allowing for flexion and extension of the forearm.
The knee joint is a modified hinge joint, allowing for more complex movements due to the rotation of the tibia bone on the femur bone. The knee also has additional structures like ligaments and menisci that provide stability and cushioning.
Hinge joints are crucial for a wide range of activities in our daily lives, such as walking, running, and lifting. The proper functioning of hinge joints depends on various factors like muscle strength, joint stability, and flexibility.
When hinge joints are damaged or suffer from conditions like arthritis, it can limit movement and cause pain. Therefore, it is important to maintain healthy joints through regular exercise, proper nutrition, and avoiding activities that can cause joint damage.
Pivot joints are a type of joint in the body that allow rotational movement around a single axis. They are sometimes also referred to as rotary or trochoid joints. The unique feature of pivot joints is that one bone surface has a ring-like shape that fits into a corresponding notch or groove on the other bone surface. This ring-like structure allows the joint to rotate around a central axis.
There are several examples of pivot joints in the body, including the joint between the first and second cervical vertebrae, which is also known as the atlantoaxial joint. This joint allows the head to rotate from side to side. The second cervical vertebra, or C2, has a projection called the dens that fits into a ring-like structure in the first cervical vertebra, or C1, which is also known as the atlas.
Another example of a pivot joint in the body is the proximal radioulnar joint, which is located at the elbow. This joint allows the forearm to rotate around its own axis, which is important for movements such as twisting or turning the hand.
Condyloid (Ellipsoidal) Joints
Condyloid or ellipsoidal joints are a type of synovial joint found in the body, which allows for movement along two axes or planes. These joints consist of an elliptical-shaped bone surface, one of which is concave and the other is convex, and they are primarily found in the hand and wrist as well as in the foot.
The unique shape of the bone surfaces in condyloid joints allows for a variety of movements, such as flexion and extension (up and down movements) and abduction and adduction (side-to-side movements). However, rotation is not possible in these joints.
In the hand and wrist, condyloid joints are found between the distal radius bone and the bones of the hand, allowing for movements like opening and closing the hand, and moving the hand from side to side.
In the foot, condyloid joints are found between the metatarsal bones and the phalanges of the toes, allowing for movements like raising and lowering the toes and moving the foot from side to side.
Saddle joints are a type of synovial joint that allows for a higher range of non-rotary movement than condyloid joints. This type of joint is so named because the end of one bone is shaped like a saddle, while the other bone is shaped like a rider that sits in the saddle.
The joint surfaces are complementary and fit together like a puzzle, which allows for a range of movement in multiple directions.
The best-known example of a saddle joint is the joint between the thumb and wrist, which is called the first carpometacarpal joint. This joint allows for a wide range of motion, including flexion, extension, adduction, abduction, and opposition. This means that the thumb can move in many different directions, which is important for tasks like grasping and manipulating objects.
Another example of a saddle joint is the sternoclavicular joint, which is located between the sternum and the clavicle. This joint allows for movement in several different directions, including elevation and depression of the shoulder girdle, as well as protraction and retraction of the shoulder blade.
In the x-ray example given, the healthy and undamaged sternoclavicular joint of a young male is shown on the left-hand side, while the same joint after trauma and subsequent osteoarthritis is shown on the right.
Osteoarthritis is a condition in which the joint cartilage breaks down over time, leading to pain, stiffness, and reduced range of motion. The x-ray on the right shows how osteoarthritis can change the shape and structure of the joint, leading to joint dysfunction and pain.
Ball and Socket Joints
Ball and socket joints are a type of joint in the body where one bone surface is almost spherical in shape and the other bone surface is deeply concave. This allows for a maximum range of motion in multiple directions with a reduced risk of dislocation. The hips and shoulders are examples of joints in the human body that are ball and socket joints.
Unfortunately, as we age, our joints can become damaged, leading to conditions such as arthritis. In elderly populations, hip arthritis is a common problem that can cause significant pain and loss of mobility.
When arthritis affects the hip joint, the articular cartilage covering the joint surfaces wears away, leading to bone-on-bone contact and further joint damage.
To help restore function and alleviate pain, one common orthopedic surgery is a total hip replacement. This involves the removal of the damaged parts of the hip joint and replacing them with prosthetic components.
The prosthetic hip joint is designed to replicate the shape and function of a natural hip joint, with a characteristic ball and socket shape that provides a full range of motion and stability.