The tissues involved in implant procedures 49

While the parts of the callus around the two fragments and the sealing callus are being formed, the connective tissue of the bridging callus differentiates into fibrocartilage and, later, hyaline cartilage. These tissues span the fracture as a cantilevered bridge spans a river. When the fibrous "abutment" on the edge of each "bank" of the fracture has be-come spongy bone, the cartilage forming the span of the bridge ossifies. Finally, the fibrous tissue between the two fracture fragments ossifies directly, without passing through a cartilaginous phase. This unifying callus becomes firmly anchored to the main fragments of the bone. At this point, the structure is called a primary bone callus.

Secondary bone callus. In the replacement of the primary bone callus, two procedures occur simultaneously. The immature coarse fibrillar bone is re-placed by mature, lamellated bone with a greater mineral content, and the loose spongy bone of the primary callus is reconstructed into a much denser secondary callus by the formation of areas of compact bone.

During callus formation, bone was produced in excess as a protective measure. After the two fragments have been solidly united with mature bone, the excess loses its functional importance. It is then gradually removed by resorption until the original shape and outline of the fractured bone have been reestablished.

How long has the restitution of the bone to its original state taken? The first steps began seconds after the injury, but the healing process is gradual. The blood of a hematoma coagulates usually within 6 to 8 hours. The callus increases in size for from 4 to 6 weeks, then gradually diminishes. In the most favorable cases, the fracture site is not recognizable after a period of 3 to 4 months. In most cases, the bone is so completely restored that it is impossible to detect the fracture 1 year after it occurs. The healing process is summarized visually in Fig. 2-28.

Toleration of the implant

Bone, like any other living tissue, is sensitive to the introduction of a foreign substance. A great deal of work has been clone on the toleration of a foreign substance by living tissues. Most of this has been clone on soft tissues, but evidence indicates that the reactions of bone are similar.

If a foreign material is implanted in bone, an immune response may or may not occur, depending upon the composition of the implant. For example, living material taken from the body of the person

himself or from his identical twin   the so-called autogenous or isogenous transfers—are often successful. In a homogenous transplant, or one in which the material has been contributed by another person, the transplant is usually unsuccessful.

If a piece of bone contributed by one person is grafted onto the bone of another, as the transplant "settles in" proteins from it are circulated away from the graft. These proteins are formed by and reside in the transplant's cells. When these foreign proteins are circulated through the lymph to regional lymph nodes, they stimulate the production of antibodies. The foreign substance is said to be antigenic, or capable of causing the production of antibodies. The spleen also reacts in a minor role to antigens reaching it in the blood.

The antibodies circulate to the homograft and attack it, eventually causing its destruction. As long as the homograft is alive, it continues to form antigens and provoke an immune response in the host.

The antibodies are carried mainly by cells, probably lymphocytes produced in the regional lymph nodes. It is also possible that circulating cytotoxins may play a role in rejecting foreign substances.

The inflammation that results in allergic activity disrupts the healing of the site, not to mention pre-venting the repair and replacement of the transplant by the host's own tissues.

The major factor in producing an immune response is that the foreign substance must be capable of provoking the response. Usually it does this be-cause it is protein in nature. Any healthy tissue will react to the introduction of a foreign protein, whether it be isolated or associated with living cells. The substance must be able to participate in a chemical reaction. This is not the case with the foreign inert substance used as an endosseous implant.

An endosseous implant is made of a relatively inert metal, such as tantalum, Vitallium, or titanium. Because the metal is inert, it rarely participates in the chemical activities of the cells around it. Be-cause it is a nonparticipant, it is well tolerated by the tissues that grow around it, ignoring its presence. The bone cells will grow close to and often through any parts of an implant and, once the bone has calcified, the implant will be even more firmly held than when it was placed in the bone.

The total acceptance by the tissues of a metallic pin or post has been known for years; prior to their use in dental implant procedures, metallic pins have been set in bone to help the healing of fractures elsewhere in the body.