Diarthrosis: what are they, types and anatomical characteristics

The musculoskeletal system is made up of the osteoarticular system (bones, ligaments and joints) and the muscular system (muscles and tendons). The human body has more than 650 voluntary muscles (40% of our weight) and a total of 206 bones, whose functionality and anatomy together we allows us to move and relate to the environment in a position that defines us as a species but which, in turn, is changing based on requirements environmental

The human body is a true work of art of biomechanics, since each bone, tendon, muscle and joint have an essential function in the movement or transmission of information. For example, a minimum group of 12 muscles participates in something as anecdotal as the production of a smile but, In addition, the number of tissues and muscle fibers involved depends on the individual message that we want to transmit.

With these facts and figures, it is impossible to deny that the musculoskeletal system is a source of functionality and scientific knowledge. We have already covered the musculature and bone system on other occasions, but we usually leave an equally important part in the pipeline: the joints. We take this opportunity to tell you all about these anatomical structures of union, especially of the type

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diarthrosis.

Related article: "Locomotor system: what is it, parts and characteristics"

What is a joint?

We cannot start by building the house from the roof and therefore we must briefly explore the world of joints in the first instance before diving into diarthrosis. A joint is defined as a place of union of firm consistency of two or more bones, whose function is to allow the movement of the components of the locomotor system.

In addition to assuming the “axes” of the extremities, they also give the body stability without interfering with the ability to move. General articular elements can be collected in the following list:

Joint capsule: made up of connective tissue, it surrounds the joint and gives it stability, firmness and flexibility.
Subchondral bone: the soft area of the bones that is located from the articular cartilage to the bone marrow.
Articular cartilage: hyaline cartilage that cushions the load of the contact surfaces and allows the displacement of the bones during movement. It covers the surface of the bones involved in joint movement.
Joint cavity: contains synovial fluid and is protected by the synovial membrane.
Synovial fluid: thick fluid that is located in the joints. Its function is to cushion the forces and reduce possible friction resulting from joint movement.

Normally, when we think of these joining structures, elbows and knees come to mind, but they do not represent more than a tiny part of the articular world: of the total of 360 joints in the human body, 86 of them are located in the skull.

On the other hand, we present 32 joint formations in each of our upper extremities (64 total) and 31 in the lower ones (62). If we pay attention to the trunk, we find 76 joints in the pelvis and spine, 66 in the thoracic section and 6 in the throat. As you can see, the location of these very special structures is at least heterogeneous, about all if we take into account that many are in places that would never occur to us at first instance.

If we classify the joints based on their functionality, we will obtain a minimum of 3 groups: diarthrosis, synarthrosis and amphiarthrosis. We tell you everything you need to know about the first of the categories in the following lines.

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What is a diarthrosis?

Diarthroses are a type of synovial joints. These are those that allow a wide range of movements to be executed, representing most of the articular elements of the extremities. Diarthroses have, in particular with respect to the rest, the fact of being covered by a lining of hyaline cartilage, a joint capsule and having reinforcing ligaments.

Diarthroses are the most common types of joints in mammalian organisms, and all of them have 3 essential structures. We tell you about them in detail.

1. Synovial cavity

It is a unique characteristic area in diarthrosis-type joints. It is responsible for containing the previously described synovial fluid and is lined by the synovial membrane.

Within this space is synovial fluid, essential for understanding joint movement. As far as rheological properties are concerned, this viscous fluid has viscosity, deformation rate and is It is a non-Newtonian fluid, that is, whose viscosity varies with temperature and the shear stress that is applied to it. applies.

Synovial fluid is of great clinical interest, because based on its composition, various pathologies can be detected. It generally contains few proteins and cells and is rich in hyaluronic acid, but, if observed in its compounds blood, pus or an excessive amount of the fluid, pathologies such as injuries, infections or osteoarthritis.

2. Joint capsule

It is the structure that surrounds the diarthrosis. Each joint capsule is divided into 2 distinct sections: an outer fibrous layer and an inner synovial layer. The first of the sections is not very "special", as it is composed of avascular tissue of a fibrous nature.

On the other hand, the inner synovial layer is striking because it contains synoviocytes, the cells that secrete synovial fluid that we have already named several times throughout this space. As they are responsible for secreting this fluid, synoviocytes or synovial fibroblasts (FS) have been studied to explain joint pathologies, such as rheumatoid arthritis (RA).

Curiously, when analyzing patients with rheumatoid arthritis, it has been found that synoviocytes suffer from hyperplastic phenomena (increase in number) and show an altered phenotype which, among other things, is associated with the secretion of matrix degrading enzymes. Undoubtedly, this appears to be linked to the typical inflammation in the joints that people with RA experience.

3. Articular cartilage

The bones of diarthrosis are covered by a layer of hyaline cartilage. This type of tissue is commonly found on the ribs, nose, larynx, and trachea, as well as the surface of many joint structures. The hyaline cartilage matrix is mainly composed of type II collagen and chondroitin sulfate, a complex glycosaminoglycan usually associated with protein molecules.

The cartilage of diarthrosis is found on the surface of the bones involved, inside the synovial cavity and bathed in synovial fluid. It should be noted that, although it is anatomically close to the menisci and articular discs, it is considered a completely different structure, since the latter are composed of fibrocartilage (compared to cartilage hyaline).

Diarthrosis functionality

This type of joint encompasses a large number of structures and, therefore, is found in various parts of the body, with different functionalities. In general, these can be summarized in that they perform flexion, extension and ulnar deviation work.

There are up to 7 general types of diarthrosis, among which are the following: sliding joints (wrist carpus, for example), "hinge" joints (elbow), condyloid joints (wrist joint) and many other examples.

If we want a clear idea about this type of joint so widespread in the body of vertebrates, this is the following: diarthrosis are mobile joints that are characterized by the possession of an intermediate membrane, which allows the existence of a synovial cavity and, therefore, the accumulation of synovial fluid, so important for the movements of our appendices.

Resume

With this little tour we have shown you the most common joints in the skeleton of vertebrates, but you must bear in mind that these are not the only ones. On the other side of the coin are the synarthrosis, very little mobile, such as the unions between the bone structures that make up our skull. Nor can we forget the amphiarthrosis, which are perfectly exemplified between the vertebral discs of our spine.

When we think of a joint, the elbow, wrist, fingers and knees quickly come to mind, but you must bear in mind that a point of union between two flat bones is also considered a structure articulate. Ultimately, the "joint" is any structure that allows the union of two bones between them, whether there is movement or not.

Bibliographic references:

Notes on Anatomy. Joint types: synovial and solid, Elsevier. Picked up on March 14 in https://www.elsevier.com/es-es/connect/medicina/anatomia-tipos-articulaciones-sinoviales-y-solidas
Classroom of anatomy, diarthrosis. Picked up on March 14 in https://www.auladeanatomia.com/novosite/es/sistemas/sistema-articular/diartroses/
Carrere, M. T. TO. (2010). Clinical biomechanics. Joint biomechanics. REDUCA (Nursing, Physiotherapy and Podiatry), 2 (3).
Diarthrosis, Navarra University Clinic (CUN). Picked up on March 14 in https://www.cun.es/diccionario-medico/terminos/diartrosis
Izquierdo, E., & Pablos, J. L. (2013). Synovial fibroblasts. Seminars of the Spanish Foundation of Rheumatology, 14 (4), 121-128.
Rheumatoid, A. (2008). 2. Relationship between synovial fibroblast and T lymphocyte in LA. Study of the cellular interactions involved in the pathogenesis of rheumatoid arthritis, 7.
Articular System, amicivirtual. Picked up on March 14 in https://www.amicivirtual.com.ar/Anatomia/04SistArticular.pdf