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:
Let's go over each, briefly, below...
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.
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.
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.
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.
© 2011 STCC Foundation Press