Joints

Joints are anatomical structures, sometimes complex, which put two or more bones into mutual contact. To avoid degenerative phenomena due to wear, in most cases it is a non-direct contact, but mediated by fibrous or cartilaginous tissue and / or by liquid.

The joints of the human body are very numerous, counting on average 360, and structurally very dissimilar from each other. This diversification reflects the type of function required at that particular joint. Taken together, the task of the joints is to keep the various bone segments together, so that the skeleton can perform its function of support, mobility and protection.

CLASSIFICATION OF THE JOINTS ON A STRUCTURAL BASIS

The joints are divided, from a structural point of view, into:

fibrous joints: the bones are joined by fibrous tissue;
cartilage joints: the bones are linked by cartilage;
synovial joints: the bones are separated by a cavity, as well as being linked by means of structures that we will describe better later.

The best known subdivision, however, is that on a functional basis. The bones of the human skeleton are in fact connected by means of joints which are allowed movements of various types and degrees. We then speak of immobile (synarthrosis), semi-mobile (amphiarthrosis) and mobile (diarthrosis) joints.

CLASSIFICATION OF THE JOINTS ON A FUNCTIONAL BASIS

The joints are divided, from a functional point of view, into:

immobile or synarthrosis joints: they tightly bind the bone heads, like a closed zipper, so as to prevent them from moving.
Low-mobility joints or amphiarthrosis: they bind two articular surfaces, covered by cartilage, through interosseous ligaments; between the two surfaces there is a fibrocartilaginous disc which allows only limited movements. In the vertebrae, for example, flat bony surfaces are joined by a cartilaginous interosseous disc that acts as a shock absorber.
Mobile joints or diarthrosis: allow a wide range of motion, in one or more directions of space (knee, shoulder, fingers ...)

The structure of a joint influences its degree of mobility:

Functional name

Structural name

Degree of movement

Example

synarthrosis

fibrous

fixed

skull

amphiarthrosis

cartilage

not very mobile

vertebrae

diarthrosis

synovial

very mobile

shoulder

Synarthrosis (immobile joints) are divided into:

Synostosis: the degree of movement is zero, since they join the joints through bone tissue (as in the adult skull).
Synchondrosis: the degree of movement is poor, since they join the joints via dense cartilage tissue (such as the first ribs of the sternum).
Syndesmosis or symphimbrosis: the degree of movement is limited, since they are held together by fibrous connective tissue (such as the pubic symphysis).

Mobile or semi-mobile joints differ in shape and allowed movements. In this regard, there are slightly different classifications between them. One of these involves the subdivision of diarthrosis based on the differences in the shape of the articular surfaces:

Arthrodia

Permitted movements: simple scrolling

Arthrodias, which join the carpal bones in the hand and the tarsus in the foot, allow only small gliding movements.

Flat bone surfaces just slide over each other to allow for minimal movement. The carpal bones, for example, slide between them during the movements of the hand. They have the task of absorbing shocks.

Further examples: costo-vertebral joints.

Trocleoarthritis (angular ginglimo)

Permitted movements: flexion / extension

The articular surfaces that face each other have the shape of a cylinder segment, one of which, with a concave groove (trochlea), is inserted into the convex face of the other. The axes of the cylinders are orthogonal (at right angle).

The movement occurs in a plane according to a single axis (uniaxial), like a door in the hinge.

Example: elbow, knee

Trochoid (lateral / parallel ginglimo)

Permitted movements: pronation and supination

The two articular surfaces have the shape of a cylinder segment, one of which, with a concave groove (trochlea), is inserted into the convex face of the other. The axes of the cylinders are parallel.

It is a uniaxial joint.

Example: between the radius capital and the ulna (proximal radio-ulnar joint).