Types of synovial joints
The synovial joints are those in which the bone elements that contact each other are separated by a cavity. They are surrounded by a sleeve-shaped joint capsule that encloses the synovial cavity and joins the bones of the joint.
Characteristics of synovial joints
First, they have a layer of cartilage that covers the joint surfaces of the skeletal elements. In this way, the bone surfaces do not contact each other directly. Therefore, when these joints are seen on radiographs, there seems to be a large space between the adjacent bones.
A second characteristic of synovial joints is the presence of an articular capsule consisting of an internal synovial membrane and an external fibrous membrane.
The synovial membrane
This membrane is attached to the margins of the articular surfaces at the interface between the cartilage and the bone and wraps the joint cavity.
The synovial membrane It is highly vascularized and produces synovial fluid, which accumulates in the joint cavity and provides lubrication to the joint surfaces.
There are also closed sacs of synovial membrane outside the joints, where they form synovial pouches or tendon sheaths. The pockets are often interposed between structures such as tendons and bone, tendons and joints, or skin and bone, and reduce the friction of a structure when moving over another. The tendon sheaths surround the tendons and also decrease friction.
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The fibrous membrane
It is formed by dense connective tissue and surrounds and stabilizes the joint. Parts of the fibrous membrane can be thickened to form ligaments, which further stabilize the joint. Ligaments external to the capsule usually provide additional reinforcement.
Other characteristic structures of synovial joints
Another common feature of the synovial joints is the presence of additional structures within the area encompassed by the capsule or synovial membrane, such as:
- Articular discs: usually fi brocartilage compounds. They absorb the compression forces, adjust the changes in the contour of the joint surfaces during movements and increase the range of movements that can occur in the joints.
- Fat pads: fat pads are usually found between the synovial membrane and the capsule. In addition, they enter and leave these areas as the joint contour changes during movement.
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Types of synovial joints
- Flat joints: allow sliding movements when one bone moves on another. Among them, there is the acromioclavicular joint that is in the shoulder.
- Hinge joints: are the joints that allow the articulation to move around a transverse axis. In this way, they regulate fl exion and extension movements. A hinge joint is that of the elbow.
- Pivot joints: they are the ones that allow the movement around an axis that crosses in a longitudinal direction the diaphysis of the bone. The pivotal synovial joints regulate the rotation.
- Bicondylar joints: they mainly allow movement around an axis, with limited rotation around a second axis. They are formed by two convex condyles. These are articulated with concave or flat surfaces. In this type is the knee joint.
- Condylar joints: are those that allow movement around two axes that are at right angles to each other. In this way, they regulate movements of flexion, extension, abduction and circumduction. This is the case of the wrist joint.
- Joints in saddle: they allow movement around two axes that are at right angles to each other. Its name is due to the fact that the articular surfaces are saddle shaped. They regulate fl exion, extension, abduction, adduction and circumduction movements.
- Spherical joints: they allow movement around multiple axes. These articulations regulate fl exion, extension, abduction, adduction, circumduction and rotation movements. In this type of synovial joint the hip is included.
If it were not for the joints, the bones of our body could not have mobility. Therefore, thanks to them, we can perform the different movements such as bending, rotating and flexing the different limbs of the body.