SKELETAL SYSTEM (JOINTS)

Joints

An articulation, or joint, is a point of contact between bones, between a bone and cartilage, or between a bone and a tooth. Synovial joints are the most common type of articulation and feature a small gap between the bones. This gap allows a free range of motion and space for synovial fluid to lubricate the joint. Fibrous joints exist where bones are very tightly joined and offer little to no movement between the bones. Fibrous joints also hold teeth in their bony sockets. Finally, cartilaginous joints are formed where bone meets cartilage or where there is a layer of cartilage between two bones. These joints provide a small amount of flexibility in the joint due to the gel-like consistency of cartilage.

Types of joints

At the end of the post there's a video about the types of joints so that you can watch it instead of reading all the information (or you can do the opposite)

Synovial Joints

There are 6 types of synovial joints. They have varying shapes, but the important thing about them is the movement they allow. Joints determine what positions our bodies can take. We learn them to invent poses. And we learn the limits to stretch the limits.

The 6 types of synovial joints are: Hinge, Pivot, Ball & Socket, Ellipsoid, Saddle, and Plane. Let’s go through them one by one.

Hinge

The hinge is a very simple joint. It allows movement only on one axis. It’s structure prevents rotation this way, or this way. The head of one bone wraps around the cylindrical head of the other, allowing a very stable rotation this way.

The hinge joint allows flexion and extension. That's all it does, but it does it well. Like the hinges on a door, allow it only to open or close. The best example of it is the elbow.

Pivot

The pivot joint also allows rotation at only one axis. However, it rotates along the long axis. A cylindrical bone fits into a ring of bone and ligament, like with the radio-ulnar joint just below the elbow. The cap on the radius bone fits nicely into this notch on the ulna bone. Ligaments complete the ring, holding the bone in place and allow the radius only to rotate inside of it.

The result on the forearm is what we call pronation and supination. During pronation, the base of the radius rotates over and around the head of the ulna. The ulna stays relatively still. Remember, the hinge joint at the elbow, prevents the ulna from twisting. So all of that twisting happens at the radius.

And by the way, the distal joint of the ulna and radius is also a pivot joint. The combination of the pivot at the top and at the bottom creates that twisting motion for pronation and supination.

Ball & Socket

It’s structure is just like how it sounds: a ball inside of a socket. This simple and effective structure allows it to move in all axes – flexion, extension, abduction, adduction, rotation, and circumduction.

The two ball and socket joints of the body are at the hip and the shoulder. The hip has a deep socket, which gives it stability, but limits some range of motion. The shoulder joint has a shallower socket, which gives it greater range of motion, but takes away some stability. Maybe that’s why a dislocated shoulder is so common.

Ellipsoid

The ellipsoid joint is very similar to a ball & socket. However, the ligaments and its oval shape prevent rotation. But it still has the ability to rotate on two axes, which allows flexion, extension, abduction, adduction, and circumduction. Circumduction is just a combination of all the others in a circular motion.

The ball, or oval head also slides inside the socket. When it rotates along the wider plane, it slides to stay inside the socket.

A great example of an ellipsoid joint is the wrist. The group of carpal bones rotate inside the socket of the radius.

Saddle

The saddle joint is similar to the ellipsoid, but the rotation is limited mostly because of the bone structure. The structure of the saddle is very interesting. Both bones have a concave and convex surface. Convex means the surface sticks out, like a hill. Concave means the surface curves in, like a hole or a cave. The concave plane of one fits on the convex plane of the other.

The legs of the top piece, which wrap around the body of the bottom piece allow a rotation this way. The body of the top piece can glide inside of the legs of the bottom piece.

So, this unique structure allows the joint to flex, extend, abduct, adduct, circumduct, and very slightly rotate.

An example of a saddle joint on the body is the carpometacarpal joint of the thumb.

Plane

Finally, the plane joint.
It’s basically two flat-ish surfaces, one on top of the other. These surfaces can glide or rotate.

They usually come in groups, like the carpals of the hand and the tarsals of the foot. Ligaments hold these bones together, but might allow some rotation and gliding.

Another plane joint is the acromioclavicular joint. That’s the one between the clavicle and acromion process of the scapula. When we elevate the shoulder, the angle in here will adjust to keep the scapula vertical.