The events that occur during normal wound healing of soft-tissue injuries (e.g., inflammation, fibroplasia, remodeling) also take place during the repair of an injured bone. However, in contrast to soft tissues, osteoblasts and osteoclasts are also involved to reconstitute and remodel the damaged ossified tissue.
Osteogenic cells (osteoblasts) important to bone healing are derived from the following three sources: (1) periosteum, (2) endosteum, and (3) circulating pluripotential mesenchymal cells. Osteoclasts, derived from monocyte precursor cells, function to resorb necrotic bone and bone that needs to be remodeled. Osteoblasts then lay Gown osteoid, which, if immobile during healing, usually goes on to calcify.
The terms primary and secondary intention are appropriate for descriptions of bone repair. If a bone is fractured” and the free ends of the bone are more than 1 mm or so apart, the bone heals by secondary intention; that is, during the fibroplastic stage of healing a large amount
of collagen must be laid. down to bridge the bony gap (Fig. 4-6). The fibroblasts and osteobIasts actually produce so much fibrous matrix that the healing tissue extends circumferentially beyond the free ends of the bone and forms what is called a callus (Fig. 4-7). Under normal ,conditio s the fibrous tissue, including the callus, ossifies. During the remodeling stage, bone that was haphazardly produced is resorbed by osteoclasts, and osteoblasts lay down new bone directed to resist low-grade tensions
placed on the bone (Fig. 4-8).
Healingof bone by primary intention occurs when the bone is either incompletely fractured so that the fractured ends do not become separated from each other (greenstick fracture), or when a surgeon closely reapproximafes and rigidly stabilizes the fractured ends of a bone
(anatomic reduction of the fracture). In both of these situations,
little fibrous tissue is produced and reossification of the tissue within the. fracture area occurs quickl with minimal callus formation. The surgical technique that comes closest to allowing bone to heal by primary intention
is anatomic reduction of the application of bone plates that rigidly hold the ends of the bone together. This minimizes the distance between the ends of a fractured bone so that ossification across the fracture gap can occur with little Intervening fibrous tissue formation.
“The term fracture used with respect to bone repair includes not only
traumatically injured bone but also bone cuts purposely made by a surgeon during reconstructive surgery.
Two factors are important to proper bone healing: (1) vascularity and (2) immobility. The fibrous connective . tissue that forms in a bony fracture site requires a high degree of vascularity (which carries blood with normal oxygen content) for eventual ossification. If vascularity
or oxygen supplies are sufficiently compromised, cartilage forms instead of bone. Furthermore, if vascularity or oxygen supplies are poor, the fibrous tissue does not chondrify or ossif.
Placing bone under continuous or repeated cycles of some tension stimulates continued osteoblastic bone formation. Bone is formed perpendicular to lines of tension to help withstand the forces placed on it. This is the basis of the functional-matrix concept of bone remodeling.
However, excessive tension or torque placed on a healing fracture site produces mobility at the site. This mobility compromises vascularity of the wound and favors the formation of cartilage or fibrous tissue rather than bone along the fracture line; in a contaminated fracture it promotes wound infection (see Fig.4-8).