 This video is on articulations or joints. Most of you are familiar with the more mobile joints, the knee joints, the elbow joints, and so forth. These are formerly called articulations. Bones start out forming as individual units, and then they are bound together into larger functional units by different types of joints. The joints are classified according to structure, and then there's a function that goes along with that particular structure. In this case, the main function that we're interested in is the movability of the joint, how much range of motion is associated with that particular joint. So structurally, they're classified as either being fibrous, where you actually have fibers extending from one bone to the next, and then that becomes calcified and essentially becomes bone. We can find some examples of these along the axial skeleton, for instance, if we turn to this skeleton now and look at it, up here in the cranium, and I have the calvaria here to look at, there is a joint between two bones of the cranium here. These are the parietal bones, and they are joined by what's called the sagittal suture. This suture is essentially solid bone. It has no mobility to it. So this is a fibrous type of bone joint, and it's classified as being a synarthrosis, which today is associated with its mobility. Essentially, there's a synthesis of these two bones. They become one, and they are fixed then in mobility. So we have some examples here. The bones are joined. They're forming a nice continuous space for the soft sensitive brain tissue that they have. So you don't want a lot of mobility associated with the cranium, obviously. The next type of joint is called the cartilaginous joint, and this functionally is cartilaginous, and they have a piece of cartilage, a softer tissue than bone, but still very tough connective tissue that joins two bones together. And there's some examples of those also along the axial skeleton. In brown here we see the cartilages that join the ribs to the sternum. So these would be cartilaginous joints that are made out of cartilage. They're joining a bone on one side to a bone on the other side, and then functionally they're called amphiarthrosis, which nowadays means that they're slightly movable joints, slightly movable. So when you're ventilating your lungs, breathing in and out, there's some mobility allowed between the ribs and the sternum. If you are subject to some trauma to the sternum, it has some bend before it breaks. Other examples here, I've highlighted it in some brown here, the joints in between the vertebrae. The intervertebral disc here have a cartilaginous component, linking one vertebra to the next gives you some slight mobility to your vertebral column. And another example here is where the two pubic bones are being joined together at the pubic synthesis by a piece of cartilage and allows for some mobility in the pelvis, limited amount of mobility. And then finally the one that you're most familiar with then is the synovial joint. And the synovial joints are called diarthrosis functionally, and they are the highly movable joints of your elbow, your wrist, your knee. All of these things you're familiar with as being synovial joints. One way to remember the relationship between structure and function is to take the first letters of these things and make a little mnemonic trick out of it. For instance, FCS, whatever that, you might mean like something like Versailles County Schools to you if you are a parent. And SAD spells sad. So you might think, oh, I, without the Versailles County Schools, I would be sad as a parent, right? Or you can, if you'd rather, you can make it Versailles Tech Community College System. You'd be sad without that, right? We're going to go on now and look at one of these synovial joints in more detail. It's one that causes a lot of clinical problems. And that is this knee joint here. And I've got a picture, I've got a piece of cloth wrapped around it to represent connective tissue that goes around the knee joint. So this is called a capsule. It's a fibrous capsule that goes around synovial joints. All your synovial joints will have a capsule that is something like this. And you can see it extends from the side of this bone called the patella all the way around the joint. And if we take this off, then we see that the bones that are articulating here, forming a joint with each other. There's no physical link bone to bone or bone to cartilage between the two. They're very free to move. What keeps these in place are the shapes of the ends of the bones here, and then various ligaments and muscle tendons that stabilize the joint. So in this joint, it's the tibia, oh, sorry, the tibia, and then the femur here and the patella that are going together to form this particular joint. This bone on the lateral side of your leg is the fibula for reference. Now, let's look at some of the tendons that stabilize this joint a little bit more of the substructure of the joint. So here's the same structure. And now we're looking at this joint being represented. We're seeing it in an anterior to posterior view. So lateral is over on this side and medial is over on that side. And we can see that there's a number of structures being cartooned here as joining the femur to the tibia. So firstly, I have in red here, so this would be within that capsule, or in this case just outside that fibrous capsule. There are ligaments that join one bone to the next. And these prevent lateral deviation, then, of the tibia in one direction from the other. Those red ligaments are called collateral ligaments. And this one is over here on the medial side. So it's called the medial collateral ligament. This one is on the lateral side. So it's called the collateral ligament. And they're going to stabilize the knee in this coronal plane, preventing lateral movements. You can see what would happen if one of them breaks. So if this one were to be damaged in an injury, what's going to happen now is this will be allowed, the tibia will be allowed to deviate to the lateral side like that. So this is stabilizing the joint in that particular plane. There's another set of ligaments that cross through this space. So the word for these joints, the synovial part, comes in here, right here. This is the synovial cavity. And there is fluid produced by a membrane that limes this cavity. It has high viscosity. It looks sort of like egg white. Thus, it got its name synovial like an oval, like an egg, like egg white. And that's what limes this cavity. It's a fluid that helps lubricate this particular joint. Also, we can see here that there are some other structures within here, little pieces of cartilage that help stabilize the joint. They are called menisci, or singular of being meniscus. So there's a lateral meniscus here, a little piece of cartilage, forms a little well for these bones to ride in and become stabilized. And then there's another one on the medial side. Now, if you turn this and look at the knee joint from a lateral view, you see there's some other important ligaments that are crossing right through the synovial cavity. So now we're looking at this joint like that. These things are called cruciate ligaments because they cross. They form a cross when you look at them from the lateral side. And what these things do is they prevent displacement of the tibia relative in the anterior-posterior direction. So they're preventing this type of motion of the tibia relative to the femur. And if you damage one of these things, what happens? Let's see if we damage this red one. See if we can leave the green one intact. But you see what happens here if we have a ligament that's connecting this bone to the next here. And now only the green one is present. We're fine as far as anterior displacement of the tibia, right? It's pulling on it. It won't let me pull it out. But it'll let me move it all the way back like that. So let me push it back. So clinically, this would be called a posterior drawer sign. It's like you're pushing a drawer to the posterior side. It's an indication of damage then to that particular cruciate ligament. And that particular cruciate ligament is called the posterior cruciate ligament that I just damaged here. So let's put that one back in. This one's posterior, okay? And that prevents displacement of the tibia into the posterior direction. So with that one, it prevents that kind of movement. It's blocked. If you damage it, posterior drawer sign. If it's intact, no posterior drawer sign. Okay, let's damage this other one now and see what happens. So we'll damage the green one. Now the posterior is intact. We've damaged the green one and you can see what happens. We're fine for displacement to the posterior side. It's stabilized in that direction. But I can pull this thing all the way out. So it's like I'm pulling out a drawer. It's called the anterior drawer sign clinically and it indicates that there's damage to this green ligament. It's called the anterior cruciate ligament. Now if you're trying to figure out which one's anterior and which one's posterior, think of where they attach on the tibia. You can see the anterior attaches to the anterior portion of the tibia. The posterior attaches to the posterior portion of the tibia. So this makes for the, you know, being able to recall the names. Remember where they attach to the tibia because that's what their name for anterior-posterior cruciate ligaments. So this is just a general overview of the joints. And I hope you've learned something.