Quantitative morphologic study of intimal thickening at the human carotid bifurcation: I. Axial and circumferential distribution of maximum intimal thickening in asymptomatic, uncomplicated plaques.

The spatial distribution of intimal thickening was determined for each of 42 carotid bifurcations removed at autopsy from patients with no clinical or anatomic evidence of cerebrovascular disease. Both right and left specimens were available for six of the individuals. Each bifurcation was removed intact and included a 1.5-2.9-cm length of the common carotid artery and a 1.5-2.5-cm length of the internal carotid artery. The specimens were restored to in situ length, fixed under conditions of controlled-pressure perfusion at 100 mmHg, filled with a radio-opaque mixture, radiographed and sectioned at 0.5-cm intervals. Computer assisted contour tracing of projected images of histologic sections was used to determine intimal thickness, intimal cross sectional area and lumen area within each of eight equal 45 degrees polar sectors with 0 degree indexed at the flow divider, 90 degrees at the outside wall, 180 degrees opposite the flow divider and 270 degrees at the inner side wall. Intima occupied 0.9-42% of the area encompassed by the internal elastic lamina, i.e. the potential lumen area if no intimal thickening were present, but there was no lumen narrowing on lateral X-ray projections. Intimal thickening was eccentric at each level of section but the circumferential location of maximum intimal thickness (MIT) shifted in a continuous helix from level to level. At the common carotid artery level 1.0 cm proximal to the bifurcation, MIT tended to be at the flow-divider side at 15 +/- 59 degrees. Immediately proximal to the flow divider, MIT was at the lateral side wall. In the mid-sinus region of the internal carotid artery MIT was opposite the flow divider at 179 +/- 64 degrees. At the distal internal carotid just beyond the sinus, MIT was at the inner side wall. The distal internal carotid was minimally involved or free of intimal thickening. Comparison of right and left bifurcations revealed that the helical spatial distribution of MIT was in mirror-image symmetry for the two sides. The findings correspond closely with previous demonstrations of a helical flow pattern in the region of the bifurcation. Although locations of MIT just proximal and just distal to the bifurcation are similar and tend to be at the 'far wall', individual differences in the shifts of MIT with axial location should be taken into account when sites of interrogation by non-invasive clinical methods are selected for detection of intimal thickening.