The joints, as mentioned in the introductory article, are areas in which two or more bones come into contact with each other. In the human body there are many joints (about 360), which are distinguished by shape and degree of mobility. Some of them, like those that make up the cranial vault, they have no possibility of movement.
However, most joints fall into the category of diarthrosis, mobile joints characterized by a particular anatomical structure. They are in fact made up of different elements: the articular surfaces of two bones; the layer of cartilage tissue; the joint capsule; the joint cavity; the synovial membrane; the synovium and intrinsic ligaments. Let's see them in detail.
ϒϒ ARTICULAR CARTILAGE
The joint heads are lined with a layer of hyaline cartilage, also known as encrusting cartilage or joint cartilage; it is soft, compressible, stretchable and deformable.
Its function is comparable to a shock absorber bearing, capable of safeguarding joint relationships and allowing movement.
ϒϒ SYNOVIAL MEMBRANE AND SYNOVIAL LIQUID (or synovium)
Articular cartilage, despite being a living tissue, is devoid of blood vessels; moreover, alone, it would be insufficient to significantly decrease the friction between the two bony ends.
The synovial fluid has a cushioning and nourishing function, facilitates the sliding between the two joint surfaces and is secreted by the synovial membrane. Basically, it has the same function as a lubricant on a bearing.
The synovial membrane, which internally covers the joint capsule, delimits the joint space immersed in the viscous liquid it produces (called, precisely, synovial fluid). It covers the bony portions contained within the joint cavity, but stops along the contours of the encrusting cartilages, which are devoid of coating (the perichondrium is also missing (a membrane of connective tissue that surrounds the cartilages, except the joint portions). , thus, a closed cavity, known as the joint cavity.
The synovial membrane is innervated and rich in blood and lymphatic vessels (to facilitate the production of the synovium and the reabsorption of any intra-articular effusions).
ϒϒ ARTICULAR CAPSULE
Proceeding from the inside towards the outside, we see that the joint is circumscribed peripherally by a fibrous membrane, called joint capsule (or fibrous capsule), which fits between the connecting bone segments. The insertion points on the bone are located at a certain distance from the hyaline cartilage that lines the articular surfaces.
The joint capsule is made up of fibrous connective tissue that completely covers the two external bone segments. More specifically, it consists of two layers, of which:
- a fibrous, external, dense and continuous connective with the periosteum and also known as a fibrous capsule;
- a synovial one, internal to the previous one, thinner and more elastic; it covers the non-cartilaginous surfaces and is none other than the synovial membrane described in the previous point.
Muscles, tendons and ligaments also participate in the joint.
ϒϒ LIGAMENTS: they are connective cords that firmly join the bone heads to which they are connected and prevent them from moving away beyond a certain extent. They are very resistant and can be located inside or outside the joint capsule, preventing or limiting dangerous movements.
ϒϒ TENDONS: while the ligaments join two bone heads together, the tendons connect the muscles to the bones; they serve to stabilize the joint and to transmit the forces between the elements they connect.
In addition to the structures just mentioned, still others can take part in the constitution of diarthrosis. They are: i cercini, discs and menisci, interarticular ligaments and distant ligaments. Let's see them briefly:
- Cercine: ring-shaped fibro-cartilage structure with the function of increasing the articular surface of the concave head to allow it to better accommodate the convex articular head; famous is the glenoid one in the scapula humeral joint.
- Discs and menisci: disc-shaped (discs) or crescent-shaped (meniscus) fibrocartilaginous structures. They stabilize the joint, increase the congruence of the joint surfaces and cushion shocks.
- The interarticular ligaments: connect two skeletal segments internally to the joint capsule. However, they are not considered intra-articular because they are enveloped by the synovial membrane.
- Distant ligaments: they are located in the vicinity of the joint, but have no intimate relationship with the joint capsule.
More articles on "Joints: anatomy structure"
- joints
- Bone
- bones of the human body
- bone tissue
- osteoblasts osteoclasts
- spongy bone compact bone
- periosteum endosteum
- bone marrow
- bone remodeling
- bone mass
- height growth
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