Fibrous, cartilage, and synovial joints

Definitions & Key takeaways

There are three types of joints in the human body: fibrous, cartilaginous, and synovial. Fibrous joints are held together by dense connective tissue, cartilaginous joints are held together by cartilage, and synovial joints are free-moving and enclosed in a capsule. Each type of joint has different features and performs different functions.

Fibrous joints include the sutures between the bones of the skull. These are very strong joints but with limited to no motion. Cartilaginous joints include the disks that separate the vertebrae. These joints allow for some movement but are mostly stable. Synovial joints are the most common type of joint in the body and include all joints that move, such as the shoulder, elbow, hip, and knee. Synovial joints are surrounded by a capsule filled with synovial fluid, which lubricates the joint and allows for smooth movement.

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Introduction0:00–0:26

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.

Fibrous Joints0:26–1:54

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.

Cartilaginous Joints1:54–3:16

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.

Synovial Joints3:16–4:26

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.

Hinge Joints4:26–4:59

Synovial joints are grouped into six types based on their structure and how they move. Let's start with the hinge joints which allow movement only in one axis, like a door hinge.
There’s a hinge joint between the humerus and the ulna, and it allows flexion and extension of your forearm at the elbow.
Other hinge joints include the interphalangeal joints in the fingers and toes. The knee is a modified type of hinge joint because, in addition to flexion and extension, it also allows some degree of rotation.

Pivot Joints4:59–5:36

The second type of synovial joint is the pivot joint, which allows for rotation. There’s a pivot joint where the head of the radius fits into a groove of the ulna within the annular ligament, and it allows the the forearm to rotate, like when you turn a door knob.
There’s another pivot joint at the distal radioulnar joint, as well as one at the atlantoaxial joint. The atlantoaxial joint is between the two top vertebrae of your spine, the atlas, and the axis, right at the junction of your head and neck, and it’s the one that allows you to shake your head when you say “no”.

Plane or gliding Joint5:36–6:04

The third type of synovial joint is the plane or gliding joint. There are plane joints between the carpal bones that make up your wrist or tarsal bones in the foot.
These bones have flat surfaces that allow them to glide across one another in any direction along the plane. That’s why your wrist can extend or flex, and abduct or adduct, moving in both the horizontal and vertical axes.

Ball and socket Joint6:04–6:38

The fourth type of synovial joint is the ball and socket joint, like your shoulder or hip joint. Your shoulder joint, for example, consists of the head of the humerus - which is the ball - fitting into the glenoid cavity of the scapula - which is the socket.
This allows almost all kinds of joint movements such as flexion, extension; abduction, adduction, and rotation. Ball and socket joints allow the widest range of action, but this also makes them prone to frequent dislocations.
The fifth type of synovial joint is a condyloid or ellipsoid joint. Examples include your metacarpophalangeal joint - which is the knuckle joint in your hand, and your metatarsophalangeal joints which is the joint between the metatarsal bones of the foot and the proximal phalanges of your toes - your foot knuckles.

Condyloid or Ellipsoid Joint6:38–7:32

This joint is basically a modified version of ball and socket because one of the bones has a rounded protuberance which fits into a hollow ellipsoid space of the opposite bone.
This allows movements in two axes; flexion-extension, and abduction-adduction. But unlike the ball and socket type, condyloid joints can’t rotate around an axis, because the socket has an ellipsoid shape.
It’s the shape you get when you squash a ball so that it’s no longer perfectly round. That change limits movement more than a round socket.

Saddle Joint7:32–8:22

The sixth type of synovial joint is a saddle joint. An example is the carpometacarpal joint of the thumb which is between the trapezium and the first metacarpal bone.
Other saddle joints include the one between the malleus and the incus bones in the middle ear and the sternoclavicular joint between the sternum and the clavicles.
Here, the bones have concavo-convex surfaces, which look a bit like a cowboy sitting on a horse saddle. Saddle joints allow for flexion-extension which is where the cowboy leans forward or backward, and abduction-adduction which is where the cowboy tilts to the right or left of the horse.
Unlike the condyloid joint, the saddle joint allows for some rotation, but much less than the ball and socket joint. Alright, as a quick recap, fibrous joints tightly bind bones together, while cartilaginous joints allow for some degree of movement.

Review8:22–9:10

Synovial joints have six main types. There’s the hinge joint which is like your humerus and ulna joint and is like a door hinge.
There’s the pivot joint which is like your radius and ulna and allows for rotation. There’s the plane joint which is like your wrist and allows for movement along a plane.
There’s the ball and socket joint which is like your shoulder or hip joint and allows for all kinds of joint movements. There’s the condyloid joint which is in your knuckle and doesn’t allow rotation around an axis.
Finally, there’s the saddle joint which is found in your carpometacarpal joint and is shaped like a horse saddle.