The tissues involved in implant procedures 37

osteocytes. It consists of an organic framework and inorganic mineral salts. The organic portion consists chiefly of a portein called bone collagen, or ossein, which gives the bone a viscoelastic characteristic. It occurs in the form of fibers associated in bundles. Between the fibers is a fluid whose main component is chondroitin sulfate, a mucopolysaccharide. This fluid is a cementing substance. Within the soft organic matrix is a complex of mineral salts responsible for the hardness of bone. Commonly designated as bone salts, the deposits consist of a complex of calcium carbonate and calcium phosphate, plus small amounts of sodium, magnesium, potassium, chlorine, fluorine, and some other elements.

Throughout life bone is continually being reconstructed or remodeled. Because osteocytes cannot undergo mitosis and multiply, they must be replaced by new osteocytes. This involves the destruction of the existing bone matrix and its replacement with a new matrix. The new matrix is brought about by maturing osteoblasts usually originating from membranes. The destruction of the old matrix is effected by osteoclasts and probably starts with the removal of the organic components of the matrix by a proteolytic action of these osteoclasts. The removal of the organic parts liberates the inorganic calcium salts, which may be removed, as in the rapid destruction of bone by injury, by macrophages or by the slower process of being made soluble by the action of sub-stances produced by the osteoclasts.

The resorption of bone is strictly localized in those areas in which osteoclasts occur, thus refuting assumptions that bone destruction may be humoral. The site of bone removal is distinguished by a sharply bitten surface. These bites, or Howship's lacunae, are probably formed by the erosive action of the osteoclasts.

The fate of the osteocytes is somewhat obscure. Occasionally they have been observed to have been incorporated into osteoclasts, probably being engulfed by them. They may revert to fibroblasts or osteoblasts or join with other osteocytes to form osteoclasts. However, good evidence does not exist for these latter claims.

Although the primary constituents of all bone are the three kinds of bone cells embedded in a matrix, these constituents may be combined in different ways. The size and orientation of fiber bundles, the proportion of cement to fibers, and the number, size, shape, and orientation of the cells are variable. These distinctions deal with how the elementary constituents of bone are combined, a study that must be conducted under high power magnification of a

microscope. There is still another variable, the pat-tern assumed by a particular combination of constituents. This pattern is frequently obvious to the naked eye. Thus one type of bone may assume one pattern in a particular part of the skeleton and look entirely different in another. Before going into gross patterns, it is preferable to examine the types of bone.

Types of bone. Unfortunately, because of all the variables that can operate in classifying bones, several different classification systems exist. However, the most widely used is that of F. Weidenreich, who has typed bones according to the bundles formed by the fibers. According to Weidenreich, there are three main types of bone.

Type 1. The first type of bone is distinguished by coarse fiber bundles. The orientation of these bundles serves as a basis for distinguishing two varieties. The coarse fiber bundles of one variety are parallel to one another; in the other they are irregular, woven, or plaited.

Parallel coarse fiber bundled bone is not a major constituent of the human adult, but woven coarse fiber bundled bone, or woven bone, is. This latter variety of bone is distinguished by coarse bundles running in a random, interlacing fashion through a cement-rich background. Its cells are usually large, densely packed, irregular in shape, and with few processes. Woven bone is found in the alveolar bone of the jaws around tooth sockets, where strong fasciae, tendons, and ligaments attach it.

Woven bone is the first kind of bone formed whenever hone formation is violently reactivated. It may arise as an ossification of a preexisting stout structure, such as a tendon or a ligament, in which case the tendinous or ligamentous fibers are carried into bone almost unchanged and form the major component of the matrix. It may form as an ossification in mesenchyme, in which all the fibers of the hone matrix are newly formed ones, or as an ossification in membrane, in which most of the fibers of the bone are newly formed but a portion is carried in from the preceding fibrous membrane.

Type 2. Another type of bone is formed of fine fiber bundles. This type also occurs in two varieties, distinguished by the direction in which the bundles lie. In the human, fine fiber bundles running parallel are most important in the fetus. They form the simple systems that are replaced in the adult by the second variety of fine fiber bundled bone, lamellar bone.

Lamellar bone is found throughout the adult skeleton arranged in parallel or concentric curving