Effects of simulated weightlessness on bone properties in rats

Abstract

This paper addresses two questions: (1) are reductions in bone apposition rate during disuse due to increased periods of ‘resting’ during bone formation? and (2) are diaphyseal strength changes due to disuse caused by changes in the quality as well as the quantity of the bone matrix? These problems were studied using the tail-suspended rat model. Seventy 210 g female Sprague-Dawley rats were divided into control and suspended groups and studied for 1 or 2 weeks. Tetracycline labels at varying intervals were used to measure bone formation rates and to assess the duration of rest periods. The mechanical properties of the femoral midshaft, neck, and distal metaphysis, and of the proximal tibia, were studied. The composition of the entire femur and the geometry of the femoral midshaft were also analyzed, and beam theory was used to compare changes in these variables with those in bending strength. Contrary to previous findings in the tibia, suspension did not result in a decrease in the mean mineral apposition rate at the femur midshaft. Neither was there an effect on rest periods, so the answer to the first question was no, at least for the femur. Femur composition, cross-sectional geometry, and strength were significantly altered by disuse after 2 weeks, but not after 1 week. Femur mineralization was affected by time, but not by suspension, and bending yield stress was affected by suspension but not by time. Thus, the answer to the second question remains unclear, but is probably yes.