Types of joints

Do you feel like all of your anatomy knowledge hinges on this tutorial? Maybe you're worried that it won't be plain sailing. But don't be anxious, because we're going to have a ball – and socket. Saddle up for our tutorial on the types of joints found in the human body.

A joint is defined as a connection between two bones in the skeleton and there are many ways in which joints can be classified. For example, they can be classified according to their structure, by their mobility, and according to their range of motion. Okay, let's begin with the structural classification of the joints.

There are three distinct types – the synovial joint, the fibrous joint, and the cartilaginous joint. Let’s start with the synovial joint as it's the most common joint found in the human body.

Synovial joints have several characteristic features which we can see using this sagittal illustration of the knee joint. First of all, synovial joints are surrounded by an articular capsule. This capsule consists of an outer fibrous layer which helps us to stabilize the joint and an inner synovial layer which absorbs and secretes synovial fluid lubricating the joint. Next, it's important to know that the articular surfaces of a synovial joint are covered in hyaline cartilage. This cartilage is also known as articular cartilage and acts to reduce friction and assists in shock absorption.

Additional structures may also be found within synovial joints such as articular discs. We can see examples of these in the knee joint – the medial and lateral menisci. We may also see bursae, which are small sacs lined by synovial membrane and filled with synovial fluid. The bursae act to reduce friction caused by muscles and tendons which are located over bony joints.

Before we move on to the next type of joint, it's worth mentioning that synovial joints can be further subclassified according to the shape of their articular surfaces and their range of motion. These include the ball-and-socket joint, the hinge joint, the pivot joint, the condylar joint, the saddle joint, and the plane joint. We’ll talk about these joints in more detail later on in our tutorial.

Ok, let's move on to our next joint type, which is the fibrous joint. In a fibrous joint, the bones are bound by a tough fibrous connective tissue. These joints exhibit little to no mobility. The types of fibrous joints we’re going to talk about today are sutures, gomphoses, and syndesmoses.

Firstly, we have sutures, which are fibrous joints found exclusively between the bones of the skull. We can see a nice example in our illustration here, which is the coronal suture. This suture connects the frontal bone to the parietal bones via suture ligaments.

Next, we have a gomphosis, which is found in the mouth where the roots of the teeth articulate with the dental alveoli at the dentoalveolar joints. The tooth is bound into its socket by the strong periodontal ligament, which we can see here highlighted in green.

Lastly, we have a syndesmosis, which is formed by ligaments and a strong membrane that holds two bones in place. We can see an example of this here where the interosseous membrane runs between the radius and the ulna.

The last type of joint we're going to talk about is the cartilaginous joint. In a cartilaginous joint, the bones are connected by a fibrocartilage or hyaline cartilage. These joints can be subclassified into two types – synchondroses and symphyses.

First, we'll talk about a synchondroses or primary cartilaginous joint where the bones are connected by hyaline cartilage. In our histological image, we can see that this type of joint is found between the diaphysis and the epiphysis of a growing long bone and will eventually become completely ossified in adulthood.

The second type of cartilaginous joint is known as a symphyses or secondary cartilaginous joint where the bones are connected by fibrocartilage. This type of joint is found primarily along the midline of the body; for example, the pubic symphysis which we can see here highlighted in green.

Alright, time to move on to our next mode of classification, which is mobility. What are the key terms? A diarthrosis is a freely mobile joint, and an example of this is the knee joint which is a synovial hinge joint. It’s worth noting that every synovial joint is a diarthrosis. An amphiarthrosis is a slightly mobile joint, and an example of this is the pubic symphysis which is a secondary cartilaginous joint. The syndesmosis, which is a fibrous joint, is also an example of an amphiarthrosis. Lastly, we have a synarthrosis, which is an immobile joint and an example of this is the coronal suture of the skull. Other examples of this type of joint include the gomphosis, which is a fibrous joint, and the synchondrosis, which is a primary cartilaginous joint.

Keep in mind that although a joint may be slightly mobile, it does not perform functional movements; therefore, when we talk about range of motion, we'll be talking about our freely mobile diarthroses or our synovial joints.

The final way in which joints can be classified is according to their range of motion. When classifying a joint based on its range of motion, it's important to first understand the various axes of movement that the movements are occurring along. In joint movement, there are three main axes – the sagittal axis which passes horizontally from anterior to posterior, the frontal axis which passes horizontally from left to right, and the vertical axis which passes vertically from superior to inferior.

We can then describe joints by the number of axes they can move along. Joints that can move back and forth along a single axis are called uniaxial. Examples of these are the hinge joint and the pivot joint. Joints that move about two distinct axes are called biaxial; for example, the condylar joint and the saddle joint. Finally, joints that can move through all three axes are called polyaxial or multiaxial. The only example of this is the ball-and-socket joint.

Okay, time to bring together what we've learned so far and take a closer look again at the most common type of joint in the human body – the synovial joint.

Do you remember the six types? Let’s start off with the ball-and-socket joint, which is also known as the spheroid joint. This is the only polyaxial joint and, therefore, the most mobile of all joint types. The movements that occur at these joints are flexion and extension, which occur around a frontal axis; adduction and abduction, which occur around a sagittal axis; and internal rotation and external rotation, which occur around a vertical axis.

For example, ball-and-socket joints are pretty useful when you want to play in the snow and make a snow angel. Two examples of this joint are the acetabulofemoral joint, otherwise, known as the hip joint and the glenohumeral joint or the shoulder joint, which is shown here. Using the glenohumeral joint as a reference, we can see that this ball-and-socket joint has one ball-shaped articular surface – the head of the humerus – and one socket-like articular surface - the glenoid cavity.

Next, we have the hinge joint which is only one axis of motion making it uniaxial. Hinge joints allow movement around the frontal axis, which passes transversely through the joint. Therefore, the movements that occur at this joint are flexion and extension. Two examples of these joints are the tibiofemoral joint or the knee joint and the elbow joint. So we use our hinge joints when we show off – I mean flex – our biceps!

Like the hinge joint, the pivot joint is also uniaxial. So pivot joints, also known as rotary joints, allow movement around a single axis – this movement being rotation. The best example of this is the atlantoaxial joint, which occurs between the anterior arch of the atlas and the front of the odontoid process of the axis. The atlantoaxial joint allows you to shake your head when you've had enough.

Let’s move on to talk about the condylar joint, which is also known as the ellipsoid joint. Condylar joints allow movement around two axes that are at right angles to each other; therefore, they are described as biaxial joints. An example of this joint is the radiocarpal joint or the wrist joint. The movements that take place at this joint are radial deviation and ulnar deviation which occur around the sagittal axis and flexion and extension which occur around the frontal axis. These movements combine to produce circumduction of the wrist joint. So this is the joint that comes in handy when you want to wave hello.

Similarly to the condylar joint, the saddle joint is a biaxial joint. In this particular joint, we can see that the bones involved meet at the concave articular surface of one bone and the convex articular surface of the other and this is the connection that forms the saddle-shaped articulation. Again, the movements that take place at this joint are abduction, adduction, flexion, extension, circumduction. An example of such a joint is the carpometacarpal joint of the thumb, which is the joint necessary for the opposition of the thumb. So, this is the joint that allows you to text your bestie about how bad your day was.

The final type of joint we're going to have a look at in the body is the plane joint, which is also known as the gliding joint. This type of joint is unlike the other joints we've discussed in that it doesn't move around an axis and only permits movement along the plane of the articular surface. Therefore, it cannot be classified as a uniaxial, biaxial, or polyaxial joint. Instead, the plane joint performs a sliding or a gliding movement where one bone moves across the surface of another. An example of this joint is the acromioclavicular joint, which increases the flexibility of the shoulder.

Before we conclude this tutorial, I'd like to take a clinical detour and talk about joint stability.

Joint stability is of great clinical significance and helps explain why some joints are more susceptible to dislocation or injury than others. Ultimately, it's a trade-off. The more mobile a joint is the less stable it will be. However, there are specific factors that contribute towards joint stability.

The first factor to consider is the degree of contact between the two articulating surfaces. With less contact, the joint is more mobile but less stable. A good example of this is the glenohumeral joint where the humeral head is much larger than the glenoid fossa resulting in less contact between the two bones.

The second factor to consider is the presence of ligaments. Ligaments increase the stability of the joint, but they also restrict movement. In our illustration, we can see some of the ligaments associated with the knee joint.

The third and final factor to consider is the tone of the surrounding muscles. The best example of this is the rotator cuff muscles, which stabilize the glenohumeral joint. If there is a loss of tone such as seen in old age, the shoulder will be more susceptible to dislocation.

So that brings us to the end of our tutorial on the types of joints in the human body.