Human Anatomy, Part 1 - K. A. Dyubenko, A. K. Kolomiysev, Yu. B. Chaykovsky 2002

Special Part
Arthrology, artrologia [the study of bone connections] - Classification of joints and their general characteristics
Polyaxial joints

The ball-and-socket joint (articulatio spheroidea; for example, the shoulder joint) features one articular surface forming a rounded HEAD, while the other forms a correspondingly concave socket. Theoretically, movements can occur around numerous axes along the radii of the sphere, but in practice, three main axes are distinguished:

- the frontal axis, around which flexion (flexio) takes place—when the moving part forms an angle with the frontal plane opening anteriorly—and extension (extensio), when the angle opens posteriorly;

- the sagittal axis, around which abduction (abductio) and adduction (adductio) take place;

- the vertical axis, around which rotation (rotatio) occurs: inward rotation (pronatio) and outward rotation (supinatio).

When shifting from one axis to another, circular movement, or circumduction, is produced.

The ball-and-socket joint is the most freely movable of all joints in The Human Body. Because the range of motion depends on the difference in surface area between the articulating surfaces, the socket in this type of joint is small compared to the size of the head. Typical ball-and-socket joints have few accessory ligaments, which dictates their wide range of motion.

A variant of the ball-and-socket joint is the cotyloid joint, art. cotylica (from the Greek cotyle meaning cup). Its articular cavity is deep and encompasses a large portion of the femoral head. As a result, movements in this joint are not as free as in a typical ball-and-socket joint; an example of a cotyloid joint is the hip joint, where this structural design provides greater stability.

The plane joint, articulatio plana, features nearly flat articular surfaces that can be regarded as sections of a sphere with an extremely large radius. Consequently, movements can occur around all three axes, though the range of motion is limited due to the minimal curvature difference between the articulating surfaces. Examples of such joints include the intervertebral joints, articulationes intervertebrales, and the sacroiliac joint, articulatio sacroiliaca.