The effect of controlled mechanical loading upon the development of longbone architecture

An experimental model was developed to study the effects of mechanical strain upon bone growth, mass and architecture in a small mammal. The method is non-invasive and involves applying dynamic loads to the intact, growing, rat ulna in vivo. The distribution of strain induced by the loading was measured using miniature strain gauges bonded to the bone surfaces. The modelling response of the ulna diaphysis to daily exposure to dynamic strain was quantified in serial sections by fluorochrome analysis employing confocal microscopy.

The rat ulna is longitudinally curved and, when subjected to axially-directed compression by the loading system, bends in the direction of its normal curvature. In response to daily exposure to these bending strains the bone straightened and increased in mass by reducing the normal modelling drifts and increasing periosteal osteogenesis. The osteogenic response on both the tension and compression surfaces of the bone was proportional to local surface strain. The periosteal bone response was present 5 days after the first loading stimulus and had a lamellar or plexiform, rather than woven, appearance. The magnitude of the response was no different whether the bone received 12,000 or 1,200 cycles of loading per day. The size of the osteogenic response was governed by strain magnitude when strains exceeded a threshold of approximately 3,000 <IMG WIDTH=8 HEIGHT=14 ALIGN=MIDDLE SRC="/maths/mu.gif">strain. The implications of these data are discussed and future investigations to characterise cellular and molecular responses of the rat ulna to mechanical loading are proposed.