66 Theories and techniques of oral implantology

destructive and formative activities keeps the bone healthy and functional.

The destruction of bone is brought about by osteoclasts, which appear in overaged or nonfunctional bone as a consequence of chemical changes caused by the degeneration or necrosis of the osteocytes. The osteoclasts apparently produce proteolytic enzymes that destroy or dissolve the organic components of the bone matrix and chelating substances that render the bone salts soluble. This destruction occurs on the face of the bundle bone toward the supporting alveolar bone. As the bundle bone re-sorbs here, it is replaced by lamellated bone or trabeculae. The trabeculae, as elsewhere, are formed for the most advantageous distribution or deflection of stress.

Osteoblasts produce new bone on the side of the bone contacting the periodontal ligament. The osteoblasts develop from undifferentiated mesenchymal reserve cells in the loose connective tissue of the periodontal ligament, which serves not only to support the tooth but also as a periosteal membrane. The osteoblasts become aligned along the surface of the bundle bone and produce osteoid tissue. This consists of collagenous fibrils cemented by mucopolysaccharides but containing no mineral salts. As the matrix calcifies, some osteoblasts become embedded in it as osteocytes, as do Sharpey's fibers from the periodontal ligament.

Occlusal trauma

If occlusal forces become abnormally intense or misdirected because of a change in the position of contacting tooth surfaces, the supporting periodontal tissues usually suffer. Occlusal disharmony results in creating areas of intensive pressure and tension, more than the physiologic limits of the periodontal ligament and bone can tolerate (Fig. 3-22).

When pressure is excessive, the collagen fibers of the periodontal ligament are compressed and be-come necrotic. The blood vessels in the ligament thrombose, and osteoclasts differentiate on the bone surface and begin the resorption of bone. As long as the excessive pressure continues, bone resorbs. When the pressure is relieved, a small amount of new bone is formed and the fibers of the periodontal ligament are reembedded.

While damage to the periodontal ligament is done in areas of pressure by compression, it results in areas of undue tension from stretching the fibers. If these fibers are extended too far, they separate along their interproximal junction, deepening the

Fig. 3-22. Occlusal trauma creates areas of excessive pressure as well as of excessive tension. (From Bhaskar, S. N.: Synopsis of oral pathology, ed. 3, St. Louis, 1969, The C. V. Mosby Co.)

periodontal pocket and permitting the invasion of bacteria and harmful chemicals. The broken fibers no longer stimulate bone regeneration, and the bone resorbs from disuse. This is seen in radiographs as a V-shaped breakdown of the alveolar crest with a concomitant widening of the periodontal ligament. If the bone breakdown occurs rapidly enough, with no stimulation of new bone apposition, marrow spaces sometimes come in contact with the periodontal ligament.

If the fiber bundles are stretched but not broken, they stimulate the differentiation of osteoblasts and the formation of new bone. It is by this process   a removal of bone in certain areas and a deposition of

bone in others   that a tooth drifts. Its movement is clearly marked in appositional areas by resting lines separating parallel layers of bone, and in re-sorption areas by reversal lines.

When bone is resorbed on the mesial alveolar wall because of excessive pressure and apposed on the distal wall in response to tension, the tooth drifts mesially. The mesial wall exhibits Howship's lacunae, which contain the osteoclasts known to bring about bone destruction. The bundle bone here is very thin and irregular or exists only in certain areas. Its very existence is caused by the fact that periods of re-sorption alternate with periods of rest and repair. During the repair period, some bundle bone is