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Quantitative studies of factors affecting bone blood flow based on bone clearance of radiostrontium (Sr85) Shim, Sun Shik

Abstract

Factors affecting bone blood flow were studied in rabbits and dogs. A method for estimating bone blood flow was developed using the initial bone clearance of radioactive strontium (Sr⁸⁵) from blood. The method is based on Fick Principle and is similar to that used in estimating renal plasma flow from clearance of PAH or diodrast. The validity of the method depends on the efficiency of or Sr⁸⁵ removal from blood by bone as indicated by the Extraction Ratio (ER). This represents the proportion of the Sr⁸⁵ which has been removed from the blood flowing through bone. It was determined in 10 dogs by injecting Sr⁸⁵ and plasma dye, T-1824, into the nutrient artery of tibia. During the next 5 minutes, 87.3 ± 2.9% of the plasma dye and 20.21 ± 1.68% of the Sr⁸⁵ were recovered from the corresponding femoral vein. The Extraction Ratio calculated from the data was 0.764 ± 0.066 (SE) and is comparable to the ER of 0.90 for extraction of PAH by kidney. The high ER appears to justify the use of initial (0-5 min) bone clearance of Sr⁸⁵ as a measure of effective bone blood flow. Divided by the Extraction Ratio, the clearance gives an indirect measure of total bone blood flow. Using the above technique, the mean effective bone blood flow for 270 bones from 80 rabbits was found to be 9.60 ± 0.19 (SE) ml/min/100 g fresh weight, and for 46 bones from 10 dogs, the average value was 10.15 ± 0.61 (SE) ml/min/100 g fresh weight. Total skeletal blood flow was estimated to be 7.1 ± 0.25 (SE) % of the resting cardiac output in the rabbits and 7.3 ± 0.95 (SE) % of the resting cardiac output in the dogs. The nutrient artery of femur was found to supply 70% of the blood flow to the shaft and 1/3 of the blood flow to the ends. Blood flow to the ends of bone was significantly higher than that to the shaft. Various factors affecting bone blood flow were studied. Section of the sciatic nerve increased blood flow to the bones of the leg and foot, presumably due to interruption of vasomotor fibers. In contrast, infusion of epinephrine (2-4 micro-gram/ kg/min) reduced blood flow to tibia and humerus by 74-81% and sharply reduced calcium exchange between blood and the labile calcium storage pool in bone. Immobilization of the leg in a plaster cast for 2 weeks resulted in some decrease in blood flow in tibia and calcaneus but more prolonged immobilization (2 months) caused disuse osteoporosis in these bones and a relative increase in blood flow. The surgical problem of fractures of the neck of femur was studied, and it was found that such fractures reduced blood flow to the femoral head by 52-83%. This interference with blood flow may account for high incidence of aseptic necrosis of the femoral head associated with such fractures.

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