A joint is a connection between two bones. Joints are essential for flexibility, movement and shock absorption. Although most people think of joints as bending like a hinge, there are many different types of joints. Joints are classified by the structures that compose them, as well as the types of movement they allow.
Synarthroses
These joints are found only in the skull and connect the cranial plates. They are separated by a small amount of fibrous connective tissue. They are essentially immobile but allow for growth during childhood development.
Amphiarthroses
These joints allow for limited motion. Examples include the pubic symphysis (where the two sides of the pelvis connect anteriorly), intervertebral discs in the spine and parts of the sacroiliac joint (where the sacrum connects with the two sides of the pelvis posteriorly). Amphiarthroses are bound by ligaments that prevent excessive motion but allow for some flexibility. In these joints, the two bones may be connected by either fibrocartilage or articular cartilage. Fibrocartilage is a dense and strong type of cartilage. Articular cartilage is smooth, can absorb shock well, and is ideal for mobility.
Diarthroses
These are the most common joints. Bones are connected by articular cartilage and surrounded by synovium, which is a tissue composed of cells that produce lubricating fluid, or synovial fluid. With the smoothness of the articular cartilage and the lubrication of synovial fluid, diarthroidal joints allow for ease of movement. These joints can be further divided by the types of movement they allow:
1. Ball and socket: allows movement in all three axes of motion. This is the most versatile type of joint. Examples include the hip and shoulder.
2. Hinge: allows movement in one axis. Examples include the elbow and knee.
3. Saddle: allows movement in two axes. One example is the base of the thumb (note how your thumb can move away from the palm when you spread out your fingers and can also cross over your palm and touch your pinky).
4. Plane: allows limited gliding motion. One example is the patellofemoral joint (note how you can glide your kneecap up and down when your thigh is relaxed).
Although these joints allow for flexibility and movement, they are also prone to injury and mostly commonly affected by arthritis.
References
- Biology of the Normal Joint, in Kelley's Textbook of Rheumatology, 8th edition; Steven R. Goldring and Mary B. Goldring; 2009
- Rheumatology Secrets; Sterling West, MD; 2009
