Bone properties are dynamic and change due to physiologic strains generated during loading. Measurement of physiologic strains has been accomplished in dogs and rats with the use calcium phosphate ceramic (CPC) coated strain gauges. The purpose of this study was to determine whether TGF-β1 enhanced CPC coated strain gauges could be attached to mouse femora in vivo. Five mice were implanted with CPC-2 coated uniaxial, single element strain gauges (Vishay Micro measurements, NC) that were enhanced with 0.25 μg of TGF-β1 8 hours before surgery. One strain gauge was implanted in each mouse, and the gauges were initially secured to the right femur of each mouse using 2 resorbable sutures. The mice were sacrifice 8 weeks postop. Following sacrifice the mechanical attachment of the implanted gauges was measured using a cantilever bend test. The bone bonding and bone to CPC contact was evaluated using high resolution μCT and histology. Bone formation rate was measured using CT/SPECT imaging and histomorphometry. Cantilever bend testing indicated that strain transfer through the CPC coated gauges was 51 ± 16% of strain measured in a control gauge glued to the contralateral femora. High resolution μCT and histology demonstrated extensive contact between the bone and CPC particles. Although there was an increased amount of bone in the experimental femora, this did not result in a significant increase in bone volume (46±6% in experimental femora and 47 ± 4% in control femora). SPECT imaging demonstrated increased bone formation on the experimental femur near the implanted sensors. Histomorphometry confirmed the increased BFR (0.0022 ± 0.0011 μm3 /μm2 /day in experimental femora and 0.0006 ± 0.0002 μm3 /μm2 /day in control femora) and demonstrated that the increase was due to an increased mineral apposition rate (0.0022 ± 0.0007 μm/day in experimental femora and 0.0010 ± 0.0002 μm/day in control femora). These results indicate extensive bone to CPC bonding occurs in mice implanted with CPC coated strain gauges. The use of in vivo bone strain gauging will be a valuable tool to study strain mediated bone remodeling in mice. In addition this technique allows for investigation of the magnitudes and roles of physiologic bone strains generated in gene knockout mice.
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