Structural Class: Synovial Joints - Online Tutor, Practice Problems & Exam Prep

Synovial joints, the most common type in the human body, allow dynamic movement and are classified as diarthroses. They feature a synovial cavity filled with synovial fluid, articular cartilage for reduced friction, and an articular capsule for stability. Unique elements like bursae, fatty pads, and articular discs enhance function and protection. Weeping lubrication occurs when synovial fluid is absorbed and released during joint compression, acting as a lubricant and shock absorber. Understanding these components is crucial for grasping joint mechanics and injury prevention.

Synovial Joints

In this video, we're going to talk about the 3rd and final structural class of joint in our lesson, which is the synovial joints class. It turns out that most of the joints in the human body are actually classified as synovial joints. If you were to ask a random person on the street to point out a joint in the human body, they're probably going to point out a synovial joint, such as the shoulder, the elbow, the knees, or the knuckles in our hand. These synovial joints are distinguished from other types of joints in that they have dynamic or a free range of movement, and therefore, these synovial joints are always functionally going to be classified as diarthroses, which is why we have the symbol for diarthroses here in our lesson. Once again, notice that this symbol for diarthroses has multiple movement arrows going around it, which can hopefully help remind you that these diarthroses allow for dynamic movement.

It is important to note that different synovial joints can vary in the degree of movement they allow for. Some synovial joints allow for more free movement than other synovial joints. But in general, synovial joints allow for more movement than other types of joints. Recall that the mobility of a joint comes at the cost of the stability of a joint. Because these synovial joints allow for dynamic and free movement, that also means that they are quite unstable and prone to injuries such as dislocations, like a shoulder dislocation for example. This is why synovial joints have the following unique characteristics that you can see down below in the text to help reinforce them and make them a little bit more stable than they otherwise would be. Notice that the text is very nicely color coordinated to the image down below, which we'll get to here very shortly.

The first unique characteristic of synovial joints is the presence of a synovial cavity, also sometimes referred to as a joint cavity. Really, this is just a space that exists between the bones of the synovial joint. This cavity or space actually stores synovial fluid. Synovial fluid is a viscous or thick liquid that has many functions, including serving as a lubricant to ensure the smooth movements within the synovial joint and acting as a shock absorber to help protect the components of the synovial joint.

The next unique feature of synovial joints is the presence of articular cartilage. Really, this is just hyaline cartilage that covers the ends of the synovial joint bones. Because it covers the ends of the bones where they meet, it creates a slippery and smooth surface that reduces friction between the bones when they move in the synovial joint. The articular cartilage also serves as a shock absorber to help protect the components of the synovial joint.

The last unique feature of synovial joints is the presence of an articular capsule, which surrounds the entire synovial joint as two layers, an inner layer and an outer layer. The inner layer is a synovial membrane, which lines the synovial cavity and produces the synovial fluid. The second layer is an outer fibrous layer that's made of fibrous connective tissue or connective tissue proper, specifically, dense irregular connective tissue. Recall from our previous lesson videos that dense irregular connective tissue has fibers in its extracellular matrix that are densely packed together and arranged in an irregular or random pattern, which allows it to resist forces in multiple directions, which is really important for a synovial joint to be able to do when it has dynamic movement. These unique characteristics help to make the synovial joint more stable than it otherwise would be without them.

Let's take a look at our image down below where we can start to piece things together. Notice over here on the left-hand side, we're showing you the bones of the hand and we're focusing specifically on one of the synovial joints of the pointer finger right over here. We know that our pointer finger has quite a lot of dynamic movement. This joint is considered a synovial joint. Notice that here we are highlighting the bones of the synovial joint and notice that the space that exists between these bones highlighted here in pink, is the synovial cavity or the joint cavity. We can go ahead and label it as so over here. Recall that the synovial cavity stores the synovial fluid, which is a very viscous or thick liquid that has many functions, including acting as a lubricant to ensure the smooth movements of the synovial joint and also serving as a shock absorber to help protect the components of the synovial joint.

We can also clearly see the articular cartilage, which is going to be the hyaline cartilage that covers the ends of the bones in the synovial joint highlighted here in blue. Recall the articular cartilage creates a very smooth and slippery surface that reduces friction when these bones are moving in the synovial joint, and it also acts as a shock absorber to help protect the components of the synovial joint. And then last but not least, we can also see the articular capsule over here, which again consists of two layers, the inner synovial membrane, which you can see highlighted in green, which is lining the synovial cavity, and it produces the synovial fluid that fills the synovial cavity. You can also see the outer fibrous layer, which is immediately outside of the synovial membrane, highlighted here. It turns out that this outer fibrous layer is actually a thickened continuation of the periosteum, which is the outermost layer of the bone. This outer fibrous layer is going to allow for the flexible movements of the synovial joints, but it's also going to help reinforce that synovial joint to make it more stable than it would be without it.

Although ligaments are not a unique characteristic of synovial joints, ligaments are present in synovial joints. Again, they help to reinforce the synovial joints to make them more stable than they would be without them. Although it's not shown, it's really important to note that synovial joints are nourished by blood vessels. That's important for the synovial membrane to continuously produce synovial fluid. Synovial joints are also innervated by nerves, which is important for us to be able to sense pain and also to be able to sense the position and the movements of the synovial joints. Although those are not shown here, it's important to keep that in mind. They have blood vessels, and they also are innervated with nerves. This concludes our brief introduction to synovial joints, and as we move forward in our course, we'll be able to learn more and apply these concepts in practice problems. I'll see you in our next video.

Structural Class: Synovial Joints Example 1

So here we have an example problem that asks, which of the following is not a unique characteristic of synovial joints? And we've got these 4 potential answer options down below. Now option A says dynamic movement. Recall from our last lesson video that synovial joints are always functionally classified as diarthroses because they allow for dynamic or free movement. And so really, this is a unique feature of synovial joints because the other structural classes of joints, including the fibrous joints and cartilaginous joints, do not allow for this dynamic movement. And so really synovial joints are the only ones classified as diarthroses, and they're the only ones that are going to allow for this dynamic movement. So dynamic movement is a unique feature of synovial joints, and for that reason, we can cross this option off of our list because we're looking for the feature that is not unique to synovial joints.

Now moving on, option B says ligaments. And so again, ligaments can be found reinforcing synovial joints. However, ligaments are not unique to synovial joints because ligaments can also be found in the other structural classes of joints, including some fibrous joints and some cartilaginous joints. And so for that reason, ligaments is a characteristic that is not unique to synovial joints. And for that reason, option B must be the option that we are looking for. So we can go ahead and indicate option B as the correct answer to this problem. But just in case, let's check the last two options here.

Option C says articular capsule, and recall that this consists of 2 layers, the inner synovial membrane and the outer fibrous layer. And so the articular capsule is a unique feature or a unique characteristic of synovial joints. So for that reason, we can cross it off this list. And then option D says synovial fluid, which acts as a lubricant and a shock absorber in the synovial cavity. And so it is again a unique feature of synovial joints, so we can cross it off this list. And so option B, ligaments, is the only option here that is not unique to synovial joints, so that is the correct answer. That concludes this example, and I'll see you all in our next video.

Which of the following is a unique feature of synovial joints?

• A. Presence of ligaments.
• B. Joint movement.
• C. Presence of a joint cavity.
• D. Presence of hyaline cartilage.

Weeping Lubrication

In this video, we're going to talk briefly about weeping lubrication of synovial joints. Weeping lubrication is the name of a process where synovial fluid in articular cartilages is forced out of the articular cartilages upon compression of the synovial joint. And when pressure on that synovial joint is relieved, the synovial fluid that was forced out of the articular cartilages is actually soaked back into the articular cartilages, kind of like how water soaks into a sponge. And so really, articular cartilage is analogous to a sponge in this weeping lubrication process. Let's take a look at our image to understand this a bit better. And so notice on the left-hand side, we're focused then on a relaxed knee joint since we can see this person relaxing on this beach chair over here. We know that the knee joint is an example of a synovial joint due to its dynamic movement. And so zooming into this relaxed knee joint over here, notice that the articular cartilage, which covers the ends of the bones, serves as a sponge to absorb the synovial fluid. And so notice here we have an image of a sponge where again the sponge is analogous to the articular cartilage, and we can see that the synovial fluid shown here in this reddish-pinkish color is being absorbed into the sponge or the articular cartilages. And again, this happens during the relaxed state of the knee joint. And this allows the synovial fluid to nourish the cells or the chondrocytes found within the articular cartilage.

Now on the right-hand side over here, we're focused on a compressed knee joint. And so notice that the person no longer is laying down on a beach chair relaxing. The person is now running. And so you can see that their knee joint is being compressed in this fashion. And so zooming in on that compressed knee joint, again, you can see the compression here with these arrows. Notice that the articular cartilage is also going to be compressed, and that means that the synovial fluid is going to be forced out upon compression, and that allows the synovial fluid to serve as a lubricant and a shock absorber to help protect the articular cartilage during this compression and this movement, and also protect the other components of the synovial joint as well. And so really this is all we have on the weeping lubrication of synovial joints, and we'll be able to get some practice applying these concepts and learn more about synovial joints as we move forward in our course. So I'll see you all in our next video.

Structural Class: Synovial Joints Example 2

So here we have a pretty straightforward example problem that wants us to use the words in the word bank down below to fill in the blanks and complete the sentences. And so the first sentence says, as a synovial joint compresses, the pressure from the bones coming together squeezes the articular cartilage. And so as a synovial joint compresses is the first interactive blank here, so we can cross it off the pressure from the bones coming together squeezes the articular cartilage. And so we can cross off cartilage from here. As this happens, the synovial fluid is squeezed out into the synovial cavity, lubricating the joint and reducing friction. And so we can cross these off. And this here concludes this example problem, so I'll see you all in our next video.

Which of the following statements accurately describes weeping lubrication in synovial joints?

• A. It involves the secretion of tears from the lacrimal glands into the synovial cavity.
• B. It relies on the release of mucus from the goblet cells within the synovial membrane.
• C. Synovial fluid is squeezed out of articular cartilage during compression & reabsorbed during decompression.
• D. It occurs when blood vessels within the joint capsule deliver nutrients to the articular cartilage.

What role does articular cartilage play in synovial joints?

• A. It reduces stress on the joint.
• B. It stores synovial fluid, allowing for weeping lubrication.
• C. It lines bone ends, preventing them from coming into direct contact with each other.
• D. All three answers above are correct.

Additional Features

In this video, we're going to talk about additional features that are found in just some synovial joints, but not in all synovial joints. And the first are going to be bursae. Bursae are synovial fluid-filled sacs that are strategically located in some synovial joints to reduce friction between ligaments or tendons and other tissues. By reducing friction, they also help to prevent damage. A tendon sheath is really just an elongated bursa, so it's just a longer bursa that wraps around and protects a tendon, as its name implies. It therefore helps to reduce friction and again prevent damage during the movement of some synovial joints.

Next on our list are fatty pads, which, as their name implies, are going to be made up of fat or adipose tissue. These fatty pads are going to be present in the knee joint and the hip joints, which are load-bearing joints that need to bear the weight of our body. They do need these extra layers for cushioning. The fatty pads help to provide an extra layer of cushioning between the outer fibrous layer and the inner synovial membrane of the articular cartilage, again, of some joints, specifically the knee and hip joints.

Next on our list are articular discs, which are also known as menisci. These are discs of fibrocartilage or of fat that divide the synovial cavity into two spaces. These articular discs have many different functions, including acting as shock absorbers to protect those joints. Articular discs are found in the knee and the jaw and in a few other joints as well.

Let's take a look at our image down below where we can start to visualize some of these additional features. Notice on the left-hand side, we're focusing on the knee joint. You can see that this is a zoom-in of the knee joint. What you'll notice is that there are these purple structures throughout the knee joint, and those purple structures are representing the bursae or bursa for the singular form. The image on the right over here is a zoom-in of the shoulder joint. What you'll notice is that the shoulder joint also has a bursa strategically located right here in this position. Again, the bursa is just a synovial fluid-filled sac that's going to help reduce friction within that synovial joint.

Notice that the shoulder joint has a tendon sheath, which is just an elongated bursa that wraps around a tendon. You can see that there's a tendon here, and the tendon sheath is wrapping around that tendon, again to help reduce friction and prevent damage. Next, over here, what we have is a fatty pad. You can see that this fatty pad is located right here in this position and it is strategically located in the knee joint to help provide an extra layer of cushioning. What we have is an articular disc or a meniscus located right here in this position, and you'll notice that there's another one over here as well. What it does is it separates the synovial cavity into two separate spaces, which can help to relieve stress and redirect synovial fluid into specific regions to relieve the synovial joint of stress, provide stability, and act as a shock absorber as well.

This here concludes our brief lesson on these additional features that again are found in just some synovial joints, not in all synovial joints, and we'll be able to get some practice applying these concepts moving forward. I'll see you all in our next video.

Structural Class: Synovial Joints Example 3

So here we have an example problem that asks which of these statements about bursae and tendon sheaths is false? And we've got these 4 potential answer options down below. Now option a says that the primary function of both bursae and tendon sheaths is to reduce friction at synovial joints. And so recall from our last lesson video that this is actually a true statement. And because it's a true statement, it's not the false answer option that we're looking for, so we can cross off option A. Now moving on, option B says that they are found in all synovial joints. But recall from our last lesson video, we said that those structures, such as bursae and tendon sheaths, are only found in some synovial joints, not in all synovial joints. And so what this means is that option B is actually a false statement. And so that means that it is the false answer option that we're looking for, so we can indicate option B is the option, the correct option for this problem. But let's check the other 2 just to be sure. Option C says that they can be found associated with some, but not all synovial joints in the body. And again, we know that this is a true statement, so it's not a false answer option that we're looking for. So we can cross it off. And then option D says that they are both fiber sacks filled with synovial fluid. And again, this is a true statement. Recall, really, the most significant difference between bursae and tendon sheaths is that the tendon sheaths are much longer. They are elongated structures. But, outside of that, they are both fiber sacks filled with synovial fluid. So again, because it's true, it's not the false answer option that we're looking for. So again, option B is the correct answer to this problem. That concludes this problem, and I'll see you all in our next video.

Imagine you're a doctor and a patient comes to you because of pain in their knee joint. You find that there is not enough synovial fluid present in their knee's synovial cavity, leading to friction when the knee moves. Which of the following structures in the knee joint is not functioning correctly?

• A. Bursae.
• B. Articular cartilage.
• C. Articular capsule.
• D. Synovial membrane.

Here’s what students ask on this topic:

What are the unique characteristics of synovial joints?

Synovial joints are characterized by several unique features that distinguish them from other types of joints. Firstly, they have a synovial cavity, a space between the bones filled with synovial fluid. This fluid acts as a lubricant and shock absorber. Secondly, the ends of the bones in synovial joints are covered with articular cartilage, specifically hyaline cartilage, which reduces friction and also serves as a shock absorber. Lastly, synovial joints are enclosed by an articular capsule, which has two layers: an inner synovial membrane that produces synovial fluid and an outer fibrous layer made of dense irregular connective tissue, providing stability and flexibility.

How does weeping lubrication work in synovial joints?

Weeping lubrication is a process where synovial fluid is forced out of the articular cartilage when the joint is compressed and then reabsorbed when the pressure is relieved. This mechanism is similar to how a sponge absorbs and releases water. During joint compression, the synovial fluid acts as a lubricant and shock absorber, protecting the joint components. When the pressure is relieved, the fluid is soaked back into the cartilage, nourishing the chondrocytes (cartilage cells). This cyclical process helps maintain joint health and function.

What are bursae and tendon sheaths, and what roles do they play in synovial joints?

Bursae are synovial fluid-filled sacs located in some synovial joints to reduce friction between ligaments, tendons, and other tissues, thereby preventing damage. Tendon sheaths are elongated bursae that wrap around tendons, providing similar friction-reducing and protective functions. Both structures are crucial for smooth and efficient joint movement, especially in areas with high mechanical stress, such as the shoulder and knee joints.

What are articular discs (menisci), and what functions do they serve in synovial joints?

Articular discs, also known as menisci, are fibrocartilage or fat discs that divide the synovial cavity into two spaces. They serve multiple functions, including acting as shock absorbers, distributing load across the joint, and enhancing joint stability. Menisci are found in joints like the knee and jaw, where they help to relieve stress, redirect synovial fluid, and provide additional cushioning and support during movement.

What is the role of synovial fluid in synovial joints?

Synovial fluid plays a crucial role in synovial joints by serving as a lubricant to ensure smooth movement and as a shock absorber to protect joint components. It is produced by the synovial membrane lining the synovial cavity. The fluid also nourishes the articular cartilage and chondrocytes, maintaining joint health. During activities, synovial fluid is forced out and reabsorbed by the cartilage, a process known as weeping lubrication, which further aids in joint protection and function.