European Journal of Vascular & Endovascular Surgery
Volume 32, Issue 2 , Pages 155-168, August 2006

Comparison of Carbon-impregnated and Standard ePTFE Prostheses in Extra-anatomical Anterior Tibial Artery Bypass: A Prospective Randomized Multicenter Study

  • X. Kapfer

      Affiliations

    • Department of Thoracic and Vascular Surgery, University Hospital Ulm, Ulm, Germany
    • ,
  • W. Meichelboeck

      Affiliations

    • Birkenallee 58, 82349 Pentenried, Germany
    • Corresponding Author InformationCorresponding author. W. Meichelboeck, Dipl.-Ing., Birkenallee 58, 82349 Pentenried, Germany.
    • ,
  • F.-M. Groegler

      Affiliations

    • Department of Vascular and Thoracic Surgery, Oberschwaben Klinik gGmbH, Ravensburg, Germany

Accepted 17 December 2005. published online 14 April 2006.

Article Outline

Abstract 

Objectives

The aim of this study was to find out whether carbon impregnated ePTFE vascular grafts have better long-term patency or limb salvage rates than Standard ePTFE vascular grafts in crural revascularization in patients with chronic critical ischemia.

Design

Prospective randomized multicenter trial. Study endpoints were 36 months follow-up, major amputation or death.

Materials

We used 6mm carbon ePTFE (Carboflo®) and 6mm standard ePTFE vascular grafts (both C.R. BARD Inc./IMPRA).

Methods

From June 1995 to November 1998, 283 patients were randomly assigned either to carbon (C) (n=140) or to standard (St) ePTFE (n=143) vascular grafts at 19 centres. A standard protocol was used with lateral extra-anatomic course of the graft to the anterior tibial artery and of a distal vein patch or cuff. More than 90% of the patients had rest pain or gangrene.

Results

Two hundred and sixty-five (C=130; St=135) patients could be analysed in the intention-to-treat (ITT) group. Primary patency, secondary patency and limb salvage rates after 36 months were 33, 43 and 67% in the carbon– and 30, 38 and 58% in the standard PTFE group, respectively, (log-rank test: p=0.20, 0.12 and 0.16). Additional analyses were made per protocol (PP) and as-treated (AT). The retrospective power of the study was calculated as 79 and 83%.

Conclusion

The ITT, PP and AT analysis, showed no statistically significant advantage of the carbon ePTFE vascular graft in terms of patency or limb salvage over the standard ePTFE vascular graft at 36 months.

Keywords: Prospective randomized multicentric trial, Polytetrafluoroethylene, Crural bypass, extraanatomic, Biomaterials, Carbon, Anterior tibial artery, Peripheral arterial occlusive disease (pAOD), Chronic critical ischemia

 

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1. Introduction 

The results of crural artery reconstruction in patients with critical ischemia of the lower limbs is often unsatisfactory.1, 2, 3, 4 The use of autologous great saphenous vein is recognized as gold standard.5, 6, 7 If vein is unsuitable or absent, prosthetic material is generally used when there is danger that amputation will be necessary.1, 4 A technological approach to improve the patency rate is to reduce the thrombogenicity of the implant material. Use of carbon as addition to ePTFE is one possibility. The additive use of carbon with implants has been explored since the early 1960s.8, 9, 10 Owing to its excellent biocompatibility, carbon is used in implant materials for various medical applications and has been associated with reduced thrombogenicity.11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21 In animal studies carbon-coated vascular prostheses demonstrate better patency rates18, 22, 23, 24, 25 but the coating of PTFE with carbon has proven impractical due to the detachment of carbon particles from the surface.26, 27

The ePTFE carbon vascular prosthesis (Carboflo®) used in the present study is a modification of the standard ePTFE vascular prosthesis (both C.R. Bard Inc./IMPRA, USA). Carbon particles have been permanently integrated into ePTFE structure by means of a special manufacturing process based on co-extrusion. The carbon thus becomes an integral insoluble constituent of approximately 25% of the ePTFE structure.27, 28, 29 Several animal experiments have been carried out with this new ePTFE carbon vascular prosthesis.27, 30, 31, 32 The results of Tsuchida et al.27 and of Babatasi et al.31 revealed a significantly reduced thrombogenicity in the early post-operative phase. However, Akers et al. (1993)30 and Ao et al.(2000)32 did not detect a difference.

Up to the beginning of the present study, one prospective randomized trial of distal bypass and two in vascular access had been reported utilising Carboflo®.33, 34, 35, 36 Bacourt et al. reported encouraging results of the patency and limb salvage rates for the ePTFE carbon vascular prosthesis, however, a statistically significant difference could not be shown.33, 34 Trials of Carboflo® in vascular access have not found a difference in outcome.35, 36

The objective of the present study was to test whether the carbon prosthesis has a clinical advantage with regard to cumulative primary and secondary patency or limb salvage rates in a larger very homogenous patient population with peripheral arterial disease (pAOD) and chronic critical ischemia.

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2. Materials and Methods 

Participating centers of various sizes from Germany and Austria were chosen (ranging from university to county hospitals) in order to ensure a representative cross section (Table 1). To include a homogenous patient population, special care was taken to standardize the outflow tract exclusively via the anterior tibial artery and to standardize operation technique by restricting to lateral extra-anatomic course of the bypass only,37, 38 with obligatory application of a distal vein patch/cuff.

Table 1. Participating centers and number of randomized patients (n)
NameCityClinicn
Nestlé, SalzmannBad NauheimWilliam Harvey Klinik19
Albiker, StockmannBerlinFranziskus Krankenhaus60
Kindl, SchwildenEsslingenStädtische Kliniken Esslingen23
AltstädtGüterslohStädtisches Krankenhaus Gütersloh3
Schröder, Riepe, ImigHamburg–HarburgAllgemeines Krankenhaus Harburg5
DragojevicHannoverKlinikum Hannover Heidehaus1
BruckschleglHeidenheimKlinikum Heidenheim14
Müller–Reinartz, HusfeldtKarlsruheDiakonissenkrankenhaus Karlsruhe7
Hiemer, Uy, GrußKasselDiakonissen-Krankenhaus5
Baumann, MangoldLahrKlinikum Lahr-Ettenheim16
Munteanu, HamannLeonbergKreiskrankenhaus Leonberg15
Naundorf, MaurerMunichKlinikum rechts der Isar4
Teßarek, TorselloMünsterSt Franziskus Hospital14
Grögler, ReinhuberRavensburgKrankenhaus St Elisabeth24
HaugRemscheidSana Klinikum Remscheid25
Djibey, GlücklichRendsburgKreiskrankenhaus Rendsburg17
SchareckRostockKlinikum der Universität Rostock1
Kapfer, Orend, Sunder-PlassmannUlmKlinikum der Universität Ulm21
Straßegger, HagmüllerViennaWilhelminen Spital9
Total283

2.1. Inclusion criteria 


•Age over 18 years (adults only).

•Critical ischemia with rest pain and or necroses/gangrene (Fontaine stage III and IV) or severe claudication (Fontaine stage IIb) after unsuccessful conservative treatment over at least 2 months.

•Indication for femoro-crural bypass with anterior tibial artery as only or best suitable distal vessel.

•No suitable veins available to perform a venous bypass (unsufficient length, diameter less than 3mm or varicose).

•Arterial vascular inflow tract without hemodynamically relevant stenoses.

The following further technical inclusion criteria and post-operative anticoagulation regime has been defined for both groups:

•Use of Carbon or Standard ePTFE vascular grafts (Carboflo®/Standard C.R. Bard Inc./IMPRA) to eliminate any other technical difference but the carbon impregnation.

•Proximal anastomosis to either: the limb of an aorto-femoral or cross-over prosthetic bypass, external iliac artery, common femoral artery, superficial femoral artery or deep femoral artery.

•Lateral extra-anatomical course of the bypass.37, 38

•Distal anastomosis to the anterior tibial artery.

•Obligatory vein patch technique at the distal anastomosis with the following techniques: Linton patch,39 Miller cuff,40 Taylor patch41 or St Mary's boot.42

•Intraoperative or post-operative angiogram for quality control.

•Anticoagulation regime: perioperatively (primary) with heparin, afterwards with warfarin or, when contraindicated, Acetylsalicylacid (ASA).

2.2. Exclusion criteria 


•Proximal anastomosis to an axillo-femoral bypass.

•Acute ischemia.

•Life expectancy less than 3 years.

Exclusively ePTFE vascular prosthesis of 6mm diameter and thin-walled were chosen. The choice of using prosthesis with or without external spiral reinforcement was left to the operating surgeon due to the extra-anatomic course of the bypass.

The randomization sequence was generated as a block randomization (block size 2) using an SAS macro generated by a contract biometric institute (METRONOMIA GmbH, Munich). The randomization to either a carbon or standard PTFE vascular graft was carried out directly in the operating theatre by the surgeon just before the operation who opened the double-sealed, numbered envelopes in ascending order.

Post-operative follow-up examinations were made at the time of discharge from hospital and after 3, 6, 12, 18, 24 and 36 months.

The following study endpoints of post-operative follow-up were defined:

•Follow-up at 36 months,

•A major amputation or

•Death of the patient.

Additional follow-up examinations could be carried out at any time following adverse events and at the discretion of the managing physician. Furthermore, details on revision operations were documented. The data were entered on a triplicate print-through form and sent at regular intervals to an external independent contract biometric institute (METRONOMIA GmbH, Munich). This contract institute fulfils the quality norm ISO 9002 and works in accordance with the usual GCP guidelines. Data input was in a data bank environment with entry masks specifically adapted to the documentation forms. The data were entered twice by two members of staff independently of each other, to rule out typing errors. Discrepancies detected by tests of plausibility were clarified by written queries to the investigators. The data analysis was made by an experienced biometrician using the SAS system (version 8) under WINDOWS 2000.

Investigators and patients were not blinded to treatment assignment after the sealed envelopes have been opened for the further duration of the study. The external independent study statisticians/biometrician saw unblinded data.

The number of cases needed was calculated on the basis of the following assumptions:

In view of the encouraging results with carbon-impregnated prostheses reported in the studies of Bacourt, Tsuchida und Babatasi,27, 31, 34 a one-sided hypothesis was formulated: ‘carbon-impregnated prostheses have better patency rates and attain better limb salvage rates than standard ePTFE prosthesis’. Data reported in the literature1, 3, 4, 5, 6, 34, 37, 38 indicate that the primary patency rate of the standard ePTFE grafts to crural arteries is about 40% after 36 months. A difference of 15% was considered to be clinically relevant with regard to the main parameter ‘primary patency rate’. A power of 80% (β-error=0.2) and an α-error of 5% enabled an evaluation of 135 patients per group. The loss to follow-up rate that was assumed with 10% per year. It was therefore, planned to recruit a total of 330 patients in two groups of 165 patients. The study protocol was approved by ethics committees and informed consent was delegated to the participating centres. An interim analysis at 1 year did not show a significant difference, thus the study was continued.

Differences between groups for nominal data were tested with the chi-square test or Fisher's exact test. Continuous variables were investigated for group differences using the t-Test for equality of means or with the Mann–Whitney-U test. The primary outcome measure primary and secondary patency and the limb salvage rates of the groups were compared on a univariate basis using life-table analysis and the log-rank test. The primary analysis was made on an intention-to-treat (ITT) basis. A per protocol (PP) and an as-treated (AT/or treatment received) analysis were also calculated. The ITT, AT and PP-analysis sets included only patients who comply with the inclusion criteria and have received one of the two graft types evaluated (Fig. 1).43 A retrospective power analysis of the final study has been performed.

  • View full-size image.
  • Fig. 1. 

    Flow diagram of the study acc. to CONSORT statement: randomization and analysis groups (C, carbon PTFE; St, standard PTFE; ITT, intention-to-treat; PP, per protocol; AT, as-treated; m, month; LTFU, lost-to-follow-up/did not reach defined study endpoints).

The reporting standard according to Rutherford et al.44 and the CONSORT statement45, 46 were followed.

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3. Results 

3.1. Randomization, study flow, treatment groups 

In the period from June 1995 to November 1998, 283 patients in 19 centres were allocated on a random basis to bypass with either Carbon PTFE (n=140 C) or Standard PTFE (n=143 St) prosthesis. Eight patients were not further documented due to lack of compliance of the participating centers (six for unknown hospital-internal reasons, two received an ePTFE prosthesis of another brand), one patient underwent an amputation intra-operatively before receiving the prosthesis.

Eleven patients (C=2/St=9) did not receive the prosthesis assigned by the randomization due to surgeon preference (n=7) or availability of the assigned graft (n=4).

A further nine patients were excluded from all analyses of the principal parameters owing to contraventions of the inclusion criteria (C=5: proximal anastomosis at an axillo-femoral bypass n=1, patients already with contralateral extremity in study n=4; St=5: no vein patch at distal anastomosis n=2, patch technique with PTFE n=1, a priori candidate for amputation n=1). A complete flow diagram of the study according to the CONSORT statement45, 46 is shown in Fig. 1.

A total of 265 patients entered the ITT (C=130/St=135) and AT (C=137/St=128) population. Fourteen patients were lost to follow-up (LTFU). Thus two hundred and fifty-one patients reached the defined study endpoints in the ITT and AT analysis groups. The per protocol (PP) population comprised 254 patients. Thirteen patients were LTFU in the PP population, so that 241 patients reached the defined evaluation endpoints in this group.

3.2. Demographic and clinical data 

Table 2 show the demographic and clinical data of the ITT population. More than 90% of the patients were in Fontaine stages III (33%) and IV (60%) with rest pain and/or necroses/gangrene. The parameter ‘walking distance’ was not evaluated in 58 patients. This was due to the patient being bedridden, requiring wheelchairs or because of a pre-existing contralateral amputation (n=15). The ankle-brachial-index (ABI) according to the American Heart Association (AHA) standards47 could be calculated for the affected side in 239 patients and for the unaffected side in 206 patients. Pseudohypertensive values of more than 1.5 (e.g. media sclerosis) were not considered48, 49 (Table 2).

Table 2. Baseline demographics and vascular risk factors of the intention-to treat (ITT) groups at the date of operation. Pre-operative Fontaine stage, pain-free walking distance and ankle-brachial-index
Carbon (n=130)Standard (n=135)p-valued.f.
Gender: n (%)
Male60 (46%)54 (40%)0.32*1
Female70 (54%)81 (60%)
Median age: (years) (range)73 (37–91)75 (49–90)0.14†1
Height: (cm) mean±SD168±9167±90.33†1
Weight: (kg) mean±SD70±1267±130.05†1
BMI: (kg/m2) mean±SD24.6±3.723.7±4.00.07†1

Vascular risk factors: n (%)

CHD80 (63%)84 (62%)1.00‡1
Hypertension85 (65%)89 (63%)1.00‡1
Hyperlipidemia61 (47%)52 (39%)0.17‡1
Diabetes mellitus60 (46%)63 (47%)1.00‡1

Smoking (acc. to Rutherford44): n (%)

None86 (66%)93 (69%)0.91‡3
Not current6 (5%)7 (5%)
Current37 (29%)34 (25%)
No data1 (1%)1 (1%)

pAOD stage acc. to Fontaine: n (%)

IIb (severe claudication)11 (8.5%)7 (5.2%)0.55‡2
III (rest pain)43 (33.1%)44 (32.4%)
IV (necrosis/gangrene)76 (58.5%)84 (62.2%)
Pain-free walking distance: (m) median (range) (n=205)10(0–115)10(0–160)0.73§1

ABI index:

ABI treated limb (n=239) median (IQR)0.36 (0.13–0.50) (n=114)0.35 (0.14–0.53) (n=125)0.68§1
ABI untreated limb (n=206) median (IQR)0.75 (0.59–1.00) (n=100)0.70 (0.53–0.93) (n=106)0.06§1

(BMI, body-mass-index; CHD, coronary heart disease; ABI, ankle-brachial-index; SD, standard deviation; IQR, inter quartil range; d.f., degrees of freedom.

*Pearson chi-square test.

†t-test for equality of means.

‡Fisher's exact test.

§Mann–Whitney-U test.

Only ABI indices below 1.5 included.

Previous ipsilateral arterial operations were reported in 150 patients (57%), 69 of these in the carbon group and 81 in the standard group (Fisher's exact test, p=0.27). More than one-third of patients (n=99/37%, C=44/St=55) had undergone previous infrainguinal bypass. An aorto-iliac operation had been previously carried out in 23 (9%, C=10/St=13) patients. Seventeen patients (7%, C=10/St=7) had undergone aorto-iliac and infrainguinal reconstructions previously. No location has been specified in 11 (4%, C=5/St=6) of the cases. (Fisher's exact test, 4 d.f., p=0.64). Bypass implantation (n=102) including combinations with angioplasty/Stent and endarterectomy were the most frequent form of previous ipsilateral therapy. The remaining 48 patients had angioplasty (n=24), stents (n=3) or endarterectomy (n=12) or non-specified procedures (n=9), (Fisher's exact test, 5 d.f., p=0.28).

3.3. Outflow tract 

Table 3 shows a complete overview on the run-off status of the overall patient population, a more detailed description of the outflow tract for the two groups is shown in Fig. 2. The prognostic favourable situation with distal patent anterior tibial artery and simultaneous complete plantar arch was found in only 60 patients (23%). There was no difference in run-off between the two groups in any of the four subgroups shown in Fig. 2. (Fisher exact test, p>0.05 for the groups and subgroups).

Table 3. Run-off: overview of all patients in the ITT group (n=265)
Distal anterior tibial arteryPlantar arch
Total n (%)Complete n (%)Incomplete n (%)Not assessed/no assessment possible n (%)No data n (%)
Patent, without stenosis169 (63.8%)60 (22.6%)65 (24.5%)38 (14.3%)6 (2,3%)
Patent, but stenosed89 (33.6%)6 (2.3%)68 (25.7%)12 (4.5%)3 (1.1%)
Occluded5 (1.9%) 2 (0.8%)3 (1.1%)
No complete data2 (0.8%) 1 (0.4%) 1 (0.4%)

Total

265 (100.0%)66 (24.9%)136 (51.3%)53 (20.0%)10 (3.8%)

Results of pre-operative angiographic assessment of the distal anterior tibial artery and the plantar arch.

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  • Fig. 2. 

    Pre-operative angiographic run-off status of the anterior tibial artery and plantar arch in the carbon and standard PTFE group (n=265; ITT-analysis; Arcus, plantar arch; patent, patent-no relevant stenosis; stenosed, patent-but relevant stenosed; occluded, distal occlusion; values below 2% not numbered; Fisher's exact test p>0.05 for all groups and subgroups).

3.4. Operation data and post-operative anticoagulation/thromobocyte aggregation regime 

There were no significant differences with regard to the data of the surgical technique, such as operation time, external spiral reinforcement used, location of the proximal anastomosis, the kind of distal vein patch technique, intraoperative administration of Heparin, intraoperative angiogram, immediate bypass occlusion and hospital stay (Table 4). Simultaneous procedures on the inflow tract in the iliac region and the bifurcation of the femoral artery were performed in 18 patients (7%). This involved 11 endarterectomies, three bypasses, one endarterectomy and bypass, two angioplasties and one type not specified. In nine patients (C=3/St=6), 11 intraoperative complications were documented. These included five venous injuries, three dissections, two perforations and one plaque disruption. The two treatment groups did not differ with regard to the occurrence of simultaneous procedures (Fishers exact test, 4 d.f., p=0.66) and intraoperative complications (Fisher's-exact test, 3 d.f., p=0.33).

Table 4. Surgical technique, operative data, hospital stay–ITT
Carbon (n=130)Standard (n=135)p-valued.f.
Operation time: (min) median (IQR)140 (115–185)140 (106–180)0.79*1
Prosthesis with external spiral reinforcement: n (%)
Yes79 (61%)78 (58%)0.71†1
No51 (39%)57 (42%)

Location of proximal anastomosis: n (%)

Limb of aorto-femoral prosthesis14 (11%)15 (11%)0.94†5
External iliac artery8 (6%)7 (5%)
Common femoral artery87 (70%)94 (70%)
Superficial femoral artery11 (9%)13 (10%)
Deep femoral artery9 (7%)5 (4%)
No data1 (1%)1 (1%)

Type of distal vein patch technique: n (%)

Taylor patch78 (60%)80 (59%)0.85†3
Linton patch45 (35%)49 (36%)
St Mary's boot6 (5%)4 (3%)
Miller cuff1 (1%)2 (2%)

Intraoperative Heparin, admin.: n (%)

Systemic52 (40%)57 (42%)0.81†3
Local19 (15%)23 (17%)
Systemic and local57 (44%)54 (40%)
No data2 (2%)1 (1%)
Intraoperative Heparin, dosage: I.U. median (IQR)5.000 (5.000–7.500)5.000 (5.000–7.500)0.93*1
Intraoperative angiogram: n (%)86 (66%)93 (69%)0.60†1
Immediate bypass occlusion (<24h): n (%)10 (8%)14 (10%)0.67†1
Hospital stay, post op: (days), median (IQR) (n=204); (C=26, St=35 no data)26 (16–35)24 (17–41)0.12‡1

d.f., degrees of freedom.

*Mann–Whitney-U test.

†Fisher's exact test.

‡t-test for equality of means.

The status of the post-operative anticoagulation was evaluated in 253 patients at the time of discharge from hospital to check for group differences (five patients were already deceased and seven had prior major amputations within the hospital). In 55% (n=138) patients were receiving warfarin, 30% (n=75) of the patients were on Acetysalicylacid (ASA) and 11% (n=27) on heparin at this time. Five patients have received ASA in combination with other anticoagulation drugs (two ASA+Heparin, three ASA+Tiplopidin). Five received other types of anticoagulation (one LMWH, one Clopidogrel, one Danaparanoid-Natrium, two Ticlopidin). No data on the applied anticoagulation regime has been recorded for three patients. There were no differences between the groups (Fisher's exact test, 10 d.f., p=0.44).

3.5. Redo procedure, infections, 30-days mortality rate: 

Redo procedures were performed in 15 cases (nine bypass explantations (2<30d/7>30d), six new bypasses) (Fisher's exact test, 2 d.f., p=0.62). These patients were rated as secondary occluded at the time of the redo operation, even if the redo procedure was successful.44 Thirteen (4.9%) bypass infections of which 3 (1.1%) have been before 30 days and five (1.9%) seroma formations were recorded during the follow-up period. 30-days mortality rate was 3.4% (n=9).

3.6. Patency and limb salvage rates analysis 

The detailed data of the life table analysis of the primary (PrimPat), secondary patency rates (SecPat) and the limb salvage rate (LSalv) for the two groups are shown in the figures (Fig. 3, Fig. 4, Fig. 5) and tables (Table 5, Table 6, Table 7). At 36 months, 33% of grafts were still primary patent in the carbon group and 30% in the standard group. The SecPat at 36 months was 43% in the carbon group and 38% in the standard group. The LSalv was 67% in the carbon group and 58% in the standard group. The 95% confidence intervals (95% CI) of both groups did not overlap in SecPat between 6 and 12 months. During all other intervals for SecPat and in all intervals for PrimPat and LSalv the 95% CI did overlap (Table 5, Table 6, Table 7 and Fig. 3, Fig. 4, Fig. 5). The obvious nominal differences between 6 and 18 months in favour of the carbon group did not lead to a statistically significant difference. In our life table analysis results have been curtained at 36 months as the number of remaining patients at risk have reached only 20% (n=54) of the original patient population (n=265). Results in these later intervals are also biased by the high degree of loss to follow-up.

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  • Fig. 3. 

    Life table curves for primary patency rate acc. to Rutherford et al.44, carbon- vs. standard-PTFE vascular grafts; ITT analysis (log-rank test χ2=1.65, 1 d.f.; p=0.20); standard-error<5% at all times; table indicates number of patients (n) at risk at begin of each intervall.

  • View full-size image.
  • Fig. 4. 

    Life table curves for secondary patency rate acc. to Rutherford et al.44, carbon- vs. standard-PTFE vascular grafts; ITT analysis (log-rank test χ2=2.46, 1 d.f.; p=0.12); standard-error<5% at all times; table indicates number of patients (n) at risk at begin of each interval.

  • View full-size image.
  • Fig. 5. 

    Life table curves for limb salvage rate acc. to Rutherford et al.44, carbon- vs. standard-PTFE vascular grafts; ITT analysis (log-rank test χ2=1.96, 1 d.f.; p=0.16); standard-error<5.3% at all times; table indicates number of patients (n) at risk at begin of each interval.

Table 5. Life tables primary patency rate according to Rutherford et al.44, carbon vs. standard PTFE vascular grafts, ITT (log-rank test χ2=1.65, 1 d.f.; p=0.20) (LTFU, lost-to-follow-up; CI, confidence interval)
Interval (month)No. at risk at start of interval (n)No. failed during interval (n)Withdrawn during interval (n) due toInterval failure rateCumulative patency rate, (95% CI%)Standard error (%)
DeathLTFUDuration
Carbon PTFE
0–1130275200.213478.7% (72.4–84.9)3.2
1–396106200.108770.1% (62.4–77.8)3.9
3–67851200.065465.5% (57.0–74.1)4.4
6–127077100.106158.6% (49.8–67.4)4.5
12–185583000.149549.8% (40.5–59.1)4.8
18–244491000.206939.5% (30.4–48.6)4.6
24–363453000.153833.4% (24.3–42.6)4.7
Total 712670

Standard PTFE

0–1135312300.234076.6% (70.3–82.6)3.2
1–399144100.145165.5% (57.9–73.1)3.9
3–680152000.189953.1% (45.1–61.0)4.1
6–1263115000.181843.4% (35.4–51.5)4.1
12–184762200.133337.6% (29.1–46.1)4.3
18–243731100.083334.5% (25.5–43.5)4.6
24–363240000.125030.2% (21.4–38.9)4.5
Total 841670
Table 6. Life tables secondary patency rate, according to Rutherford et al.44, carbon vs. standard PTFE vascular grafts, ITT (log-rank test χ2=2.46, 1 d.f.; p=0.12) (LTFU, lost-to-follow-up; CI, confidence interval)
Interval (month)No. at risk at start of interval (n)No. failed during interval (n)Withdrawn during interval (n) due toInterval failure rateCumulative patency rate, (95% CI%)Standard error (%)
DeathLTFUDuration
Carbon PTFE
0–1130155200.118688.1% (82.9–93.4)2.7
1–310896200.086580.5% (73.8–87.2)3.4
3–69153200.056576.0% (68.3–83.6)3.9
6–1281107100.129966.1% (57.7–74.5)4.3
12–186373000.113858.6% (49.3–67.9)4.8
18–245371000.133350.8% (41.2–60.4)4.9
24–364573000.160942.6% (33.2–52.0)4.8
Total 602870

Standard PTFE

0–1135232300.173682.6% (76.8–88.5)3.0
1–3107114100.105373.9% (66.8–81.1)3.7
3–691192000.211158.3% (50.6–66.1)4.0
6–1270105000.148149.7% (41.4–58.0)4.2
12–185553200.095245.0% (36.1–53.8)4.5
18–244531100.068241.9% (32.6–51.2)4.8
24–364042000.102637.6% (28.4–46.8)4.7
Total 751970
Table 7. Life tables limb salvage rate according to Rutherford et al.44, carbon vs. standard PTFE vascular grafts, ITT (log-rank test χ2=1.96, 1 d.f.; p=0.16) (LTFU, lost-to-follow-up; CI, confidence interval)
Interval (month)No. at risk at start of interval (n)No. failed during interval (n)Withdrawn during interval (n) due toInterval failure rateCumulative patency rate, (95% CI%)Standard error (%)
DeathLTFUDuration
Carbon PTFE
0–1130105100.078792.1% (87.7–96.6)2.3
1–311466300.054887.1% (81.3–90.9)2.9
3–69935100.031384.4% (77.8–90.8)3.4
6–129069100.070678.4% (70.9–85.9)3.8
12–187424100.028076.2% (67.7–84.7)4.3
18–246751000.075270.5% (61.3–79.7)4.7
24–366135000.051366.7% (57.2–76.5)4.9
Total 353570

Standard PTFE

0–113593300.068293.2% (89.1–97.3)2.1
1–3120127000.103083.6% (77.6–89.6)3.1
3–610185100.081676.8% (69.5–84.0)3.7
6–1287911000.110468.3% (60.2–76.4)4.1
12–186745200.063064.0% (54.8–73.2)4.7
18–245622100.036761.6% (51.6–71.6)5.1
24–365134000.061257.9% (47.6–68.2)5.3
Total 473770

3.7. Comparison of the group evaluations and power analysis 

The PP and AT analysis did not show any qualitative difference from the ITT analysis (Table 8). The alternative hypothesis according to which ‘carbon-impregnated prosthesis have better patency or limb salvage rates than standard ePTFE prostheses’ hence could not be accepted. The null hypothesis (‘carbon-impregnated prostheses do not have better patency or limb salvage rates’) is valid for all three analysis groups.

Table 8. Log-rank test analysis at 36 months of the life table curves for the three different analysis groups. (ITT, intention-to-treat; PP, per protocol und; AT, as-treated)
Analysis groupPrimary patency p-valueSecondary patency p-valueLimb salvage p-value
ITT (n=265)0.200.120.16
PP (n=254)0.230.180.23
AT (n=265)0.290.280.34

The retrospective power analysis of the ITT set showed a power of 79%. For this computation the sample size 265 patients (C=130, St=135) and the observed lost-to-follow-up rate (5.3%, 14/265) was used. That means that a study with 135 patients in the control group and 130 in the treatment group has a chance of 79% (=power) to show a difference between a patency rate of 40% in the Standard PTFE group and a patency rate of 55% in the Carbon PTFE group, using a one-sided log-rank test with alpha equals 5%; given a lost-to follow-up rate of 5.3%. When using the final patency rate of 30.7% for the Standard group with a 15% difference for the Carbon group a power of 83% was calculated for the ITT set. This means that the study was large enough in order to detect a difference of 15% points between carbon and standard PTFE grafts in case this difference is true (but unknown). The power analysis computations were performed with n Query Advisor Version 5.0.

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4. Discussion 

One of the main problems in crural bypass surgery is choice or availability of suitable bypass material. The autologous great saphenous vein (GSV) is the material of first choice, but is unavailable in up to 28% of patients owing to prior coronary bypass surgery, varicose veins surgery, prior arterial reconstructions, insufficient vein calibre and prior thrombophlebitis.50

In this prospective, randomized and multicenter study, the effect of a physically and chemically insoluble carbon impregnation of ePTFE prosthesis on clinical outcome was investigated by comparison with standard ePTFE prosthesis of the same kind. The principal parameters evaluated were the primary (PrimPat) and secondary patency rate (SecPat) and the limb salvage rate (LSalv). Important factors affecting these parameters are the outflow tract and the kind of bypass procedure, which were tightly controlled. Use of a vein patch technique at the distal anastomosis39, 40, 41, 42, 51, 52 was an indispensable prerequisite in the present study, but the technical implementation was left to the surgeon's discretion in order to rule out additional sources of technical error. With these restrictions we have compared two very homogeneous patient groups.

As suggested by Meichelböck in 2002,53 the outflow tract was described in terms of the condition of the anterior tibial artery and the hollow of the foot (Fig. 2). Descriptive angiographic data enable a differentiated evaluation of the outflow tract. Schweiger et al.3 also point to the crucial significance of the outflow tract for the prognosis of bypass extending beyond the knee joint. The method suggested by Meichelboeck53 appeared to us to be more effective for practical clinical use than the well-known appraisal of the outflow tract according to Rutherford.44

By using a material with a reduced thrombogenicity we would have expected differences in the early post-operative phase up to 30 days. But we did not see any difference in the immediate occlusion up the 24h (Table 4) and in the further early course up to 30 days. In total 39% of all occlusions in the Carbon and 36% of all occlusions in the Standard grafts occurred in the early period up to 30 days, suggesting that a significant number of graft failures could be related to technical problems. Less occlusions in favour of the carbon prosthesis were recorded in the period between one and 6 months. In the interval between 6 and 18 months the life table curves of both groups run almost parallel, while the standard PTFE prosthesis seems to have a quantitative advantage later than 18 months (Fig. 3, Fig. 4, Fig. 5).

The life tables for PrimPat, SecPat and LSalv presented are shown consistently in accordance with the revised Rutherford recommendations.44 All cumulative patency calculations were made not at the beginning of an interval, but at the end. This leads to a formal worsening of the results, but gives a more precise picture of the actual course. Consequently, our results cannot be compared directly with ‘former’ literature, which has been calculated according the recommended standards of Rutherford 198654 at the beginning of each interval.

In a meta-analysis, Albers et al.55 showed pooled results for PTFE bypass to infrapopliteal arteries for primary patency (n=40 studies), secondary patency (n=35 studies) and limb salvage (n=31 studies). The results at 36 months were 41, 51 and 66%, respectively. However, it was shown that there is a marked difference in the results for PrimPat, SecPat and LSalv between ‘qualitatively good’ and ‘qualitatively less good’ studies. In a total of 43 publications investigated, only ten reported on a patient number in excess of 100. Of these, only five3, 56, 57, 58, 59 attained a rating of more than 10 points in quality scoring used (maximum 14 points). In their extensive retrospective study of 1993, Schweiger et al.3 were able to demonstrate a cumulative PrimPat and SecPat of 37 and 45%, respectively, at 3 years. If these calculations are considered in terms of the latest Rutherford criteria,44 they would be reduced to 31 and 38%, respectively, comparable with our own results. The cumulative LSalv was 63% after 2 years and 51% after 5 years.

The results of Bacourt33, 34 for Carbon vs. Standard PTFE at 24 months were 45 vs. 35% (PrimPat), 53 vs. 36% (SecPat) and 57 vs. 47% (LSalv). These results compare with our 24 months data with 40 (C) vs. 35% (St), 51 vs. 42% and 71 vs. 62%.

In 2001 Lang et al.60 presented results of a prospective randomized study with more than 200 patients of Carbon PTFE vs. Standard PTFE bypass, but confined exclusively to the below knee popliteal segment. No statistically significant difference could be found in the per protocol analysis in this study. The results for PrimPat, SecPat and LSalv for carbon vs. standard PTFE at 36 months were 36 vs. 35%, 46 vs. 45% and 60 vs. 63%, respectively. The detailed final results are currently being prepared for publication.61

While the coating with Carbon in mechanical heart valves is standard today as surface treatment to achieve higher mechanical stability, it did also not lead to clinical improved results in other implants with blood contact such as stents,62, 63, 64 although the results for carbon coating were encouraging in terms of biocompatibility, hemocompatibility and decreased thrombogenicity evaluated in vitro, in animal trials and in non-randomized clinical settings.65, 66, 67, 68, 69

The apparent advantage in the carbon group in our study in the period from six to 18 months is noteworthy and might be of clinical importance for individual patients, but seems to be a temporary effect. However, any single point on a survival curve is not a particularly reliable representation of survival at that time; it is the entire curve which is a reliable representation of survival, thus the entire curves should be compared, not a single point on the curve.

Our hypothesis that PrimPat, SecPat and the LSalv rate are better for the Carbon prosthesis was refuted. At 36 months, Carbon PTFE prosthesis and Standard PTFE prosthesis had similar outcomes. This finding was confirmed with additional calculations based on the ‘per protocol’ (PP) and ‘as-treated’ (AT) groups, with reasonable power to show an effect (79 and 83%, respectively). Overall this suggests that reduction of graft thrombogenicity with a non-pharmacological method will not result in a crucial clinical advance in treatment of this group of patients evaluated in the long term without further additional measures. Alternative mechanisms and conduits needed to be evaluated in proper prospective randomized studies for the group of patients with no autologous vein available.

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Acknowledgements 

We would like to specially thank all participating centres of the study as listed in Table 1, as without their contribution this data collection and analysis could not be realized. Financial support. The study was in part financially supported by C.R. BARD Inc./IMPRA. The data entry and data analysis were financially supported by C.R. BARD Inc./IMPRA, but was carried out by METRONOMIA GmbH, (Munich, Germany), an independent biometric institution. C.R. BARD did not influence the data analysis. Conflicts of interest. There is no conflict of interest with the authors Xaver Kapfer and Friedrich-M. Groegler. Wolfgang Meichelboeck was an employee of C.R. BARD Inc./IMPRA from 1993 to 2002 and is now an employee of Edwards Lifesciences.

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PII: S1078-5884(06)00008-6

doi:10.1016/j.ejvs.2005.12.015

European Journal of Vascular & Endovascular Surgery
Volume 32, Issue 2 , Pages 155-168, August 2006

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