Joints are the point where two bony elements come together, giving movement or holding them together. Let’s see how they are classified according to morphology and functions.
Joints are body structures that have the function of joining bones together and thus allowing them a certain degree of movement. Different types of joints have been classified divided into two categories according to their structure, that is to say, what tissue forms them, where we will find the synovial, fibrous and cartilaginous joints and according to the function they perform, allowing more or less movement where we will find the diarthroses, the amphiarthroses and synarthroses, named these from more to less mobility. In this article you will be able to better understand what we understand by articulation as well as the different types of these that exist and how they are classified.
- We recommend you read: “Knee joint (anatomy): tendons, muscles and ligaments”
What are joints?
Joints are anatomical structures, soft and hard, whose main function is to join the bones together , or the bones with cartilage, which is a connective tissue that surrounds joints. In this way, through these joints they allow the body to acquire elasticity and have greater mobility, although not all joints are equally mobile, for example those that form the skull, do not have mobility as such, although it does allow the skull to grow during childhood, also acting as protectors.
Other functions that the joints also carry out are to give consistency and favor the maintenance of the structure and reduce the wear and tear of the bones, keeping them together. Thus, the part of the study of anatomy that is responsible for knowing the joints is arthrology.
How are joints classified?
There are different types of joints that we will classify according to two criteria, according to the type of structure, which takes into account the kind of material that makes up the joints, and according to the function, also known as range of motion.
1. Types of joints according to their structure
As we pointed out before, this category takes into account the type of structure, material or tissue that forms the joints. This will be divided into three, which are fibrous, cartilaginous and synovial, the latter being divided into more subcategories.
1.1. synovial joints
Synovial joints are so named because they are joined by capsules called synovial which in turn contain synovial fluid, it is a liquid with a thick consistency that tries to cushion and reduce friction when the joints are in motion . In this way, the liquid makes the joints lubricated, allowing different types of movement to be carried out.
This type of joint mainly makes up the limbs, and it is mainly in this area where, if we study it using X-rays, we see that a space appears between the bones. This is because the type of cartilage that covers the joints is not visible in this class. of techniques.
1.1.2. flat joints
Flat joints or arthrodia are structures or flat surfaces that the only function that we can perform will be moving , without the possibility of making another movement.
Thus, a particular class of these joints are the intercarpal joints, which are located at the ends of the hands, allowing the fingers to move, and the acromioclavicular joints, which, as their name indicates, serve as a union between the clavicle and the medial area of the acromion, which It is a bone that is located in the shoulder, in the scapula and has a curved shape.
1.1.3. hinge joints
Hinge or trochlear joints only allow movement in one axis , in the transverse, appearing only two types of activity: flexion and extension, these are two types of opposite movements that allow the different parts of the body to move towards back when bending, in this case the angle formed between the bones decreases or they move forward, thus producing an extension where the angle will then increase.
As examples of the hinge joints we have the one located in the elbow that receives the name of humero-ulnar and the one that is in the knee called femoro-patellar.
1.1.4. pivot joints
The pivot joints, also called trochoids, would have the opposite function to those previously raised, the hinge joints, since they only allow movement in one axis and this is the horizontal one, in the same way we can only carry out one kind of movement that is rotation, both medial, which occurs close to the midpoint of the body, and lateral, which moves away from the midpoint or plane of the body. An example is the atlantoaxial joints found in the neck, which are what will allow us to move it.
1.1.5. condyloid joints
The condyloid joints, which are also called ellipsoidal joints, allow the movement of the bones that are irregularly joined forming a right angle , join a concave or backward bone with a convex or forward bone.
The activity they allow is flexion and extension, already mentioned, abduction, a movement that produces separation from the midline of the body, and circumduction, which refers to a circular movement. An example of condyloid joints would be the temporomandibular joints located on both sides of the head and, as its name indicates, it allows us to move the jaw. We also find this type of joint in the wrists.
1.1.6. saddle joints
Saddle joints, also called sellar joints, owe their name to their shape that resembles a saddle, they allow movement in two axes that form a right angle between them . In this way, the movements that can be performed are extension and flexion, abduction and adduction, which approximates the medial plane of the body, and circumduction.
The sternoclavicular joint that joins the sternum to the first costal cartilage and the carpometacarpal joint found in the thumb are two types of saddle joints.
1.1.7. ball joints
Ball-and-socket joints are shaped similar to a balloon, thus allowing maximum mobility. They can be moved in any direction , favoring flexion, extension, abduction and rotation movements. Two typical examples of this type of joint are the scapulohumeral, which is the one that constitutes the shoulder, and the coxofemoral, which, as we can deduce, will be found in the hip.
1.2. Fibrous joints
Fibrous joints receive this name since they are made up of fibrous tissue and, like any other joint, are responsible for joining two bones, in this particular case the movement that is allowed is minimal, very scarce. There are three subtypes of fibrous joints.
1.2.1. Gomphosis joints
Gomphosis joints are found in the oral area, specifically in the teeth and maxillary bones, and are short fibers of collagen tissue. In this particular type, one of the bones is embedded inside the other, for this reason the movement will be minimal.
1.2.2. Joints Sutures
Suture joints are made up of collagen fibers , also called Sharpey fibers, which are the ones found in the skull. These allow very little mobility, especially movement occurs during childhood when the skull is growing, however in old age it decreases, turning everything into bone.
1.2.3. Syndesmosis joints
The syndesmosis joints have the function of joining two widely separated bones, for this purpose membranes or fibrous ligaments are used and they are also responsible for controlling the movements of the spine, serving as reinforcement.
As examples, of these joints we have the bones of the forearm that would be joined specifically by fibrous membranes and the vertebral arches, which are the lateral and posterior part of the vertebral foramen which constitutes a vertebral canal that protects the spinal cord, in this case the spinal cord. Union will be by fibrous ligaments.
1.3. cartilaginous joints
As its name suggests, this type of joint is made up of cartilaginous tissue, thus allowing greater resistance to the bones it joins, maintaining flexibility and giving rise to small movements.
An example of a cartilaginous joint is the one found in the vertebral discs, which has the function of joining the vertebral bodies of the spine, thus allowing it to be resistant to the force exerted on it but also maintaining a certain degree of flexibility so that it can be can make a visible movement.
2. Types of joints according to their function
As we move forward, another way to classify joints is by the function they perform or the degree of movement they allow, in this case three different categories will be formed.
2.1. Diarthrosis Joints
Diarthrosis joints are the type of joints that most appear in the body , allowing it to have great mobility. They are made up of cartilage and join the already named synovial capsule. A typical example of this kind of joint is the glenohumeral joint that joins the humerus with the scapula, also known as the shoulder blade.
2.2. Amphiarthrosis Joints
Amphiarthrosis joints are also formed by cartilage, but in this case mobility is much less, it is low. An example would be the joints that we find in the vertebral discs, as we have already said, they allow the spine to be resistant and acquire a certain flexibility.
23. Synarthrosis Joints
The synarthrosis joints are those that allow less mobility, and can be divided into two classes: the synchondroses that are formed by a cartilaginous union, an example would be the coststernal joint that joins the ribs with the sternum and the synfibroses that, as expected, will be formed by fibrous tissue and are found primarily between the bones that make up the skull .
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