What is a joint?
A joint is where two or more bones meet. The formal word is articulation, which is why the cartilage inside is called articular cartilage.
What are the three groups of joint?
- Fibrous joints barely move, like the seams in the skull.
- Cartilaginous joints move slightly, like the ones between vertebrae.
- Synovial joints move freely, like the knee and shoulder. Most joints you train are synovial joints. They're full of synovial fluid.
What are the six types of synovial joint?
Synovial joints come in six shapes. The shape decides how the joint can move.
| Joint | How it moves | Example in the body |
|---|---|---|
| Ball and socket | A rounded head sits in a cup and can move in every direction, including rotation. The most mobile type. | Hip and shoulder |
| Hinge | Moves in one direction only, bending and straightening, like a door | Elbow and knee |
| Pivot | One bone turns around another | The top two vertebrae let you shake your head. The radius turning on the ulna rotates your palm. |
| Saddle | Two saddle-shaped surfaces fit into each other | The base of the thumb, which gives it its grip |
| Condyloid (KON-dih-loyd) | An oval head in an oval cup. Bends forwards, backwards and side to side, but can't truly rotate. | The wrist |
| Gliding (also called a plane joint) | Flat surfaces slide a little over each other | Between the small bones of the wrist and ankle |
Which joints are ball and socket joints?
The hip and the shoulder. The shoulder is the most mobile joint in the body. The hip has a deeper socket, trading some range for stability.
What are the parts of a synovial joint?
- Articular cartilage (ar-TIK-yoo-ler KAR-tih-lij): the smooth, slippery layer covering the ends of the bones. It cuts friction and absorbs shock. It has no blood supply of its own, so it heals poorly. (Also accepted in the app: hyaline cartilage.)
- Synovial fluid (sy-NOH-vee-ul FLOO-id): the thick, slippery liquid that fills the space in the joint. It lubricates the joint and feeds the cartilage. Movement and warmth help it work.
- Synovial membrane (also called the synovium): the thin lining inside the capsule. Its job is to make synovial fluid.
- Joint capsule: a tough, fibrous sleeve that seals the joint and holds the fluid in.
- Ligaments: strong bands that join bone to bone and keep the joint stable.
What is the difference between a ligament and a tendon?
Ligaments join bone to bone. Tendons join muscle to bone and pass the muscle's pull onto the skeleton. Ligaments link bone to bone: L and L. Tendons tie muscle to bone: T and T.
What are the five types of bone shape?
The shape tells you the job.
| Bone shape | What it is like | Examples |
|---|---|---|
| Long bone | Longer than it is wide. Acts as a lever that muscles pull on. | Femur, humerus, tibia, fibula, radius, ulna |
| Short bone | About as wide as it is long. Gives stability with a little movement. | Carpals in the wrist, tarsals in the ankle |
| Flat bone | Thin and broad. Protects organs and gives muscles wide surfaces to attach to. | Sternum, ribs, cranium, scapula |
| Irregular bone | A complex shape that fits no other group | The vertebrae of the spine |
| Sesamoid bone | Grows inside a tendon. Improves the leverage of the muscle pulling over it. | The patella, the biggest one |