Your body is made of around 206 bones, which are connected together by about 360 joints.
These joints can be classified into three main groups based on their structure and how they move.
Fibrous joints which generally don’t move at all, cartilaginous joints which allow some movement, and synovial joints which are freely movable.
Let’s begin with the fibrous joints, which are also called synarthrosis or fixed joints.
In fibrous joints, bones are connected by ligaments and they fall into three main categories based on their location.
First, we have sutures, which are the joints between the bones of the skull.
Skull bones are supported by their interlocking design and short ligaments that connect adjacent bones together.
Adult sutures are stiff and completely fixed. But in the fetus and the babies, the sutures are more widely spaced and therefore partially movable.
During labor, there’s a process called molding where the baby’s skull bones slightly overlap, temporarily reducing the baby’s head diameter, so that it can pass through a mother’s pelvis.
Second, there’s the syndesmosis which is the joint between the radius and ulna in the forearm. Along their shafts, they are attached by long bands of ligaments called the interosseous membrane.
Unlike the interlocking sutures, syndesmoses are slightly mobile throughout life.
The third category of fibrous joints is a gomphosis, which is a joint between the roots of a tooth and its socket within the jawbone - either the maxilla or mandible.
A tooth is anchored in its socket by periodontal ligaments, which allow light movements to lessen the impact, like when you are chewing on corn nuts.
Next are the cartilaginous joints, which are joints surrounded by hyaline cartilage which can stretch to allow some movement.
One type of cartilaginous joint is a synchondrosis, and an example of a synchondrosis is the costochondral joint where a bit of cartilage attaches the rib to the sternum.
The elastic cartilage allows your chest to expand and relax, allowing air in and out when you’re breathing.
A synchondrosis is also found between the diaphysis, which is the shaft of a long bone, and the epiphysis, which is the rounded tip of a long bone.
The hyaline cartilage in the synchondrosis allow the bone to grow in length, and eventually this cartilage accumulates more calcium and undergoes ossification - which is when the synchondrosis turns into bone.
Another type of the cartilaginous joint is the symphysis, which consists of a tough, fibrous cartilage.
Unlike the synchondrosis, the symphysis doesn’t undergo ossification.
Symphyses are stronger but less flexible than synchondroses, so they hold bones together tightly and only stretch a little bit. One example is the pelvic bone which is held together by the symphysis pubis and only stretches apart slightly - just enough to let the baby’s head slip through during delivery.
Next up, are the most common joints - the synovial joints.
Unlike the fibrous and cartilaginous joints, synovial joints allow a wide range of movements like flexion and extension of various body parts like an arm, a finger, or a toe.
Synovial joints allow abduction, like when you move your limbs away from the midline of your body, and adduction, when you bring them closer to the midline. They also permit rotation around an axis, like when you turn your head.
Synovial joints have a joint capsule which has an outer fibrous capsule, and an inner synovial membrane filled with synovial fluid.
Synovial fluid is a viscous fluid that looks like the white of an egg, and it helps lubricate the joint and absorb shock.
The inner synovial membrane also has blood vessels that supply the cells in the joint with nutrients and oxygen.
The tips of the bones that come together to from the joints are covered by an articular cartilage, which is a slippery smooth layer of hyaline cartilage that also absorbs shock and reduces friction during movement.