In a simple fracture the vessels of the periosteum and the marrow being torn at the same time as the bone is broken, blood is poured out, and clots around and between the fragments. This clot is soon permeated by newly formed blood vessels, and by leucocytes and fibroblasts, the latter being derived from proliferation of the cells of the marrow and periosteum. The granulation tissue thus formed resembles in every particular that described in the repair of other tissues, except that the fibroblasts, being the offspring of cells which normally form bone, assume the functions of osteoblasts, and proceed to the formation of bone. The new bone may be formed either by a direct conversion of the fibrous tissue into osseous tissue, the osteoblasts arranging themselves concentrically in the recesses of the capillary loops, and secreting a homogeneous matrix in which lime salts are speedily deposited; or there may be an intermediate stage of cartilage formation, especially in young subjects, and in cases where the fragments are incompletely immobilised. The newly formed bone is at first arranged in little masses or in the form of rods which unite with each other to form a network of spongy bone, the meshes of which contain marrow.
Fig. 4.—Excess of Callus after compound fracture of Bones of Forearm.
The reparative material, consisting of granulation tissue in the process of conversion into bone, is called callus, on account of its hard and unyielding character. In a fracture of a long bone, that which surrounds the fragments is called the external or ensheathing callus, and may be likened to the mass of solder which surrounds the junction of pipes in plumber-work; that which occupies the position of the medullary canal is called the internal or medullary callus; and that which intervenes between the fragments and maintains the continuity of the cortical compact tissue of the shaft is called the intermediate callus. This intermediate callus is the only permanent portion of the reparative material, the external and internal callus being only temporary, and being largely re-absorbed through the agency of giant cells.
Detached fragments or splinters of bone are usually included in the callus and ultimately become incorporated in the new bone that bridges the gap.
In time all surplus bone is removed, the medullary canal is re-formed, the young spongy bone of the intermediate callus becomes more and more compact, and thus the original architectural arrangement of the bone may be faithfully reproduced. If, however, apposition is not perfect, some of the new bone is permanently required and some of the old bone is absorbed in order to meet the altered physiological strain upon the bone resulting from the alteration in its architectural form. In overriding displacement, even the dense cortical bone intervening between the medullary canal of the two fragments is ultimately absorbed and the continuity of the medullary canal is reproduced.
The amount of callus produced in the repair of a given fracture is greater when movement is permitted between the broken ends. It is also influenced by the character of the bone involved, being less in bones entirely ossified in membrane, such as the flat bones of the skull, than in those primarily ossified in cartilage.
If the fragments are widely separated from one another, or if some tissue, such as muscle, intervenes between them, callus may not be able to bring about a bony union between the fragments, and non-union results.
Bones divided in the course of an operation, for example in osteotomy for knock-knee, or wedge-shaped resection for bow-leg, are repaired by the same process as fractures.