Reductions in bone strength after fluoride treatment are not reflected in tissue-level acoustic measurements

Acoustic velocity measurements are used to estimate tissue-level bone strength after fluoride therapy for osteoporosis. However, acoustic measurements provide information about elasticity, not strength, and bone elasticity does not necessarily correlate with bone strength at a tissue level. The current study was undertaken to evaluate the effects of fluoride treatment on tissue-level acoustic velocities, and to determine the relationship between acoustic velocity and bone strength measured in the femur, femoral neck, and spine. Young adult rabbits were treated with either 0 or 100 parts per million of fluoride in their drinking water for six months. After treatment, the bones were harvested for measurement of tissue fluoride, bone strength, and acoustic properties. Acoustic velocities were measured in the femoral midshaft using an acoustic microscope with a 50 MHz transducer. Both longitudinal and transverse velocities were measured. After the initial acoustic measurements the bone specimens were treated to remove either the organic matrix or mineral, and the acoustic measurements were repeated. Fluoride treatment increased bone fluoride levels 7-8 fold and reduced all biomechanical parameters. Most notably the fracture force of the femoral neck was reduced by 25% (p < 0.005), and the fracture stress of the L-5 vertebra was reduced by 19% (p < 0.05). Fluoride treatment had no significant effect on any of the measured acoustic velocities. The elastic anisotropy of the bone was decreased by demineralization (p < 0.0001) and increased by removal of the organic matrix (p < 0.0001), but unaffected by fluoride treatment. Acoustic measurements were not correlated with bone strength in the femoral neck or femoral midshaft. There was a positive correlation between the longitudinal velocity measured in the femur and the vertebral fracture stress, but this was the only positive association between acoustic velocities and strength measurements. These data cast doubt on the utility of high frequency (>2 MHz) acoustic measurements for evaluating the efficacy of fluoride therapy, especially in the hip.