ARTICULATIONS

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An articulation (joint) is the point of contact between bones, cartilage and bones, or between teeth and bone. Aside from the hyoid bone, each bone in the human organism has some contact with another bone. The function of a particular joint is determined by its structure. Some joints, called synarthrotic joints, afford no movement; others called amphiarthrotic joints, afford only slight movement; and others, the diarthrotic joints, afford free movement. Joints can also be classified as to whether or not they contain a synovial cavity, which is a space between articulating bones, and fibrous connective tissue or cartilage.

In this exercise you will examine the various types of joints, the structure of synovial joints and the numerous types of joint types.

INTRODUCTION

SYNARTHROTIC JOINTS

Immovable

Synarthrotic joints are those which afford no movement and characteristically contain fibrous connective tissue.. The four types of synarthrotic joints are the• sutures, • gomphoses,• synchrondroses and • synostosis

In the skull, it is extremely easy to see the presence of the sutures. These are the points at which two or more bones are interlocked with one another and are surrounded by fibrous connective tissue. This connective tissue is easily seen in the fontanels or "soft spots" of the skull, as in the anterior or frontal fontanel. In the adult, these bones fuse to form an immovable joint called Synostosis. The irregularity with which the bones interlock, illustrated by the coronal suture between the frontal and parietal bones, provides for added strength and reduces the possibility of fracture.

A gomphosis is a joint between the tooth root and the surrounding fibrous connective tissue. The membrane comprises the periodontal ligament, and affords no movement.

A synchondrosis, unlike the other synarthrotic joints, contains hyaline cartilage. These joints are found at the epiphyseal plates and between the first rib and sternum. They can be considered immovable since the cartilage will ossify to bone at the conclusion developmental growth.

AMPHIARTHROTIC JOINTS

Slightly Immovable

These slightly movable joints include syndesmoses and symphyses.

A syndesmosis contains more fibrous connective tissue than in an amphiarthrotic joint. As a result there is more flexibility and limited movement. An example of this type of joint is the distal end of the tibia and fibula which are connected by fibrous bands.

A symphysis is a line of fusion between paired bones where the articulating surfaces of the bones are connected by disks of fibrous cartilage. These can be found at the intervertebral disks, the adjoining the two pubic bones of the os coxae, called the pubic symphysis.

A syndesmosis occurs where there is much more fibrous connective tissue than in a suture, and so the fit between the bones is not so tight and allows some movement at the joint.(eg. the distal articulation of the tibia and fibula)

A symphysis is a line of fusion between paired bones where the articulating surfaces of the bones are connected by disks of fibrous cartilage. These can be found at the intervertebral disks, the adjoining the two pubic bones of the os coxae, called the pubic symphysis.

A symphysis occurs where there is much more cartilage present than in a synchondrosis, and so the fit between the bones is not so tight and allows some movement at the joint. The pads between bones in a symphysis are compressible, allowing the joint to act as a shock absorber and to move slightly. The vertebral joint allows slight movements between individual vertebrae.

DIARTHOTIC JOINTS

Freely Immovable

These are the freely movable joints. The points of articulation on the bone are covered by articular cartilage, appropriately shaped for the movement required (to be discussed later) and enclosed by a fibrous connective tissue envelope called the joint ( articular) capsule.. The capsule consists of an outer fibrous layer and an inner fluid secreting layer called the synovial membrane. In some joints, such as the knees which support most of the body's weight, there is an articular disk called the meniscus, found between the articulating cartilages. This affords further protection to the joint. In addition, if the joint capsule is reinforced with overlying muscles and ligaments to provide the varied movement, fluid-containing sacs, called bursae, are also present. They are found between the sliding surfaces to reduce friction. All bursae are lined with synovial membranes which secretes the synovial fluid. Be sure to review the structure of the knee in the text.

synovial fluid.

JOINT MOVEMENTS

The basic structure of diarthrotic joints remains fairly constant. However, the articulating surfaces between the numerous diarthrotic bones of the human organism are varied. Depending upon the shape of the articulating surfaces, the diarthrotic joints can be subdivided into six major groups. These are gliding, hinge, pivot, ellipsoidal, saddle and ball-and-socket.

Gliding (planar) joints contain articulating surfaces which are generally flat and thus allow a gliding movement where one of the articulating bones moves back-and-forth or side-to-side- over the other articulating bone. There is no circular or angular movement. Examples of gliding diarthrotic joints are between the bones of the carpals and tarsals, sternum and clavicle, and the clavicle and scapula.

Hinge joints allow for movement in one plane. The angle between the two or more articulating bones can be decreased (flexion) or can be increased (extension). Hyperextension is also possible, which is extension beyond normal anatomical position. Other examples are the elbow and knee.

Pivot joints are characterized by have an articulating bone with a pointed or rounded surface articulated with a ring-like bone as is found between the axis, containing the odontoid process (dens) and the atlas. This movement allows for rotation. Another example is the proximal end of the radius with the humerus which allows for the supination and pronation of the palm.

Ellipsoidal (condylloid) diarthrotic joints are characterized by having an oval condyle on one bone articulating with an ellipsoidal depression in another bone such as atlas and occipital condyles. These joints allow for movement in two planes as exemplified in the wrist joint and the metacarpal-phalangeal joints or knuckle joints. These joints allow for abduction, or movement away from the organism's midline, and adduction, movement toward the midline.

Saddle diarthrotic joints are characterized where the articular surface of one bone is saddle-shaped and the other articulating bone has a complementary form. An equestrian on a horse would be similar to a saddle joint. These joints also provide movement in two planes and are exemplified by the articulation between the thumb of the metacarpal and the trapezium of the carpus.

Ball-and socket joints are the most diverse in terms of their overall movement. In these diarthrotic joints a ball-shaped structures, such as the head of the femur, articulates with a cuplike depression in another bone, as the acetabulum of the ox coxa. This type of joint can also be found in the shoulder between the head of the humerus and the glenoid fossa of the scapula. These joints allow for movement in all planes, pivotal rotation and circumduction.

Specialized Joint Movements