CHAPTER 3 The implant site

An endosseous implant is set in the same tissues that invest and support a natural tooth. For this reason the nature of these tissues in health and disease deserves intensive study when they are occupied both by a tooth and by an implant. Although there are certain distinct differences between the periodontia of a tooth and the tissues supporting an implant, their similarities far exceed their differences in number and importance. Those factors that con-tribute to the health of a tooth contribute to the success of an implant, and those that lead to tooth loss also adversely affect an implant's prognosis.

PERIODONTIA OF A NATURAL TOOTH

The following section is devoted to the periodontal tissues and their relationships to a natural tooth. Because the anatomy of the tooth itself has little bearing on the discussion in the following section, only those parts of the tooth most closely involved v'-ith the supporting tissues will be discussed. The major stress will be on the alveolus, the periodontal ligament, and the gingiva and their anatomy in health and disease. What affects these tissues when they are occupied by a tooth will influence them when they contain an implant.

Alveolus

The alveolus is the socket formed by alveolar bone around a tooth during its eruption (Fig. 3-1). That part nearest the root is the alveolar bone proper, a thin layer of bone characterized by numerous holes through which small blood vessels and nerves pass. The numerous perforations give it a sieve-like appearance, and for this reason it is often called the cribriform plate, after the woven cribs used as sieves in ancient times.

Alveolar bone proper consists of both lamellated and bundle bone. The lamellae are generally parallel to the surface of adjacent marrow spaces or form

haversian systems. The bundle bone is of greater interest because it is here that the principal fibers of the periodontal ligament are anchored and pass into the alveolus as Sharpey's fibers (Fig. 3-2). Bundle bone contains relatively few collagen fibrils in its intercellular substance, and those present are oriented at right angles to the Sharpey's fibers. The matrix of bundle bone contains more cementing sub-stance, making the entire alveolar bone proper appear in radiographs as a thin, continuous, dense line--the lamina dura (Fig. 3-3).

Radiating outward from the alveolar bone proper are the trabeculae of the supporting alveolar bone. These serve as buttresses for the alveolar bone proper and direct stress away from the socket (Fig. 3-4). They may be fairly regularly arranged, forming ladder-like trajectories with distinct horizontal cross-bars, or lack distinct trajectory patterns. Both arrangements seem to function successfully as stress distributors, with the irregularity of one pattern compensated for by an increase in the number of trabeculae. Whereas the stronger, more distinct pat-tern appears to direct stress away from the socket, the second dissipates it by deflecting it in numerous directions.

The trabeculae terminate in denser compact bone, which is characterized by longitudinal lamellae and haversian systems. This compact bone forms the vestibular or buccolingual and oral or lingual cortical plates, which are continuous with the compact layers of the maxillary and mandibular bodies. Both the spongy bone with its trabeculae radiating from the alveolar bone proper and the compact bone form the supporting alveolar bone.

The amount and character of the supporting bone depend upon the tooth's location. Supporting bone in the maxilla is generally thinner than in the mandible, with a greater proportion of cortical bone to cancellous bone. Forces brought to bear on maxillary teeth are partially dissipated by being directed

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