Spin-resonance modes of the spin-gap magnet TlCuCl_3

Three kinds of magnetic resonance signals were detected in crystals of the spin-gap magnet TlCuCl_3. First, we have observed the microwave absorption due to the excitation of the transitions between the singlet ground state and the excited triplet states. This mode has the linear frequency-field dependence corresponding to the previously known value of the zero-field spin-gap of 156 GHz and to the closing of spin-gap at the magnetic field H_c of about 50 kOe. Second, the thermally activated resonance absorption due to the transitions between the spin sublevels of the triplet excitations was found. These sublevels are split by the crystal field and external magnetic field. Finally, we have observed antiferromagnetic resonance absorption in the field-induced antiferromagnetic phase above the critical field H_c. This resonance frequency is strongly anisotropic with respect to the direction of the magnetic field.Comment: v.2: typo correction (one of the field directions was misprinted in the v.1

Knee loading stimulates cortical bone formation in murine femurs

BACKGROUND: Bone alters its architecture and mass in response to the mechanical environment, and thus varying loading modalities have been examined for studying load-driven bone formation. The current study aimed to evaluate the anabolic effects of knee loading on diaphyseal cortical bone in the femur. METHODS: Using a custom-made piezoelectric loader, 0.5-N loads were laterally applied to the left knee of C57/BL/6 mice at 5, 10, 15, and 20 Hz for 3 minutes per day for 3 consecutive days. Animals were sacrificed for examination 13 days after the last loading. The contralateral femur was used as a non-loading control, and the statistical significance of loading effects was evaluated with p < 0.05. RESULTS: Although diaphyseal strains were measured as small as 12 μstrains, bone histomorphometry clearly demonstrated frequency-dependent enhancement of bone formation. Compared to a non-loading control, bone formation on the periosteal surface was significantly enhanced. The loading at 15 Hz was most effective in elevating the mineralizing surface (1.7 x; p < 0.05), mineral apposition rate (1.4 x; p < 0.001), and bone formation rate (2.4 x; p < 0.01). The loading at 10 Hz elevated the mineralizing surface (1.4 x; p < 0.05), mineral apposition rate (1.3 x; p < 0.01), and bone formation rate (1.8 x; p < 0.05). The cross-sectional cortical area and the cortical thickness in the femoral diaphysis were significantly increased by loading at 10 Hz (both 9%) and 15 Hz (12% and 13%, respectively). CONCLUSION: The results support the anabolic effects of knee loading on diaphyseal cortical bone in the femur with small in situ strain, and they extend our knowledge on the interplay between bone and joints. Strengthening the femur contributes to preventing femoral fractures, and the discovery about the described knee loading might provide a novel strategy to strengthen osteoporotic bones. Further analyses are required to understand the biophysical and molecular mechanism behind knee loading

Effect of risedronate on the risk of hip fracture in elderly women

Background: Risedronate increases bone mineral density in elderly women, but whether it prevents hip fracture is not known. Methods: We studied 5445 women 70 to 79 years old who had osteoporosis (indicated by a T score for bone mineral density at the femoral neck that was more than 4 SD below the mean peak value in young adults [-4] or lower than -3 plus a nonskeletal risk factor for hip fracture, such as poor gait or a propensity to fall) and 3886 women at least 80 years old who had at least one nonskeletal risk factor for hip fracture or low bone mineral density at the femoral neck (T score, lower than -4 or lower than -3 plus a hip-axis length of 11.1 cm or greater). The women were randomly assigned to receive treatment with oral risedronate (2.5 or 5.0 mg daily) or placebo for three years. The primary end point was the occurrence of hip fracture. Results: Overall, the incidence of hip fracture among all the women assigned to risedronate was 2.8 percent, as compared with 3.9 percent among those assigned to placebo (relative risk, 0.7; 95 percent confidence interval, 0.6 to 0.9; P=0.02). In the group of women with osteoporosis (those 70 to 79 years old), the incidence of hip fracture among those assigned to risedronate was 1.9 percent, as compared with 3.2 percent among those assigned to placebo (relative risk, 0.6; 95 percent confidence interval, 0.4 to 0.9; P=0.009). In the group of women selected primarily on the basis of nonskeletal risk factors (those at least 80 years of age), the incidence of hip fracture was 4.2 percent among those assigned to risedronate and 5.1 percent among those assigned to placebo (P=0.35). Conclusions: Risedronate significantly reduces the risk of hip fracture among elderly women with confirmed osteoporosis but not among elderly women selected primarily on the basis of risk factors other than low bone mineral density. (N Engl J Med 2001;344:333-40.) Copyright (C) 2001 Massachusetts Medical Society