Joint Mobility Types

Your textbook does not have a separate section on joint mobility. But I think it is important to remember to consider this separately, since it is so important and doesn't quite match the tissue make-up.

First of all, we think about joints as places where we move. But that is not really the case. Everywhere two bones come together is a joint. Not all bones come together in order to allow for movement.

Think about it... in order to have such a large and carefully-designed skull, it requires that many bones develop in many places around our (developing) brain at the same time. As the bones come together, they are going to be important for protection of our brains, not mobility of our skull! So the joints between these bones will become (eventually) immobile. They actually maintain some mobility until after we are born, in order to help our heads fit through the birth canal... however, after birth, the bones come together and become joined via suture joints.

There are three types of joints that can be described on the basis of mobility:

	synarthrotic joints
	amphiarthrotic joints
	diarthrotic joints

Let's go over each, briefly, below...

Synarthrotic joints

These joints offer no mobility. "Syn-" means together, and "arthr-" stands for joint. So, this is a joint that is stuck together. It doesn't move. The sutures in your skull are an example of this type of joint.

Amphiarthrotic joints

These joints of some mobility. "Amphi-" means both or dual, and you may remember this term from when you learned about how phospholipids were amphipathic (both hydrophilic and hydrophobic). The term amphiarthrotic means then, that these joints offer both a bit of mobility and a bit of sturdiness.
An example of an amphiarthrotic joint is found in your vertebral column. Every vertebra can move against the next vertebra through their joint together which contains an intervertebral disk. This offers the vertebrae limited mobility. You should realize, though, that your entire vertebral column allows for pretty good movement (like when you go to bend down and touch your toes)... that is because you have many of these limited-movement joints along the entire vertebral column, so all of them combined total up to a fair amount of movement.

Diarthrotic joints

These joints are considered to be freely movable. The term diarthrotic means (I would guess) that the joint acts as if there are two ("di-") separate bones within the one joint. However, keep in mind that just because a joint is diarthrotic does not mean that it can move in any direction or that it can bend completely.
All synovial joints are diarthrotic joints.

Note:

When diarthrotic joints stop working efficiently, we end up with problems in movement. Some of these problems can be rectified, like by performing a hip replacement... But other problems, like in certain types of arthritis, cannot be fixed.

A nice set of images of the hip bone, clearly showing the ligaments that hold it in place, as well as diagrams on how hip replacement is performed, can be found at Body Online. Go to this link and click on the button underneath all the systems on "images" and scroll down to the images of the hip. There are a few. You could also take a peek at the knee joint, too. Knee (cut view) shows epiphyseal disks, too, as well as the guts of a synovial joint.