40 Theories and techniques of oral implantology

these bars is called a line of trajectory or a stress line, for obvious reasons.

Relieving or dissipating stress by distributing it over a greater area is a sound engineering principle. During the latter half of the nineteenth century, a Swiss mathematician-engineer named Culmann recognized in Herman von Meyer's description of the bar-like structure of a spongy bone the similarity in function between the structures and stress lines in bone and beam mechanics. This resulted in the Culmann-Meyer theory, which states that there is a functional relationship between the stress lines in bone and its anatomic shape. This concept was further developed by Julius Wolff, who formulated his

law of trans f ornzation of bone in 1892. This law

states that wherever stresses occur in bone, whether they are tensile or compressive stresses, there is a formulation of bone so that the strength of bone in that area is increased. In other words, osteogenesis is stimulated by pressure or tension. If functional stresses shape bone, changes in the direction or strength of these stresses also cause changes in the form and structure of the bone. It also follows that if there is no stress, there is no stimulation for the growth of bone.

Each jaw is adapted differently, both externally and internally, to resist and distribute stress. Al-though the basic architecture of each is inherited, it is modified by function and has strikingly different stress patterns based on its relative position in the

skull. Recognizing these differences will help the operator plan the best sites for the implants. Knowing that stress encourages bone formation, according to Wolff's law, the operator can anticipate where dense bone is normally located. In order to create the stimulation needed for osteogenesis, an implant must be placed inside the bone and stresses similar to those provided by a natural tooth restored. Conventional dentures, which rest on top of the bone, cannot offer the bone this stimulation; therefore, the hone resorbs.

Both the mandible and the maxillae bring about occlusion, with one form of stress affecting their structure. Their structures are remarkable engineering feats, each adapted in its particular way to its position in the head and to the common function that they share.

The mandible is the only movable bone in the skull. It is attached to the skull by muscles and tendons, and it must be sufficiently strong in itself to withstand opening and closing the mouth and movements of the head and chin in general. It is structurally adapted to this by the thick layer of compact bone that forms a kind of shell, or armor, around it. It is strengthened by even more compact hone along its lower border, in the mental area, and where it articulates with the rest of the skull (Fig. 2-18).

Inside the compact bone shell is cancellous bone, which forms and surrounds the sockets of the teeth

Fig. 2-18. The density of bone through various levels of the mandible differs, as seen in these horizontal cross sections. The cross sections begin superiorly through existing teeth and continue down beyond the apices of the roots, finally going into the inferior floor of the mandible. (From Updegrave, W. J.: New horizons in periapical and interproximal radiography, Elgin, Illinois, 1966, Rinn Corporation.)