Human Anatomy - Kotsan I. Ya. 2009

Bones and Their Joints
Classification of Joints

Due to The complexity of The Structure of synovial joints, various classifications exist, based on either the structural or functional Features of the joints.

Based on their structure, joints are classified as simple, compound, combined, or complex.

A simple joint (articulatio simplex) is formed by only two articular surfaces of bones (for example, the shoulder and hip joints).

A compound joint (articulatio composita) is formed by three or more articular surfaces. Sometimes a compound joint consists of several independent simple joints that share a common Joint Capsule (for example, the elbow joint).

Combined joints are those that are anatomically completely separated, located apart, and sometimes even at a distance from one another, yet movements in them always occur simultaneously; it is impossible for one to move while the other remains motionless. Examples include the temporomandibular joints — the right and left ones — which always move in unison.

Complex joints are characterized by the presence of a disc or meniscus within the joint cavity. The disc divides the joint cavity into two compartments (chambers) and increases the variety of movements within it. Examples include the sternoclavicular and temporomandibular joints. Menisci compensate for bone incongruency (for example, the lateral and medial menisci in the knee joint).

The range of mobility in a given joint depends on its structural features and, above all, on the shape of the articular surfaces. The shapes of the articular surfaces allow movements around one, two, or three axes, or circular movements at the intersection of these axes.

Functionally, joints are divided into uniaxial, biaxial, and triaxial (multiaxial) joints.

Depending on the shape of the articular surfaces, the following TYPES OF JOINTS are distinguished: cylindrical, ginglymus (hinge), trochoid (pivot), ellipsoid, bicondylar, saddle, spheroidal (ball-and-socket), plane, and cotyloid (nut-like) joints.

Cylindrical, hinge, and pivot joints permit movements around a single axis. Based on the number of axes of rotation, they are called Uniaxial joints.

A cylindrical joint (articulatio trochoidea) is formed by articular surfaces, one of which approximates a segment of the outer surface of a cylinder, while the other approximates a segment of its inner surface (for example, the distal and proximal radioulnar joints).

A hinge joint (ginglymus) resembles a cylindrical joint, but features a guiding groove or ridge running around the circumference of the articular surface, which prevents sliding movements along the geometric axis of the joint. Examples of hinge joints are the interphalangeal JOINTS OF THE hand. A variation of the hinge joint is the pivot-like spiral joint. The difference between a spiral joint and a standard hinge is that the groove is arranged along a spiral rather than perpendicular to the axis. An example of a spiral joint is the humeroulnar joint.

Ellipsoid, bicondylar, and saddle joints permit movements around two axes of rotation. They are referred to as Biaxial joints.

An ellipsoid joint (articulatio ellipsoidea) is formed by articular surfaces that approximate segments of an ellipsoid (for example, the radiocarpal joint).

A bicondylar joint (articulatio bicondylaris) is structurally similar to hinge and ellipsoid joints; however, unlike the former, the articular surface is located on a condyle. Examples include the knee and atlanto-occipital joints.

A saddle joint (articulatio sellaris) is formed by articular surfaces that resemble two saddles. An example is the carpometacarpal joint of the thumb.

Spheroidal, cotyloid, and plane joints permit movements around three axes of rotation. They are called triaxial (multiaxial) joints.

A ball-and-socket joint (articulatio spheroidea) is the most mobile type of joint. The articular surface of one articulating bone approaches the shape of a sphere, while the other features a correspondingly concave articular fossa (for example, the shoulder joint). Spheroidal joints also include the cotyloid or cup-shaped joint (articulatio cotylica), in which the HEAD of the bone is deeply embedded in the articular socket. An example of a cotyloid joint is the hip joint. Movements in it are the same as in a ball-and-socket joint, but their range is considerably smaller.

A plane joint (articulatio plana) has nearly flat articular surfaces, which can be viewed as surfaces of a sphere with a very large radius. The range and amplitude of movements in such joints are negligible, manifesting only as slight gliding of one articular surface against another. Therefore, these joints are classified as having limited mobility. An example is the sacroiliac joint.

As evident from the foregoing, the variety of movements in joints depends on the shape of the articular surfaces. As for the range of movement, it depends primarily on the difference in size between the articular surfaces. The greater the difference between the size of the articular head and the articular socket, the freer and wider the movement, unless restricted by other factors, such as a tightly stretched capsule and strong ligaments. The most mobile joint in The Human Body is the shoulder joint. It exhibits a large difference in the dimensions of its articular surfaces because the glenoid fossa is small and shallow, the joint lacks strong restraining ligaments, and the capsule is quite loose, not tightly stretched between the bones. The hip joint, much like the shoulder joint, is also triaxial, but the disparity between its articular surfaces is much smaller, the capsule is tightly taut, and it possesses strong ligaments. Consequently, movements in this joint cannot be as unrestricted as in the shoulder. Mobility and the variety of movements in joints also depend on individual, age, and sex characteristics of a person, the level of physical conditioning, and other factors. In early childhood, joints develop intensively, and the final formation of all joint elements is completed by the age of 13 to 16 years. Joint mobility is greater in children and young adults, and greater in women than in men. With Aging, mobility decreases due to sclerosis of the fibrous membrane and ligaments, as well as diminished muscular activity. The best means to achieve high joint mobility and prevent age-related changes is regular Physical Exercise.