When the bone become fractured there are many mechanism or pathophysiology process which start after the bone become fractured to allow it to become healed as follow

When a bone break there is disruption of periosteum, cortical bone trabecular bone and the blood vessels which run in the periosteum and the medulla
There is haemorrhage and immediate release of cytokines this signals to cells locally that dam­age has occurred

 These cytokines attract macrophages, which start the clearing-up process

They also attract undifferentiated stem cells, which migrate in and start differentiating into fibroblast and bone -producing cells

These stem cells probably come from the periosteum and the endosteum, and normally lie latent

The haematoma around the fracture is invaded with small
capillaries while the macrophages remove the haematoma itself

 At the same time connective tissue is laid down the connec­tive tissue slowly organises

 This pattern of layers of organised tissue appear, first as a collar arising from the periosteum close to the end of each broken bone

The collars appear to grow towards the collar on the other bone Eventually, the spurs of callus meet and bridge the fracture site

   They become increasingly thick, and strong fibrocartilage stabilises the fracture. this period, which in the adult occurs over the first few weeks after the fracture, is described as the fracture becoming sticky

It may still be possible to angulate the fracture but it is no longer possible to translate the fracture (move it from side to side

Meanwhile in the fracture cleft Itself, osteoclasts con­tinue to resorb haematoma and other dead tissue and to eat away the broken bone ends

This can result in the fracture becoming more obvious on radiographs over the first few weeks and, indeed, can make visible fractures that were initially invis­ible (e.g. the scaphoid

 The callus of fibrous cartilage around the fracture cleft becomes calcified and then ossified (so that it is visible on radiographs

 Ossification starts not at the bone ends but in the centre of the fracture cleft, where oxygen lev­els may be very low

 Cartilage may be laid down initially rather than bone this cartilage is then replaced by bone (endochondral­ ossification).  the callus is either derived from the haematoma or from the periosteum by movement stimulates the production of a callus.

When the fracture can no longer be angulated with normal
loads, and it is not painful to try, the fracture is said to be clinically united

On radiographs, when the Strands of ossified callus can be seen to be stretching continuously from one bone end to another the fracture is said to be radologically united , in neither case is the fracture at full strength yet, but at this stage limited activity can be undertaken safe

 Finally, the callus forms a fat cuff of woven bone from one bone end to the other

This callus is at least as strong as the bone around it, because it has widened the diameter of the tube and this confers extra strength

This stage is called consolidation Over the next months the woven bone is replaced by Haversian cortical bone which remodels over the following years, until it is almost impossible 
to see where the fracture was in the bone

Types of bone union

Clinically:united pain free to pressure not full strength

Radiologically :united bone cross the fracture cleft

Consolidatiaon :osteoblastic activity has returned to near normal or full strength

Clinical union

A bone is clinically united when putting load on the fracture produces no detectable movement and no pain

The fracture site will not yet be as strong as the bone around it, but it is united

Radiological union

This is not the same as clinical union

 It occurs when the callus around the fracture can be seen to pass from one broken bone end to the other without a gap between

 The fracture across the medulla of the bone may still be visible, but the callus around the bone is continuous

 The bone should now be able to cope with normal loads but will not be as strong as the bone around it

 From a management point of view, it is the time when movement and loading of the limb should be increased to build up muscle power, mobility and proprioception

 If the patient plays sport or works in a job involving heavy labour he or she should not return to this unless the bone is protected or until the fracture has consolidated 


Consolidation takes much longer than union

 And is defined as the time when the process of fracture healing is complete and the strength of the bone has risen to normal levels or even beyond

The formation of callus around a fracture creates a strong cuff The diameter of this cuff is greater than the diameter of the bone itself, and so a consolidated fracture can be stronger than 
the orig­inal bone

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