Classification of Joints
A joint is defined as the point at which two or more bones articulate. Not all joints move, and different classes of joint contain different tissues. The structure and tissue makeup of a joint will define its properties, including the mobility, strength and stability.
Joints can be easily classified by the type of tissue present. Using this method, we can split the joints of the body into fibrous, cartilaginous and synovial joints.
Joints can also be classified by whether they move a lot, a little, or not at all. In this article, we’ll be going through the joints by tissue classification; but we’ll also make mention of the mobility classification to which they belong.
A fibrous joint is where the bones concerned are bound by tough, fibrous tissue – these are strength joints. Fibrous joints can be further subclassified into sutures, gomphoses and syndesmoses.
These are immovable joints (called a synarthrosis), only found between the flat, plate-like bones of the skull.
There is limited movement until about 20 years of age, after which they become fixed. They are most important in birth, as at that stage the joints are not fused, allowing deformation of the skull as it passes through the birth canal.
These are also immovable joints, and can be found where the teeth articulate with their sockets in the maxillae (upper teeth) or the mandible (lower teeth).
The fibrous connection that binds the tooth and socket is the periodontal ligament.
These are slightly movable joints (called an amphiarthrosis). Their structure is comprised of bones held together by an interosseous membrane. They are key joints in providing strength along the length of long bones, preventing them from separating.
The middle radio-ulnar and middle tibiofibular joint are examples of syndesmosis joints.
In cartilaginous joints, the bones are attached by fibrocartilage or hyaline cartilage. There are two main types – primary cartilaginous and cartilaginous secondary joints.
Cartilage is an excellent shock absorber, as it is essentially a thick gel. The pelvis and spine are hugely important in this respect.
Also known as primary cartilaginous joints, they only involve hyaline cartilage. The joints can be immovable (synarthroses) or slightly movable (amphiarthroses).
The joint between the diaphysis and epiphysis of a growing long bone is a synchondrosis. This is interesting in that it is a temporary joint with no movement, and can be thought of as a bone-cartilage-bone sandwich. On an X-ray, cartilage is not visible; have a look at some growing bones, it looks like the bony end plate is floating.
Also known as a secondary cartilaginous joint, it can involve fibrocartilage or hyaline cartilage. These joints are slightly movable (amphiarthroses), an example of a which is the pubic symphysis.
A synovial joint is a joint filled with synovial fluid. These joints tend to be fully moveable (known as diarthroses), and are the main type of joint found around the body. They allow a huge range of movements, which are defined by the arrangement of their surfaces and the supporting ligaments and muscles.
It is commonplace to classify synovial joints by their movement. To understand some of the terms of movement, have a look at our movement article which describes what the terms mean.
Permits flexion and extension. Elbow joint is a notable example.
Concave and convex joint surfaces unite at saddle joints, e.g. Metatarsophalangeal joint (toes).
Permit gliding or sliding movements, e.g. Acromioclavicular joint (collarbone to shoulder blade).
Allows rotation; a round bony process fits into a bony ligamentous socket. Examples include the atlantoaxial joint & proximal radio-ulnar joint (top of the neck and elbow).
Permits flexion, extension, adduction, abduction and circumduction e.g. Metacarpophalangeal joint (in the middle of your hand).
Ball & Socket
Permits movement in several axis; a rounded head fits into a concavity. An example is the glenohumeral joint (shoulder).