Strain shielding 12 weeks after femoral reaming and nailing in rats.

The purpose of this study was to investigate the in vivo strain-shielding in rat femora 12 weeks after intramedullary nailing with either flexible or rigid implants. Five days prior to death, unidirectional strain gauge units were implanted on the anterior bone surface bilaterally. Median strain values of reamed only and polyacetal-nailed femora ranged from 67 to 90 percent of the intact side. Removal of polyacetal nails and sham operations gave negligible changes in strain. Strain of the steel-nailed femora was 51 percent of the intact side. Removal of nails increased strain to 91 percent of the intact side. Testing of femora after removal of implants revealed stiffness of 116 and 103 percent of normal in reamed-only and polyacetal-nailed groups, respectively. Stiffness in the steel-nailed group was reduced to 85 percent of normal. None of these changes were statistically significant. This study indicates that rigid intramedullary nails cause strain shielding at the anterior, mid-diaphyseal surface of rat femora 12 weeks after insertion. Removal of rigid implants seems to restore the loading configuration immediately.