Hey everyone! This is Nicole from Kenhub, and in this tutorial, we will be discussing the types of joints found in the human body, the basic principles of joint mechanics and associated terminology.
So as you've probably already observed within your own body, the bones of the human body work together to create a structural scaffolding to protect the internal organs and to facilitate movement. And as we can see in our image, every bone in the body articulates with at least one other bone with the hyoid bone which is floating up around the neck around here being the only exception. Articulation which is the joining together of two bone structures is what makes movement possible and the point of articulation between two bones is called a joint. Joints are classified according to the range of motion they exhibit and the types of tissue that holds the bones together and, in our tutorial, we're going to begin with talking about the latter.
So in classifying joints according to the types of tissues holding the articulating bones together, we can describe three distinct types of joints and these are the synovial joint, the fibrous joint and the cartilaginous joint. And we're going to begin by looking at the synovial joint. And synovial joints are also called true joints. But what makes the joint a true joint? Let's have a look at some of the requirements below.
So a true joint must have a discontinuous connection or separation between the bones called a joint space, the articular surfaces of the joint must be covered in hyaline cartilage, it must have a closed joint capsule and therefore a joint cavity within filled with synovial fluid, and most importantly, it must have ligaments and muscles that act as primary joint stabilizers. The acetabulofemoral joint – or hip joint – pictured here is an example of a synovial joint. And a synovial joint is capable of a wide range of movements and consequently maybe susceptible to dislocation. Some synovial joints may also contain intraarticular surfaces that help with joint function such as the menisci of the tibiofemoral joint otherwise known as the knee joint.
So we just discussed how a synovial joint is considered a true joint and, in contrast, a fibrous joint is often considered a false joint, and this is due to the continuous connections between the articular surfaces of the bones created by intervening fibrous tissue and fibrous joints also exhibit low to moderate mobility.
A fibrous joint can be further classified into three subgroups. First, we have the synarthrosis joint which can be found between the flat bones of the skull and the synarthrosis joint is represented in this image by the cranial sutures. The cranial sutures act as the continuous fibrous connections occupying the joint space and the synarthrosis joint is a fairly immovable joint. Next, we have the syndesmosis joint which is formed by the interosseus ligaments holding the two bones in place and an example of this is the tibiofibular joint. And finally we have the gomphosis joint which can be found in the mouth where the roots of the teeth articulate with the dental alveoli, and this is called a dental-alveolar joint.
And now we're going to talk about our cartilaginous joints which, like our fibrous joints, are also called false joints. Cartilaginous joints are characterized by the continuous cartilage connection between two bones. And although this type of joint is classified as a false joint, it actually exhibits a moderate range of mobility which is greater than that exhibited by fibrous joints.
And there are two types of cartilaginous joints. The first type known as a synchondrosis joint or primary cartilaginous joint is where the hyaline cartilage meets with bone. For example, at the costochondral joints of the anterior thoracic cavity, we can find a synchondrosis joint between the ribs and the highlighted costal cartilage. The second type of cartilaginous joint is known as the symphysis joint or secondary cartilaginous joint. And this type of joint is found primarily along the midline of the body and is characterized by adjacent bone surfaces lined with hyaline cartilage and connected by fibrous tissue. For example, the pubic symphysis which is shown just here.
So let's move on to talk about joints and motion. So in classifying a joint based on its range of motion, it's important to first understand the various axes that the movements are occurring along and in joint movement, there are three main axes – the x-axis for up and down movement, the y-axis for side to side movement, and z-axis which is for three-dimension movement. We can also describe joints by the number of axes they can move along, and there are three of these joints. Joints that can move back and forth along a single axis are called uniaxial, joints that move about two distinct axes are called biaxial, and joints that can move through all three axes are called polyaxial or multiaxial. And as we continue, do keep this axes in mind to help you visualize the motions being described.
There are six types of joints that are classified by their range of motion – the ball and socket joint, the hinge joint, the pivot joint, the condylar joint, the saddle joint, and the plane joint. Note that all these joints are synovial joints. The ball and socket joint also known as the spheroidal joint is the only polyaxial joint and therefore the most mobile of all joint types, and 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. And 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.
The hinge joint has one axis of motion which is along the x-axis making it uniaxial and of course this results in only two types of movement – flexion and extension – and examples of these joint are the knee joint or the tibiofemoral joint shown on the left and the elbow joint shown on the right. And like the hinge joint, the pivot joint is also uniaxial. However, instead of movements along the x-axis, the pivot can perform rotational movement only along the y-axis. And the pivot joint is given its name because it acts like a pivot in a lever system. So if you can imagine how a pivot wrench works which is by rotating a nut or a bolt around the y-axis and you can also visualize how a pivot joint works with its up and down movements. And examples of a pivot joint are the atlanto-axial joint between the axis and atlas bones of the cervical spine which is the joint that's on the right here and the proximal and distal radioulnar joints which is on the left. And this joint with its uniaxial movements would therefore allow you to do an action such as unscrewing the lid of a jar.
The condylar joint also known as the ellipsoid joint is a biaxial joint that has four primary movements, and an example of this type of joint is the radiocarpal joint. And this joint which is found at the wrist permits independent radial or ulnar deviation as well as flexion and extension of the hand. And just to note on the radiocarpal joints, the combination of these movements can give the false impression that it is a polyaxial joint.
The saddle joint is another type of biaxial joint, and 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. The saddle joint allows movement along two axes, however, not independently. And an example of such a joint is the carpometacarpal joint of the thumb which has an inverted saddle shape. And this joint is necessary for the opposition of the thumb.
And the final type of joint we're going to have a look at the body is the plane joint. And this type of joint unlike the other joints we've discussed only permits movement along the plane of the articular surface and therefore it cannot be classified as a uniaxial, biaxial or polyaxial joint. Instead, the plane joint performs what is known as a translational movement which is where a bone of the joint slides or glides over another point of the joint. And an example of the plane joint is the acromioclavicular joint which increases the flexibility of the shoulder.
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