A joint or articulation is a connection between two bones. Joints can have a large range of motion (a healthy shoulder joint that lets you swing your arm around) or very immobile (the sutures between the bones of the skull allow no movement). Joints are classified by both structure and by range of motion. I’ll go through the structural categories first.
Structural Classification of Joints:
In terms of structural categories there are synostosis (bony) joints, fibrous joints, cartilaginous joints, and synovial joints.
synostosis (bony) joints: The two bones are connected by bone. This type of connection between bones can occur normally during development (Example: in a baby the hips are made up of three separate bones that will fuse together to form a complete hip bone) and as part of normal aging (the skull plates tend to fuse together as we get older), but synostosis can also occur as part of a disorder (sometimes a joint that should be mobile can fuse after injury or due to arthritis)— this type of abnormal synostosis is often called ankylosis.
fibrous joints: In fibrous joints two bones are joined by dense connective tissue (lots of collagen fibers). This type of joint allows very little to no movement. Examples include the joints between the skull plates called sutures and the joints between your teeth and jaw bones (called gomphosis joints).
cartilagenous joints: The bones in a cartilagenous joint are joined by cartilage. Examples of cartilagenous joints include the joints between vertebrae in the spinal column and the joint between the pubic part of the hip bones (called symphysis pubis).
synovial joints: Synovial joints are the most complex joints. In a synovial joint the bones are connected to each other by dense connective tissues (including a fibrous capsule and ligaments) and there is a synovial cavity between the two bones that is filled with synovial fluid. The surfaces of the two bones are covered with cartilage (usually hyaline cartilage).
Range of Motion Classification of Joints:
In terms of range of motion the categories of joints consist of synarthrosis joints, amphiarthrosis joints, and synovial joints.
Synarthrosis joints: Synarthrosis joints allow very little to no movement between the bones involved. Bony joints are synarthrosis joints along with most fibrous joints and some cartilagenous joints.
Amphiarthrosis joints: Amphiarthrosis joints allow a little bit of movement. Most amphiarthrosis joints are cartilagenous joints.
Synovial joints: Synovial joints allow for the greatest range of motion. The most mobile of the synovial joints is the shoulder joint.
More on Synovial Joints:
Synovial joints have their own categories based on structure and range of motion. These different types of synovial joints are: ball and socket joints, condyloid joints, hinge joints, pivot joints, and plane joints. Understanding the structure of these joint types allows you to really understand the ways in which each allows movement to happen.
A ball and socket joint consists of one long bone having a ball like head that is inside of a rounded socket in the other bone. This kind of joint has the largest range of motion. Two examples of ball and socket joints are the hip joint and the shoulder joint. The shoulder joint has the greatest range of motion, allowing us to swing our arms around in a large circle. The hip joint has a large range of motion too, but it needs to be stronger in order to hold the body’s entire weight and the structure needed to give that strength limits the hips range of motion compared to the shoulder.
Hinge joints can only move on one plane (your knee can only bend one way– if it bends sideways you know there is problem). Examples of hinge joints include the knee, the joints between the finger bones (but not the base of the fingers), and the elbow joint (you can move your lower arm sideways, but that movement is happening as you rotate your shoulder).
Condyloid joint‘s have an oval shape to the articular surfaces. This allows a large motion through one plane (like a hinge joint), and a bit of movement through another. We find condyloid joints connecting the base of our fingers. The condyloid joint at the base of our fingers allows us to open or close our hands into a fist (large motion possible in that plane) and it allow is to wag our fingers — move them sideways a bit.
A pivot joint only allows one type of movement, it allows a bone to twist or rotate. One example of a pivot joint is between the radius and ulna near the elbow. This particular joint is called the radioulnar joint and it allows the radius to twist alongside the ulna so that we can twist our lower arm/hand– think of the way your hand twists when using a screwdriver. There is another pivot joint in your neck, this one is between the first two vertebrae and it allows your head to rotate in a silent “No” movement.
In a saddle joint the two bones meet with surfaces that are kind of like saddles in shape. This allows movement similar to the condyloid joint but with a wider range of motion. The saddle joint at the base of the thumb is important for allowing us to have opposable thumbs.
In a plane joint the articular surfaces of the two bones are flat in shape allowing the two bones to slide over one another. Plane joints have relatively tight joint capsules, so the movement of each individual joint is quite limited. However, in places like the wrist multiple gliding joints work together to allow for a larger range of motion.