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A joint connects two bones in the skeletal system. Joints can be classified in two ways – the type of tissue present, or the degree of freedom permitted.
There are three tissue types: Fibrous, cartilaginous, and synovial. Fibrous joints connect bones with fibrous tissue, typically because the joint needs to be stable over a range of movement. Cartilaginous joints connect bones with cartilage, either fibrocartilage or hyaline cartilage. Finally, synovial joints are articulating surfaces enclosed within a fluid-filled joint capsule.
The classifications based on degree of movement are the following: synarthrosis if the joint is immovable, amphiarthrosis if the joint is slightly moveable, and diarthrosis if the joint is freely moveable.
Let’s look at examples of each of these types of joints!
Fibrous joints include sutures, gomphoses, and syndesmoses. Sutures are an example of synarthrosis. The only example of such joints are between bones of the skull. The soft fontanelle between cranial bones fuse by three years of age, and then ossify. The fontanelle are there for the birth process, when the skull deforms as the baby passes through the birth canal, and so the brain and skull can grow during the baby’s first year.
Gomphoses are also immovable. The one example of this type of joint is where the teeth articulate into the dental alveoli of the maxilla and mandible, bound by strong periodontal ligament.
Syndesmoses are amphiarthroses, so they are slightly moveable joints. They are held together by an interosseous membrane. One example is the middle radioulnar joint.
There are two main types of cartilaginous joints – synchondroses, or primary cartilaginous joints, and symphyses, or secondary cartilaginous joints.
Synchondroses are immovable joints connected by hyaline cartilage, such as the joint between the diaphysis and epiphysis of a growing long bone. Symphysis joints are slightly moveable, connected by a layer of fibrocartilage. An example is the pubic symphysis.
Synovial joints are the most common type of joint in the body. The articulating surfaces of these free-moving joints have no direct contact with each other. Except for the temporomandibular joint, which is lined with fibrous cartilage, synovial joints are lined with hyaline cartilage. A capsule encases the joint cavity, and a synovial membrane covering the capsule’s interior secretes synovial fluid that lubricates the joint. This reduces the friction of bone movement. Joints are further reinforced by tendons and ligaments, as well as by skeletal muscle.
Synovial joints can be classified based on the movements they are allow and the shape of their articulating surfaces into hinge, saddle, plane, pivot condyle, and ball and socket joints.
Hinge joints, such as the elbow joint, allow movement along one plane, typically flexion and extension.
Saddle joints, such as the carpometacarpal joint, have opposing articular surfaces with reciprocal concave-convex shapes.
Plane joints, such as the subtalar joint, are flat, allowing bones to glide over one another.
Pivot joints, or rotary joints, such as the atlantoaxial joint, only allow rotation. The moving bone rotates within a ring formed by the concave surface of the other bone.
Condyloid joints, also called ellipsoid joints, such as the wrist joint, have a convex surface that articulates with a concave elliptical cavity.
Finally, ball and socket joints, such as the shoulder joint, feature a ball-shaped surface on one bone fitting into a concave socket in another bone. This allows for free movement in many directions.


Brain model by:
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