Examination of compact bone microdamage using back-scattered electron microscopy

A new staining technique using heavy metals (lead-uranyl acetate) has been developed to allow visualization of bone microdamage using both light microscopy and scanning electron microscopy operated in its back-scattered mode (BSE). At the light microscopic level, the number of microcracks counted in sequential sections of human ribs is the same for both the traditional basic fuchsin method of differentially staining microcracks and the new lead-uranyl acetate procedure. With BSE study, however, the number of microcracks observed is significantly reduced in all samples, because of the reduction of projection effect error associated with surface based imaging techniques. Application of the lead-uranyl acetate staining technique to ex vivo-loaded crack propagation specimens showed an extensive ultrastructurally disrupted region associated with the crack path through bone, consistent with the damage process zone around cracks in toughened composite materials.