Reflection ultrasound velocities and histomorphometric and connectivity analyses: Correlations and effect of slow-release sodium fluoride

To better understand how structural and functional bone properties contribute to the changes in bone biomechanical properties revealed by ultrasound critical angle reflectometry (UCR) analysis, we measured both UCR velocities and histomorphometric properties in bone biopsy specimens from 33 osteoporotic patients before and following intermittent slow-release sodium fluoride (SRNaF) and continuous calcium citrate administration. Mean skeletal fluoride exposure was 17 months, and mean skeletal fluoride content was 0.203 ± 0.088 SD% bone ash. Intermittent SRNaF and continuous calcium citrate promoted significant increases in trabecular thickness (122 ± 18 SD μm to 131 ± 20, p = 0.020), mineral apposition rate (0.79 ± 0.26 to 1.05 ± 0.40 μm/day, p = 0.014), and a significant decline in eroded surface (3.9 ± 1.6 to 2.8 ± 1.4%, p = 0.002). There were also significant increases in node number (0.193 ± 0.100 to 0368 ± 0.245, p < 0.01) and node-to-node strut length (0.076 ± 0.087 to 0.191 ± 0.173, p < 0.01) relative to total cancellous area. Cortical UCR velocity did not change but cancellous velocity significantly increased by 97 m/s following therapy (p = 0.0005). When compared against the significant changes in bone histomorphometry and connectivity, the sum of both cancellous and cortical ultrasound velocities was significantly correlated with node number/area (R² = 0305, p < 0.0001) and node-to-node strut length/area (R² = 0372, p < 0.0001) and to a lesser extent with mineral apposition rate (R² = 0.106, p = 0.032). Multiple regression analysis demonstrated that 40% of the variance in the sum of the UCR velocities can be accounted for by the variability in these histomorphometric and connectivity parameters. There were no significant correlations between the sum of cortical and cancellous ultrasound velocities and cancellous bone volume (R² = 0.014, p = 0.533), trabecular thickness (R² = 0.012, p = 0.47), or hone mineral density (R² = 0.003, p = 0.80). These observations indicate that velocity measurements with the UCR methodology show an improvement in bone elasticity associated, in part, with an improvement in the rate of bone mineralization and an improvement in bone quality at the structural level as shown by microarchitecture.