Immediate strain shielding after femoral reaming and nailing. An in vivo study in rat femora.

Following intramedullary reaming and nailing of rat femora, in vivo changes in dynamic strain were correlated with in vitro measurements of the bones. Reaming and nailing procedures were performed 2 days after implantation of unidirectional strain gauges at the anterior, mid-diaphyseal level of the femur. Structural stiffness of polyacetal nails were three times as stiff as intact bone. Reaming only decreased the median strain value by 26 percent, and this value was not reduced by insertion of polyacetal nails. Steel nailing reduced the strain by 74 percent. Tested by three-point bending, reaming increased stiffness by 5 percent at the anterior aspect. The presence of nails gave stiffness values that were 9 percent (polyacetal) and 56 percent (steel) higher than the reamed only condition. Our results indicate that steel nailing following reaming causes marked reduction in strain at the anterior, mid-diaphyseal surface, whereas reduction in strain caused by polyacetal nails is negligible.