Mechanically adaptive bone remodelling.

Removal of the ulna in mature sheep causes a slight increase in peak principal walking strains in the radius which can be recorded by rosette strain gauges. The overstrain on the cranial surface of the radius (20%) was more than twice that on the caudal surface (8%) yet over the 50 weeks following ulnar osteotomy new bone was deposited predominantly on the bone's caudal periosteal surface. The total amount of new bone deposited on the radius replaced the area of bone in the removed ulna, thus equilibrating strains due to compression between osteotomised and non-osteomised limbs. Strains due to bending, and consequently total strains, were reduced to below normal suggesting that mechanically adaptive bone remodelling may not be related to absolute strain levels but to the relative distribution of strain. New bone formation can therefore be stimulated as the result of a mechanical reorganization in which total strains are lower than those which normally occur. The new bone deposited on the caudal cortex of the radius became intensively remodelled with secondary osteons while that on the cranial surface remained in its primary form. This suggests that osteonal remodelling may not always be a simple reparative process but may be one influenced by the strain situation possibly to improve the structure and physical properties of the tissue.