Bone microdamage from creep in vivo

During life, repetitive loading causes bone to experience loads that typically repeat in the same overall direction, imposing a greater-than-zero mean stress on bone. Consequently, during repetitive loading both creep and cyclic loading components can contribute to bone failure. Bone failure at the matrix level results from the formation and propagation of microscopic damage within matrix. Recent studies show two predominant damage forms in repetitively loaded bone, presumably resulting from distinct damage mechanisms: (i) Linear Microcracks (Mck), which are sharp cracks ~ 30-100 μm long with well-defined edges, (ii) Diffuse Damage (DifDx): comprised of clusters of small (<;1 μm) sublamellar-sized cracks. Whether fatigue and creep, which are both present during normal cyclic loading, contribute differentially to these types of matrix damage in bone is not known, nor is there an understanding of how bone matrix damage processes operate in living bone. In the current studies, we imposed pure creep conditions on bone in vivo and examined the specific damage patterns that result.