European Journal of Vascular & Endovascular Surgery
Volume 32, Issue 5 , Pages 523-528, November 2006

Incidence and Effect of Bare Suprarenal Stent Struts Crossing Renal Ostia Following EVAR

  • A. England

      Affiliations

    • Corresponding Author InformationCorresponding author. A. England, BSc, Department of Medical Imaging and Radiotherapy, University of Liverpool, Johnston Building, Brownlow Hill, Liverpool, L69 3GB, UK.
    • ,
  • J.S. Butterfield
    • ,
  • R.J. Ashleigh

Department of Radiology, South Manchester University Hospitals, Southmoor Road, Manchester, United Kingdom

Accepted 20 January 2006. published online 29 August 2006.

Article Outline

Objectives

The incidence and effect of bare stent struts crossing the renal ostia following endovascular aortic aneurysm repair with the Talent stent-graft is not known. The study aims to establish the incidence in which bare stent struts cross the renal ostia and to assess any associated effects on renal function.

Methods

Fifty-five patients (51 men, mean age 73 years, range 57–90) who had endovascular repair of their abdominal aortic aneurysms with a Talent suprarenal stent-graft were included in the study. Patients were scanned at a variety of follow-up periods (median 24 months, range 3–102). The relationship between the bare stent struts and the renal ostia, together with renal function were retrospectively recorded. The presence and location of the bare stent struts was assessed using CT virtual intravascular endoscopy (CT VIE). Struts were defined as being absent, peripherally located or in the central channel of the renal ostia. Renal function was assessed from glomerular filtration rates (GFR) derived from serum creatinine levels and the Cockcroft and Gault formula.

Results

A total of 109 renal ostia were evaluated by CT VIE with one patient having a previous nephrectomy. Bare stent struts crossed 1 renal ostium in 22 (40%) patients and bilateral ostia in 5 (9%) patients. Of the 109 ostia assessed, 15 (14%) ostia were crossed centrally and 17 (16%) had struts crossing the ostium peripherally. There were no statistically significant differences in the change between pre-operative GFR and latest GFR in the group without any strut involvement (6mLs/min±7mLs/min) and the group with struts crossing one or both renal ostia (2mLs/min±9mLs/min; p>.05).

Conclusion

Peripheral or central coverage of renal ostia by bare stent struts occurs in a third of all renal arteries following EVAR. Crossing of renal ostia by bare stent struts does not affect follow-up GFR.

Keywords: Stent-graft, Renal artery, Strut involvement, Renal function

 

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Introduction 

Endovascular repair is fast becoming an acceptable and challenging alternative to open surgery for the treatment of abdominal aortic aneurysms (AAA). There is clear short-term survival benefit of EVAR when compared to open repair.1, 2 Despite the reported advantages there is concern regarding the long-term durability of aortic stent-grafts.1 Fabric tears,3 stent fractures4 and graft migrations5, 6 have been reported. A large proportion of commercially available stent-grafts used in clinical practice have a bare suprarenal component to facilitate proximal fixation. The behaviour of these stent struts crossing the renal ostia and their effect on renal function is not well understood.7 Assessment of the relationship between bare stent struts and the renal ostia is problematic even with modern day radiological imaging. There are increasing numbers of reports in the literature of CT virtual intravascular endoscopy being used in the assessment of the aorta and aortic stent-grafts.7, 8, 9, 10, 11, 12, 13

The aim of this study is to investigate the incidence of bare stent struts crossing the renal ostia in patients treated with a Talent suprarenal stent-graft (Medtronic AVE, Santa Rosa, CA). Additionally, we also wish to investigate the associated effect on renal function of these bare stent struts crossing the renal ostia.

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Materials and Method 

Between January 2004 and January 2005 there were 82 patients in our endovascular follow-up programme. Five patients had US/MR follow-up, four patients repeatedly failed to attend and four patients were lost to follow-up. Sixty nine consecutive patients with a Talent suprarenal stent-graft attended for follow-up with contrast-enhanced CT angiography. The study consisted of two parts, retrospective inspection of the renal ostia for stent coverage using CT virtual intravascular endoscopy (CT VIE). The second part of the study was the retrospective comparison of renal function in patients with and without bare suprarenal stent struts crossing the renal ostia. Clinical records, radiographic studies and laboratory data of all patients undergoing endovascular repair with suprarenal stent-grafts were reviewed. Follow-up studies were conducted at discharge, 3 months and annually thereafter. We have been performing EVAR procedures since 1997, this study is cross-sectional in design with patients attending at a variety of follow-up periods (median 24, range 3–102 months). Patient assessment included routine laboratory studies, clinical assessment and radiological imaging (duplex ultrasound and contrast-enhanced CT scanning). The specific technique used to implant the Talent stent-graft has been described in the literature.14 Informed consent for EVAR was obtained for all patients and our local research and ethics committee approved the study.

CT scanner and parameters 

Multislice CT examinations were performed on a Siemens Somatom Sensation 16 (Siemens, Erlangen, Germany) 16 slice CT scanner. The detector collimation of the scanner was set to 0.7mm. The rotation time of the CT scanner was set to 0.5 seconds, the tube voltage was 120kVp, and the current was 200mAs. One millimetre images were reconstructed with a 0.7mm reconstruction interval (Kernel 30f very soft). A topogram was first obtained to define the range of the scan, scans were take from the inferior border of the twelfth thoracic vertebrae to the superior border of the acetabulum. All patients received an intravenous injection of iodinated contrast medium (iohexol 240mgI/mL, 100mLs at 3mLs−1, GE Healthcare, Cork, Ireland). Data was acquired during arterial and late (60 seconds) phases of contrast enhancement using the in-built bolus tracking software for automatic scan triggering. After scanning, CT data was transferred to a workstation (Siemens Leonardo, Siemens, Erlangen, Germany) for analysis. All data was reviewed for clinical abnormality by a consultant vascular radiologist at the time of scanning. Only patients with an arterial phase follow-up CT scan with a reconstructed slice thickness of 1.0mm were included in the study. Patients with reconstructed slice thickness of >1.0mm or non-arterial phase scans were excluded as the CT data did not facilitated examination of the renal arteries by CT VIE. A total of fourteen patients were excluded from the study as a result of violations to the scanning protocol (alternative scanned used, scanned as part of another imaging protocol, single non-arterial phase acquisition).

CT virtual intravascular endoscopy 

Following acquisition all CT data for each patient was loaded onto the Siemens Leonardo workstation for fly-through analysis. All images were loaded into the MPR mode, where they were reconstructed into orthogonal planes (transaxial, coronal and sagittal). The upper limit for the endoscopic navigator system was set at 100 Housfield units, and the lower threshold at the lowest level possible (−1024). This allowed subtraction of the endoluminal contrast from the aorta and renal arteries. The data obtained was displayed as volume rendered intraluminal images. Once generated, the dataset can be navigated through real-time, with simultaneous update of the 3 orthagonal planes in order to allow the user to identify the exact position within the vessels. The technique used to generate CT VIE images has been previously described in the literature.7, 11 Using the fly-through software the renal ostia were assessed for coverage by suprarenal stent struts. The relationship of the stent struts was defined as ‘no involvement’, ‘peripheral coverage’ and ‘central coverage’ (Fig. 1a,b and c).

  • View full-size image.
  • Fig. 1 

    (a) CT virtual intravascular endoscopic image of a patient with no struts crossing the renal ostium. (b) CT virtual intravascular endoscopic image of a patient with a stent strut peripherally crossing the renal ostium. (c) CT virtual intravascular endoscopic image of a patient with a stent strut crossing the renal ostium in a central position.

Assessment of renal function 

All patients had serum creatinine values taken pre and postoperatively and during each of the annual CT follow-up visits as part of our routine endovascular follow-up program. Using serum creatinine together with physical patient data it was therefore possible to determine renal function changes either by changes in serum creatinine or by glomerular filtration rate (GFR) as defined by the Cockcroft and Gault formular.15 The National Kidney Foundation guidelines on renal dysfunction were used to classify renal dysfunction; serum creatinine levels ≥170μmol/L were considered to be indicative of renal dysfunction.16 We also considered a decline in GFR of equal to or greater than 20% as being indicative of worsening renal function when comparing pre-operative to latest GFR levels; this definition was taken from the study by Grego et al.17

Statistical analysis 

The results were analysed with SPSS 11.0 for Windows (SPSS Inc, Chicago, IL) to determine the relationship between patients with and without struts crossing the renal ostia and renal function. Continuous data is reported as mean values plus or minus its standard deviation. A P value of <.05 was considered to be statistically significant. For analysis of renal dysfunction the cohort was divided into two groups based on renal ostial involvement by the bare suprarenal struts. Two groups were generated; CT evidence of struts crossing renal ostia and patients with no evidence of involvement. Relationship between continuous variables was assessed using the student t test or Mann-Whitney U test, relationships between categorical variables were assessed using the Fisher exact tests.

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Results 

A total of fifty-five patients (51 men, mean age 73 years, range 57–90) who had endovascular repair of their abdominal aortic aneurysms with a Talent suprarenal stent-graft were included in the study. Demographic information is presented in Table 1. From the Cockcroft and Gault formula15 mean GFR was 61mL/min±18mL/min pre-operatively for the whole study group. There was no patient on dialysis before the procedure, one (2%) patient had a serum creatinine of 264μmol/L which satisfied the National Kidney Foundation definition of renal dysfunction.16 Mean aneurysm size for the study group was 64mm (range, 41–95mm). The mean contrast volume injected during the initial endovascular procedure was 254mL±74mL. No suggestion of preoperative renal artery stenosis was reported in the radiological or clinical case notes in any patient.

Table 1. Patient characteristics by coverage group
Struts crossing
No renal ostia≥1 renal ostia
Total patients28 (51%)27 (49%)
Agea (years)72±874±7
Gender/male27 (96%)24 (89%)
ASA GradeII8 (29%)8 (30%)
III19 (68%)15 (56%)
IV1 (4%)4 (14%)
Pre-Op AAA diametera (mm)67±1461±6
Procedural contrast usagea (mL)274±80232±61
Follow-upa (months)35±2029±21

aMean.

CT VIE assessment 

A total of 109 renal ostia were assessed by CT IVE with one patient having a surgically absent kidney. Crossing of renal ostia by bare suprarenal stent struts occurred in 32 (29%) vessels (fifteen struts were centrally located and seventeen peripherally). Bare stent struts crossed 1 renal ostium in 22 (40%) patients and five (9%) patients had bare struts crossing both renal arteries.

Postprocedural renal insufficency 

The mean post-operative GFR was 60mL/min±18mL/min. Ten (18%) patients had serum creatinine levels equal or above 130μmol/L. Three (5%) patients were classified as having post-operative renal dysfunction according to the National Kidney Foundation classification system.16

Follow-up renal insufficency 

The mean follow-up GFR was 57mL/min±19mL/min Fifteen (27%) patients had serum creatinine levels equal or above 130μmol/L at latest follow-up. Consequently, four (7%) patients were classified as having follow-up renal dysfunction according to the National Kidney Foundation (NFK) classification system.16 Three out of the four patients with NKF classified renal dysfunction had struts crossing one renal ostium. Of the four patients with NKF renal dysfunction, one patient had renal dysfunction in both the pre and postoperative periods, the remaining three patients first had NKF renal dysfunction classified on the serum creatinine taken during follow-up.

Comparison of renal function and suprarenal bare strut coverage 

There were no statistically significant difference in the change between pre-operative and latest GFR in patients with and without struts crossing the renal ostia (2mL/min±9mL/min and 6mL/min±7mL/min respectively, P>0.05; Table 2). Of the four patients with NKF derived renal dysfunction three patients had struts covering one renal ostium, one of these patients had NKF renal dysfunction prior to the EVAR procedure. There was one patient with NKF renal dysfunction detected during follow-up without struts covering any renal ostia. Analysis of GFR indicated that there were equal numbers of patients with and without struts crossing renal ostia who had GFR ≤20% during follow-up (Table 3).

Table 2. Assessment and comparison of renal function between coverage groups
Struts crossingP Value
No renal ostia≥1 renal ostia
Pre-OpSCra (μmol/L)101±23117±370.020
GFR (mLs/min)67±1756±170.020
Post-OpSCra (μmol/L)109±26114±310.341
GFR (mLs/min)63±1757±170.206
LatestSCra (μmol/L)113±25130±600.247
GFR (mLs/min)61±1854±190.144
Change – Pre to LatestSCra (μmol/L)12±1712±360.319
GFR (mLs/min)6±72±90.102

aSCr–serum creatinine, displayed as median values; GFR–glomerular filtration rate, displayed as mean values.

Table 3. Assessment and comparison of classified changes in renal function
Struts crossingP value
No renal ostia≥1 renal ostia
Renal dysfunctionPre-Op0 (0%)1 (4%)0.510
Post-Op1 (4%)1 (4%)0.745
Latest Follow-Up1 (4%)3 (11%)0.320
GFR ≥20% declinePre-Op–Latest F/U4 (14%)4 (15%)0.648

Renal dysfunction as per NKF definition16 and GFR decline according to Grego et al.17

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Discussion 

Multi-slice CT scanning is the preferred imaging modality for the follow-up of stent-grafted patients.18, 19 CT VIA is currently being investigated as a potential addition to the post-procedural follow-up currently being utilised today.7, 8, 9, 10, 11, 12, 13 CT VIA has been reported has having distinct advantages in assessing renal ostia in relation to bare suprarenal stent struts.10 The crossing of renal ostia by bare stent struts occurs in 29% of renal arteries when using the Talent stent-graft. Sun and Zheng investigated the incidence of strut coverage when using the Zenith stent-graft, they reported struts involving 90% of renal ostia.10 This incidence of coverage with the Zenith device is higher than in our series with the Talent (29%). This factor could be attributed to the closer proximity and larger number of stent struts present in the Zenith device when compared to the Talent.

The investigation of renal function following endovascular repair is not new. Mehta et al., compared renal function in patients with infrarenal and suprarenal stent-graft devices.20 They concluded that there was no difference in renal decline when using a transrenally fixed aortic stent-graft. However, in a study using fenestrated stent-grafts there was a significant risk for adverse renal events when using such devices.21 Our findings are in keeping with other studies that coverage of renal ostia by bare stent struts does not affect renal function.7 A report by Burks et al., concluded that transvisceral coverage by bare stent struts occurs frequently after EVAR and is associated with no early morbidity.22 It is generally accepted that renal function can deteriorate following both EVAR and open repair.23, 24, 25 This is a feature supported by data from our study, where mean GFR fell from 61mL/min pre-operatively to 57mL/min at latest follow-up.

There are numerous reports in the literature debating the use of serum creatinine levels as an indicator of renal function. Creatinine clearance is considered to be the gold standard for the assessment of renal function. However, from this retrospective review, serum creatinine was the only parameter available to assess renal function. Therefore we opted for estimated GFR using the Cockcroft and Gault formula15 which is a more superior indicator of renal function than serum creatinine alone. A more definitive study would require the prospective measurement of creatinine clearance. Data from this study uses a median follow-up period of 24 months, the true effect of bare struts crossing renal ostia may not become apparent until studies with longer-term follow-up data report. Furthermore, this study was cross-sectional in design with data taken from a variety of follow-up periods. This places additional limitations on the study, a more ideal approach would be to perform a baseline assessment of renal ostial coverage at 1-month post-treatment and then compare this with annual follow-up scans.

The use of iodinated contrast media is controversial in patients will elevated serum creatinine levels. In our study there were several patients who had serum creatinine levels >170μmol/L. It is our policy to use isomolar radiographic contrast in patient's with serum creatinine levels above 120μmol/L and reserve MR imaging for patient's with serum creatinine levels in excess of 300μmol/L.

The location of graft fabric with respect to the renal ostia is a variable that can potentially contribute to changes in renal function following EVAR. In was not our intention from this study to investigate the distance between graft fabric and the renal ostia and changes to renal function. This study was undertake to outline the incidence of struts crossing renal ostia when using the Talent stent-graft. However, we accept that no investigating fabric coverage in addition to strut coverage is a limitation of this study. There are other complications associated with bare struts such as aortic wall erosion that again have not been investigated by this study.

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Conclusion 

Coverage of renal ostia by suprarenal stent struts from an aortic stent-graft frequently occurs. Bare stent struts crossing renal ostia does not affect follow-up serum creatinine levels or glomerular filtration rates.

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Conflict of Interest 

I would like to point out several possible conflicts of interest. Medtronic Ltd has provided funding for a research radiographer (Andrew England) at South Manchester University Hospitals. Additionally, Dr Raymond Ashleigh has acted as a proctor for Medtronic and has received several educational support grants from Medtronic.

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References 

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PII: S1078-5884(06)00351-0

doi:10.1016/j.ejvs.2006.01.027

European Journal of Vascular & Endovascular Surgery
Volume 32, Issue 5 , Pages 523-528, November 2006

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