Fracture Healing

The basic processes involved in the healing of bone fractures bear many resemblances to those seen in skin wound healing. Unlike healing of a skin wound, however, the defect caused by a fracture is repaired not by a  fibrous “scar” tissue, but by specialized bone-forming tissue so that, under favorable circumstances, the bone is restored nearly to normal.

Structure of bone

 Bone is composed of calcified osteoid tissue, which consists of collagen fibers embedded in a mucoprotein matrix (osteomucin). Depending on the arrangement of the collagen fibers, there are two histological types of bone:

1. Woven, immature or non-lamellar bone

This shows irregularity in the arrangement of the collagen bundles and in the distribution of the osteocytes. The osseomucin is less abundant and it also contains less calcium.

2. Lamellar or adult bone

In this type of bone, the collagen bundles are arranged in parallel sheets.

 Stages in Fracture Healing (Bone Regeneration)

Stage 1: Haematoma formation.  Immediately following the injury, there is a variable amount of bleeding from torn vessels; if the periosteum is torn, this blood may extend into the surrounding muscles. If it is subsequently organized and ossified, myositis ossificans results.

Stage 2: Inflammation. The tissue damage excites an inflammatory response, the exudate adding more fibrin to the clot already present. The inflammatory changes differ in no way from those seen in other inflamed tissues. There is an increased blood flow and a polymorphonuclear leucocytic infiltration. The haematoma attains a fusiform shape. 

Stage 3: Demolition. Macrophages invade the clot and remove the fibrin, red cells, the inflammatory exudate, and debris. Any fragments of bone, which have become detached from their blood supply, undergo necrosis, and are attacked by macrophages and osteoclasts. 

Stage 4: Formation of granulation tissue. Following this phase of demolition, there is an ingrowth of capillary loops and mesenchymal cells derived from the periosteum and the endosteum of the cancellous bone. These cells have osteogenic potential and together with the newly formed blood vessels contribute to the granulation –tissue formation.

Stage 5:  Woven bone and cartilage formation. The mesenchymal “osteoblasts” next differentiate to form either woven bone or cartilage. The term “callus”, derived from the Latin and meaning hard, is often used to describe the material uniting the fracture ends regardless of its consistency. When this is granulation tissue, the “callus” is soft, but as bone or cartilage formation occurs, it becomes hard.

Stage 6: Formation of lamellar bone. The dead calcified cartilage or woven bone is next invaded by capillaries headed by osteoclasts. As the initial scaffolding (“provisional callus”) is removed, osteoblasts lay down osteoid, which calcifies to form bone. Its collagen bundles are now arranged in orderly lamellar fashion, for the most part concentrically around the blood vessels, and in this way the Haversian systems are formed. Adjacent to the periosteum and endosteum the lamellae are parallel to the surface as in the normal bone. This phase of formation of definitive lamellar bone merges with the last stage.

Stage 7:  Remodelling. The final remodeling process involving the continued osteoclastic removal and osteoblastic laying down of bone results in the formation of a bone, which differs remarkably little from the original tissue. The external callus is slowly removed, the intermediate callus becomes converted into compact bone containing Haversian systems, while the internal callus is hollowed out into a marrow cavity in which only a few spicules of cancellous bone remain.

References

Bezabeh ,M. ; Tesfaye,A.; Ergicho, B.; Erke, M.; Mengistu, S. and Bedane,A.; Desta, A.(2004). General Pathology. Jimma University, Gondar University Haramaya University, Dedub University.