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The synovial joints are the main type of joint found around the body.
Well, this person doesn’t look like she’s doing very well. What’s going on? Any guesses as to today’s tutorial might be on? Any guesses? A lack of something that might lead to her not moving properly. Alright, here’s a clue. That’s right. Joints! But more specifically, we’re going to be looking at synovial joints and the types of synovial joints in your body. So let’s get to it!
We’re going to be starting by looking at what is a synovial joint including its structural anatomy, and then we’ll dive into looking at the types of synovial joints starting with ellipsoid joints, saddle joints, pivot joints, hinge joints, gliding joints, and finally, ball-and-socket joints. And then we’ll wrap up with a clinical scenario to illustrate why these joints are so important.
Synovial joints are one of three main classifications of joints in the human body, but before we get into that, I just want to first define what a joint is. So, what’s the definition of a joint? Well, a joint itself is where two bones meet, and some joints allow movement and some don’t. So, one way to classify joints is based on the range of motion allowed at that joint or that can occur at the joint. The other way is by the type of tissue that holds the bones together. So, in terms of the first classification, synovial joints are capable of a large range of motion, and in terms of the second, a joint must made a few criteria to be considered a synovial joint.
So, firstly, both bones forming the joint must be lined with hyaline cartilage at the articulating surfaces. Secondly, the joint must be enclosed within a capsule which creates the joint cavity. Within this joint cavity is synovial fluid which lubricates the joint. So, let’s take a look at these details a little bit more closely.
So here’s a classic example of a synovial joint, the elbow, and the image that we’re looking at now on the screen is a sagittal section through the arm, the elbow, and the forearm, and over here, we have the trochlea of the humerus and distal to that is the trochlear notch of the ulna. So, let’s revisit our first criteria, that the articular surfaces must be lined with hyaline or articular cartilage. So we can see hyaline cartilage on both the humerus and the ulna. Next, the joint is enclosed within an articular capsule and the articular capsule is highlighted in green right now.
So for an external view, the articular capsule can be seen in the image on the right of the screen, and this capsule is made up of fibrous connective tissue and provides a lot of the strength and stability for the joint. On the deep side of the articular capsule is a membrane called the synovial membrane and the capsule lined by the synovial membrane helps to form the synovial cavity, which is very characteristic of synovial joints.
The final thing to point out is that this cavity is filled with synovial fluid which is secreted by the synovial membrane to keep the articular surfaces nice and lubricated.
So where can we find synovial joints in our body? Well, synovial joints are the most common joints that we have, so your shoulder, your knee, the sternoclavicular joint, and proximal tibiofibular joint are all examples of synovial joints. However, that is, that they all have different ranges of motion, and we’ll get into that as we look at axis of movement and run through the types of synovial joints.
So joints that move are classified based on the number of axis that they move around. So imagine a graph – I know, high school maths, not the most fun, but it is helpful sometimes. So, do you remember this vertical axis? And if you recall, this is the y-axis. So, I’d like you to imagine that the y-axis is a pin. Now, this door can swing around the pin of the y-axis and move in one of two directions. It can either open the door or it can close the door. So, just to bring it back to our joints. Some joints in the body are like these. They move around one axis point and one axis only and this classifies them as uniaxial joints.
Now for joint can move around the y-axis and another one, say the x-axis, it then has two axes around which it can move. So this image, for example. Let’s put a pin in one axis and the blue part can then move in this direction and this direction. Now, if we put a pin in the second axis like this. We add another axis of movement to the left and to the right, and joints such as these are called biaxial joints.
If the joint is even more flexible than that and can move around more than two axes, so three or more, it’s called a multiaxial joints and we’ll run into some examples of these as we look at the various types of synovial joints coming up.
So let’s finally jump into what the types of synovial joints actually are, and to remind ourselves, here’s the list that we’re going to go through. So we have the ellipsoid, saddle, pivot, hinge, gliding, and finally, ball-and-socket.
So first up are our ellipsoid joints, also known as condyloid joints. In an ellipsoid joint, the rounded end of one bone articulates with a shallow depression in the other bone and these are biaxial joints meaning that they can move on two different axes or planes. If you think of your knuckles or more precisely your metacarpophalangeal joints, these joints can move from anterior to posterior, bending and straightening your fingers, and they can also move side to side medial and laterally spreading your fingers apart and then bringing them back together. And you can see the rounded end of the metacarpal bones articulating with the shallow depression in the proximal phalanx of each digit.
Next up is the saddle joint, and the way the bones fit together here is similar to the ellipsoid joint, just a bit more dramatic. The blue part of this image is very concave and articulates with the convex part of the gray structure, and this is characteristic of a saddle joint.
A classic example of a saddle joint is the first carpometacarpal joint which we can see highlighted in the image on the right, and saddle joints just like ellipsoid joints are biaxial. And these planes along which this joint can move is what allows for the opposable movement of our thumbs, which I think is pretty cool.
Another type of synovial joint is a pivot joint, and this joint is named primarily for the movement it allows – pivoting. In order to accommodate this, these joints usually have a rounded portion of bone acting as the axis for another bone to pivot around which in this image is the vertical grey bit. The other bone will form an enclosed ring around this rounded bone with the help of a ligament.
A good example of a pivot joint is the atlantoaxial joint which is between C1, the atlas, and the dens of C2, the axis. So, C2 is also called the axis because of the pivot point it creates for the atlas and C1 forms an enclosed ring around the dens with the help of the transverse ligament of the atlas. The rotation of the atlas around the dens is the movement we make when we shake our heads no, and because this movement is around a single atlas, pivot joints are considered uniaxial joints.
Another uniaxial joint is a hinge joint which is what we’ll look at next. Hinge joints involve a convex end of one bone articulating with a concave end of another bone allowing movement along only one axis. A great example of a hinge joint is the elbow where the convex trochlea of the humerus slots nicely into the concave trochlear notch of the ulna, and movement at the elbow only occurs along one axis and is our ability to flex and extend the forearm at the elbow joint.
Alright, four down and two to go. So next stop is our gliding joints, also called a plane joint. And the gliding joints do exactly that. They permit a gliding movement within the joint capsule. To be able to do this, the surfaces of the bones that articulate have to be quite flat as you can see in the images.
Between the carpal bones, the intercarpal joints is where we can find a few gliding joints, and because the articulating surfaces are fairly flat, the bones can move against each other in many directions making gliding joints multiaxial. Other examples of gliding joints include the intertarsal joints and the acromioclavicular joint.
And the last but not least is the synovial joint type most people know about and this is the ball-and-socket joint. The name of this joint gives away its structure exactly, so one bone which in this image is the blue structure has a ball-shaped articular surface while the other in this image, the gray structure has a concave socket-shaped articular surface.
And the shoulder, of course, is an excellent example of this ball-and-socket joint and the huge range of motion this type of joint allows. So the head of the humerus acts as the ball while the glenoid cavity of the scapula forms the socket. The shoulder is capable of flexion and extension, abduction and adduction as well as rotation making it a multiaxial joint.
So what if something goes wrong in a synovial joint? What can happen then?
So remember the articular cartilage that lines the articular surfaces of bones within synovial joints, this cartilage can get damaged even just by general wear and tear which can lead to quite a lot of pain, and as the cartilage wears away, the surfaces of the bones are exposed and can start to grind against each other. Now, that that doesn’t sound very nice, does it? So this condition is called osteoarthritis and can occur in any synovial joint in the body.
The main symptoms include joint pain and stiffness while some people also experience swelling and crackling noises when moving the affected joint. Unfortunately, osteoarthritis is a long-term condition that can’t be cured, but there are various treatments available to manage pain and other symptoms such as special devices designed to reduce strain on the affected joint and exercise and losing weight. In serious cases, pain medication and/or surgery might be suggested.
So now you’re an expert on types of synovial joints. But before I let you go, let’s have a quick review of what we looked at today.
So before getting into the types of synovial joints, we first looked at what they are in general including their own anatomy. So synovial joints have hyaline cartilage including the articular surfaces of the bones involved and they are enclosed by an articular capsule which is lined internally by the synovial membrane, and this enclosure forms the joint cavity which is filled with synovial fluid secreted by the synovial membrane for joint lubrication.
We then jumped into the various types of synovial joints starting with the ellipsoid joint, and these biaxial joints have a slightly rounded articular end of a bone articulating with a shallow depression in the other bone allowing movement around two axis.
Saddle joints came up next with the convex saddle-shaped articular surface sitting on the concave articular surface of the adjacent bone and this is also a biaxial joint. Pivot joints are uniaxial allowing movement without a single axis with an enclosed ring formed of bone and a ligament rotating around a rounded pivot point such as the dens of C2. Next up were hinge joints such as the elbow, and here are very concave articular surface sat within a very convex surface of the adjacent bone and this deep articulation means movement is only able to happen along one axis making hinge joints uniaxial.
Gliding joints are present between bones that have relatively flat surfaces, and because of this, they’re able to move in many directions making them multiaxial joints. And the final type of synovial joint we looked at was the ball-and-socket joint, and these are exactly as they sound. A convex ball-shaped articular surface and a concave socket-shaped one, and these allow for a high range of motion in all directions making them multiaxial joints.
And we finished off by looking at damage wear and tear to synovial joints and how that can lead to osteoarthritis.
And that brings us to the end of our tutorial. I hope you enjoyed it. Thanks for joining me.