The external callus is converted into bone by passing through the intermediary condition of cartilage. Between the broken-bone ends granulation occurs more slowly, and repair at this point is delayed, partly because of poor circulation and nutrition; but the internal callus acting as a bobbin within, and the external callus acting as a solder on the outside, give sufficient support and strength to effect a final and absolute ossification of all the interfragmentary granulation tissue. When the time comes when callus is no longer necessary it begins to disappear by absorption. When everything proceeds normally callus is absorbed in a proportion commensurate with its loss of utility. When bone ends have badly united considerable callus remains permanently. When apposition has been ideal it almost completely disappears, even the medullary cavity being restored.
Fragments which are completely detached may be reunited by practically the same primary process, but fragments of considerable size usually become surrounded by granulation tissue, by which they are nourished and may be finally reunited, with more or less departure from their original shape and location. It is in this way that a comminuted fracture may heal. Fragments that are separated sometimes necrose and have to be removed.
Fig. 278
Fig. 279
Compound fractures resulting from arm being caught in belting and wound around shafting. End of radius united to ulna and lower end of ulna to the radial fragment. Pseudarthrosis of humerus, thrice operated, the third time in the Buffalo Clinic. (Skiagram by Dr. Plummer.) (Arch. Phys. Therap., May, 1905.)
The repair of the flat bones is effected by a similar process, which is referred to as callus formation. In the skull it is brought about chiefly through the agency of the diploë, whose powers in this direction are somewhat limited. Cancellous bone tissue usually throws out but little callus. Its repair occurs from within. Cartilage heals by a very similar process, though it is not now ossific tissue but fibrous which reunites the fractured surfaces. Instances of both kinds can be seen when a fracture has crossed a joint surface.
In a compound fracture much will depend upon the existence or absence of septic complications. In a clean wound, whence blood and fluid may have escaped, there will be little but granulation tissue. Should this wound suppurate the exposed bone surfaces will undergo at least a superficial necrosis, necrotic particles being removed by the same granulation tissue which will later bind the bone ends together. Here, too, the internal callus plays the largest role in the process of repair. The bone tissue first formed is always coarse and soft. Complete calcification and restoration of original density and vascularity occur slowly. Neither cartilage nor bloodvessels alone appear capable of forming bone; the latter is produced only under the influence of the osteoblasts, which penetrate from the periosteum and the bone itself along the course of the bloodvessels.
The process is one of conversion of blood clot into provisional callus, which then changes into granulation tissue or into cartilage, both of these materials undergoing subsequent conversion into bone through the medium of the osteoblasts and osteoclasts (or giant bone cells), the neighboring bone itself undergoing a rarefying ostitis, to change back into its original condition with the final changes of the callus.
Repair of intra-articular fractures has already been described as influenced by the presence of synovial fluid and cartilage. The latter does not proliferate, and the line of fracture usually appears as a groove on its surface. At epiphyseal junctions union is usually rapid and satisfactory, for the changes taking place at this point are in the direct line of what is needed for repair.