Summary:Sixty-two percent of cortical blood flow in the tibial diaphysis is supplied by the tibial nutrient artery, and thus the normal blood flow is centrifugal in direction. Intramedullary reaming destroys the nutrient artery and injures the endosteal surface of the cortex. For this procedure to be physiologic, blood flow to the cortex and fracture site must be recruited from a different, nonendosteal, compensating source. Trueta suggested that blood-flow direction can reverse from centrifugal to centripetal, depending on the physiological conditions, and in particular, after breakdown of the normal endosteal blood supply. We examined this hypothesis after intramedullary reaming of the intact and osteotomized ovine tibia. The cortical and periosteal blood flow to intact reamed tibiae was measured in eight sheep. The cortical and periosteal blood flow to tibiae that had undergone an experimental osteotomy before intramedullary reaming was measured in six sheep. 57Cobalt-radiolabeled microspheres were used to measure blood flow. The unoperated contralateral tibiae served as controls. Immediately after reaming of the intact tibial medullary canal, there was no significant change to cortical blood flow, but there was a significant sixfold increase in the periosteal flow. After reaming of the osteotomized tibiae, there was a significant threefold increase in periosteal flow. Our study confirms Trueta's hypothesis that after trauma or other pathologic states, cortical blood flow direction can be reversed.