Changes in the architecture of cancellous bone in the femora of developing mice as a result of short duration hypergravity

Abstract

SYNOPSIS. Mice were used as a model system to demonstrate the effects of increased loading on the trabecular bone in the developing proximal femur. A two-dimensional free body diagram revealed that because the femur is normally held in a horizontal position during support, functional loading resulted in a net posterior bending moment and a muscle generated axial compressive load that was 7.7 times the normal support weight for that limb. Experimental treatment consisted of four times normal gravity for 10 min, six times per day for 30 days in addition to unrestrained normal exercise. Unrestrained normal exercise served as the control. An animal whose foot was amputated by his mother shortly after birth was used as a less than normal load example for comparison with the hypergravity and control groups. All groups were treated for 30 days. Two areas of trabecular bone were examined: a proximal central webbing area that may be under predominantly tensile loading much of the time, and a more distal area near the third trochanter that presumably experiences bending stresses when the animal is exposed to normal activity or hypergravity. Results indicate that: 1) The central webbing area appears to have a similar mass of bone regardless of loading conditions; however, with increased loading the trabeculae become oriented in directions that approximate directions of presumed principal tensile stresses. 2) There is no genetically predetermined amount of bony tissue in the third trochanter area; when the limb is relatively unloaded no trabeculae are present, as the loading increases trabecular bone is formed. In the hypergravity group, trabeculae in the area of the third trochanter increased in length and complexity and became more centrally positioned in the lumen of the femur, a position that presumably would allow the femur to better resist bending moments. These results demonstrate that short duration, high intensity functional loading can stimulate growth of trabecular bone, and further, specific types of functional loading can be correlated with specific architectural changes. © 1989 by the American Society of Zoologists.