Author + information
- Received January 5, 1996
- Revision received May 21, 1996
- Accepted June 3, 1996
- Published online October 1, 1996.
- CHRISTODOULOS STEFANADIS*
- ↵*Address for correspondence: Dr. Christodoulos Stefanadis, 9 Tepeleniou Street, Paleo Psychico, Athens 15452, Greece
- KONSTANTINOS TOUTOUZASb,
- CHARALAMBOS VLACHOPOULOSb,
- COSTAS STRATOSb,
- IOANNIS KALLIKAZAROSb,
- PANAYIOTIS KARAYANNAKOSb and
- PAVLOS TOUTOUZASb
Objectives. A new type of coated stent, consisting of a conventional stent covered by an autologous vein graft, was developed at our institution.
Background. Coated stents are under investigation to address stenting limitations. However, experimental implantation of coated stents covered by autologous tissue has not been reported.
Methods. An autologous vein graft was removed and carefully prepared. Subsequently, a Palmaz stent was covered by the vein graft both internally and externally. Twenty-seven stents were implanted in the normal iliac arteries of 27 pigs weighing 18 to 33 kg. In 15 of the pigs, 15 noncoated Palmaz stents were implanted in the contralateral artery; these animals served as the control group. The animals were followed up angiographically for a period ranging from 7 days to 6 months. At the time of death, the stented segments were removed, and histomorphometric analysis was performed.
Results. Autologous vein graft-coated stent preparation and implantation was feasible and uncomplicated. In both stents, angiographic follow-up revealed the absence of thrombosis, except for two cases of subacute thrombosis in the control group. The thickness of the intimal layer was greater in the coated stents and seems to be due to the existence of the internal vein layer ([mean ± SD] 0.57 ± 0.12 vs. 0.27 ± 0.13 mm, p = 0.001). The arterial media of the coated stent segments was thinner than that in the control group (0.14 ± 0.03 vs. 0.18 ± 0.01 mm, p = 0.02).
Conclusions. The autologous vein graft-coated stent seems to be nonthrombogenic, and only minimal hyperplasia was observed in the pigs. Further studies are needed to explore the efficacy of this technique in humans.
In an effort to address the limitations of stenting [1–7], several types of coated stents have been reported and are under current investigation [8–13]. In our laboratory, a new type of coated stent, consisting of a conventional stent completely covered by an autologous vein graft, was developed [14–16]. We report the results of the implantation of the autologous vein graft-coated stent (AVGCS) in normal porcine iliac arteries. The purposes of this study were to determine 1) whether AVGCS preparation and percutaneous implantation are feasible, compared with conventional stenting; 2) the short-term and long-term angiographic patency; and 3) the histologic response of the arterial wall.
Experimental animal preparation and procedure. The study consisted of two phases, in which 27 domestic nonatherogenic pigs were studied . In phase I (12 pigs weighing 19 to 30 kg), only an AVGCS was implanted in the right iliac artery of each animal. In phase II (15 pigs weighing 21 to 30 kg), both an AVGCS and a conventional stent were placed in the right and left iliac arteries, respectively.
Stented segments. Of the animals in phase I, 12 AVGCSs were implanted (group A). In phase II, 15 AVGCSs (group B1) were placed in the right iliac artery, and 15 conventional stents (group B2) were implanted in the contralateral artery, serving as a control group. The study was approved by the Review Committee of our institution. All animals were cared for, premedicated, anesthesized and monitored as previously described  and in accordance with the guidelines of the American Heart Association.
Experimental protocol.Autologous vein graft-coated stent preparation. After surgical exposure, an appropriately sized vein graft was removed from the animals' neck. The wall of the vessel was carefully cleared from the surrounding adipose tissue (Fig. 1A). Subsequently, the autologous vein graft was inserted with the use of surgical forceps through the lumen of a Palmaz stent (P100 and P150 for phase I, and P100 for phase II, Johnson & Johnson Interventional Systems Co.), so that the endothelial surface of the vein graft became the internal surface of the stent (Fig. 1B). When the middle point of the autologous vein graft was in line with the middle point of the stent, the two ends of the vein graft were reversed to cover the exterior surface of the stent. A polymerizing agent (0.5 ml of formalin [9:10] and glutaraldehyde [1:10]) was added to a biologic glue (G.R.F., Laboratories European Hospital Supplies, Paris, France), and the compound was thoroughly mixed for 30 to 45 s with forceps to provide an adhesive agent. The mixture was then applied to form a very thin film between the two folds of the vein graft [19, 20]. Finally, the two ends of the vein graft were sutured on the external surface of the stent with four stitches (7-0 prolene, Fig. 1C). Thus, the stent was totally covered on both the internal and external surface by the wall of the autologous vein graft (Fig. 1D, E).
In vitro test (Fig. 1). After AVGCS preparation, in vitro testing was performed by crimping six AVGCSs on a balloon catheter (Cook, Europe or Cordis, Europe). Then the balloon was inflated to 8 bar, and the AVGCSs were examined macroscopically.
Stent implantation. The right carotid artery was exposed surgically and an arteriotomy was performed. Under fluoroscopic guidance, a 260-cm (0.038 in.) heparin-coated guide wire was introduced down the target vessel. Then, a 7F diagnostic catheter was advanced over the guide wire to the distal abdominal aorta, and arteriography of the iliac arteries was subsequently performed by hand injection of contrast media (76% Urografin [diatrizoate meglumine and diatrizoate sodium] Schering). The internal diameter of the artery was then calculated after calibration, based on the known internal diameter of the angiographic catheter (Image Vue Workstation, Nova Microsonics).
A balloon catheter (Cook, Europe or Cordis) whose diameter was ∼10% to 20% greater than the baseline arterial diameter was selected for stent implantation. The diameters of the balloons ranged from 5 to 8 mm. Before inserting the AVGCS, a 5F introducer sheath (10 cm, Cordis) was passed over the balloon catheter with its distal tip approximating the proximal end of the balloon. The AVGCS was then mounted on the balloon and crimped down manually with extreme care to avoid rupture of the vein. During insertion of the balloon catheter over the guide wire, the introducing sheath was in contact with the AVGCS, and both the introducing sheath and the balloon catheter were advanced through the lumen of the carotid artery. In this way, the distal part of the introducer sheath acted as a supportive device and prevented the AVGCS from sliding backward. As soon as the introducer sheath was totally inserted into the carotid artery, the balloon catheter was advanced farther down the iliac artery. The conventional stent was already mounted on the delivery balloon. Insertion through the carotid artery and advancement to the target vessel were performed without a sheath. Both types of stents were expanded at the target vessel by a dilation of 6 to 8 bar for 1 min, and in case of suboptimal expansion, a second dilation was performed. All AVGCSs in phase II were implanted in the right iliac artery, whereas the conventional stents were implanted at the same level of the contralateral artery. A final arteriography was then performed to evaluate the immediate result. Throughout the experiment 2,000 U of heparin was administered (intravenous bolus) to maintain an active clotting time >300 s.
After the procedure, the right carotid artery was sutured and the animals were closed and allowed to recover. No further anticoagulant or antiplatelet treatment was given to the animals. Cefamandole nafate was administered (40 mg/kg intravenously) immediately before the procedure and 1 day after (40 mg/kg intramuscularly).
Follow-up.Angiography (Table 1). phase i. One animal in group A died from septicemia 3 days after the procedure (animal 12), and thus 11 animals remained for the follow-up study (see Results section). Of the remaining 11 animals, 6 were euthanized 2 months after AVGCS implantation and 5 animals at 6 months. In these 11 animals, angiography was performed 2 months after AVGCS implantation. In the five animals that lived until 6 months, additional angiography was performed before death.
phase ii. Ten animals were killed 7 days after the procedure, three animals after 2 months and two animals after 4 months. In all animals, angiography was performed at 7 days. Except for the animals killed at 7 days, angiography was performed in the remaining five animals at 2 months. Also, angiography was performed at 4 months in the two animals before the death.
The lumen diameter was calculated as previously described (see previous Stent implantation discussion). Also, the following variables were calculated: immediate gain (minimal lumen diameter [MLD] after AVGCS implantation - MLD before AVGCS implantation) and late loss (100 × [MLD at follow-up - MLD after implantation]/MLD after implantation, %).
Histologic studies. The animals were euthanized in accordance with the Animal Welfare Regulations outlined by the American Physiologic Society. The pigs were given potassium chloride under deep anesthesia. Tissue preparation for light and scanning electron microscopic observation was performed as previously described .
Morphometric studies. The following variables were measured: lumen area, maximal intimal thickness (defined as the maximal distance from the lumen site of the stent wire to the lumen) and arterial medial thickness of the artery (defined as the maximal distance from the external to the internal elastic membrane, measured at the sites between the stent wires). The measurements were performed by two observers who had no knowledge of the groups.
Statistical analysis. Data are expressed as mean values ± SD. For changes in MLD throughout the experimental protocol, analysis of variance was used. For comparison of variables between the AVGCS segments of group B1 and the conventionally stented segments of group B2, the paired t test was used. Statistical significance was set at p < 0.05.
Autologous vein graft-coated stent preparation, in vitro testing and implantation. The process for coating the stent with an autologous vein graft was easy and never exceeded 20 min. After in vitro mounting and deployment of the AVGCS, the vein graft completely covered the stent without flapping or rupture of the vein (Fig. 1). The autologous vein graft was appropriate in all cases in the in vitro studies. The AVGCS was mounted on the balloon catheter and delivered without complication. Deployment of the stents was successful in all pigs after one or two dilations, as viewed on immediate angiography. The AVGCS was stabilized on the wall of the artery and did not migrate from the original position. No flapping or dissection of the vein graft was observed. There were no cases of rupture, dissection or peripheral embolization of the artery. Arterial spasm, when presented, was treated successfully by intravenous administration of nitroglycerin.
Follow-up.Angiography (Fig. 2). On the third day, one animal in group A died from septicemia. Autopsy revealed no regional septic inflammation in the sites of AVGCS. In both groups during follow-up angiography, all vessels remained patent except two in the control group, which were totally occluded on 7-day angiography. These two animals were excluded from the study. The mean angiographic diameter did not change during the follow-up period (Table 1). Also, the angiographic diameter of the five animals that were killed at 6 months (group A) did not change during the follow-up period (before: 6.1 ± 1.8 mm; immediately after: 6.4 ± 1.7 mm; at 2 months: 6.1 ± 1.8 mm; and at 6 months: 6.0 ± 1.8 mm, p = 0.99).
In MLD comparisons between groups B1 and B2, no statistical difference was observed during the follow-up. The immediate gain was similar between groups B1 and B2 (n = 15, 0.15 ± 0.26 mm vs. 0.25 ± 0.13 mm, respectively p = 0.15). Late loss was similar between the two groups at 2 months (n = 5, 5.0 ± 2.77% in group B1 vs. 8.34 ± 3.54% in group B2, p = 0.13). The MLD did not change during the follow-up in either group (group B1 [n = 5], p = 0.89; group B2 [n = 5], p = 0.89; Table 1).
Histologic studies.Gross observations (Fig. 3). In both groups, 7 days after the procedure, the stent was visible under the translucent lining of the new endothelium. After 2 months, the stents were incorporated into the wall of the artery. The lumen surface was smooth and without evidence of thrombus during the follow-up period, except for the two cases of thrombosis in the control group.
Light microscopic observations (Fig. 4Fig. 5). In all specimens, no signs of arterial injury were detected. The internal and external elastic membranes of the artery were intact. Both stents were incorporated in the arterial wall 2 months after implantation.
Seven days after AVGCS implantation, the vein covering was detectable both in the lumen and external aspect of the stent. However, in two specimens, 1 week and 2 months after the procedure, a small concentration of red blood cells, organized in an eccentric hematoma in the latter specimen, was observed within the two layers of the vein graft. The lumen surface of all AVGCS segments was smooth, although there were occasional ridges, consisting of smooth muscle cells and extracellular connective tissue matrix. In AVGCS sections, in the location of the arterial intima, minimal smooth muscle proliferation, along with extracellular matrix, was revealed. This reaction was seen between stent wires and at the stent wire sites. Also, smooth muscle cells with extracellular matrix were observed in the intima of the internal portion of the vein graft.
Scanning electron microscopic observations (Fig. 6). Seven days after the procedure the lumen surface of all AVGCS specimens was covered by an endothelial layer, in contrast to partial coverage of the control segments. However, in both stents the endothelium displayed a mixture of polygonal and elongated morphology, with an occasional leukocyte adherence in AVGCS segments. Also, after 2 and 4 months all stents were incorporated into the vessel wall. No rupture of the internal vein layer was detected. All specimens showed a smooth lumen surface without evidence of attached microthrombi or platelets.
Morphometric studies (Table 2). The mean lumen area was similar between groups B1 and B2 at 2-month and 4-month follow-up (n = 5, p = 0.13). The mean maximal intimal thickness was greater in group B1 compared with that in group B2 (n = 5, p = 0.001). However, by definition, intimal thickness included the thickness of the internal fold of the vein graft. The mean thickness of the arterial media was less in the AVGCS segments compared with conventionally stented segments (n = 5, p = 0.02).
The results of this study showed that AVGCS implantation is a feasible procedure. During the short-term follow-up no thrombosis was observed, and during the 6-month follow-up minimal hyperplasia appeared. These encouraging results are in accordance with the preliminary experience of AVGCS implantation in humans [15, 16].
Coronary stenting: current status. Despite the established benefits of stent implantation, several limitations need to be addressed [1–7, 21, 22]. In an attempt to solve these problems, while retaining the benefits of stenting, coated stents have been proposed. Several agents have been used as sleeves for the stents, both experimentally and in clinical practice [8–12]. However, despite encouraging results, clinical experience is limited and further investigation is warranted .
Autologous vein graft-coated stenting.Thrombosis. In all AVGCS segments, no thrombus was detected on the lumen surface. Occlusion of a stent by thrombus occurs primarily during the period between the fourth and eighth day after implantation. Also, in experimental studies involving the implantation of polyurethane-coated stents, a high rate of thrombosis occurred during the first 48 h . In contrast, neointimal coverage of the internal surface of the stent is completed after the third week of implantation . Therefore, early formation of thrombus after stenting may be ascribed to the incomplete coverage of the stents, because after re-endothelialization no thrombosis is observed. The neointima seems to protect against the thrombogenous reaction of blood and metal. In the case of AVGCS, the presence of an autologous vein graft from the time of implantation may protect against the thrombogenous reaction of blood and metal during the high risk period of the first days after stent implantation . Also, the re-endothelialization of the stented segments may be accelerated by covering the stent by autologous tissue. Our scanning electron microscopic observations demonstrate an accelerated endothelialization in AVGCS segments 7 days after the procedure. This may be an important antithrombotic factor. In addition, covering the stent results in avoidance of embedding the stent struts into the vessel wall, and thus injury of the vessel wall is minimized . This hypothesis is supported by our current experience with AVGCS implantation in 40 human coronary arteries, in which neither acute nor subacute thrombosis was observed [15, 16].
Intimal hyperplasia. All vessels remained patent, and both angiographically and histologically, the intimal hyperplasia was minimal. This may be ascribed to the experimental protocol of this study, because an overexpansion of the stented artery was avoided to minimize arterial damage. However, during implantation of AVGCS, the arterial wall expansion was greater compared with that of conventional stents, owing to the thickness of the vein layers, since the same inflated balloon diameter provokes, theoretically, a greater injury in AVGCS segments. Covering the metallic surface of the stent and thus avoiding contact with the arterial wall may provide an antirestenotic mechanism, because injury to the artery may be avoided. Also, by covering the edges of the stent, the stress concentration at the interface of the stent and adjacent endothelium is avoided. In the case of AVGCS, the intima was thicker compared with that of the conventional stent because the measurements from the stent wire to the lumen also include the thickness of the internal fold of the vein graft.
Morphometric analysis showed that the thickness of the arterial media of the AVGCS artery was thinner than that of the control group, indicating medial atrophy of the stented arterial segment. This observation may be ascribed to the reduced wall stress caused by the existence of vein layers [26–28]. Another explanation is the impairment of nutrition to the arterial wall. Addition of the vein linings may lead to compromise of diffusion of nutrients from the lumen to the arterial wall. Consequently, nutrition to the media may be disturbed, thus leading to medial atrophy [29, 30].
Methodologic considerations. The autologous vein graft was appropriate for the diameter of the iliac artery. Also, the crimping and implantation procedure did not induce any vein graft damage, as confirmed by in vitro studies and scanning and light microscopic observations during follow-up. Insertion of the AVGCS into the carotid artery was performed with extreme care to avoid any injury to the vein graft, because no introducer sheath was used.
Another consideration is the increased profile of AVGCS, owing to the double layer of the vein graft. Thus, implantation in low caliber arteries, such as the coronary arteries, is prohibited. This limitation may be solved by covering only one aspect of the stent by the vein graft, although ideally the stent should be completely covered. Also, occlusion of arterial side branches at the target lesion after AVGCS delivery appears to be a limitation of this method. Implantation of the AVGCS in curves was avoided, because the Palmaz stent was used. However, if the vein graft is used for covering flexible stents, then AVGCS may be applied in curves.
Implications. Stent coating by an autologous vein graft may be used in patients with coronary artery disease as an elective treatment by sequestration of the atherosclerotic plaque. Indeed, the preliminary results after AVGCS implantation in human coronary arteries are encouraging [15, 16]. Additionally, in the clinical application of this type of stent, several lengths of Palmaz-Schatz stents were used [15, 16]. The procedure of AVGCS preparation is not prolonged and thus may be used as an emergency procedure in cases of complications of currently used invasive techniques, although the increased profile of the device may be prohibitive. Autologous vein graft-coated stenting may also be used for the closure of arteriovenous fistulas, because flow through the wall of the stent is excluded.
Conclusions. Covering the stent with an autologous vein graft may help in eliminating the limitations of stenting and may appear to be an appealing treatment for coronary artery disease. However, the encouraging results after the experimental application of AVGCS must be confirmed by large clinical trials, which will reveal the efficacy of this approach.
A.1 Abbreviations and Acronyms
AVGCS = autologous vein graft-coated stent
MLD = minimal lumen diameter
↵1 This investigation was supported by a grant from the Hellenic Heart Foundation, Athens, Greece.
- Received January 5, 1996.
- Revision received May 21, 1996.
- Accepted June 3, 1996.
- THE AMERICAN COLLEGE OF CARDIOLOGY
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