Bone remodeling due to continuously applied loads.

It has long been known that the stress history of bone tissue influences its structure; however, the nature of this relationship remains largely uncharacterized. The objective of this work was to induce a quantifiable change in the stress history of in vivo bone tissue and examine subsequent changes in structural and material properties that might occur. Continuous compressive loads were applied to the diaphysis of adult mongrel dogs for 2 months. The loads, ranging from 12-130 N, were superposed on the normal activity of the animals by implanting spring loading devices on the diaphysis of the femur. After the animals were sacrificed, mid-diaphysial specimens were subjected to compression testing to determine a structural bulk stiffness. The cross-sectional areas of original bone tissue and new bone deposition were then determined. The ash weights of selected specimens were also determined. The results indicate that a positive correlation between the increase in cross-sectional area and the superposed stress does exist. The new bone apposition was found almost entirely on the periosteal surface. Very little evidence of internal remodeling or endosteal movement was observed. The new tissue was found to have a lower ash weight and appeared to have a disorganized microstructure. Mechanical testing also suggests that the newly deposited tissue is far less stiff than the mature original bone.