0552 - in Vitro Sodium Fluoride Exposure Decreases Strength and Increases Ductility of Mouse Femora

Introduction: Fluoride has been advocated for the treatment of osteoporosis because of its anabolic effect on osteoblasts. Fluoride can also alter bone mineral structure, and several investigators have inferred that the alteration in bone mineral causes a reduction in bone tissue strength that can negate the positive effects of increased formation [1-3]. However, the direct effects of fluoride-related mineral alteration on the mechanical properties of bone are largely unknown. Walsh and Guzelsu [4] reported that bone strength was reduced by in vitro exposure to sodium fluoride, although a lack of physiological buffering may have confounded their findings [5]. In addition, experimental studies have not led to a consensus on the effects of fluoride in vivo, perhaps because of differences in testing techniques between studies. It has been suggested that whole bone torsional and bending tests may produce different relative findings [6]. Our objective was to investigate the effects of in vitro sodium fluoride exposure on whole bone properties. We asked: 1) Does in vitro sodium fluoride exposure affect the mechanical properties of mouse femora? 2) Is this effect dependent on the presence or absence of physiological buffers? 3) Is this effect different for torsional and bending loading? Methods: Sixty femora from 30 C57BL/6 female mice were obtained at 10 weeks of age and assigned to one of four treatment groups: A) Control-Buffered, B) Control-Non Buffered; C) Fluoride-Buffered; D) Fluoride-Non Buffered. The buffered solution was standard phosphate buffered saline with calcium chloride added to achieve a physiological concentration of calcium [5]; the non-buffered solution was distilled water only. Sodium fluoride was added to groups C and D to a concentration of 1.5 M. All bones were flushed of marrow and cleaned for 24 hours using a mild detergent solution [4], a step we previously determined has no effect on mechanical properties. Bones were soaked in the treatment solutions for 12 hours and then tested mechanically (Instron 8500R, Canton, MA). Eight femora in each group were prepared for torsional testing by potting the ends in plastic tubes; they were then loaded to failure at 1 deg/sec. Seven femora were loaded to failure in four-point bending at a displacement rate of 0.03 mm/sec, resulting in a similar time to failure as for torsion. Torsional stiffness, bending stiffness, ultimate torque, ultimate bending moment, ultimate rotation and ultimate displacement were measured. Following testing we determined that the average fluoride concentration was 0.9 and 26.4 (mg F/g of ash) in the control and fluoride bones, respectively (p<0.001). Results: Fluoride exposure significantly reduced torsional stiffness, bending stiffness, ultimate torque and ultimate moment, and increased ultimate rotation and ultimate displacement (p<0.001) (Figs. 1 and 2). The non-buffered specimens had significantly decreased maximum torque and increased average ultimate rotation (p<0.01) compared to the buffered specimens. Average ultimate torque for the fluoride group was only 38% of the control value, while ultimate moment was 66% of control (Table 1).