Joints ( ESSAY )

Joints are the meeting places between two or more bones. And even though it sounds mathematically possible, you actually have more joints than bones, which is weird. In a lot of places, like your hands and feet, each individual bone is part of more than one or two joints. And then what is the job of those joints? To help you move. Body movements happen when muscle contracts across joints, moving one bone toward another. As is often the case in anatomy, we classify joints both by what they do and by what they're made of. Because form follows function, we can't really talk one without talking the other. So the structural classification of your joints is all about what kind of material make up those joints, and it is made of three categories :

• Fibrous joint  
• Cartilaginous joint
• Fluid-Filled or synovial joint

While the functional classification of your joints focuses on how much that particular joint can move, and it is divided into several more groups :

• Synarthroses; non moving joints
• Amphiarthroses; partly-moving joints, like shock-absorbers.
• Diarthroses; fully movable

Fibrous joint connects bones with dense, fibrous, connective tissue and are mostly immovable, so they are mostly classified synarthroses joints.

Cartilaginous joints unite bones using cartilage, they don't move much, and therefore classified as ampiarthroses joint. These come in two types : synchondroses and symphyses.

Synovial joint are freely movable, so they are in the class of diarthroses. Most of your joint fall into this category. Although they do make use of cartilage and fibrous connective tissue to connect bones, they're different in that the bones that they joint are separated by a fluid-filled joint cavity, which is good, because they move a lot, and if all our joints use cartilage or fibrous connective tissue to connect all your bones, a walk down the block would make your joints so hot that it would essentially cook the surrounding tissue and leave your legs smoking like a desperate Looney Tunes character. They also have six special features :

• band-like ligaments
• articular cartilage that covers the opposing bone surface
• a joint cavity
• synovial fluid lubricant
• a fibrous joint capsule
• sensory nerve fibers and blood vessels.

These synovial joints have six different configurations that allow you do all the things that you are capable of doing, like a subtle head nod to a vigorous-whole-body shake :

• Plane
• Hinge
• Condylar
• Pivot
• Ball-and-socket
• Saddle

So that is pretty much the anatomy and physiology of your joints, and Matthew is signing off.