DIRECT MEASUREMENT OF TRANSVERSE RESIDUAL STRAINS IN AORTA

Residual strains were measured in the porcine aorta. Segments were cut from the aorta perpendicular to its longitudinal axis. Microdots of w ater-insoluble black ink were sprinkled onto the transverse sectional surface of the segments in the no-load state. The segments were then c ut radially, and sectional zero-stress states were approached. The coo rdinates of selected microdots (2-20 mu m) were digitized from photogr aphs taken in the no-load state and the zero-stress state. Residual st rains in the transverse section were calculated from the displacement of the microdots. The circumferential residual strains on the inner wa ll and outer wall were calculated from the circumferential lengths in the no-load state and the zero-stress state. Results show that the cir cumferential residual strain is negative (compressive) in the inner la yer of the aortic wall and positive (tensile) in the outer layer, wher eas the radial residual strain is tensile in the inner layer and compr essive in the outer layer. This residual strain distribution reduces t he stress concentration in the aorta under physiological load. The exp erimental results compared well with theoretical estimations of a cyli ndrical model. Regional difference of the residual strain exists and i s significant (P < 0.01), e.g., the circumferential residual strains o n the inner wall of the ascending, descending thoracic, and abdominal regions of the aorta are -0.133 +/- 0.019, -0.074 +/- 0.020, and -0.04 6 +/- 0.017 (mean +/- SD), respectively. More radial cuts of a segment produced no significant additional strains. This means that an aortic segment after one radial cut can be considered as the zero-stress sta te.