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Two methods of intramedullary fixation of fractures of the middle of the femoral shaft were evaluated in a sheep model to determine the effect of femoral reaming on pulmonary function. The effect of a modified reamer was also studied. A second experiment with the same model was performed to evaluate the relationship between embolization and pulmonary dysfunction. This experiment involved two groups of sheep-those with normal lungs and those with contused lungs-divided into subgroups-those that had nailing with reaming and those that had nailing without reaming. Intracardiac ultrasound was used to measure the magnitude and duration of transvenous particulate embolization during the operations. Both experiments involved hemodynamic monitoring during and after the nailing. The pulmonary tissue was examined histologically after the animals were killed.The hemodynamic monitoring revealed only a transient increase in pulmonary vascular resistance in the animals that had femoral nailing with reaming in both experiments.The modified reamer had no effect on the pulmonary response. Histological analysis of pulmonary tissue demonstrated a significant increase in the number of fat emboli in both the animals that had nailing with reaming and the animals that had nailing without reaming compared with the control animals. Intravascular ultrasound revealed that the operative maneuver associated with the greatest number of emboli was opening of the intramedullary canal with the awl. The first two passes of the reamer produced more emboli and embolism of longer duration than did the later passes. Pulmonary contusion did not increase the risk of pulmonary dysfunction due to intramedullary nailing in this model.Pulmonary dysfunction as a result of intramedullary nailing was minimum in our fracture model. There was no significant difference, between the animals that had reaming and those that did not have reaming, with regard to the adverse effects on pulmonary function. Reaming had a minor transient effect on pulmonary vascular resistance that was not seen in the animals that did not have reaming. The minor pulmonary effects in the two groups were not worsened by the presence of a pulmonary contusion. We concluded that, with regard to their effects on pulmonary function, there was no distinct advantage either to nailing with reaming or to nailing without reaming for fractures of the femoral shaft.