The tissues involved in implant procedures 39

forming to the external patterns set by the blood vessels, not to an internal pattern determined by the accumulation of similar structural units orienting themselves in crystal fashion.

Coarse cancellous bone. Coarse cancellous bone looks like a sponge to the naked eye, and for this reason it is often called spongy bone. In a given volume of this bone there is at least as much, if not more, vascular connective tissue than bone matrix. Haversian canals are few, the matrix evidently receiving sufficient nourishment via its canaliculi from superficial vessels lying in the marrow spaces. The bars, or trabeculae, of the sponge are covered with a few areas of resting osteoblasts, active osteoblasts, and in other places osteoclasts, indicating sites where deposition and resorption are still taking place. The abundant, irregularly indented cement lines indicate a complete past history of the resorption and redeposition cycle of this type of bone.

Only the thicker trabeculae are complete osteons; the others consist of an irregular, haphazard arrangement of osteon fragments, similar to the interstitial lamellae of compact bone. Scattered calcified cartilage may be found embedded between the lamellar systems, apparently remnants that have escaped the complex remodeling process.

Fine cancellous bone. Although characteristic of the fetal skeleton and of secondary centers of ossification in the epiphysis of growing bone, fine cancellous bone is of particular interest because it is found in the early healing stages of a fracture.

When first formed, fine cancellous bone superficially resembles wire netting. It has thin trabeculae of woven hone alternating in a regular fashion with spaces filled with loose embryonic connective tissue containing a central blood vessel. A regular palisade of active osteoblasts covers the surface of its trabeculae. The ends of those trabeculae that project into the surrounding connective tissue are covered with a single layer of osteoblasts except at the tip, where they form a rosette. Coarse fiber bundles pass from the surrounding connective tissue into the bone matrix. Many bundles contain osteogenetic fibers.

The woven mesh stage is of a short duration, for remodeling begins almost simultaneously with deposition. Many centrally placed trabeculae are re-sorbed, with osteoblasts being replaced by osteoclasts. Other trabeculae, however, become thickened by surface addition, first of more woven bone and later of fine-fibered bone. Eventually simple lamellar systems develop over the scaffolding of residual woven bone. In this way, the very fine, irregular network of woven bone is replaced by a coarse, more irregu-

lar network of mixed woven and lamellar bone. Eventually, almost all the original bone is replaced by lamellar bone organized into haversian systems.

Classification for implant procedures

The way in which bone is normally organized as compact or cancellous tissue affects the success of an implant, because the implant must be held by the regrowth of bone around it. Using x-rays, the structure of the area contemplated for the implant should be revealed and evaluated. It generally falls into one of three categories.

1. Class I bone structure. This is the ideal type of bone: it consists of fairly evenly spaced trabeculae with small cancellated spaces. It makes a very satisfactory foundation for implant prostheses because the bone will tend to regrow compactly around the implant.

2. Class II bone structure. This bone has slightly larger cancellated spaces with less uniformity of the osseous pattern than Class I bone, but it is still satisfactory for implantation.

3. Class III bone structure. This is the least desirable of all for endosseous implant proceclures. Here large marrow-filled spaces exist between the bone trabeculae, resulting in a loose-fitting implant.

It is highly recommended that all areas to bear implants be closely observed in x-rays before starting any procedure. In the same jaw the implant sites may vary considerably; generalizing from a single observation is often misleading.

When bone density looks unfavorable, it is advisable to drill horizontal holes through the labial plate of bone into the cancellous areas and to deter-mine the osseous structures with the use of the oral scanner (a highly intensified fiberoptic light with a magnified eyepiece for viewing the site: made by Vicon Co.) .

Stress adaptations

Generally a bone receives its maximum strength from its overall shape and by a thickening of those areas that receive the greatest stress. Deposits of dense cortical bone, for example, are found in the chin, which tends to be pulled outward by the masticatory muscles and downward by the neck muscles. In addition to thick deposits of dense bone, stress is distributed interiorly by the way in which spongy bone is deposited. The bars, or trabeculae, of spongy bone are arranged to distribute stress over the greatest possible distance and toward areas that are strengthened to bear the stress. The orientation of