Histopathologic analysis of in-stent neointimal regression in a porcine coronary model

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Abstract
Animal and clinical studies have demonstrated late regression of in-stent neointima. This study was performed to identify the temporal changes in the in-stent neointimal constituents responsible for late regression. NIR stents were implanted in porcine coronary arteries (size of stent (in mm) to size of artery (in mm) ≅ 1.1) and harvested at 2 months and 6 months (n = 6 stents/group). Histopathologic analyses included morphometric analysis, smooth muscle cell density, and extracellular matrix contents. Compared with the findings at 2 months, at 6 months there was a significant reduction in area stenosed (from 21 ± 3% to 14 ± 1%, P < 0.05) and neointimal thickness (from 0.2 ± 0.03mm to 0.03 ± 0.02mm, P < 0.05), despite similar injury scores (0.05 ± 0.06 at 2 months and 0.36 ± 0.29 at 6 months). This regression was accompanied mainly by a reduction in proteoglycan (from 24 ± 19% to 5 ± 8%, P = 0.05), with no change in smooth muscle cell density (71 ± 7 compared with 76 ± 23/high power field) or collagen content (25 ± 19% compared with 25 ± 19%). The study confirmed the regression of in-stent neointima, which was mainly attributable to a reduction in proteoglycan content, resembling the natural healing response.