What happens when I break a bone?

Since bone is living, it usually mends itself if it breaks.  It ultimately heals through physiological processes. 

The are four steps involved in mending a fracture:


1. Inflammation stage – formation of fracture haematoma

2. Fibrocartilaginous callus formation

3. Bony callus formation

4. Bone remodelling


The healing is mainly determined by the periosteum (the connective tissue membrane covering the bone).  The periosteum is one source of precursor cells which develop into chondroblasts and osteoblasts which are essential for the healing of bone.  Other sources of precursor cells include the bone marrow, endosteum, small blood vessels and fibroblasts.

 Step 1: Inflammation Stage – formation of Haematoma

The inflammation stage begins the minute the fracture has occurred and lasts for approximately five days.


When a fracture occurs, there is massive injury to the blood supply to the bone which results in a large amount of bleeding from the fracture segments.  The initial mend is a ‘haematoma’.


The damaged bone tissue at the edges of the fracture segments die back and the dead cells release inflammatory chemicals called “Cytokines” which initiate the healing process. 


Osteoclasts work to remove the dead bone cells.  It is imperative the fracture is reviewed by a doctor to determine what treatment is necessary to “reduce” (push back in place), the dislocated bones.  This may or may not require an anaesthetic.  It then needs to be stabilised via a plaster cast or surgery whereby plates and screws are inserted.  It is important to recognise that it is not only the bones which are affected, but there is also damage to soft tissues (muscles and tendons and blood vessels so there is usually significant swelling.  Once the two ends of the fracture are reduced, this stems the blood-flow and reduces the pain. 


Within hours of a fracture, the blood from the fracture fragments forms a mesh of clotted blood which is the first link between the two fragments.  The mesh contains special cells called fibroblasts, which begin to lay down granulation tissue.  This process occurs between 4-10 days post-fracture.  The granulation tissue forms a scaffold between the two fragments, from which the formation of a soft callus can begin.

Step 2:  Formation of soft callus

The chemical and metabolic reactions that produce the soft callus begin a few days after the fracture has occurred. 


Fibroblast cells that are contained within the granulation tissue begin to form cartilage and fibrocartilage.  This is the spongy material that fills the gap between the two fracture fragments.


After about two weeks, despite being quite fragile, the soft callus provides sufficient stability at the fracture site to allow the formation of new blood vessels, and for osteoblasts at the periosteum (outer layer) to begin laying down “woven bone”.  This woven bone at the margins of the fracture is quite soft and disorganised, but is the first bone contact between the two fracture fragments.


However, the site remains weak for around six weeks. 

Step 3:  Formation of hard callus

From 2-3 weeks onwards, a process begins whereby the delicate cartilage material of the soft callus is transformed completely into woven bone.  This process usually takes between 6-12 weeks (depending on the site and severity of the fracture).  Hard callus formation is a complex process that is guided by the release of mineral compounds such as calcium and phosphate into the cartilage tissue. This process transforms into a bridge of hard callus over the fracture site which signifies fracture union.  Fracture union can be seen on x-ray at approximately 6 weeks post-fracture for upper limbs and 12 weeks post-fracture for lower limbs.  One of factors that encourage hard callus formation in lower limbs is gentle weight bearing exercises.                   

Step 4: Bone remodelling 

This stage can continue over several years. 


The body will naturally lay down more hard callus than what is actually required to unite the fracture segments.  On x-ray the fracture site will look larger because of this deposit. 


Bone remodelling begins as a continuum of normal bone function.  Bone is laid down where it is needed by osteoblasts and removed by osteoclasts depending on the stresses that are placed on the bone during usual daily and sporting activities.  The loosely arranged woven bone is gradually replaced by lamellar bone. This bone is highly organised matter which forms along lines of stress and therefore is much stronger than woven bone. 


Ultimately, once the fracture healing process is complete, the bone should be at least as strong as it was originally.


For pictures and diagrams go to http://www.physioroom.com/injuries/bone_fracture/1_fracture_introduction.php

http://www.slideshare.net/jfreshour/the-fracture-healing-process Various slideshows about fracture healing

What is the difference between bone and cartilage?

Unlike bone, cartilage is a bendable substance that doesn’t contain calcium.  It has no blood vessels or nerves.


The end of every long bone has a smooth layer of cartilage on it where it forms a joint with other bones. Cartilage can be found in other parts of our bodies too. Our ears and nose for example are stiffened with cartilage, making them flexible and enabling them to withstand knocks.