38 Theories and techniques of oral implantology

sheets, with successive sheets differing in their pre-dominant fiber direction, giving an area of lamellar bone a stratified appearance. The cells of lamellar bone are regular in size and shape but are generally flatter and smaller than those of woven bone. They are widely spaced in the matrix at fairly regular intervals, and their orientation conforms with that of the lamellar system as a whole.

Lamellar bone is formed slowly on preexisting bony or cartilaginous tissues. It is essentially a re-placing, rather than a primary, tissue. For a given volume of material, lamellar bone probably has greater strength than any other type of bone.

Type 3. The third type of bone consists of inter-mixed coarse and fine fiber bundles. This may be a permanent type of bone, as in the bone underlying the tooth sockets that anchors a great number of Sharpey's fibers, or temporary, as a passing phase in the replacement of coarsely bundled by finely bundled bone.

Patterns assumed by the types of bone. As bone accumulates, it assumes a pattern and a grain that is independent of the much finer pattern and grain of the cells, fibers, and crystals that constitute it. This coarse pattern is brought about by a number of factors, including the relationship of the bone matrix to vascular spaces, the relationship of bone matrix to the honeycomb of calcified cartilage at the epiphyseal line, the intermittency of matrix disposition and removal, and the conformity of bone architecture with the prevailing external forces of pressure and tension acting upon it.

On the basis of the vascular–connective tissue spaces and canals alone, bone can be divided into three main varieties: compact, coarse cancellous, and fine cancellous. Because bone is an active tissue, the type of bone and the pattern assumed by it are not absolute. Transformations of fine cancellous bone to coarse cancellous bone and back again are not rare. Usually, however, the adult human is characterized by compact and coarse cancellous bone composed for the most part of lamellar bone, and the human fetus by fine cancellous bone. Fine cancellous bone may reappear in the adult in the healing of a fracture and in pathologic bone conditions generally. It may be composed of woven bone alone, woven bone overlaid with lamellar bone, or lamellar bone surrounding spicules of calcified cartilage.

Compact bone. The compact bone pattern is distinguished by relatively inconspicuous interruptions in an almost solid matrix. The long bones, such as those of the arms and legs, and the shells of flat bones are compact bone. The organ most frequently

used as an example in describing the pattern of compact bone is the femur. In cross section, the femur is seen to consist of a broad girdle of bone surrounding a marrow cavity. Through the bone itself run numerous blood vessels arranged on a longitudinal axis. These supply the living bone cells with the nutrients needed for normal metabolism and carry away metabolic wastes.

The organization of bone begins with these vessels and their associated nerves. One association of blood vessels and a nerve and the cavity in which they lie is called a haversian canal. Around the canal are concentric layers of bone, the lamellae. There may be from five to twenty lamellae around each canal, and the unit they form is called a haversian system. The system is clearly bounded by a cement line. Arranged in a fairly even pattern in the lamellae are small pockets, or lacunae, in which lie the osteocytes, the living bone cells. Between lacunae and extending to the lining cells of the haversian canal are the canaliculi, small grooves in which the branching process of the osteocyte's cytoplasm lie. These processes anastamose with neighboring cells, thus forming a continuous pathway for the inter-change of materials between the cells and the canal of the system.

This description of lamellar compact bone organized into distinct haversian systems is strictly a two-dimensional concept. These haversian systems are actually cylinders extending for considerable distances in the long axis of the bone, following the vessels they enclose. When referring to the cylinder—the haversian system as a three-dimensional unit—the term osteon is used. The osteons are interconnected by Volkmann's canals, which house the blood vessels that perforate the lamellae of adjacent systems directly to the central axial canals. They also communicate with the vessels of the periosteum or bone marrow.

Between osteons, or haversian systems, are angular spaces packed with interstitial lamellae, the remnants of past, largely eroded haversian systems. Toward the free surface of the bone are circumferential lamellae whose curvature corresponds to that of the surface of the bone as a whole.

Haversian systems are always formed by appositional growth, the deposit of new osseous tissue upon the free surfaces of older bone. This occurs on the inner surfaces of bone lamellae surrounding blood vessels until there is a marked decrease in the diameter of the haversian canal, with a concomitant in-crease in the thickness of the bone.

Bone is thus a continuum, with its features con-