Dilative characteristics of Microvel and Vasculour-II aortic bifurcation grafts

Long-term dilatation of polyester and expanded polytetrafluoroethylene tube grafts after open repair of infrarenal abdominal aortic aneurysms

OBJECTIVES

Since 1995, expanded polytetrafluoroethylene (ePTFE) grafts have been implemented in open surgical repair (OSR) of abdominal aortic aneurysms (AAA) and are supposed to show less dilatation than polyester grafts. This study examined differences in graft dilatation and clinical outcome.

METHODS

This single-center long-term, prospective randomized study monitored 99 consecutive patients after OSR of AAA. Implanted were 90 tube ePTFE Gore-Tex Stretch grafts, 56 tube Dacron grafts (Uni-Graft KDV, polyester, B. Braun, knitted), and 51 tube Dacron grafts (Gelseal Plus, polyester, Vascutek, triaxial knitted). Follow-up with ultrasound examination was performed at discharge, at 12 months, and at 6 years.

RESULTS

Patients were a mean age of 67 years. Thirty-day mortality was 2.5% (n = 5 of 199), without significant differences among the groups. The Kaplan-Meier analysis showed 5-year survivals of 0.82 (ePTFE/Gore), 0.81 (polyester/Braun), and 0.83 (polyester/Vascutek). Mean ± standard deviation dilatation of the midgraft segment was 1% ± 5% (ePTFE/Gore), 10% ± 9% (polyester/Braun), and 7% ± 8% (polyester/Vascutek) (P ≤ .001) at discharge; 8% ± 11% (ePTFE/Gore), 24% ± 7% (polyester/Braun), and 20% ± 13% (polyester/Vascutek; P ≤ .001) after 12 months; and 19% ± 21% (ePTFE/Gore), 33% ± 22% (polyester/Braun), and 23% ± 19% (polyester/Vascutek; (P ≤ .001) after 6 years. No graft failure or rupture occurred. Graft patency was 100%.

CONCLUSIONS

After a mean implantation of 6 years, the ePTFE/Gore, polyester/Braun, and polyester/Vascutek tube grafts presented with significant differences. The ePTFE grafts showed a stronger resistance against dilatation than the two types of polyester grafts. Owing to similar perioperative and postoperative courses, no advantage could be identified in any group concerning the overall outcome. Vascular implants for OSR of AAA made of ePTFE and polyester are safe, even after a long implantation time. Therefore, the choice of the suitable graft does not depend on its postimplantation dilative characteristics. The outcome is not likely to be connected with dilatation of the implanted graft, because a causal connection between graft dilatation and death cannot be made. The study does not offer a basis for the preference of one of the three graft types. Nevertheless, continuous ultrasound examinations should be performed after implantation of an aortic tube graft to identify possible problems arising from changes in the graft and the residual vascular branches over time.

Dilatation of aortic grafts over time: what to expect and when to be concerned

Dilatation of aortic prosthetic grafts is commonly reported, but most reports are anecdotal, with little objective data in the literature. We performed a prospective trial of 303 patients who underwent prosthetic graft repair for aortic aneurysm or occlusive disease, randomizing patients between insertion of a woven polyester or expanded polytetrafluoroethylene (ePTFE) graft. Patients were followed with computed tomography and ultrasonography for up to 5 years in order to assess the frequency and magnitude of postoperative dilatation. Graft dilatation was documented in patients with polyester grafts at 12 months. Thereafter and up to 60 months, polyester grafts did not dilate further. After 5 years, polyester prostheses had dilated by 25% and ePTFE by 12.5%, as determined by computed tomography imaging. These observations suggest that dilatation of prosthetic grafts is more frequent with knitted polyester grafts compared with ePTFE. Dilatation occurs within the first year after implantation and can be, in part, explained by a discrepancy between the initial nominal graft diameter and its diameter after clamp release, probably due to an in vivo adaptation of the textile structure. Interestingly, graft dilatation did not appear to be associated with an increased frequency of graft-related complications.

In vitro approach to the dilative behavior of knitted vascular prosthetic grafts

The purpose of this report is to propose an in vitro approach to predicting the long-term dilative behavior of knitted polyester prosthetic grafts. Various techniques were applied to five warp knitted fabric prosthetic grafts in order to determine the following fabric properties: knitted fabric structure, textile structure, number and respective linear density of threads and strands, and length of yarn in each stitch. Following these investigations, the prosthetic grafts underwent testing to determine specific strength at break, breaking extension, and stress-strain curve. On two prosthetic grafts, image analysis was performed during circumferential tensile strength testing in order to monitor changes in structural features as a function of stress. Changes in the distance between two wales and two courses of stitches and stitch surface were measured. In addition to surface deformation, thickness was measured, using an induction sensor. Study of fabric structure showed many differences between the five models made by different manufacturers. Knit fabric structure was Indeforma in three cases and half-tricot in two. Strand number and size varied greatly from one model to another. Pattern also varied from one model to another, with knit stitch density varying from 1 to 3. Specific strength at break testing showed great differences in the mechanical properties of the grafts. These differences were especially obvious in the first part of the rheograms, which reflects the ability of the graft to comply in response to low-strength forces, i.e., much lower than those necessary to cause rupture. Image analysis of stitch behavior under stress further confirmed differences in graft behavior depending on the fabric structure adopted by the manufacturers. The in vitro approach proposed in this study to analyze the fabric characteristics of knitted prosthetic grafts effectively revealed differences in construction and behavior. These differences could account for differences in the dilative behavior of grafts in vivo.

Knitted dacron grafts used for abdominal aortic reconstruction: sizing references

This study was undertaken to analyze immediate and mid-term knitted Dacron graft dilation and to establish which parameters should be taken as a reference when aortic graft dilation is evaluated. A Dacron knitted microvel double velour vascular graft (Hemashield Gold) was implanted in 30 patients with aneurysmal (19 cases, 63%) or occlusive (11 cases, 37%) aortic disease. The stems of bifurcated prostheses (27 patients, 90%) and tube grafts (3 patients, 10%) were measured. The package sizing (labelled size) was compared with the external diameter measured intraoperatively with a slide caliper, prior to implantation and after complete clamp release. Additional measurements were obtained by ultrasound 1 and 6 months after implantation, and in 16 cases (53% of the patients) ultrasound and computed tomography (CT) were performed at the end of the first year. The means of the measurements were compared using Student's t test for matched pairs. The statistical significance level was set at p values < 0.05. There was a statistically significant difference between the package sizing (15.3 +/- 1.1 mm) and the external diameter measured prior to implantation (18.7 +/- 1.3 mm); and with the external diameter following implantation (19.6 +/- 1.4 mm), (p < 0.01). External diameters measured prior to grafting and following implantation (after complete clamp release), when compared with the manufacturer's size, showed a mean increase in graft diameter of 3.4 mm (22%) and 4.3 mm (28%), respectively. There were no statistical differences between the external diameter measured after clamp release (19.6 +/- 1.4 mm) and the size determined by ultrasound 4 weeks (19.3 +/- 1.2 mm) and 6 months (19.8 +/- 1.5 mm) following surgery (p values 0.11 and 0.56, respectively). Considering size after clamp release as a reference (19.6 +/- 1.4 mm), an almost significant (p = 0.08) increase in the diameter (0.7 +/- 1.5 mm) was obtained at the end of the first year when the measurement was performed with ultrasound. However, when the measurement was performed by CT at the end of the first year, the differences (0.9 +/- 1.6 mm) revealed statistical relevance (p = 0.04). There was no statistically significant difference between the sizes obtained by ultrasound (20.3 +/- 2.1 mm) and by CT (20.5 +/- 2.2 mm) at the end of the first year (p values 0.07). The package sizing is not a reliable parameter for choosing the size of knitted Dacron grafts. Immediate increase in diameter noted in Dacron grafts is caused by discrepancies between the package sizing and the measured diameter after clamp release during implantation, and by an initial adaptation of the textile structure. This must be taken into account for an accurate investigation of the immediate graft dilation rate, and if further follow-up is contemplated, a measurement to be taken as a reference should be performed by ultrasound or CT in the immediate postoperative period.

Can knitting structure affect dilation of polyester bifurcated prostheses? A randomized study with the use of helical computed tomography scanning

PURPOSE

The aim of this study was to prospectively evaluate the postoperative dilation of two types of knitted polyester arterial prostheses with the use of helical computed tomographic scanning.

METHODS

Thirty-four patients who underwent aortoiliac or aortofemoral bifurcation grafting were randomized to receive a collagen-sealed warp-knitted polyester graft (n = 16 patients) or a gelatin-sealed Köper-knitted polyester graft (n = 18 patients). Alterations in size of all parts of the grafts were evaluated by helical computed tomographic scanning at postoperative day 8, at 3 months, and at 6 months.

RESULTS

On postoperative day 8, the mean dilation of the Köper-knitted grafts was 18% +/- 8% for the stem and 15% +/- 12% for the limbs. At the same time period, the mean dilation of warp-knitted grafts was 27% +/- 13% for the stem and 33% +/- 18% for the limbs. No increase in graft dilation was observed at 3 and 6 months. Despite the wide range of values among patients with the same graft type, at each time interval, the Köper-knitted grafts dilated significantly less than the warp-knitted grafts (P <. 05).

CONCLUSION

In this randomized study, helical computed tomographic scanning was an accurate technique with which to assess graft dilation. For a 6-month follow-up interval, the Köper-knitted polyester structure dilated less than the warp-knitted structure. Longer-term serial scans should allow a better understanding of the clinical significance of graft dilation.

Stent attachment site-related endoleakage after stent graft treatment: An in vitro study of the effects of graft size, stent type, and atherosclerotic wall changes

OBJECTIVE

Perigraft endoleakage is a major complication of the endovascular treatment of abdominal aortic aneurysms. The factors that cause this form of endoleakage are not completely identified. The effect of sizing of the prosthesis in combination with either self-expandable or balloon-expandable stents is evaluated in this study. Further, the influence of atherosclerotic changes on endoleakage is evaluated.

METHODS

Eight human abdominal aortas were assessed macroscopically at 11 sites for the presence of atherosclerotic changes with intravascular ultrasound scanning (IVUS) and with computed tomography (CT). Five aortas were placed in in vitro circulation with physiologic parameters. After the determination of the proximal and distal landing site of the stent graft, the diameter and surface measurements of the cross sections were taken. The stent graft diameters were chosen from 4-mm undersizing to 6-mm oversizing, both for Gianturco stent grafts (William Cook Europe A/S, Bjaeverskov, Denmark) and for Palmaz stent grafts (Cordis/Johnson & Johnston Co, Warren, NJ). After placement of the stent graft, the diameter and surface measurements of the aortic cross section were determined at the proximal and distal stent attachment sites. The presence and size of the folds at the stent attachment site and the interface with the aortic wall were determined with IVUS and angioscopy. Endoleakage was evaluated with angiography. After angioplasty of the stent attachment site, IVUS, angioscopy, and angiography were repeated.

RESULTS

Regarding atherosclerotic changes of the aortic wall, the correlations between clinical impression and CT, clinical impression and IVUS, and CT and IVUS were high (r = 0.77, r = 0.79 and r = 0.79, respectively). For the Gianturco stent grafts, no significant relationship existed between the diameters measured before and after stent graft placement, leading to great differences in intended and achieved oversizing. The achieved oversizing was less in the case of minimal atherosclerotic changes of the aortic wall. The Gianturco stent graft followed the aortic wall closely during the heart cycle. The sizes of the folds of the fabric were clearly correlated with the achieved oversizing (r = 0.83; P =.04) and the grade of endoleakage (r = 0.88; P =.022). Angioplasty after stent graft placement had no effect on the diameter and the grade of endoleakage. Palmaz stent grafts did not follow the aortic wall during the heart cycle. A significant correlation existed between oversizing and both space between aortic wall and stent graft (r = -0.88; P =.02) and grade of endoleakage (r = 0.84; P =.036). Grade of endoleakage in the Palmaz stent graft group was less than in the Gianturco stent graft group.

CONCLUSION

With the use of Gianturco stents, a great difference between intended and achieved oversizing is accomplished. The atherosclerotic changes of the aortic wall possibly affect this finding. The configuration of the Gianturco stent results in the formation of fold in the case of oversizing, which is associated with endoleakage. However, the self-expandable character of the stent leads to a close relation to the aortic wall during the heart cycle, and this may possibly accommodate future aortic neck dilation. The Palmaz stent grafts do not follow the aortic wall during the heart cycle, but they do lead to better interface between the graft and the aortic wall, which results in less endoleakage.

Bleeding through the fiber interstices of a knitted dacron graft 12 years after its implantation: report of a case

We present herein the case of a 65-year-old man who suddenly developed fresh perigraft bleeding into the space between a knitted Dacron bifurcated graft and the aneurysmal sac, 12 years after undergoing graft replacement. Intraoperative findings did not show Dacron fiber degeneration at the nonanastomotic sites or a pseudoaneurysm at the anastomotic site. Widened Dacron fiber interstices resulting from graft dilation without sufficient graft healing was thought to have caused delayed hemorrhage through the macroscopically intact graft material, followed by massive clot retention between the graft and the aneurysmal sac. This case report serves to demonstrate that careful long-term follow-up is essential for patients who have undergone vascular graft replacement.

Aortoduodenal fistula arising from the dilatation of a knitted Dacron graft: report of a case

A fatal aortoduodenal fistula occurred in a 72-year-old man who underwent a repair of an abdominal aortic aneurysm 16 years previously with a 20 x 10 mm bifurcated knitted Dacron graft. The aortic part of his bifurcated graft had dilated to 40 mm in diameter, with a discrepancy of 20 mm in the diameter between the graft and infrarenal aorta. The fourth portion of the duodenum adhered to the left side of the anastomosis, where the aortoenteric fistula had occurred. We believe that the graft dilatation was the cause of the anastomotic failure, although other factors such as atherosclerotic degeneration of the host aorta should also be considered. Knitted Dacron grafts that have been implanted for more than 10 years should therefore be monitored carefully because they have an inherent tendency to dilate, especially those manufactured before 1981.

Graft-related complications after abdominal aortic aneurysm repair: reassurance from a 36-year population-based experience

PURPOSE

Graft-related complications must be factored into the long-term morbidity and mortality rates of abdominal aortic aneurysm (AAA) repair. However, the true incidence may be underestimated because some patients do not return to the original surgical center when a problem arises.

METHODS

To minimize referral bias and loss to follow-up, we studied all patients who underwent AAA repair between 1957 and 1990 in a geographically defined community where all AAA operations were performed and followed by a single surgical practice. All patients who remained alive were asked to have their aortic grafts imaged.

RESULTS

Among 307 patients who underwent AAA repair, 29 patients (9.4%) had a graft-related complication. At a mean follow-up of 5.8 years (range, < 30 days to 36 years), the most common complication was anastomotic pseudoaneurysm (3.0%), followed by graft thrombosis (2.0%), graft-enteric erosion/fistula (1.6%), graft infection (1.3%), anastomotic hemorrhage (1.3%), colon ischemia (0.7%), and atheroembolism (0.3%). Complications were recognized within 30 days after surgery in eight patients (2.6%) and at late follow-up in 21 patients (6.8%). These complications were observed at a median follow-up of 6.1 years for anastomotic pseudoaneurysm, 4.3 years for graft-enteric erosion, and 0.15 years for graft infection. Kaplan-Meier 5- and 10-year survival free estimates were 98% and 96% for anastomotic pseudoaneurysm, 98% and 95% for combined graft-enteric erosion/infection, and 98% and 97% for graft thrombosis.

CONCLUSIONS

This 36-year population-based study confirms that the vast majority of patients who undergo standard surgical repair of an abdominal aortic aneurysm remain free of any significant graft-related complication during their remaining lifetime.