Fracture healing is the healing of the bone fracture & attaining its original architecture & strength- takes months to years to complete.
Fracture healing can be divided into two types:
1.Primary/Direct healing (in stable fracture by internal remodeling):-direct filling of fracture-site with bone, without formation of mechanically relevant periosteal/endosteal callus(No callus), No resorption of fracture ends; e.g. Rigid non-gliding implants like Compression Plate, Lag Screws etc.

2.Secondary/Indirect healing (in unstable fracture by callus formation):- formation of an intermediate callus prior to bone formation (3 overlapping phases – Inflammation/Repair/Remodelling).
Amount of callus formed depends upon stability of fracture & increases with greater instability.

The stages of secondary fracture healing:
Grossly, 4 main stages of fracture healing-
1.Stage of inflammation
2.Stage of soft callus formation
3.Stage of hard callus formation
4. Stage of remodelling
Although the stages have distinct characteristics, there is a seamless transition from one stage to another. These stages can be grouped into the following three phases.

Three Major Phases:– I. Reactive Phase, II. Reparative Phase & III. Remodelling Phase. Although the stages have distinct characteristics, there is a seamless transition from one stage to another.

I. Reactive Phase: 3 stages- 1) Haematoma formation– within hours, due to ruptured blood vessels by the injury; 2) Stage of Inflammation– within 48 hours, invasion of inflammatory cells(due to release of TGF-β, PDGF & other signaling molecules from haematoma); 3) Stage of Granulation tissue formation– 2 to 12 days, angiogenesis & routing of appropriate cells to the fracture site & formation of a granulation tissue bed.

II. Reparative Phase: 4) Stage of Soft Callus formation– invasion of pluripotent mesenchymal cells- differentiating into fibroblasts, chondroblasts & osteoblasts with formation of soft fracture callus; 5)Stage of Hard callus formation ( last to several months)-mineralization of soft callus leading to the transition into woven bone & stiffening & strengthening the newly formed bone;

III. Remodelling Phase:  Remodelling to original bone contour- continues for several months to years- woven bone is replaced by lamellar bone, the medullary canal is restored, & the bone is restored to normal or near normal morphology & mechanical strength.

Factors affecting fracture healing:

• 1. Systemic: a. Age, Activity, Nutritional Status, Hormonal factors
     b.Diseases- diabetes, anemia, neuropathies, tabes, Vitamin(A,C,D,K) deficiencies
     c. Drugs (NSAIDs, anti-coagulants, CCB, cytotoxins, phenytoin etc) & substance abuse(nicotine, alcohol)
     d.Others- CNS trauma, Hyperoxia, environmental temperature, Systemic growth factors…
  2. Local: I. Independent factors– a.Type of bone, b. Abnormal bone(Radiation necrosis, infection, tumours & pathologic conditions), c. Denervation.
     II. Injury dependent: a. Degree of local damage- Open fracture, comminuted fracture, velocity of injury, severity of injury
     b. Extent of disruption of vascular supply to bone & soft tissues
     c.Type & location of fracture
     d.loss of bone, loss of overlying soft tissue, soft tissue interposition
     e. Local growth factors
     III. Treatment dependent: a. Extent of Surgical trauma,
     b.Implant induced altered blood flow,
     c.Degree & kind of rigidity of fixation & the influence of timing
     d.Load inducd deformation of bone & soft tissue
     e.Contact between fragments(gap, displacement, overdistraction)
     f.Posttraumatic Osteogenetic factors- bone graft, BMP, electrical stimulation, etc
     IV. Complications dependent: Infection, Venous stasis, Metal allergy etc.
     (From Campbell’s Operative Orthopaedics)