How Are Joints Classified in The Human Body?

By Chandan Sekhon - Medicine Student @ Peterhouse, Cambridge

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A joint is defined as a part of the body where two or more bones articulate to allow movement. There are many different types of joint and their functions are different. Generally, the greater the degree of movement allowed by the joint, the greater the risk of injury to the joint.

Joints found in the body can be classified by the type of tissue connecting the bones involved in the formation of the joint. Synovial joints are when the articulating surfaces of the bones involved are enclosed within a capsule filled with synovial fluid which helps reduce friction between the surfaces allowing the easy and smooth movement of the bones, making synovial joints freely movable. This is the most common type of joint found in the body and synovial joints can be further sub-classified into numerous types.

A fibrous joint is when the bones involved are connected by fibrous tissue. This is very tough tissue, and these joints are commonly found at joints which generally require a lot of strength and stability to allow a large range of movement in the individual. There are three types of fibrous joint in the body – sutures are immovable joints; syndesmoses are slightly movable and gomphoses are also immovable. An example of a fibrous joint is the joint securing teeth into its socket. These are bound to the socket by the periodontal ligament – an extremely strong connective tissue ligament.

The third type of joint found in the body is a cartilaginous joint, where the bones are united by either hyaline cartilage or fibrocartilage. Those joints which are connected using fibrocartilage are called symphysial joints and are slightly moveable. These are connected by hyaline cartilage are called synchondroses. These joints are immovable. An example of a cartilaginous joint is the manubriosternal joint between the manubrium of the sternum and the sternal body. This is an example of a synchondrosis.

Synovial joints can be further sub-divided into six joint types. Hinge joints permit movement in one plane only. Here, protective cartilage covers the bones and synovial fluid lubricates the articular surfaces. An example of a hinge joint is in fingers.

Saddle joints occur when the articulating surfaces are reciprocally convex and concave in shape. This type of joint is biaxial, allowing movement in 2 planes. An example of a saddle joint is the carpometacarpal joint in the thumbs.

Pivot joints allow rotation. In these joints, one end is cylindrical in shape and the other surface is a ring shape composed of bone and ligament. For some joints, the ring rotates around the cylinder and in other pivot joints, the cylinder rotates in the ring. An example of a pivot joint is at the wrist which allows the palm of the hand to be turned up and down.

Ball and socket joints are where one articular surface is rounded and fits into a depression in the other bone, allowing the free movement of the bones in multiple axes with one common centre. An example of a ball and socket joint is the glenohumeral joint where the ball-shaped projection in the upper humerus fits into the glenoid fossa of the scapula.

Plane joints are where the bones are able to glide over one another, due to the articulating surfaces being relatively flat. The bones are permitted to move in the direction of the plane of the joint. An example of a plane joint is the joints between the metacarpal bones in the hand.

The final type of synovial joint is condyloid joints. This is where a convex surface articulates with a concave elliptical cavity. This type of joint permits movement in two lanes, allowing flexion, extension, circumduction, abduction and adduction. An example of condyloid joints is metacarpophalangeal joints in the hands.

Further reading:

1. A great article about the anatomy of joints, good if you’re interested in learning about basic anatomy: https://teachmeanatomy.info/the-basics/joints-basic/classification-of-joints/

2. An article about osteoarthritis – a common cause of joint pain: https://journals.lww.com/pain/fulltext/2006/07000/osteoarthritis_and_joint_pain.3.aspx?casa_token=qW5RBY_5WXsAAAAA:WsjlxEaPCLdZHSdOrCbk8iuWYxLEhfFwRYrBZB-6YA12sqUCD3tzGgLaVPQItLUCfIKooL93z7D9GNy9hgb6fjiTYa8Y

3. A great article giving a good introduction to arthritis and joint pain. Slightly similar to the article above but still a good read: https://link.springer.com/article/10.1186/ar2069

4. An article about sacroiliac joint pain (in the pelvic region): https://journals.lww.com/jaaos/Fulltext/2004/07000/Sacroiliac_Joint_Pain.6.aspx?casa_token=WOZcWiBlGOUAAAAA:RVFru6rLsRFvr8VpOUma8QcPUfwx75QW6AN3u8aw1bN3oMWynPZOot6Us22gGzPkVncA7WOeYFkYBPlpPFEPtB22h1UH

5. A very long and detailed article about joint pain and its management. Take what you find interesting from this rather than all of it as it does go into great detail and is a lot of information: https://www.sciencedirect.com/science/article/pii/030439599390183P?casa_token=XkLVQnpwQV8AAAAA:P2ZhyUSn0EO1Yp2b2Tjn2hYGNxLPghvG87MbidhhrLtRvhjPkWAlcZXOcqZJLRXKbD6T2NvNzCY