Moderate-Intensity Rotating Magnetic Fields Do Not Affect Bone Quality and Bone Remodeling in Hindlimb Suspended Rats

<div><p>Abundant evidence has substantiated the positive effects of pulsed electromagnetic fields (PEMF) and static magnetic fields (SMF) on inhibiting osteopenia and promoting fracture healing. However, the osteogenic potential of rotating magnetic fields (RMF), another common electromagnetic application modality, remains poorly characterized thus far, although numerous commercial RMF treatment devices have been available on the market. Herein the impacts of RMF on osteoporotic bone microarchitecture, bone strength and bone metabolism were systematically investigated in hindlimb-unloaded (HU) rats. Thirty two 3-month-old male Sprague-Dawley rats were randomly assigned to the Control (<i>n</i> = 10), HU (<i>n</i> = 10) and HU with RMF exposure (HU+RMF, <i>n</i> = 12) groups. Rats in the HU+RMF group were subjected to daily 2-hour exposure to moderate-intensity RMF (ranging from 0.60 T to 0.38 T) at 7 Hz for 4 weeks. HU caused significant decreases in body mass and soleus muscle mass of rats, which were not obviously altered by RMF. Three-point bending test showed that the mechanical properties of femurs in HU rats, including maximum load, stiffness, energy absorption and elastic modulus were not markedly affected by RMF. µCT analysis demonstrated that 4-week RMF did not significantly prevent HU-induced deterioration of femoral trabecular and cortical bone microarchitecture. Serum biochemical analysis showed that RMF did not significantly change HU-induced decrease in serum bone formation markers and increase in bone resorption markers. Bone histomorphometric analysis further confirmed that RMF showed no impacts on bone remodeling in HU rats, as evidenced by unchanged mineral apposition rate, bone formation rate, osteoblast numbers and osteoclast numbers in cancellous bone. Together, our findings reveal that RMF do not significantly affect bone microstructure, bone mechanical strength and bone remodeling in HU-induced disuse osteoporotic rats. Our study indicates potentially obvious waveform-dependent effects of electromagnetic fields-stimulated osteogenesis, suggesting that RMF, at least in the present form, might not be an optimal modality for inhibiting disuse osteopenia/osteoporosis.</p></div

Effects of 4-week RMF exposure on trabecular bone microarchitecture in the distal femora and cortical bone thickness in the mid-diaphyseal femora.

<p>(<b>A</b>) The selected trabecular volume of interest (VOI) with yellow color in 2.0 mm height, which is represented with yellow color and only contains the secondary spongiosa. (<b>B</b>) 3-D µCT images of trabecular bone microarchitecture determined by the VOI. (<b>C</b>) 2-D µCT images of trabecular bone microarchitecture from the axial, coronal and sagittal plane observation in the distal femora, and cortical bone images in the femoral mid-diaphysis. The rat femur in the HU group exhibited significant decrease in the trabecular number, trabecular area and cortical thickness as compared with that in the Control group, whereas RMF exposure did not exhibit remarkable effects on trabecular bone microarchitecture and cortical bone thickness in HU rats.</p

Schematic representation of the treatment device with RMF exposure used in the present study.

<p>(<b>A</b>) The therapeutic device mainly consists of a treatment table, two opposite anti-parallel arrays of NdFeB permanent magnets, and a signal display and control module. (<b>B</b>) Each magnet array comprises a total of 20 disc-shaped NdFeB magnets. The maximum magnetic flux density for each magnet is 400 mT. The right panel in (<b>B</b>) shows the network topology of the lower NdFeB permanent magnet array (<b>N</b> and <b>S</b> in the figure indicate the north pole and south pole of the magnet, respectively). The lower magnet array is rotated at 7 Hz driven by a high-power spinning motor, and thus driving the rotation of the upper magnet array. The rotation of both magnet arrays generates non-uniform RMF in the space between the arrays. The cage is placed coaxially with the upper and lower magnet arrays. The magnetic flux density distribution in the position of the cage region was determined to be 0.60–0.38 T.</p

Comparisons of body mass and soleus muscle mass of rats in the three experimental groups.

<p>Values are expressed as mean ± S.D.</p><p>*Significant difference from Control group with <i>P</i><0.05.</p

Effects of 4-week RMF exposure on tibial static and dynamic bone histomorphometric parameters in HU rats, including (A) osteoblast numbers per millimeter of trabecular bone surface (N.Ob/BS), (B) osteoclast numbers per millimeter of trabecular bone surface (N.Oc/BS), (C) mineral apposition rate (MAR) and (D) bone formation rate per bone surface (BFR/BS).

<p>Control, the control group (<i>n</i> = 10); HU, the hindlimb unloading group (<i>n</i> = 10); HU+RMF, the hindlimb unloading with RMF exposure group (<i>n</i> = 12). Values are all expressed as mean ± S.D. *Significant difference from the Control group with <i>P</i><0.05.</p

Effects of 4-week RMF exposure on µCT indices of femoral trabecular and cortical bone microstructure in HU rats, including (A) trabecular bone mineral density (BMD), (B) trabecular number (Tb.N), (C) trabecular thickness (Tb.Th), (D) trabecular separation (Tb.Sp), (E) bone volume per tissue volume (BV/TV), (F) structure model index (SMI), (G) cortical area (Ct.Ar) and (H) cortical thickness (Ct.Th).

<p>Control, the control group (<i>n</i> = 10); HU, the hindlimb unloading group (<i>n</i> = 10); HU+RMF, the hindlimb unloading with RMF exposure group (<i>n</i> = 12). Values are all expressed as mean ± S.D. *Significant difference from the Control group with <i>P</i><0.05.</p