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If it were not for the joints, the bones would not have any mobility.  Thanks to the joints various parts of the body can bend.  There are three types of joints.

Movable:  also called synovial or diarthrosis, are the joints that have the most range of movement.  They are the ones that connect the limbs with the torso, the shoulders and the hips.

Semi-movable:  also called amphiarthrosis, they permit slight mobility, like the joints of the vertebrae.

Fixed:  also known as synarthrosis. Most of them are found in the skull and do not need to move because their main function is to protect internal organs.

The most common, the synovials, are characterized by having certain elements in common, these are:

Bone surfaces, which are the ends of the bones involved in the determined joint.

Cartilage joint , a soft and smooth tissue made up of collagen, which increases the ease of motion of the joints.

Synovial Membrane, a layer that internally covers all of the joints and secrets synovial fluid, a lubricant for the joints.

Meniscus, flat semi-moon shaped structures, protecting and cushioning the cartilage, among other things.

Ways of joining, made up of collagen fibers, forming complete envelops called joint capsules and a form of reinforcement called ligaments.

Types of synovial junctions

These are according to the type of specific movement the joints perform:

Spheroidal joint: the head of a bone fits into the concave cavity of another. They are found in the hips and shoulders.
Saddle joint: they only exist at the base of the thumbs and they allow movement in two directions (front to back and side to side).
Hinge joint: like the ones in the elbow and fingers, they are less mobile and allow movement in one direction.
Pivot joint: is the one in which an osseous cylinder spins around its own axle, being in contact with another surface that forms a ring for it (part bone, part ligament) like the superior radius-cubital joint (forearm).
Ellipsoidal joint: the end of an ovoid (egg) shaped bone, moves in an ellipsoidal cavity. They are found in the radius of the forearm and the scaphoid bone of the hand.
Sliding joint: some bone surfaces are almost flat and slide one on top of the other. They are found at some joints of the hand and feet.