In vivo analysis of microcirculation following closed soft-tissue injury

Major loss of tissue is an almost invariable consequence of severe closed s oft-tissue injury. Clinically, the extent of soft-tissue trauma determines the outcome of complex injuries and significantly influences bone healing. With use of a new animal model, this study quantitatively analyzed microcir culation. i.e., nutritive perfusion and leukocyte-endothelial cell interact ion, in skeletal muscle after standardized closed soft-tissue injury. By me ans of a computer-assisted controlled-impact technique, a severe standardiz ed closed soft-tissue injury was induced in the left hindlimb of 28 rats. T he rats were assigned to four experimental groups (n = 7 per group) that di ffered by time of analysis (1.5, 24, 72, and 120 hours after injury); rats that were not injured served as controls (n = 7). Intramuscular pressure wa s measured, and microcirculation in the rat extensor digitorum longus muscl e was analyzed by in vivo fluorescence microscopy, which allowed assessment of microvascular diameters, functional capillary density, number of rollin g and adherent leukocytes in venules, and microvascular permeability. Edema weight gain was quantified by the ratio of wet to dry weight of the extens or digitorum longus muscle. Microvascular perfusion of the skeletal muscle was characterized by a significant reduction in functional capillary densit y, which was paralleled by an increase in capillary diameter throughout the 120 hours of observation when compared with the controls. Trauma-induced i nflammatory response was reflected by a markedly increased rolling and adhe rence of leukocytes, primarily restricted to the endothelium of postcapilla ry venules; this was accompanied by increased microvascular permeability, i ndicative of a substantial loss of endothelial integrity. The microcirculat ion surrounding the core of the damaged tissue area resembled that of ische mia-reperfusion injury in skeletal muscle, i.e., heterogeneous capillary pe rfusion, pronounced microvascular leakage, and adherence of leukocytes. Enh anced vascular leakage and leukocyte adherence (24-72 hours after injury) c oincided with the maximum intramuscular pressure (which was not indicative of compartment syndrome) and edema formation. These results demonstrate tha t initial changes, leading to ultimate tissue death, after closed soft-tiss ue injury are caused on the microcirculatory level. This standardized model provides further insight into microvascular pathophysiology and cellular i nteractions following closed soft-tissue injury Thus, it is an adequate too l for testing novel therapeutic interventions.