Elevated solute transport at sites of diffuse matrix damage in cortical bone: Implications on bone repair.

Diffuse matrix damage in rat cortical bone has been observed to self-repair efficiently in 2 weeks without activating bone remodeling, and unlike the case with linear cracks, the local osteocytes at the sites of diffuse damage remain healthy. However, the reason(s) for such high efficiency of matrix repair remains unclear. We hypothesized that transport of minerals and other compounds essential for damage repair is enhanced at the damaged sites and further increased by the application of tensile loading. To test our hypothesis, diffuse damage was introduced in notched bovine wafers under cyclic tensile loading and unloading. Using the Fluorescence Recovery After Photobleaching (FRAP) approach, we measured the transport of a small fluorescent tracer (sodium fluorescein, 376 Da) in damaged versus undamaged regions and under varying tensile load magnitudes (0.2 N, 10 N, 20 N, and 30 N), which corresponded to nominal strains of 12.5, 625, 1,250, and 1,875 microstrains, respectively. We found a 37% increase in transport of fluorescein in damaged regions relative to undamaged regions and a further ∼18% increase in transport under 20 N and 30 N tension compared to the non-loaded condition, possibly due to the opening of the cracking surfaces. The elevated transport of minerals and other adhesive proteins may, at least partially, account for the highly effective repair of diffuse damage observed in vivo. CLINICAL SIGNIFICANCE: Diffuse damage adversely affects bone's fracture resistance and this study provided quantitative data on elevated transport, which may be involved in repairing diffuse damage in vivo. 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:692-698, 2018.