The implant site 67

Cementum

Bundle bone

Periodontal ligament

Fig. 3-23. As the tooth drifts mesially, bundle bone is apposed and resorbed. A, The distal alveolar wall shows apposition of bundle bone as evidenced by resting and reversal lines. B, The mesial alveolar wall shows resorption. The scalloped pattern is indicative of Howship's lacunae. (From Weinmann, J. P.: Bone changes related to eruption of the teeth, Angle Orthodont. 11: 83, 1941.)

formed on the surface facing the marrow spaces, and the detached periodontal fibers become reanchored. These newly formed areas tend to be destroyed as further occlusal trauma exerts its overload.

As the mesial wall of the bony socket is being re-sorbed, the distal wall is undergoing thickening (Fig. 3-23). New bundle bone is deposited along the face of the periodontal ligament in response to tension. As this distal bundle bone attains a certain thickness, that side facing the supporting alveolar bone is resorbed by osteoclasts in the adjacent marrow spaces. This apposition on one face and resorption on the other maintains the functional width of the bundle bone. The resorbed bundle bone is replaced by lamellated bone.

Because a tooth rarely drifts into a more advantageous occlusal relationship, further occlusal trauma is created by drifting.

Disuse atrophy

When a tooth loses its functional opponent, it is no longer in functional occlusion. This means

that the major stimuli that encourage the continual structural changes necessary for the health of the tooth and its periodontal tissues are absent. As a result the tissues begin to resorb from disuse.

The periodontal ligament becomes narrow, and the regular arrangement of its principal fibers is lost (Fig. 3-24) . As the collagenous fibers lose their functional stimulation and assume haphazard orientation, Sharpey's fibers disappear from both the bone and the cementum. The cementum thickens and finally becomes aplastic.

The alveolar bone, which relies on the tension created by Sharpey's fibers for appositional growth, begins to resorb. This is a gradual process. The spongy bone around the alveolar bone proper shows marked rarefaction, while the trabeculae become thinner and less numerous (Fig. 3-25). Because some stimulation is still transmitted by the remaining periodontal tissues, the alveolar bone proper re-mains fairly well preserved. Its support, however, is lost. The poorly supported tooth may drift labially, rotate, or extrude. Any of these tendencies, corn-