Diarthroses: what are they, types and anatomical characteristics?

Diarthroses are one of the main types of skeletal joints. Let's see what they look like.

The locomotor system consists 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 allow us to move and relate to the environment in a posture that defines us as a species but which, in turn, is changeable based on environmental requirements.

The human body is a true work of art of biomechanics, as each bone, tendon, muscle and joint has an essential function in the displacement or transmission of information. For example, a minimum group of 12 muscles is involved 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 we want to convey.

With these facts and figures, it is impossible to deny that the locomotor system is a source of functionality and scientific knowledge. We have already covered the musculature and skeletal system on other occasions, but we tend to leave an equally important part uncovered: the joints. We take this opportunity to tell you all about these anatomical joint structures, especially those of type diarthrosis.

What is a joint?

We cannot start by building the house from the roof up, and therefore we must first explore the world of joints in a cursory manner 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 being the "axes" of the limbs, they also provide the body with stability without interfering with the ability to move. The general articular elements can be gathered in the following list:

• Joint capsule: formed by 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 on the contact surfaces and allows the bones to move during movement. It covers the surface of the bones involved in joint movement.
• Articular cavity: contains synovial fluid and is protected by the synovial membrane.
• Synovial fluid: thick fluid located in the joints. Its function is to cushion forces and reduce possible friction resulting from joint movement.

Normally, when we think of these joint structures, elbows and knees come to mind, but they represent only a tiny part of the joint world: of the 360 joints in the human body, 86 of them are located in the skull.

On the other hand, we have 32 joint formations in each of our upper limbs (64 total) and 31 in our lower limbs (62).. If we turn our 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 heterogeneous to say the least, especially if we take into account that many of them are found in places that would never occur to us at first.

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

What is a diarthrosis?

Diarthroses are a type of synovial joints.. These are those that allow to execute a wide range of movements, representing most of the articular elements of the extremities. The diarthroses have a special feature compared to the others in that they are covered by a hyaline cartilage coating, an articular capsule and have reinforcing ligaments.

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

1. Synovial cavity

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

Within this space is found synovial fluid, essential for understanding joint movement.. In terms of rheological properties, this viscous fluid has viscosity, strain rate and is a non-Newtonian fluid, i.e. its viscosity varies with temperature and shear stress applied to it.

The synovial fluid is of great clinical interestIt is of great clinical interest, because based on its composition various pathologies can be detected. Generally it contains few proteins and cells and is rich in hyaluronic acid, but if blood, pus or an excessive amount of the fluid is observed in its composition, pathologies such as injuries, infections or osteoarthritis may be suspected.

2. Joint capsule

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

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

Interestingly, When analyzing patients with rheumatoid arthritis, it has been found that synoviocytes undergo hyperplastic phenomena (increase in number) and show an altered phenotype, which, among other things, is associated which, among other things, is associated with the secretion of matrix degrading enzymes. This certainly seems to be linked to the typical inflammation in the joints experienced by people with RA.

3. Articular cartilage

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

The cartilage of the diarthroses is found on the surface of the bones involved, within the synovial cavity.The cartilage of diarthroses is found on the surface of the bones involved, within 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 (as opposed to hyaline cartilage).

Functionality of diarthroses

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

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

If we want to have a clear idea about this type of joint so widespread in the vertebrate body, this is the following: diarthroses are mobile joints 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 correct movements of our appendages.

Summary

With this short tour we have shown you the most common joints in the vertebrate skeleton, but you should keep in mind that these are not the only ones. On the other side of the coin are the synarthroses, which are not very mobile.such as the joints between the bony structures that form our skull. Nor can we forget the amphiarthroses, 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 should keep in mind that a point of union between two flat bones is also considered a joint structure. In the end, "joint" is any structure that allows two bones to join together, whether movement occurs or not.

Bibliographic references:

• Anatomy notes. Types of joints: synovial and solid, Elsevier. Retrieved March 14 from https://www.elsevier.com/es-es/connect/medicina/anatomia-tipos-articulaciones-sinoviales-y-solidas.
• Anatomy classroom, diarthrosis. Retrieved March 14 from https://www.auladeanatomia.com/novosite/es/sistemas/sistema-articular/diartroses/.
• Carrere, M. T. A. (2010). Clinical biomechanics. Joint biomechanics. REDUCA (Nursing, Physiotherapy and Podiatry), 2(3).
• Diarthrosis, Clínica Universidad Navarra (CUN). Retrieved March 14 at https://www.cun.es/diccionario-medico/terminos/diartrosis.
• Izquierdo, E., & Pablos, J. L. (2013). Synovial fibroblasts. Seminarios de la Fundación Española de Reumatología, 14(4), 121-128.
• Rheumatoid, A. (2008). Relationship between synovial fibroblast and T lymphocyte in LA. A study of cellular interactions involved in the pathogenesis of rheumatoid arthritis, 7.
• Articular System, amicivirtual. Retrieved March 14 from https://www.amicivirtual.com.ar/Anatomia/04SistArticular.pdf.