Synovial Joint

Synovial Joint Definition

A synovial joint is a connection between two bones consisting of a cartilage lined cavity filled with fluid, which is known as a diarthrosis joint. Diarthrosis joints are the most flexible type of joint between bones, because the bones are not physically connected and can move more freely in relation to each other. In synarthrosis and amphiarthrosis connections between bones, the bones are directly connected with fibrous tissue or cartilage, limiting their ultimate range of motion. The image below shows several different joints in the human body, each of which are an example of a synovial joint.

Types of Synovial Joints

Structure of a Synovial Joint

Although the exact structure of a synovial joint may vary depending on the function of the joint and the animal it is in, the general structure of each joint is the same. Extending from the periosteum of the bone, an articular capsulemade of cartilage and other fibers encapsulates the entire joint. Inside the articular capsule, a synovial membrane creates a sac which holds a specialized hydraulic fluid, called synovial fluid. This fluid creates a lubricating cushion between the two bones, allowing them to glide past one another. The ends of the bones are protected in another layer of articular cartilage.

Synovial Joints structure

The exact shape of the bone is determined by evolution and the bone’s function in the animal. For instance, ball-and-socket joints have evolved in the human hip to allow humans to have full rotation of their legs. This allows us to run, dance, bend over, climb trees, and even swim.

Function of a Synovial Joint

The main purpose of a synovial joint is to allow to bones to rotate freely about each other. Some synovial joints, like the hip joint mentioned above, are meant to give the greatest flexibility around the joint. Other joints, like the joints found in the ankle, have a slightly more limited range of motion, but provide an enormous cushion for the repeated impacts from running and jumping. A synovial joint may vary slightly in function based on design, but the main purpose of a synovial joint is to provide a range of motion between two or more bones and to cushion the impacts of those bones against each other. Other types of joints provide less range of motion and are susceptible to the cartilage and fibers that connect bones directly breaking down under the stresses of motion.

Although there are many classes and types of synovial joint, the main classifications in function depend on the range of motion conferred by the synovial joint. A uniaxial joint can only move in one direction, such as the elbow. While this is slightly limited in range, it allows the muscles to make extremely powerful levers of the bones connected to these synovial joints. A biaxial joint can move in two directions, which is important for joints in the wrist and ankle. A multiaxial joint can move in a variety of directions, and is seen in the hip and shoulder synovial joints.

Related Biology Terms

• Periosteum – A special layer of tissue that covers bones, creating both new bone tissue and the connections between bones.


• Synovial Cavity – A fluid-filled cavity which exists in every synovial joint and allows the bones to glide smoothly past each other.


• Synarthrosis – A type of joint between bones which is completely immobile, such as those in the skull.


• Amphiarthrosis – Joints which allow a range of motion, but nonetheless keep the bones immobile, like the joints between most vertebrae in a spine.

Quiz

1. One of the benefits of a synovial joint over other types of joints is the ability of the joint to recover after trauma. A synovial joint can undergo an event called dislocation, when the bones become misaligned. However, the bones can usually be forced back into place. Why is this not possible with more fixed forms of joints?
A. Only synovial joints connect bones without firmly connecting their ends
B. More fixed types of joints cannot heal after a tear
C. The fiber used in fibrous joints cannot reform

Answer to Question #1

A is correct. Because of the disconnected nature of synovial joints, the bones do not actually touch. This creates a situation in which the cartilage and fibrous capsule can move and stretch. When bones are dislocated, they are displaced from the proper position they rest in. Other types of joints, if over extended or pulled, will tear or rupture the cartilage and fibers that connect the bones. If tearing happens, the cartilage can heal, but it will still take a lot longer than simply popping the joint back into place.

Synovial Joint