To investigate the effects of mechanical loading on the rat and mouse forelimb, male Sprague Dawley rats and male C57/BL-6 mice forelimbs were cyclically loaded in axial compression. Two studies were performed, one to examine woven bone formation in both the rat and mouse forelimb, and one to examine lamellar bone formation in the rat forelimb. Both types of bone formation were induced through an acute loading event.
Repeatable levels of fatigue damage were induced in both the rat and mouse ulna, yielding a consistent woven bone response. Rats formed more woven bone than mice. Woven bone formed by the rats was evenly distributed along the medial cortex versus woven bone formed by the mice was more abundant towards the posterior side of the medial surface.
150 cycles of rest inserted loading to a peak load of 30 N was found to induce lamellar bone formation with the greatest apposition rate. Apposition rate was more strongly influenced by peak loading rather than number of loading cycles. At the 30 N loading level, 300 loading cycles induced woven bone formation in two of the five animals.
Finite element models were created to examine the mechanical environment of both the rat and mouse ulna during compressive forelimb loading. Stress distribution in the rat ulna was found to be distributed along the medial surface of the rat ulna. Stress distribution was biased to the posterior side of the medial surface on the mouse ulna. These stress distributions supported the bone formation observed. Maximal stress occurred in both the rat and mouse ulna at the same location failure occurred during monotonic and fatigue to failure tests conducted on both rat and mouse ulnas.
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