视频：Types of synovial joints

Well, this person doesn't look like she's doing very well. What's going on? Any guesses as to what today's tutorial might be on? Any guesses? A lack of something that might lead to her not moving properly? All right, 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!

In this tutorial, we'll start by looking at what a joint is and which classes of joints exist. Then we'll examine the structure of a synovial joint and describe its main components like the articular cartilage, articular capsule, synovial membrane, joint or articular cavity, and reinforcing ligaments. After that, we'll dive into looking at the six types of synovial joints – plane joints, pivot joints, hinge joints, ellipsoid joints, saddle joints, and finally, ball and socket joints. We'll wrap up with a clinical scenario to illustrate why these joints are so important. But first and foremost, let's start with the basics and see what exactly a joint is and which types of joints we have in our body.

A joint itself is a site where two bones meet. Some joints allow movement and some don't. Based on the range of motion allowed at the joint and the type of tissue holding the bones together, joints are classified into three main types: Fibrous joints, like the sutures in the skull. In this type of joint, the bones are tightly linked by fibrous tissue and very little movement is possible. Cartilage joints, like the fibrocartilaginous intervertebral discs. Here the space between the bones is filled with a cartilaginous pad and they are a bit more movable than fibrous joints. And synovial joints, the most common type of joint in the body. Synovial joints are capable of a large range of motion between the bones they join. Big joints of the limbs like your shoulder, elbow, wrist, hip, knee, and ankle are all examples of synovial joints. Today's tutorial is all about synovial joints, so let's focus on the structure of this type of joint.

To describe the structure of a synovial joint, we're going to use the knee as an example. Here we have a sagittal section of the knee. Notice that we have three bones articulating here – the femur, the tibia, and the patella. The knee is therefore a complex joint composed of two articulations – the tibiofemoral joint between the tibia and the femur and the patellofemoral joint between the patella and the femur. So let's explore the different components of a synovial joint.

In synovial joints, the articular surfaces of the bones are lined with a thin layer of hyaline cartilage. This is known as articular cartilage. It provides a smooth surface for articulation which decreases the friction between bones and also facilitates the transmission of load from one bone to another. The articular capsule is highlighted in green now.

From an external view, the articular capsule can be seen in these anterior and posterior views of the knee. This capsule is made of fibrous connective tissue and provides a lot of strength and stability to the joint. Internally, the articular capsule is lined with what's known as a synovial membrane. The synovial membrane attaches to the margins of the joint surfaces at the interface between the cartilage and bone. The synovial membrane encloses the articular cavity or 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.

Parts of the capsule may thicken to form accessory ligaments which further stabilize the joint. Extracapsular ligaments outside the capsule usually provide additional reinforcement. In this image, we can see the patellar ligament of the knee. In certain synovial joints like the knee or the temporomandibular joint, discs or wedges of fibrocartilage are interposed between the articular surfaces of the bones. These are referred to as articular discs. In the case of the knee, we have two of these and they are called menisci. Articular discs absorb compression forces, adjust the changes in the contours of joint surfaces during movements, and increase the range of movement that occur at these joints.

In the case of the knee, they also make the articulating surfaces more congruent, meaning they help the bones of the joint fit better. Without the menisci, the rounded femoral heads would sit on top of a flat tibial plateau. By increasing congruence in this way, the joint stability of the knee is improved. As the knee joint is the most stressed joint in the body, it's not surprising that it needs 2 menisci and that they are often damaged.

Some synovial joints contain fat pads like this one here lying between the synovial membrane and the fibrous capsule or bone. They provide protection by absorbing the impact, shear forces, and pressure applied to the joint during movement.

Besides ligaments, further reinforcement of the capsule can be provided by surrounding tendons and muscles as is the case of the quadriceps femoris tendon in the knee. Closed sacs of synovial membrane also occur outside joints where they form synovial bursae. Bursae often intervene between bony prominences, muscles, tendons, and ligaments and work to reduce friction helping them glide and slide past one another while movement occurs.

Okay, so before jumping into the different types of joints, let me just explain a concept – the axes of movement of a joint.

So joints that move are classified based on the number of axes they can move around. Imagine a graph – I know, high school maths, not the most fun, but it's helpful. Remember this vertical axis? Yup, the y-axis. Imagine 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 – to open or close the door. Some joints in the body are like this. They move around one axis point and one axis only. This classifies them as uniaxial joints.

Now, if a joint can move around the y-axis and another one, say the x-axis, it then has two axes around which it can move. This image, for example. Let's put a pin on one axis. 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 right. Joints such as this are called biaxial joints. If a joint is even more flexible than that and can move around more than two axes, so three or more, it is called a multiaxial joint. We'll run into some examples of these as we look at the various types of synovial joints coming up.

And now it's finally time to look at the different types of synovial joints.

To remind ourselves, here's the list we'll go through – plane joints, pivot joints, hinge joints, ellipsoid joints, saddle joints, and finally, ball and socket joints.

Let's start with the plane joint, also called a gliding joint. These joints do exactly that. They permit a gliding movement within their joint capsule. To be able to do this, the surfaces of the bones that articulate have to be relatively flat as you can see in the image. Between the carpal bones, intercarpal joints is where we can find a few gliding joints. Because the articulating surfaces are fairly flat, the bones can move against each other in many directions; however, the fact that no rotation occurs makes plane joints non-axial. In this image, you can see the acromioclavicular joint – another example of a plane joint.

Another type of synovial joint is a pivot joint. 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 through which the axis of rotation is located for another bone which pivots around. A good example of a pivot joint is the median atlantoaxial joint which is between vertebra C1 – the atlas – and the dens of vertebra C2. C2 is also called the axis because of the pivot point it creates for the atlas. 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”. Because this movement is around a single axis, pivot joints are considered uniaxial joints. Other examples of pivot joints are the radioulnar joints which allow the movement of pronation and supination of the forearm.

Another type of uniaxial synovial joint is a hinge joint which is what we'll look at next. Hinge joints involve a convex end of one bone articulating with the concave end of another bone allowing movement along only one axis that passes transversely through the joint. Hinge joints resemble a hinge of a door. These joints permit flexion and extension.

Another great example of a hinge joint is the elbow where the trochlea and capitulum of the humerus slots nicely into the concave trochlear notch of the ulna and the head of the radius. Another example of a hinge joint is the ankle or talocrural joint. The joint capsule of hinge joints is thin and lacks anteriorly and posteriorly where movement occurs; however, the bones are joined by strong laterally placed collateral ligaments.

Unlike the elbow and ankle, the knee joint is not a true hinge since it has a rotational component – an accessory motion that accompanies flexion and extension; hence, it is termed a modified hinge joint. It is also sometimes called a bicondylar joint.

Ellipsoid joints are also known as condylar or condyloid joints. In this type of joint, the convex end of one bone known as the condyle articulates with a concave surface on the other bone. These are biaxial joints, meaning they can move on two different axes or planes. They permit flexion and extension as well as abduction and adduction. Simultaneous movement around both axes is usually possible in a limited manner producing a circular motion known as circumduction.

Think of your knuckles, or more properly, your metacarpophalangeal joints. You can see the rounded end of the metacarpal bones articulating with the shallow depression in the proximal phalanx of each digit. These joints can move anterior-posterior, bending and straightening your fingers. In other words, they are capable of flexion and extension. They can also move side to side, lateral and medial, spreading your fingers apart and bringing them back together which represents the movement of abduction and adduction respectively. Here we have another example of an ellipsoid joint – the wrist or radiocarpal joint.

Next up is the saddle joint, also known as a sellar joint. The way the bones fit together here is somewhat similar to the ellipsoid joint just a bit more dramatic in that both participating bones have reciprocally convex and concave articular surfaces which articulate like two saddles opposed against one another. This type of joint allows movements around two axes that are right angles to each other.

Just like ellipsoid joints, saddle joints are biaxial. They permit flexion and extension as well as abduction and adduction. Circumduction is also possible. A classic example of a saddle joint is the first carpometacarpal joint or carpometacarpal joint of the thumb which we can see highlighted in this image. The planes along which this joint can move is what allows for the opposable movement of our thumbs. Pretty cool, right?

Last but not least is the synovial joint type most people know about – the ball and socket joint. The name of this joint gives away its structure exactly. One bone – in this image 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. This arrangement of the articular surfaces allow movement around multiple axes so these are multi-axial joints. They permit flexion and extension, abduction and adduction, internal and external rotation, and circumduction.

The shoulder is an excellent example of the ball and socket joint and the huge range of motion this type of joint allows. The head of the humerus acts as the ball while the glenoid cavity of the scapula forms the socket. The hip joint is also a ball-and-socket joint in which the spherical head of the femur rotates within the socket formed by the acetabulum of the hip bone.

So what if something goes wrong in a synovial joint? What can happen there? 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. As the cartilage wears away, the surfaces of the bones are exposed and can start to grind against each other. Doesn't sound very nice, does it?

This condition is called osteoarthritis and can occur at any synovial joint in the body. 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 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.

And now you're an expert on types of synovial joints. Before I let you go, let's have a quick review of what we looked at today.

We started by looking at the definition of a joint – a site where two skeletal elements come together – and described the three classes of joints – fibrous joints, cartilage joints, and synovial joints, the most movable and common type of joint in the body.

We then looked at the structural anatomy of a synovial joint. Synovial joints have hyaline cartilage lining the articular surfaces of the bones involved. They are enclosed in an articular capsule which is lined internally by the synovial membrane. This enclosure forms the joint or articular cavity which is filled with synovial fluid secreted by the synovial membrane for joint lubrication. Synovial joints are usually reinforced by accessory ligaments that are either separate or are a thickening of a portion of the joint capsule.

We then jumped to the various types of synovial joints starting with the plane or gliding joints that permits gliding or sliding movements between opposed surfaces of the bones which are relatively flat. Examples include intercarpal joints and the acromioclavicular joint. Pivot joints permit rotation around the central axis such as the dens of C2 in the median atlantoaxial joint. Other examples of pivot joints are the radioulnar joints.

Next up were hinge joints such as the elbow. They resemble the hinge of a door and allow uniaxial flexion-extension movements. Another example of hinge joints is the ankle. In ellipsoid joints like the metacarpophalangeal joints, a convex articular surface is received into an elliptical concave articular surface. Biaxial movements of flexion and extension, abduction and adduction, and limited circumduction are allowed. The wrist is also an ellipsoid joint.

In saddle joints, the opposing articular surfaces are shaped like a saddle; that is, they are reciprocally concave and convex. They are biaxial joints and allow for movements of flexion and extension, abduction and adduction, and circumduction. The carpometacarpal joint at the base of the thumb is a saddle joint. In ball and socket joints, a ball-shaped head of one bone fits into a socket-like concavity of another allowing multiaxial movement. They permit flexion, extension, abduction, adduction, circumduction, and rotation. The shoulder and the hip are examples of this type of joint.

We finished off by looking at the damage, wear and tear to synovial joints and how that can lead to osteoarthritis.

And that brings us to the end of the tutorial. I hope you enjoyed it. Thanks for joining me and happy studying!