What parts make up a synovial joint?
A joint is where two bones come together. We have different types of joints in the body. One important way that joints are classified is based on the amount of movement that is possible at that joint. The amount of movement is different depending on how the joint is connected.
We have three main types of joints based on the amount of movement that is possible:
- Synarthrotic joints
- Amphiarthrotic joints
- Diarthrotic joints
Synarthrotic joints are joints where little to no movement happens. These joints are connected with a fibrous connection that doesn’t allow for essentially any movement. One of the best examples of these type of joints are the sutures between the bones in our skull. It would be problematic if there were lots of movement happening between the bones of the skull!
Amphiarthrotic joints are joints where very small amounts of movement can happen. These joints are joined with a cartilaginous connection that allows for some small amounts of movement. One example of an amphiarthrotic joint is the pubic symphysis, the joint that connects the two sides of the pelvis on the anterior side. Another example is the joint created by the disc between each vertebra in the spine.
Diarthrotic joints or synovial joints
Finally, we have diarthrotic joints. These are the joints we’re often most familiar with where most movement happens in the body. There are lots of examples of diarthrotic joints including: the glenohumeral joint (the shoulder), the femorotibial joint (the knee), and the acetabulofemoral joint (the hip).
These joints have a special construction that allows for lots of movement to happen there.
The end of each bone that comes together at a synovial joint has a layer of smooth cartilage called articular or hyaline cartilage. A structure called a joint capsule encapsulates the area where the two bones come together to make a joint. The joint capsule is lined with a membrane, called the synovial membrane, that produces a fluid, called synovial fluid, which lubricates the joint where the two bones meet. This structure allows for the movement that we are familiar with at our joints. As the presence of synovial fluid is unique to diarthrotic joints, they are also sometimes called synovial joints.