The tissues involved in implant procedures 47

The bone surrounding the apical and lateral walls of an implant is stimulated through the fibers of the fibrous tissue that develops around and encapsulates the embedded portions of the implants, provided they are correctly designed. This fibrous tissue encapsulation acts as a suspensor ligament, or false periodontal membrane. Although it lacks Sharpey's fibers, the false membrane can transmit the vertical occlusal forces as tension to the surrounding bone. With the physiologic direction of applied forces closely restored, the implants, through their balanced mechanical action, encourage mobilization of bone-regenerating elements by restoring functional stress to the area. The inner surface of the fibrous tissue binds to the buried portions of the implant while the outer surface is attached to the alveolar hone.

Fractures and healing

Performing an implant intervention, of course, injures tissues. When bone, the most important tissue involved, suffers any form of injury, it immediately begins to heal itself. However, the healing of bone differs from the healing of other kinds of tissues in that the injured bone is replaced by healthy bone, leaving the healed site as strong as it was before injury. This, of course, is not what happens in the healing of most soft tissues, which are replaced by scar tissue.

The healing of the injured implant site closely resembles the healing of a bone fracture elsewhere. Basically, inflammation, bone resorption, then revascularization occur, and finally substitution by new bone cells derived from adjacent endosteum and periosteurn. In long bones such as the tibia, the new bone cells migrate across a network of fibrous connective tissue and cartilage. In membrane bones such as those in the skull, the cells are derived from a proliferation and extension from the old bone tissue.

Because so much of the study of fractures has centered on the healing of long bones, it seems advisable to summarize what is known about the healing processes in these bones and then to modify the information with what happens to a membrane bone when an implant intervention is performed.

Long bone fractures. As blood vessels in the soft tissues, periosteum, bone marrow, and other involved structures are injured, they rupture and re-lease blood. The blood floods the site, and factors in it begin the initial steps in repairing the injury. Blood coagulates as platelets in it disintegrate, re-leasing a substance called cephalin. The cephalin

combines with calcium and prothrombin circulating in the blood, forming thrombin. Thrombin then combines with fibrinogen, and an insoluble form of fibrinogen—fibrin   precipitates.

Fibrin is an elastic filamentous protein that is laid down as a network throughout the clot as it becomes organized. It is over this network that other elements will migrate to heal the injury. At this stage, the localized mass of clotted blood is called a henaatoma. Then the blood capillaries, and probably lymph capillaries as well, proliferate and invade the clot. Macrophages derived from undifferentiated cells of the connective tissue surrounding the hematoma and from resting wandering cells, histiocytes, or adventitial cells invade the clot.

Fibroblasts, cells believed to furnish connective tissue fibers, are also contributed by surrounding connective tissue, and they too invade the clot. Together, the macrophages and fibroblasts produce another fibrous network called granulation tissue, which will aid in the removal of debris from the injured site. The macrophages and leukocytes engulf necrotic particles of debris and remove them via the lymphatics. Osteoclasts, also derived from cells of neighboring connective tissue, remove bone fragments.

As soon as the necrotic tissues have been re-moved, the number of white blood cells diminishes and the capillaries become reduced in size. The granulation tissue develops into loose connective tissue as the fibroblasts produce numerous collagenous fibers. The resulting tissue is called a fibrous, or temporary, callus.

Fibrous callus. The fracture area is now enclosed by a broad, fibrous, spindle-shaped cuff that extends a considerable distance along the two bone fragments. This cuff completely seals the fracture. It will serve both as a cartilaginous model for osteogenesis and, it is assumed, as a mechanism for the induction of osteogenesis.

Bony callus. The fibrocartilaginous callus is re-placed by bone in a manner similar to building a bridge to span a river. First the fibrous callus surrounding the two fragments is replaced by a bony callus. In building a bridge, elevated abutments are constructed on each bank to bear the cantilevered span. The spongy bone that replaces the fibrous callus connects directly with the surface of the bone fragments and serves as an anchoring point. Next a more or less complete bony plate is formed, sealing the marrow space toward the tissue between the bone fragments.