The Basic Research Of Biodegradable Magnesium Metal Stents (MPM) After Implantation In The Abdominal Artery Of Experimental Rabbits

Objective: On the basis of existing researches at home and abroad about thebiodegradable metal stent, bioabsorbable magnesium alloy stents (MPM)were inserted into blood vessels of experimental animals (abdominal aortas ofrabbits) in order to make a preliminary evaluation of the degradationproperties including the degradation time, characteristics of vascular intimaand lumen area during its degradation process.Methods: Twelve New Zealand white rabbits were divided into four groupsrandomly and equally in our experiment. The weight of every rabbit was2.0±0.1Kg without consideration gender differences. Every New Zealand whiterabbit was underwent fluoroscopically guided placement of an absorbablemagnesium alloy balloon-expandable stent with the size of23×3mm aftercompletely released (MPM, MicroPort Scientific Corporation,Shanghai,China) in the abdominal artery one centimeter below the renal artery.Digital subtraction angiography images of abdominal aorta of all rabbits wereevaluated1,2,3and6months(30,60,90and180days) respectively after stentinsertion. Meanwhile, the segment of the artery with stent was removed inevery rabbit. The near, middle and far segment of each blood vessel with stent were underwent HE staining and immunohistochemistry. The remainingnumber of stent struts and the area of hyperplasia of intima and lumen wereobserved by HE staining. The number of PCNA-positive cells wereobserved by immunohistochemistry. Data were presented as mean±standarddeviation(SD). The degradation time was analyzed using the linear regression(SPSS version17.0, SPSS, Chicago, IL, USA).Differences of the hyperplasiaarea of intima, the remaining lumen area and mean optical density value ofPCNA-positive cells between the four groups were analyzed by the one-wayanalysis of variance (ANOVA) so as to observe the effect of the stent on thevascular intima.Reasults: All animals survived healthily and no angiostenosis of abdominalaorta was found by angiography during follow-up period after stentsplacement. Pathology revealed that stents fully expanded with complete shapecharacterization one month after the procedure，and that parts of stent strutsbegan to be degraded and fractured with decrease of the radial force after twomonths, and that most stent struts were fractured and began to be corrodedafter three months, and that stents almost vanish with only little residue aftersix months. The hyperplasia area of neointima intimal was0.87±0.60mm2,0.73±0.33mm2,1.38±0.47mm2and0.85±0.23mm2respectively after one month(30days), two months(60days), three months(90days) and six months(180days). And the corresponding residual lumen area was2.11±0.41mm2,1.85±0.44mm2,1.38±0.31mm2and1.86±0.43mm2respectively. The microscope showed that the vascular intima becameirregularly thickened, specially in the areas of the stent struts after stentplacement. Data of our research suggested that neointima hyperplasia wasstatistically significant in the third month group compared with othergroups(P＜0.05),just as well as the residual lumen area.Immunohistochemistry showed that mean integral optical density ofPCNA-positive cells (0.00917±0.0025) was statistically significantcompared with other groups after3months (P＜0.05),which furtherdemonstrated that the development of intimal hyperplasia and angiostenosisexperimental will reach a peak three months after stent insertion. Then thatneointima hyperplasia will become slow and intima will be remold by thetime of complete degradation of the stent. The area of intimal hyperplasia willalso be smaller than that in peak period while the residual lumen area willbecome larger.Conclusions: The duration of losing supportive strength and completedegradation of the MPM magnesium alloy stent in rabbit’s aorta is182dayswhich is longer than the duration reported before (60days-90days). TheMPM magnesium stent can also induce neointimal proliferation and lead toangiostenosis before total degradation, especially3months after stentplacement. Neointimal proliferation and angiostenosis will gradually go slow after3months. So it is a direction for the absorbable magnesium alloy stent toslow down the degradation time for increasing its effective supporting time toavoid the late angiostenosis in the future.