We have developed a mathematical model of microvascular network blood flow in which the nonlinear flow properties of blood and the nonuniform axial distribution of red blood cells in each vessel, as well as disproportionate cell partitioning at bifurcations, are all accounted for. The movements of red blood cells in the network are tracked; hence, the model is able to simulate temporal variations in local flow parameters in the network due to hemodynamic mechanisms. The model was applied to four rat mesenteric networks for which the topology, boundary conditions, blood velocity, and discharge hematocrit (Hctd) had been measured for each branch. Temporal variations in Hctd and blood velocity after simulation convergence were predicted. In some cases of the three vessels connected to a node, Hctd of one vessel fluctuates in a simple periodic form, Hctd of the second one oscillates in a more complex periodic form, whereas the Hctd of the third one does not oscillate at all. These variations were obtained with constant flow boundary conditions and, therefore, are due to hemodynamic factors alone. The temporal variations in flow parameters predicted by the model simulations are caused by hemorheological mechanisms and would be superimposed on variations caused by other mechanisms (e.g., vasomotion). The frequencies of the predicted fluctuations in blood velocity are in qualitative agreement with observed in vivo variations in dual-slit velocity in the arterioles of the cremaster muscle of anesthetized Golden hamster.

The aim of this study was to assess the osseointegration of copper vapor laser-superfinished titanium alloy (Ti6Al4V) implants with pore sizes of 25, 50, and 200 microm in a rabbit intramedullary model. Control implants were prepared by corundum blasting. Each animal received all four different implants in both femora and humeri. Using static and dynamic histomorphometry, the bone-implant interface and the peri-implant bone tissue were examined 3, 6, and 12 weeks postimplantation. Among the laser-superfinished implants, total bone-implant contact was smallest for the 25-microm pores, and was similar for 50- and 200-microm pore sizes at all time points. However, all laser-superfinished surfaces were inferior to corundum-blasted (CB) control implants in terms of bone-implant contact. Within the 12-week study period, remodeling of woven bone initially formed within pores occurred only in the implants with 200-microm pores. Implants with 25-microm pores showed the highest amount of peri-implant bone volume at all time points, indicating that the amount of peri-implant bone was not correlated with the quality of the bone-implant interface. At 3 and 6 weeks postsurgery, we did not find any differences in mineral apposition rates or bone formation rates between the various implant surfaces. However, the peri-implant bone formation rate at the end of the trial was 70 and 62% higher in implants with 50- and 200-microm pores compared with CB implants, respectively. We conclude that, although laser-superfinished implants were not superior to CB control implants in terms of osseointegration, our study has provided further insights into the mechanisms of bone remodeling within pores of various sizes, and may form a basis for future experiments to design optimal implant surfaces with the help of modern laser technology.

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We have developed a new in vivo microscopic technique for comprehensive measurements of vessel diameter, segment length, and red blood cell velocity in discrete arteriolar vessel trees of the lung. In anesthetized and mechanically ventilated rabbits, a transparent window was implanted into the right thoracic wall. We injected fluorescently labeled red cells to visualize blood flow and to measure red blood cell velocity. The distribution of microvascular pressures was simulated in a computer model based on morphometric and microhemodynamic data. Of the total pulmonary vascular pressure drop from pulmonary artery to left atrium, on average 2.5% occurred in distal arteriolar vessel trees with main trunk diameters of 73-111 microns. Along the pathlength from main trunk to terminal arterioles (0.18-2.79 mm), the pressure drop ranged between 0.06 and 0.94 mmHg. The pressure drop along individual pathways correlated significantly with pathlength of terminal arterioles, whereas red blood cell velocity did not. The results indicate that in terminal arteriolar vessel trees of the ventilated rabbit lung the resistance to blood flow is low, and the heterogeneity of microvascular pressures in arterioles feeding capillary networks is high.

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