Healing of fracture by callus formation depends upon some clinical considerations whether the fracture is traumatic (in previously normal bone), or pathological (in previously diseased bone); complete or incomplete like green stick fracture; simple (closed), comminuted (splintering of bone) or compound (communicating to skin surface).

Primary union of fractures occurs occasionally in a few special situations when the ends of fracture are approximated like by application of compression clamps. In these cases, bony union takes place with formation of medullary callus without periosteal callus formation.

Secondary union is the more common process of fracture healing. Though it is a continuous process, secondary bone union is described under the following 3 headings:

1. Procallus formation. There are following steps involved in the formation of procallus.

Hematoma forms due to bleeding from torn blood vessels, filling the area surrounding the fracture. Loose meshwork is formed by blood and fibrin clot which acts as framework for subsequent granulation tissue formation.

Local inflammatory response occurs at the site of injury with exudation of fibrin, polymorphs and macrophages. The macrophages clear away the fibrin, red blood cells, inflammatory exudate and debris. Fragments of necrosed bone are scavenged by macrophages and osteoclasts.

Ingrowth of granulation tissue begins with neovascularisation and proliferation of mesenchymal cells from periosteum and endosteum. A soft tissue callus is thus formed which joins the ends of fractured bone without much strength.

Callus composed of woven bone and cartilage starts within the first few days. The cells of inner layer of the periosteum have osteogenic potential and lay down the collagen as well as osteoid matrix in the granulation tissue. The osteoid undergoes calcification and is called woven bone callus. A much wider zone over the cortex on either side of fractured ends is covered by the woven bone callus and united to bridge the gap between the ends, giving spindle-shaped or fusiform appearance to the union.

2. Osseous callus formation. The procallus acts as scaffolding on which osseous callus composed of lamellar bone is formed. The woven bone is cleared away by incoming osteoclasts and the calcified cartilage disintegrates. In their place, newly-formed blood vessels and osteoblasts invade, laying down osteoid which is calcified and lamellar bone is formed by developing Haversian system concentrically around the blood vessels.

3. Remodeling. During the formation of lamellar bone, osteoblastic laying and osteoclastic removal are taking place, remodeling the united bone ends, which after sometime, is indistinguishable from normal bone. The external callus is cleared away, compact bone (cortex) is formed in place of intermediate callus and the bone marrow cavity develops in internal callus.

These are following complications of fracture healing: fibrous union, non-union, delayed union.

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