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The technical definition of ‘hardness’ is similar to our intuitive understanding of the word: it refers to the resistance of a material to indentation. As bone consists of two major components, one (mineral) which is considerably harder than the other (collagen), the microhardness of the composite material reflects their relative proportions or the degree of mineralization of the bone. This is altered by a number of processes, physiological, pathological, or pharmaceutical, and microhardness testing provides a means of assessing these changes.

microhardness.gif (59871 bytes)The operation of a microhardness tester consists of the following; a pyramidal diamond indenter, of measured geometry, is lowered onto the test sample under a known load, leaving an indentation in the surface. The depth of the indentation is related to the hardness of the sample (the harder the sample, the shallower the indentation). While the depth itself is difficult to measure, a linear relationship exists between the length of the indentation and the depth (because the diamond is pyramidal) and length can therefore be used as a proxy for depth. The length and the load are then used to calculate the hardness of the material, in Knoop hardness units, according to the following formula:

KH number = 1.451 * F / d^2

Where "F" is the test force and "d" is the length of the indentation

The Knoop hardness is a material property and is, in principle, independent of the test conditions