Infrainguinal arterial reconstructions in patients with aortoiliac occlusive disease: The influence of iliac stenting☆☆☆

Article Outline

• Abstract
• Methods
• Results
• Discussion
• Discussion
• References
• Copyright

Abstract 

Objective: Iliac artery angioplasty (IAA) is an effective adjunct when combined with infrainguinal arterial reconstructions (IARs) in appropriate patients with multilevel occlusive disease. However, the effect of iliac artery stenting (IAS) on the outcome of patients undergoing distal bypass procedures is not defined. The purpose of this study was to estimate the influence of previous IAS for iliac occlusive disease on the outcome of IARs, compared with those after IAA alone or aortofemoral bypass grafting procedures (AFBs). Methods: During a 5-year period (1995-2000), 105 patients with previous intervention for iliac occlusive disease underwent 120 IARs. The criteria prepared by the Ad Hoc Committee on Reporting Standards (Society for Vascular Surgery/International Society for Cardiovascular Surgery) were followed to define the variables. The TransAtlantic Inter-Society Consensus classification was used to characterize the type of iliac lesions. Univariate (Kaplan-Meier) and multivariate analyses (Cox proportional hazards model) were used to determine the association between preoperative variables and cumulative primary patency. Results: Forty-five IARs were performed in patients with an earlier IAS repair, 33 in patients with an earlier IAA repair, and 42 in patients with an earlier AFB repair. There were not significant differences between patients in the IAS and IAA groups, except for a more frequent use of polytetrafluoroethylene grafts for IARs in the IAS group (40% vs 15%; χ² test, P = .03). The 5-year primary patency rate for IARs was 68% in the IAS group, 46% in the IAA group, and 61% in the AFB group. Univariate analyses revealed that primary patency rates for IARs in patients with previous IAS were significantly higher than those in the IAA group (Kaplan-Meier, log-rank test, P = .02). Previous IAA repair was associated with a two-fold increased risk of IAR graft failure (relative risk, 2.2; 95% CI, 1.1-4.8; P = .04). Conclusions: IARs in patients with previous IAS have significantly improved graft patency, compared with those in patients with previous IAA alone. Such graft patency for IAR after IAS is similar to that obtained after AFB repair. (J Vasc Surg 2001;34:971-8.)

Iliac artery angioplasty alone (IAA) is an established treatment of aortoiliac occlusive disease. It is generally applied to more focal lesions.¹ Despite the safety and the reasonable results with balloon angioplasty alone, acute occlusions and early restenosis limit the long-term success. Stent placement after angioplasty has been used as a means of improving the initial technical success rates and may decrease the risk of long-term failure.², ³ However, aortoiliac reconstructive surgery continues to offer the best long-term results for patients with aortoiliac occlusive disease, compromised only by the higher perioperative morbidity and mortality and late graft limb failures.⁴

Although most patients with multisegmental arterial occlusive disease of the lower extremities only require reconstruction of the suprainguinal lesions, as many as one third of patients may require additional infrainguinal arterial reconstruction (IAR) to obtain adequate clinical improvement.⁵ Patients requiring multilevel arterial reconstruction for lower-extremity ischemia may have a higher risk for perioperative complications, in particular after extensive aortoiliac reconstructive surgery. IAA alone has been described as an effective adjunct when combined with IAR in appropriate patients.⁶, ⁷, ⁸ Few reports have described the use of iliac artery stenting (IAS) in combination with infrainguinal bypass procedures, and its effect on the outcome and long-term results of IARs is unknown.⁹, ¹⁰, ¹¹, ¹²

The TransAtlantic Inter-Society Consensus (TASC) working group introduced a new morphologic stratification of iliac lesions as an improvement over the outdated classifications, outmoded by stent use (not considered at the time of such classifications) and additional experience with lesion-specific anatomy.¹³ Endovascular procedures are the treatment of choice for type A lesions, and surgery is the procedure of choice for type D lesions. Although type B and type C lesions are more frequently treated with endovascular procedures, there is not definitive evidence to support any superiority of this approach over surgery. Such consensus statements have not yet been widely used in publications but clearly reflect the most current standard of evidence-based medicine.

This study investigated the influence of prior IAA and stent placement for iliac occlusive disease on the outcome of IARs, compared with those after IAA alone or aortofemoral bypass (AFBs) grafting procedures. Defined preoperative, procedural, and artery-specific factors that are predictive of adverse results were considered. The most current recommendations and standards were used to define the different variables.¹³, ¹⁴, ¹⁵

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Methods 

Between July 1995 and June 2000, 105 patients with previous or concomitant intervention for iliac occlusive disease underwent 120 IARs at the University of Tennessee Medical Center at Knoxville, and all were included in a retrospective cohort study. All of the patients had evidence of chronic limb ischemia. Demographic data, risk factors, complications, and outcome variables were defined according to the criteria prepared and revised by the Ad Hoc Committee on Reporting Standards (Society for Vascular Surgery/International Society for Cardiovascular Surgery [SVS/ISCVS]).¹⁴, ¹⁵ Patients who had undergone redo IAA and/or stenting or IARs before the iliac reconstructive procedure were not included. Preoperative, intraoperative, and follow-up information was available in all patients and was obtained via office and hospital chart review, dictated operative records, and telephone conversations with patients, physicians, or family members. The study protocol was approved by the local institutional review board.

The endovascular procedures were performed in the angiography suite by interventional radiologists after consultation with the attending vascular surgeon, or in the operating room by vascular surgeons. Arteriography was performed by means of an ipsilateral or contralateral femoral approach. Primary or selective stent placement was performed at the discretion of the physician performing the procedure. A balloon-expandable Palmaz stent (Cordis J and J, Warren, NJ) was used preferentially for focal lesions, severely calcified lesions, and all lesions adjacent to the aortic bifurcation. A self-expandable Wallstent (Boston Scientific Vascular, Boston, Mass) was usually placed for long-segment disease or tortuous iliac arteries and for contralateral approaches. A combination of the two stents was generally used to provide extra length if the lesion could not be completely covered by a Wallstent. Stent placement was deemed technically successful if there was less than a 30% residual stenosis and if the gradient across the treated lesion was less than 5 mm Hg. Patients in the surgical group had either aortobifemoral or aortobi-iliac bypass procedures. Polyethylene terephthalate (Hemashield, Boston Scientific, Oakland, NJ) (Dacron) or polytetrafluoroethylene (PTFE) (Goretex, WL Gore & Associates, Flagstaff, Ariz) grafts were used. Patients with extra-anatomic bypass or unilateral aortoiliofemoral bypass procedures were not included.

Assessment of aortoiliac and infrainguinal bypass grafts patency was determined by using the SVS/ISCVS criteria.¹⁴, ¹⁵ There was no strict postoperative surveillance protocol. However, the patients were usually seen within 2 weeks after the time of the procedure. Improvement and changes in clinical status were determined by history and noninvasive vascular laboratory tests. At the discretion of the attending surgeon, postoperative follow-up (clinical and serial duplex ultrasound scanning examinations) was conducted every 3 months during the first postoperative year and every 6 months thereafter. Arteriography was performed when duplex scanning revealed a decrease in ankle/brachial index of 0.15 or more or if peak systolic velocities were greater than 300 cm/second. Indications for reintervention included a stenosis greater than 60% and a gradient across the lesion greater than 15 mm Hg with papaverine or greater than 10 mm Hg at rest. All revisions performed based on these criteria or occlusion were considered a stent failure and an end of primary patency.¹⁴ Survival could be established by telephone contact, but patency and limb salvage were determined at the date of the most recent examination.

Variables analyzed included age, indication for the procedure, extent of ischemia, comorbidities (diabetes mellitus, hypertension, hyperlipidemia, coronary artery disease, cerebrovascular disease, end-stage renal disease, smoking history), runoff score, type of inflow reconstruction, location of the iliac lesion (common iliac vs external iliac), type of graft conduit, and level of distal anastomosis. The TASC classification of disease severity for iliac lesions was used to define the categories of the lesions.¹³ Runoff was classified from preoperative and intraoperative arteriograms (Ad Hoc Committee on Reporting standards, SVS/ISCVS).¹⁴, ¹⁵ The angiographic runoff score for IARs was used for subgroup analysis, categorizing scores as poor runoff (score ≥5) and good runoff (score <5). Infrainguinal bypass grafting procedures performed within 30 days of stent placement were considered “concomitant procedures” to improve distal runoff.

Demographic data and risk factors were compared between patients according to the previous iliac reconstructive procedure (IAS, IAA, and AFB groups); univariate analysis of categorical variables was performed using χ² test (χ² for independent groups, two-tailed P value). Nonparametric tests were used for continuous variables (Kruskal-Wallis one-way analysis of variance). All analyses were performed according to the intention-to-treat principle. Primary patency, limb salvage, and patient survival were determined with the Kaplan-Meier method and differences with the log-rank test.¹⁶ Cox proportional hazards model with time-dependent covariate was used for multivariate analysis to assess the influence of various risk factors on primary patency, limb salvage, and patient survival.¹⁷, ¹⁸, ¹⁹ Variables that had a P value less than .25 in the univariate analysis and those known to be important or possible confounding factors were entered into the regression model and considered significant by forward stepwise selection if P was less than .05 in the final regression equation. By this method, the relative risk (RR) and 95% confidence intervals for the different variables were estimated.¹⁷, ¹⁸ For statistical analyses, SPSS for Windows version 10.0 (SPSS Inc, Chicago, Ill) was used.¹⁹

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Results 

The median patient age of the 105 patients was 62 years (range, 35-83 years). The median follow-up period was 38 months, with a range of 3 to 63 months. The most commonly associated risk factors in the overall group were a significant history of tobacco use (86%), hypertension (85%), coronary artery disease (74%), hyperlipidemia (67%), and diabetes mellitus (52%). Other risk factors included a history of chronic obstructive pulmonary disease (29%), cerebrovascular disease (22%), and end-stage renal disease (4%). Indication for revascularization was limb salvage in 90 procedures (75%) (ischemic rest pain in 22 [18%], tissue loss in 68 [57%]) and disabling claudication in 30 (25%). Autogenous vein was used in 66 IAR procedures (55%), PTFE in 42 (35%), and 12 were PTFE-vein composite grafts (10%). Twenty-seven IARs were femoral to above-knee popliteal (22%), 43 femoral to below-knee popliteal (36%), and 50 femorotibial (42%). Most femoral to above-knee popliteal bypass reconstructions were performed with PTFE (27/30 [90%]), whereas autogenous vein was mainly used for femoral to below-knee popliteal and more distal procedures (75/90 [84%]).

Forty-five IARs were performed in patients with an earlier IAS repair, 33 in patients with an earlier IAA repair, and 42 in patients with an earlier AFB repair. Eighty-seven IARs (82%) had a concurrent aortoiliac procedure, whereas 33 had a previous inflow procedure, mostly within 2 months (range, 0-28 months). Selective iliac stenting was performed for the following indications: a residual stenosis or pressure gradient after angioplasty (75%), a dissection after balloon angioplasty (8%), or a long segment occlusion (5%). Primary iliac stenting was only performed in 6 procedures (11%). Palmaz stents were used in 18 procedures (40%), Wallstents in 25 (56%), and a combination of both in 2 (4%). Stents were placed in three TASC type-A iliac lesions, 26 type-B lesions, 12 type-C lesions and 4 type-D lesions. IAA alone was performed only for TASC types A and B, whereas AFB was mostly performed for type D lesions (74%). In the AFB group, there were 33 aortobifemoral grafts and three aortobi-iliac bypass grafts. The transperitoneal approach was used in 32 procedures, whereas the retroperitoneal approach was used in four.

Initial technical success was obtained in 75 iliac endovascular procedures (97%). Initial hemodynamic success and clinical improvement, as defined by the SVS/ISCVS reporting standards, was obtained in 95% of patients.¹⁴ Forty-eight percent of patients had improved to category +3, 38% to category +2, and 9% to category +1. Two percent of patients were unchanged, whereas three percent of patients were worse (category –1, 1%; category –2, 1%; category –3, 1%). Eighteen patients (15%) required revascularization of the contralateral extremity during follow-up, with a median interval between procedures of 9 months (range, 1-33 months). Wound infections developed in 11 patients (9%), but only three patients (3) had deep infections that required drainage. No graft infections occurred.

Univariate analyses revealed that the patients in the AFB group were older and had more comorbidities. Female sex, disabling claudication, use of PTFE for IAR, and poor runoff were also risk factors more frequent in patients in the AFB group. However, there were not significant differences between the members of the IAS and IAA groups, except for a more frequent use of PTFE grafts for IARs in the IAS group (40% vs 15%; χ² test, P = .03) (Table I). Iliac artery occlusive disease also tended to be more extensive and multifocal in the IAS group, with more TASC type-C lesions than in the IAA group (16% vs 1%; Fisher exact test, P = .03). The frequency of external iliac artery (EIA) lesions was similar between the IAS and IAA groups.

Table I. Patient characteristics

NS , Not significant; Cr , creatinine level.

Cumulative primary patency rates for all infrainguinal bypass procedures at 1, 3, and 5 years were 81%, 62%, and 59%, respectively. Primary patency rates at 1, 3, and 5 years were 89%, 75%, and 68%, respectively, for patients with previous IAS, and 62%, 46%, and 46%, respectively, for patients in the IAA group. Primary patency rates for patients in the AFB group at 1, 3, and 5 years were 85%, 64%, and 61%, respectively. IARs in patients with previous IAA alone had significantly decreased primary graft patency rates with respect to those with previous IAS and AFB (Fig 1) (Kaplan-Meier, log-rank test, P = .02).

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• Fig. 1. 

  IARs in patients with prior IAA alone had significantly decreased primary graft patency rates with respect to those with previous IAS and AFB (Kaplan-Meier, log-rank test, P = .02).

Stratified analyses also revealed that IARs in patients with EIA lesions also had decreased primary graft patency rates compared with those in patients with only common iliac artery lesions (Kaplan-Meier, log-rank test, P = .02). Primary graft patency rates were not significantly different for other risk factors analyzed such as patients' sex, diabetes, history of smoking, hyperlipidemia, indication for revascularization, type of conduit, level of distal anastomosis, and distal runoff. Cox regression analysis revealed that previous IAA was the only independent predictor of decreased primary graft patency when controlling for the presence of all other risk factors (RR, 2.2; 95% CI, 1.1-4.8; P = .04) (Table I).

Cumulative aortoiliac primary patency rates at 1, 3, and 5 years were 93%, 88%, and 88%, respectively, for patients with previous IAS, 87%, 67%, and 67%, respectively, for patients in the IAA group, and 98%, 96%, and 96%, respectively, for patients in the AFB group. Patients with previous IAA alone had significantly decreased primary iliac patency rates with respect to those with previous IAS and AFB (Fig 2) (Kaplan-Meier, log-rank test, P = .005).

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• Fig. 2. 

  Patients with previous IAA alone had significantly decreased primary iliac patency rates with respect to those with previous IAS and AFB (Kaplan-Meier, log-rank test, P = .005).

Interestingly, multivariate analyses revealed that the use of PTFE for IARs and limb salvage was the only independent predictor for decreased iliac primary patency (Table II).

Table II. Independent predictors of primary patency*

95 % CI , 95% confidence interval.

Graft thrombosis was the most frequent mechanism of primary graft failure of IARs in patients with previous IAA (10 of 13 graft failures [76%]) and was not as common in patients with previous IAS (4 of 10 graft failures [40%]) or previous AFB (7 of 15 graft failures [47%]). Significant graft stenosis was not demonstrated in patients who subsequently developed graft thrombosis. Conversely, graft stenosis accounted for most graft failures in patients with previous IAS (55%) and AFB (53%), as compared with IAA (23%).

For all patients, limb salvage at 1, 3, and 5 years was 99%, 97%, and 96%, respectively. Limb salvage analyses by Kaplan-Meier and log-rank test revealed no significant differences related to the previous aortoiliac reconstruction. Two operative (30-day) deaths occurred, one in a patient after a stroke in the AFB group and one after a myocardial infarction in the IAA group. Overall, long-term survival was 96% at 1 year, 92% at 3 years, and 84% at 5 years. Cumulative patient survival was not statistically different between patients in the three groups (Kaplan-Meier, log-rank test, P = .72). No independent predictors for decreased limb salvage or long-term survival were identified with multivariate analyses.

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Discussion 

Several observational studies demonstrate that in patients with lower-extremity ischemia and multilevel arterial occlusive disease requiring combined revascularization, aortobifemoral bypass remains the most effective procedure for inflow revascularization.⁵, ¹⁰, ²⁰ However, the associated mortality and morbidity may be unacceptable, and aortoiliac surgical reconstruction is therefore restricted to low-risk patients. New methods of improving inflow have been introduced, and several studies have shown that IAA may be performed with some success to improve inflow for subsequent distal revascularization procedures.⁶, ⁷, ⁸ Although few reports have described the use of iliac stenting as an adjunct to IARs, the long-term influence of iliac stent placement on the outcome of IARs has not been defined.⁹, ¹⁰, ¹¹, ¹²

In this study, infrainguinal bypass graft primary patency was significantly decreased in patients with previous IAA alone, compared with that in patients who had undergone IAS or AFB repair for aortoiliac occlusive disease. Moreover, IAA was the only independent predictor of decreased IARs primary graft patency. Despite the fact that most of the IARs in the IAA group were performed with autogenous vein grafts and that less severe iliac lesions were also more frequent in this group, the outcome of concurrent IARs was poor with significantly lower primary graft patency. We could not demonstrate any significant contribution of other risk factors to such results. Because IAA patency changes across time, ie, the hazard ratios are different at different time points, an extended Cox regression model, which allowed us to specify time-dependent covariates, was used for the multivariate analyses.¹⁹ We established that the patency of the inflow procedure, in particular IAA alone, was the only independent predictor of IAR primary graft failure.

Inflow failure can be implied as the mechanism of graft failures of IARs in patients with previous IAA because no previous graft abnormalities were evident apart from a lower iliac primary patency. This finding correlates with experiences previously reported that indicate that inflow failure appears to account for infrainguinal bypass graft failure.¹¹ Although IAR primary patency after IAA appeared to be lower in our series than previously reported, this may be related to the higher frequency of limb salvage as the indication for the procedures and the more frequent use of prosthetic grafts.⁷, ⁸

IAS was associated with a significant improvement of IAR primary graft patency and was comparable to that observed after AFB repair. Although iliac primary patency after IAA and stenting was reduced compared with that in patients with AFB repair, this did not appear to affect the IAR primary graft patency. In this series, selective iliac stenting was performed in most procedures for unsatisfactory or complicated IAA. Stratified analyses to determine the influence of primary iliac stenting on the outcome of IARs were subject to a type II statistical error because of the small number of patients and could not be obtained. Our results, however, favor primary iliac stenting for iliac lesions when combined IARs are also required.

Although not statistically significant, there was a trend toward more frequent stent placement for iliac lesions among men, which may account for the higher primary stent patency after IAS observed in this group than previously reported.², ³, ¹³

EIA stenting has been identified as a risk factor for iliac stent failure, in particular in patients with multisegmental iliac occlusive disease.²¹, ²² Interestingly, in our series EIA stenting was also associated with a significantly reduced IAR primary graft patency. Although in the multivariate analysis model, EIA stenting was not an independent predictor for IAR graft failure, this may be related to a type II statistical error.

Prosthetic grafts were used preferentially for above-knee popliteal bypass procedures and patients with disabling claudication and whenever an autogenous vein graft was not available. Unexpectedly, however, the use of PTFE grafts for IARs was an independent predictor of reduced iliac primary patency in all groups. Our results therefore favor the use of venous reconstructions in patients who need multilevel arterial reconstructions, even for patient with claudication who may need above-knee popliteal bypass procedures.

AFB repair was restricted to patients with lower operative risk, and this may account for the lower morbidity and mortality observed in this group. Although patients in the AFB group had a higher frequency of comorbidities, female sex, use of PTFE for IAR, and poor runoff, the outcome of IARs did not appear to be affected. Our data indicate that in carefully selected patients, AFB as an adjunct of IARs offers optimal results.

Because of the small number of patients, the possibility of a type II statistical error was considered. A post hoc power analysis was then obtained to determine the β error and was based on previous studies.¹, ², ³, ⁴, ⁵, ⁶, ⁷, ⁸, ¹¹, ¹² The sample size calculation was stratified to the data set including only patients with previous IAA and IAS and was large enough to obtain at least 80% power, with a two-sided type I error rate of 0.05, to detect a difference in the 5-year primary patency rates of IARs from 68% in patients with previous IAS to 46% in patients in the IAS group (RR, 2.2).

In conclusion, IARs in patients with previous IAS have significantly improved graft patency as compared with those in patients with previous IAA alone. Such graft patency for IAR after IAS is similar to that obtained after AFB repair. Although our results favor primary iliac stenting for iliac lesions when a combined IAR is also required, further studies are necessary to assess the efficacy of such an approach.

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Discussion 

Dr Keith D. Calligaro  (Philadelphia, Penn). The authors compared differences in lower-extremity bypass patency rates following iliac artery balloon angioplasty alone, iliac artery stenting, or aortobifemoral bypass. The 5-year primary patency rates for lower-extremity bypasses were significantly better in patients previously treated by iliac stenting or aortobifemoral bypass compared to iliac balloon angioplasty alone. Iliac artery stenting yielded higher bypass patency rates compared to balloon angioplasty alone, despite the fact that these stents were placed in patients with more advanced iliac artery disease and in patients who more frequently had a prosthetic graft in place. So that's pretty impressive. And I think the take-home message the authors are getting at is that previous reports have suggested that stenting yields better iliac patency rates than a balloon alone. And they're saying not only that, but iliac stenting will yield higher lower-extremity bypass patency rates, which is a different way to look at it.

I have three questions.

Thank you for the opportunity to discuss the paper.

Dr Carlos H. Timaran.  Thank you, Dr Calligaro, for your comments and your thoughtful questions.

The first question, why the use of prosthetic graft affected aortoiliac primary patency, is actually something that we don't know. We were also surprised when our multivariate analysis models revealed that this was an independent predictor for decreased primary patency for the aortoiliac segment. Unfortunately, our sample size was small, and although additional stratified analyses were attempted, we always had the problem of a type II statistical error that prevented us from identifying additional factors that could play a role in these findings.

About two thirds of the inflow procedures were performed simultaneously with the distal revascularization procedures in the operating room by vascular surgeons.

In regard to the case you presented to us, I think that what is really important is to have a good inflow. The problem with iliac angioplasty alone is that the risk of early restenosis is higher, and that is probably one of the factors that accounts for the decreased primary patency that we observed in our infrainguinal revascularization procedures. But in the patient you are describing, probably the inflow is good enough to just perform the distal revascularization procedure without any other additional proximal or suprainguinal procedure. Measuring a pressure gradient across the previous iliac lesion may also aid in determining the need for stent placement.

Dr Daniel Clair  (Cleveland, Ohio). I'd like to congratulate you for a nicely presented paper. I just have a couple of questions for you about techniques.

One of the things that I think is a problem when we're evaluating angioplasty results with pressure gradients across the stenosis to decide whether to place a stent, is the outflow after the vessel has been treated. If the angioplasties are done before the outflow has been restored, essentially what you're doing is measuring a pressure gradient across a closed tube, so you can't really assess as to whether there is a true pressure gradient across the lesion that you've just treated. My own preference is to perform angioplasty or the stent after the distal bypass has been done so that we get a true sense of what the outflow of that vessel is going to be. I would wonder as to whether this evaluation was done before or after the outflow procedure has been done.

Secondly, it looks like from your data that basically it's the iliac angioplasty failure that really is what you're reporting on. I may be misreading the graphs that you presented, but it appears that if iliac angioplasty fails, not very surprisingly the outflow procedure fails as well. Is there more to it than just this?

Thanks.

Dr Timaran.  We do agree that pressure gradients are really important to define if a patient needs additional stent placement. Seventy-five percent of our patients underwent stent placement for significant residual stenosis or pressure gradients.

We certainly divided primary patency rates in two different segments. One was the primary patency for the infrainguinal reconstruction grafts, and the other one was the aortoiliac primary patency. We found that the aortoiliac primary patency was always higher than the distal primary patency. However, failure of the proximal aortoiliac segment did not always account for failure of the distal bypass procedures.

Dr Timur P. Sarac  (Cleveland, Ohio). Carlos, I enjoyed your presentation. The ultimate result I think that we'd like to look at is limb salvage. And certainly improving the inflow may help the bypass stay open long enough to heal the ulcer or improve the rest pain or even develop collaterals. But can you comment at all on your long-term limb salvage for this, because that's really what the patient is looking at.

Dr Timaran.  We focused our paper on the primary patency rates. But you are right, secondary patency rates were actually much higher and similar to the limb salvage rates. And the reason is that many of the patients with procedures that fail, for example, those with iliac angioplasty alone, underwent iliac angioplasty and stenting later on, whereas other patients underwent surgical reconstructions.

We think that primary patency rates are important because they determine the initial outcome and in this way the initial procedure that's best for the patients.

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