What is bone remodelling?

This is the normal process of turnover of the bone, for example in response to changing mechanical stimuli (such as excessive weight loss or gain, athletic stresses etc.). As stresses on bones change, so the optimal configuration of the bone structure changes. The stresses begin to lie in different mechanical planes meaning more strength is need in some areas than others.

It has two main phases, resorption and formation.

• Resorption - when bone remodelling is activated, osteoclast cells initially digest the bone. These are cells (derived from a fusion of cells related to phagocytic cells called monocytes) that secrete enzymes onto the bone causing the matrix to dissolve away. This process takes about 3 weeks.
• Formation - After the bone has been resorbed by osteoclasts, osteoblast cells lay down new bone in the areas that require it. Initially they first lay down the organic matrix (osteoid, see 'What is bone made up of?'). Later, the inorganic matrix (calcium salts such as hydroxyapatite) fills out this organic matrix to make up the fully formed bone.
  After this the bone goes into a state of quiescence, where no remodelling occurs, until it is required again in the future.