Comparison of renal function after endovascular aneurysm repair with different transrenally fixated endografts
Article Outline
Objective
Transrenal fixation of abdominal aortic endografts may provide a more secure proximal attachment with few deleterious effects with respect to renal function. This study’s purpose was to determine whether different metals used in two commercially available endografts (Cook Zenith and Medtronic Talent) result in different effects on renal function when placed across renal ostia.
Methods
A total of 140 consecutive patients, between August 2003 and April 2005, who underwent elective endovascular repair of an aortic aneurysm with a nitinol or stainless steel–based endograft with transrenal fixation were reviewed with a mean follow-up period of 5.5 months (range, 1-22 months). The main outcome variable was the percentage change in creatinine clearance (CrCl), which was determined before surgery and at the most recent follow-up. Multiple regression analysis was performed to analyze the contribution of various factors to any deterioration in renal function.
Results
The 140 patients were predominantly male (86%), with a mean age of 75 years (range, 56-92) and a mean aneurysm diameter of 62 mm (range, 42-110 mm). The mean intraoperative contrast use was 67 mL (range, 45-160 mL), and after surgery these patients received a mean of 3 surveillance computed tomographic scans (range, 1-7). Nineteen cases (13.6%) required deliberate accessory renal artery coverage. CrCl did not change significantly after surgery; the mean change in CrCl at the end of follow-up was a 2.5% decrease. There was no difference in the reduction in CrCl between endograft types. Only 14.3% of patients experienced a greater than 20% decrease in CrCl. Multiple regression analysis failed to show a relationship between change in renal function and age, sex, aneurysm diameter, infrarenal neck dimensions, endograft type, coverage of accessory renal arteries, intraoperative contrast volume, preoperative CrCl, and postoperative computed tomographic scans. Length of follow-up was an independent predictor (P = .04).
Conclusions
Transrenal endograft fixation results in little deterioration in renal function during intermediate follow-up. The few patients who do experience a decline in renal function do so independently of any features related to the initial endovascular repair, and there is no difference in renal deterioration between endografts with nitinol and stainless steel transrenal bare metal stents.
Although the endovascular approach to infrarenal aneurysm repair has been widely adopted and accepted, increasingly challenging aortic anatomy has been encountered. Consequently, the use of this technique in the presence of unfavorable aortic neck features, including calcification, tortuosity, lining with thrombus, larger diameters, and shorter lengths, has raised concerns regarding the long-term durability of this procedure. In response, endograft design evolved to include bare metal transrenal fixation. Many centers have subsequently reported favorable early and mid-term outcomes with several transrenally fixated endografts.1, 2, 3
Coinciding with these reports of improved durability were concerns as to the potentially deleterious effects of metal struts crossing the renal artery ostia. As a result, several authors reviewed their experience and, reassuringly, reported no difference in renal dysfunction between infrarenal and transrenally placed endografts.4, 5, 6, 7 However, these studies grouped transrenally placed endografts together and failed to explore any differences in postoperative renal dysfunction between different devices. Furthermore, some investigators have observed different biological responses to various metals when the renal ostia are crossed. Specifically, in an animal model, nitinol stents exhibited greater collagen matrix deposition than those composed of stainless steel and had a greater effect on renal artery flow when placed across the ostia.8
This animal data and the lack of direct comparisons of the effect of different transrenally fixated endografts on renal function prompted this study. The study’s purpose was to determine whether there is any difference in postoperative renal function after the transrenal placement of a nitinol and a stainless steel endograft.
Methods
Our vascular surgery database was retrospectively reviewed to identify all patients who underwent elective endovascular abdominal aortic aneurysm repair at our university-affiliated medical center between August 2003 and April 2005. Information regarding patient demographics, aneurysm anatomy, and procedural details was reviewed. Aneurysm and aortic measurements were determined by using a General Electric Advantage (General Electric, Waukesha, Wis) workstation with semiautomatic vessel-analysis software. Data were entered into the database at the time of measurement and reviewed retrospectively. Preoperative and the most recent postoperative creatinine (Cr) levels, as well as age and preoperative weight, were applied to the Cockcroft-Gault formula,
One of two commercially available endografts, Talent (Medtronic, Santa Rosa, Calif) and Zenith (Cook, Bloomington, Ind), providing transrenal bare metal fixation was used in this series at the discretion of the operating surgeon. The stainless-steel uncovered portion of the Zenith endograft is 26 mm long with 10 to 12 struts (depending on graft diameter) with 3-mm barbs, whereas that of the Talent endograft is 15 mm high with 5 struts, each of 0.5 mm diameter, and is comprised of nitinol. Transrenal fixation of the bare metal portion of the endografts was confirmed in all patients with completion angiograms and postoperative computed tomographic (CT) scans. Both endograft groups were compared with respect to patient demographics, aneurysm anatomy, procedural details, and postoperative renal function. Group means and proportions were compared by using unpaired t tests and Fisher exact tests with a P < .05 level of statistical significance.
Multiple regression analysis was performed to analyze the contribution of various factors to a deterioration in renal function, defined as a greater than 20% decrease in CrCl. Factors included in the multiple regression modeling were age, sex, presence of hypertension, medications, aneurysm diameter, infrarenal neck dimensions, endograft type, coverage of accessory renal arteries, intraoperative contrast volume, number of postoperative CTs, and length of follow-up. Statistical analysis was performed with InStat version 3.06 (GraphPad Software, San Diego, Calif) and a P < .05 level of statistical significance. This study received approval from the University of Western Ontario Research Ethics Board for Health Sciences Research Involving Human Subjects.
Results
During this 20-month period, 140 patients underwent elective endovascular repair of an infrarenal aortic aneurysm at our center. No patient was excluded from analysis. One of two commercially available endografts was used, each with bare metal transrenal fixation. The Talent device contains a nitinol-based stent framework, whereas the Zenith device uses stainless steel. A Talent endograft was used in 59 patients, and 81 patients received the Zenith graft. There was no statistically significant difference between the endograft groups with respect to age (74.3 ± 7.6 years and 75.4 ± 8.0 years; P = .44), sex distribution (89.8% and 82.7% male; P = .33), or preoperative aneurysm diameter (61.3 ± 6.7 mm and 63.2 ± 10.7 mm; P = .23), as summarized in Table I.
Table I. Comparison between the two endograft groups
| Variable | Talent | Zenith | P value |
|---|---|---|---|
| n | 59 | 81 | — |
| Age (y) | 74.3 ±7.6 | 75.4±8.0 | .44 |
| % Male | 89.8 | 82.7 | .33 |
| AAA diameter (mm) | 61.3±6.7 | 63.2±10.7 | .23 |
| AAA neck diameter (mm) | 24.8±3.4 | 23.7±2.8 | .04 |
| AAA neck length (mm) | 27.3±11.5 | 30.0±13.8 | .23 |
| Proximal graft diameter (mm) | 29.9±3.1 | 29.0±2.3 | .06 |
| Accessory renal artery coverage (%) | 10.2 | 16.0 | .45 |
| Intraoperative contrast volume (mL) | 65.1±15.1 | 71.1±20.8 | .06 |
| Postoperative CTs | 3.2±1.5 | 2.8±1.3 | .09 |
| Length of follow-up from date of surgery (mo) | 5.4±4.2 | 5.4±4.5 | .9 |
Aneurysms repaired with the Talent graft had significantly larger-diameter aortic necks (24.8 ± 3.4 mm and 23.7 ± 2.8 mm; P = .04); this resulted in proximal endograft diameters tending to be larger than in the Zenith group (29.9 ± 3.1 mm and 29.0 ± 2.3 mm; P = .06), although this did not reach statistical significance (Table I). The length of the infrarenal aortic neck did not differ between endograft groups (27.3 ± 11.5 mm and 30.0 ± 13.8 mm; P = .23). During surgery, equivalent proportions of patients received deliberate accessory renal artery coverage (10.2% and 16.0%; P = .45), and the intraoperative contrast load did not differ between endograft groups (65.1 ± 15.1 mL and 71.1 ± 20.8 mL; P = .06). There were no cases of inadvertent main renal artery coverage. Additionally, the mean length of follow-up after surgery (5.4 ± 4.2 months and 5.4 ± 4.5 months; P = .9) and the number of postoperative CT scans (3.2 ± 1.5 and 2.8 ± 1.3; P = .09) were similar. The mean length of follow-up for the entire cohort was 5.4 months (range, 1-22 months; median, 3 months).
Table II and the Fig summarize the data pertaining to renal function. Preoperative (74.8 ± 30.8 and 65.1 ± 26.9; P = .05) and postoperative (73.8 ± 31.9 and 66.0 ± 31.4; P = .15) CrCl (mL/min) did not differ between groups, although the preoperative values approached significance. Additionally, there was no significant change in CrCl when comparing postoperative and preoperative values for each endograft (Talent, P = .86; Zenith, P = .83) and when comparing the mean decrease in CrCl between endografts (3.3% ± 2.3% and 2.0% ± 1.1%; P = .7). Similar proportions of patients with each endograft experienced a greater than 20% decrease in CrCl (13.5% and 14.8%; P = .9) during follow-up. The extent of deterioration in these particular patients was similar between endografts. No patient required postoperative dialysis.
Table II. Comparison of renal function between the two endograft groups
| Variable | Talent | Zenith | P value |
|---|---|---|---|
| Preoperative CrCl (mL/min) | 74.8±30.8 | 65.1±26.9 | .05 |
| Postoperative CrCl (mL/min) | 73.8±31.9 | 66.0±31.4 | .15 |
| Decrease in CrCl (%) | 3.3±2.3 | 2.0±1.1 | .7 |
| >20% decrease in CrCl (%) | 13.5 | 14.8 | .9 |
Fig.
Box plot displaying the change in creatinine clearance (CrCl) after endovascular aneurysm repair. Each box extends from the 25th to 75th percentile, and the central line represents the 50th percentile. Lines extending from the box are the most extreme observations not more than 1.5 times the height of the box beyond either quartile. Black dots represent outliers.
A multiple regression analysis was performed to evaluate the role of individual factors and a greater than 20% decline in CrCl. As shown in Table III, the sole predictive factor with respect to a postoperative decline in renal function was length of follow-up (P = .04).
Table III. Multiple regression for a greater than 20% decrease in CrCl
| Variable | P value | Coefficient | Lower 95% CI | Upper 95% CI |
|---|---|---|---|---|
| Age | .89 | 0.0006 | −0.008 | 0.009 |
| Sex | .82 | −0.073 | −0.249 | 0.102 |
| Hypertension | .64 | −0.036 | −0.190 | 0.118 |
| Medications | .47 | 0.057 | −0.099 | 0.213 |
| Preoperative CrCl | .85 | 0.0002 | 0.0009 | 0.002 |
| AAA diameter | .61 | −0.002 | −0.009 | 0.102 |
| Neck diameter | .11 | 0.027 | −0.006 | 0.060 |
| Neck length | .07 | 0.004 | 0.002 | 0.009 |
| Endograft | .83 | 0.013 | −0.116 | 0.143 |
| Endograft diameter | .32 | −0.019 | −0.055 | 0.018 |
| Accessory renal artery coverage | .22 | −0.111 | −0.111 | 0.067 |
| Contrast | .60 | 0.0009 | −0.002 | 0.004 |
| Postoperative CTs | .77 | −0.007 | 0.023 | 0.038 |
| Length of follow-up | .04 | 0.014 | 0.007 | 0.030 |
Discussion
The use of transrenally fixated endografts offers several potential advantages compared with infrarenally placed grafts. The transrenal bare metal component effectively lengthens the proximal attachment site, thus resulting in fewer type I endoleaks and possibly a more durable repair.1, 2, 3, 10 However, there were concerns regarding the renal effects of metal stents placed across the renal artery ostia which have been explored by a number of investigators using several models.
With the in vitro pulsatile circulation model, the Cologne group showed that renal artery flow was not acutely compromised by the transrenal placement of bare metal stents and that any progressive decline in renal function would be attributed to neointimal hyperplasia, rather than an acute disruption in flow dynamics.11 Further animal models echoed these findings with neointimal hyperplasia on bare metal struts 1 month after transrenal fixation in a porcine model, resulting in renal artery stenoses and renal dysfunction as measured by increased serum Cr levels.12 Equally concerning was the apparent increase in renal infarctions after transrenal fixation. Using a sheep model, German investigators observed an increased incidence of renal infarctions after transrenal fixation compared with infrarenally placed grafts (58.3% vs 25%; P = .04).13 Although not limited to endografts with transrenal fixation, renal infarcts were certainly more common with these grafts.
Differences between types of metal and the neointimal response have been investigated by using animal models and, at least in part, were the basis of this study. Using a porcine model, the Sheffield group investigated the differences between transrenally placed stainless steel and nitinol stents.8 They observed that after 6 to 15 weeks, all animals displayed an organized cellular matrix with endothelial cells covering the stent struts, with nitinol stents inducing a more aggressive and disorganized acellular matrix and often causing partial renal ostial occlusion. The investigators postulated that this might be attributed to the chemical properties of nitinol or its unpolished condition that could cause increased reactivity and thrombogenicity.
As a result of this concern regarding renal deterioration after endovascular aneurysm repair, postrepair renal function has been reviewed and compared between transrenally and infrarenally placed endografts and open repair groups. The Zenith investigators found no difference in serum Cr levels during follow-up in those patients receiving open or endovascular aneurysm repair with the Zenith endograft and transrenal fixation.14 They did observe a postoperative decline in CrCl in both groups which has been observed in other reports of endovascular aneurysm repair.4, 5, 15, 16 This, however, did not seem to be any different from the normal elderly population, who experience a yearly 2% decline in CrCl after 75 years of age.17 Whether this decline in renal function after transrenal fixation plateaus after several years, as suggested by some authors,14 or continues will require longer follow-up.
These findings are consistent with those of other groups who have found transrenal fixation to be safe2, 10, 18 and to not cause greater renal dysfunction compared with infrarenally fixated endografts.4, 5, 6, 7, 15, 16, 19 A gradual decline in renal function is a consistent postoperative finding in these studies, although this progressive renal insufficiency after endovascular repair is similar with transrenal and infrarenal fixation. It also seems to be independent of pre-existing renal artery stenoses1, 16 and accessory renal artery coverage.16, 20 Several of these reports described the use of different transrenally fixated endografts but failed to compare these endografts in regard to renal function deterioration.5, 7, 18
This study used CrCl as a measure of renal function. Although the Cockcroft-Gault formula may overestimate glomerular filtration rate by as much as 16%,21 it is viewed as more accurate and relevant than serum Cr alone in determining renal function. As with previous studies,7, 15 a 20% reduction in CrCl was determined to be a clinically significant decline in renal function. After approximately 6 months of follow-up, the Talent and Zenith groups showed similar mild effects on CrCl. There was no difference in several factors that might contribute to renal dysfunction, including intraoperative contrast volume, accessory renal artery coverage, and preoperative serum Cr. Both endograft groups had similar numbers of patients who experienced a greater than 20% decline in CrCl, and length of follow-up was the only significant independent predictor after multiple regression analysis. One possible factor that was not examined was the effect of pre-existing renal artery stenoses. This has been reviewed by Marin et al,1 who found that the preoperative presence of renal artery stenoses did not result in greater renal dysfunction after transrenal endograft placement.
Of course, the metal composition of the bare stents is not the only difference between the Talent and Zenith endografts. Along with being composed of stainless steel, the Zenith bare metal component has more struts than its Talent counterpart and, as a result, is more likely to cross renal artery orifices. The placement of stents across the ostia is unpredictable with endograft deployment but could result in different flow patterns across the ostia, depending on this orientation. This effect on flow patterns has been investigated by an Australian group, who found that different configurations of stents across an arterial orifice had a minimal effect on blood flow into arteries greater than 3 mm in diameter, as long as there was no buildup on the wire.22 Potentially, the effects of the possibly greater thrombogenicity of the nitinol-based stent could be balanced by the fewer stent bars in the Talent endograft protecting against greater renal dysfunction. In the mid term, however, there is no difference in renal dysfunction observed with the different endografts; whether this persists in the longer term is outside the realm of this study and will require longer follow-up.
Several shortcomings of this study have already been mentioned and include the retrospective nonrandomized design. Data pertaining to kidney size, aortic thrombus or calcification, renal artery stenoses, and renal infarcts were not available. Renal artery stenoses have not been found to contribute significantly to renal dysfunction after endovascular repair,1 and although renal infarcts are more common after transrenal fixation compared with infrarenal fixation, they seem relatively benign with respect to postoperative renal dysfunction.19 This study describes the short- to mid-term follow-up of patients, and whether these findings persist will require additional follow-up. Whether this similarity between these transrenally fixated endografts, with respect to renal function, is maintained has yet to be determined. Additionally, the possibility that there is a small difference that would have been statistically significant with greater numbers of patients does exist, although the number of patients described in this report is similar to those in others.
Regardless, this study confirms that transrenal fixation of endografts causes little deterioration in renal function and that, despite concerns of differing properties, nitinol and stainless steel–composed bare metal stents behave clinically similarly in regard to renal function.
Author contributions
References
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Competition of interest: none.
PII: S0741-5214(06)01362-0
doi:10.1016/j.jvs.2006.07.038
© 2006 The Society for Vascular Surgery. Published by Elsevier Inc. All rights reserved.
