Joints of the Human Body

The site where two or more bones come together whether or not there is movement between them. Joints are classified according to the tissues that lie between the bones as fibrous joints, cartilaginous joints, and synovial joints.

The articulating surfaces of the bones are joined by fibrous tissue, and thus very little movement is possible. The degree of the movement depends on the length of the collagen fibers uniting the bones. The sutures of the vault of the skull and the inferior tibiofibular joints are examples of fibrous joints.

Cartilaginous joints may be divided into two types, the primary and secondary. The primary cartilaginous joint is one in which the bones are united by a plate or bar of hyaline cartilage. Thus the union between the epiphysis and the diaphysis of a growing bone and that between the first rib and the manubrium sterni are examples of such a joint. No movement is possible. A secondary cartilaginous joint is one in which the bones are united by a plate of fibrocartilage, and the articular surfaces of the bones are covered by a thin layer of hyaline cartilage. Examples are the intervertebral joints and the symphysis pubis. The amount of movement possible is dependent on the physical qualities of the fibrocartilage.

Synovial Joints

The articular surfaces of the bones are covered by a thin layer of hyaline cartilage separated by a joint cavity. This arrangement permits a great degree of mobility. The cavity of the joint is lined by a synovial membrane which extends from the margins of one articular surface to those of the other.

The synovial membrane is protected on the outside by a tough fibrous membrane referred to as the capsule of the joint. The articular surfaces are lubricated by a viscous fluid called synovial fluid. In certain synovial joints, for example, in the knee joint, discs or wedges of fibrocartilage are interposed between the articular surfaces of the bones. These are referred to as the articular discs. Fatty pads are found in some synovial joints lying between the synovial membrane and the fibrous capsule or the bones. Examples are those found in the hips and knee joints. The degree of movement in a synovial joint is limited by the shape of the bones participating in the joint, the coming together of adjacent anatomical structures, and the presence of fibrous ligaments uniting the bones.

While most ligaments lie outside the joint capsule, in the knee, some important ligaments, like the cruciate ligaments, lie within the capsule.

Types of Synovial Joints

Synovial joints are somehow classified according to the arrangement of the articular surfaces and the types of movements possible. They are- the plane joints, the hinge joint, pivot joints, condyloid joints, ellipsoid joints, saddled joints, and the ball-and-socket joints. In the plane joints, the opposing articular surfaces are flat or almost flat, and this permits the bones to slide upon one another. Examples are sternoclavicular and acromioclavicular joints. The hinge joints resemble the hinge on a door, so that flexion and extension movements are possible. Like that of the elbow, knee, and ankle joints. In the ball–and–socket joints, the ball-shaped head of the bone fits into a socket-like concavity of another, and this permits very free movements, including flexion, extension, abduction, adduction, medial rotation, lateral rotation, and circumduction. The shoulder and the hip joints are fine examples of this type of joint.