4.3 Arthrology--The Study of Joints (Articulations)

A joint, or articulation, is the location where two or more bones meet.

TYPES OF JOINTS

Joints are classified according to the kind of material holding the bones together and the relative freedom and kind of motion at the particular joint.

a. Fibrous Joints. Varying degrees of motion, from none to some, are possible in fibrous joints.

(1) Syndesmosis. When the bones are held together by FCT (fibrous connective tissue), the joint is referred to as a syndesmosis.

Example: The inferior tibio-fibular joint.

(2) Suture. When the bones are quite close together with a minimum of FCT, the joint is known as a suture. Example: the joints between the cranial bones.

b. Bony Joints. Should the bones be united by bony material, the joint is referred to as a synosteosis.

Example: The frontal bone. (The frontal bone of the skull is actually a bony fusion of two bones. Approximately 10 percent of the time, this fusion fails to take place; the original suture between the bones remains and is called a metopic suture.)

c. Cartilagenous Joints. These are also nonmovable joints.

(1) Synchondrosis. A cartilagenous joint in which the bones are held together by hyaline cartilage.

Example:  Epiphyseal plate.

(2) Symphysis. A cartilagenous joint in which the bones are held together by a disc of fibrocartilage.

Example:  Pubic symphysis.

d. Synovial Joints. In the synovial type of joints, the bones move on one another so as to allow various motions of the body parts. The "ovial" part of the name refers to the fact that the fluid substance seen in this type of joint appeared to the old anatomists to be like raw egg white (ovum = egg).

A "TYPICAL" SYNOVIAL JOINT

A "typical" synovial joint is one which has parts common to all of the synovial joints. In a sense, it is imaginary. It is not actually a specific synovial joint. It is a composite. It is illustrated in figure 4-2. The "typical" synovial joint has the following parts:

a. Bones. Bones are the levers of motion. They are the site of attachment for skeletal muscles.

b. Articular Cartilages. The "contact" points of the bones are usually covered with a layer of lubricated cartilage. Where these cartilages end, the synovial membranes begin. Cartilages provide a smooth surface to reduce friction.

Figure 4-2. A "typical synovial joint:--diagrammatic.

c. Synovial Membrane, Space, and Fluid.

(1) Synovial membrane. The synovial membrane lines the inner surface of the capsule. It secretes synovial fluid into the synovial space.

(2) Synovial space. Figure 4-2 exaggerates the amount of space between the bones. The space within the capsule allows movement.

(3) Synovial fluid. Synovial fluid is a colorless, viscous fluid similar in consistency to raw egg white. It lubricates the articulation.

d. Capsule. The "typical" synovial articulation is surrounded by a sleeve of dense FCT known as the capsule. The capsule encloses the articulation.

e. Ligaments. Primarily, ligaments hold bones together. Ligaments also may help restrain motion in certain directions and stabilize the articulation.

f. Muscles. Skeletal muscles apply the forces to produce a given motion.

CLASSIFICATION OF SYNOVIAL JOINTS

Synovial joints are further classified according to the kind of motion and the number of axes of motions used.

a. Uni-Axial Synovial Joints.

(1) In uni-axial synovial joints, motion occurs in only one plane. The joints of the fingers (interphalangeal) flex and extend in the sagittal plane. These are commonly referred to as hinge joints.

(2) If a single rotatory (rotational) motion occurs around a post-like structure, the joint is a pivot joint. The atlas vertebra rotating around the dens (tooth like projection) of the axis vertebra at the top of the neck (base of the skull) is a pivot joint.

b. Bi-Axial Synovial Joints. In bi-axial synovial joints, motion between the bones occurs in two planes. Here the surface in contact is curved or rounded in two directions.

(1) The proximal phalanx of a finger can flex and extend and move from side to side on the rounded head of the metacarpal bone. This is the MP or metacarpophalangeal joint.

(2) When the two surfaces are curved in directions at right angles to each other, a shape similar to that of a cowboy's saddle is formed. This type of synovial joint is called a saddle joint. In the human body, the saddle joint is located at the base of the thumb.

Table 4-1. The tissues and functions of structures of a "typical" synovial articulation.

In multi-axial joints, motion is possible in all three planes of space. The ball-and-socket-type synovial joint has the freest motion in all directions. A spherically rounded head (ball-like) fits into a receiving concavity (socket). The hip joint is an example of the ball-and-socket type, with the spherical head of the femur fitting into the cup or socket (acetabulum) of the pelvic bone. In the plane joint, the contact surfaces of the bones are essentially flat. These flat surfaces slide on one another (also called translatory motion). The acromioclavicular joint of the shoulder region is an example of a plane joint.

THE ARTICULAR DISC

In three of the synovial joints of the human body, a special addition is seen. This addition is known as an articular disc. The joints with articular discs are the temporo- mandibular joint of the lower jaw, the sternoclavicular joint (at the sternum (breastbone)), and the ulnocarpal joint of the distal end of the forearm.

An articular disc is a fibrocartilage plate. It is inserted between the articular surfaces of the bones of a synovial joint. In this way, it divides the synovial space into two spaces. Joints having an articular disc are capable of having several different motions occurring at the same time. Mechanically, there are really two joints together here.