Human joints: anatomy and classification || Joint Structure, Blood Supply, Function and Diseases
Rolling is one of the greatest gifts of nature, carefully presented to a person. To have time to cope with hundreds of everyday tasks, you have to overcome more than one kilometer, and all this thanks to the well-coordinated work of the joints. They combine the bones of the skeleton into a single whole, forming a complex system of the musculoskeletal system.
The joints of the human body are conventionally divided into three functional groups. The first – synarthrosis – provide a completely immobile articulation of two or more bones and are formed in the human skull as the infant fontanelles overgrow.
The second – amphiarthrosis – move very limited and are represented by the spinal column. And, finally, the third – diarthrosis – the most numerous joints in the body, which are true and are completely mobile. Thanks to them, a person can enjoy an active lifestyle, do work or a favorite hobby, cope with household chores – do everything that cannot be done without movement.
Human joint structure
A joint is a place where two or more bones join into a single functional system, thanks to which a person can maintain a stable posture and move in space. The main elements of the joint are represented by the following formations:
articular surfaces covered with cartilage tissues;
synovium and fluid.
The articular surfaces are located on the articulating bones and are covered with thin cartilage with a thickness of 0.2 to 0.5 mm. These cartilages have a dense elastic structure due to the interlacing of hyaline fibers. Absolutely smooth surface, polished by constant sliding of bones relative to each other, greatly facilitates movement within the joint; and elastic cartilage provides safety, playing the role of a kind of shock absorber under load and sharp shocks.
The joint capsule forms a sealed cavity around the joint, protecting it from external influences. It consists of elastic threads that are securely intertwined, fixing at the base of the bones that form the articulation. For extra strength, fibers of adjacent muscles and tendons are woven into the capsule walls.
Outside, the joint capsule is surrounded by a fibrous membrane, from the inside by a synovial membrane. The outer fibrous layer is denser and thicker, since it is formed by longitudinal strands of fibrous connective tissue. The synovial membrane is less massive. It is here that most of the nerve endings are concentrated, which are responsible for the pain sensitivity of the joint.
The synovium and the articular surfaces form a hermetic slit-like space – the articular cavity. Inside it, menisci and discs can be located to provide mobility and support for the joint.
On the surface of the synovial membrane, there are special secretory villi that are responsible for the production of synovial fluid. Filling the inner space of the cavity, this substance nourishes and moisturizes the joint, and also softens the friction that occurs between the articular surfaces during movement.
Periarticular tissues, represented by muscle fibers, ligaments, tendons, nerves and vessels, are located directly around the joint. Muscles provide mobility along different trajectories; tendons hold the joint, limiting the angle and intensity of movement; interlayers of connective tissue serve as a place for anchoring blood vessels and nerves; and the blood and lymphatic bed nourishes the joint and adjacent tissues. As a rule, the periarticular tissues in the body are not sufficiently protected, therefore they actively respond to any external influence. In this case, the disorders that occur in the periarticular tissues also affect the condition of the joint, provoking the occurrence of various diseases.
Ligaments occupy a special place in the anatomy of the human joints. These strong fibers strengthen the bony articulation by supporting all the anatomical units of the joint and limiting the range of motion of the bones. In most diarthrosis, the ligaments are located on the outside of the bag, but the most powerful of them (for example, the hip) need additional support, therefore they also have an inner ligamentous layer.
Joint anatomy: blood supply and innervation
To maintain the physiological capabilities of the joint, it needs adequate nutrition, which is largely provided by blood circulation. The arterial networks surrounding the articular capsule usually consist of branches of 3–8 arteries of various diameters, through which oxygen and nutrient molecules enter the tissues. And the venous bed is responsible for the complete elimination of toxins and decay products from adjacent tissues.
The innervation of the joint is provided through the interlacing of the sympathetic and spinal nerves. Nerve endings are found in virtually every anatomical unit that makes up a joint, with the exception of hyaline cartilage. The perception of pain and the activation of the body’s defense mechanisms depends on their sensitivity.
The key function of joints is to combine bony formations into a single structure. Together with bones and ligaments, they form the passive part of the musculoskeletal system, which is set in motion with the participation of muscle fibers. Thanks to the joints, the bones can change position relative to each other, slide and at the same time not wear out. The slightest thinning of the joint tissue can lead to serious consequences, since the bone structures wear out very quickly during friction, cause severe pain and irreversible deformation of the skeleton.
In addition, the joints help maintain a stationary position of the body in space. Fixed joints provide a constant shape of the skull, sedentary joints allow you to take an upright position, and mobile ones belong to the organs of locomotion, that is, the movement of the body.
In anatomy, it is customary to classify joints into several groups depending on the number and shape of the articular surfaces, the functions performed and the range of motion. By the number of articular surfaces, the following types of joints are distinguished:
Simple has two articular surfaces (for example, phalanges of the fingers). Only two bones take part in its formation.
Complex includes three or more articular surfaces, since it is formed by at least three bones (for example, the ulna).
Complex has an intra-articular cartilaginous element – a meniscus or a disc. It divides the joint cavity into two independent chambers (for example, the knee).
Combined is a complex of several separate joints taking part in the same action (for example, temporomandibular). Each joint in this complex is anatomically isolated, but physiologically it cannot cope with the task without a “companion”.
The classification by function and trajectory of movement is based on the shape of the articular surfaces. Based on this criterion, the following groups are distinguished:
Uniaxial joints: cylindrical, block and helical. The cylindrical joint is capable of rotating movements. According to this principle, the articulation is arranged between the first and second cervical vertebrae. The block joint allows movement only along one axis, for example, forward / backward or right / left. A variety of such joints are helical joints, in which the trajectory of movements is performed slightly obliquely, forming a kind of screw.
Biaxial joints: elliptical, saddle, condyle. The ellipsoid joint is formed by articular surfaces, one of which is convex and the other concave. Due to this, in joints of this type, movement around two mutually perpendicular axes can be maintained. There is only one saddle joint in the human body – the carpometacarpal. The trajectory of movement in it encompasses rotation, including swinging from side to side and forward / backward. The condylar joints are able to maintain similar mobility due to an ellipsoid process (condyle) on one of the bones and a suitable cavity on the other articular surface.
Multiaxial joints: spherical, bowl-shaped, flat. Ball joints are among the most functional, as they involve the widest range of motion. The cup-shaped joints are a slightly less mobile version of the ball joints. And flat joints, on the contrary, are distinguished by a primitive structure and minimal range of motion.
Diseases of the human joints
According to WHO statistics, joint pain is familiar to at least one in seven people around the world, and among the age group from 40 to 70 years old, one or another problem can be found in 50% of cases, over 70 years old – in 90% of cases. Such prevalence of diseases of the musculoskeletal system is associated with many factors:
low physical activity, in which the joints do not function and, accordingly, do not receive the proper amount of nutrition with the blood flow;
uncomfortable, too tight shoes and clothes that limit the functionality laid down by nature;
poor heredity as one of the risk factors for the development of pathologies associated with joints;
drastic changes in temperature conditions, including both overheating and hypothermia;
infectious processes in the body, which often provoke complications associated with the work of the joints;
injuries that reduce the functionality of the musculoskeletal system;
Experts say that it is quite possible to maintain healthy joints if you start preventing diseases in time. You should avoid injuries and injuries, strengthen immunity, and include sports in your daily schedule. Yoga can be an excellent option, because static loads strengthen the muscles and ligaments that hold the joints well. Take care of your health in advance – this natural resource is much easier to preserve than to replenish!