John B. Brunski, Ph.D.

  Professor, Biomedical Engineering

Research Area

Mechanisms of Overload

The image below comes from an introductory study in which we compressed a small block of Haversian bone mounted on the stage of a reflected light microscope.  We captured images of the bone microstructure before and during straining to a strain of about 2000 microstrain (0.002, or 0.2%) as measured with a strain gage on the reverse side of the sample.  From the DIC data, we found that the effective strain at the level of the microstructure was considerably different from the  strain field that would have been predicted from the strain gauge data.  (The effective strain is a combined strain parameter involving the principal strains.)  Note that there are some regions where the effective strain exceeds 0.03, or 3%, which is rather high in view of the fact that the average axial strain from the strain gauge was 0.002 or 0.2%.

Nicolella et al. have seen similar results in tests of bovine bone, so these data are not unexpected, given bone's microstructure, which includes lamellae at various orientations, osteons having various levels of mineralization, and holes of various shapes, etc., all of which would be expected to perturb the local strain state.  One of the most interesting implications from this type of work is that our "global" estimates of strain as measured with strain gages (which give average values of strain over a large area in terms of the microstructure) may significantly underestimate the true levels of strain that occur at the microstructural level.  This could be important to consider when thinking about bone biology and the role of mechanical deformation and cells, etc.

This type of work is helping us understand the actual stress-strain conditions at real bone-implant interfaces.  It should eventually help us design better implants, so that dangerous loads and interfacial strains can be prevented.•