Stent-Graft Placement in Popliteal Artery Aneurysms: Midterm Results

The popliteal artery aneurysm is the most common peripheral aneurysm, accounting for 70–80%. It is found mainly in male patients,^10-12 with 50% of all cases being bilateral.¹³ The pathogenesis is often atherosclerotic, however, generalized dilatation and elongation of other arteries, such as the abdominal aorta, suggest a systemic abnormality.¹⁴ Mechanical stresses like hypertension, which is present in many patients^15,16 and the fixation of the vessel at the adductor hiatus can contribute to aneurysm formation in the popliteal artery. Non-atherosclerotic popliteal aneurysm is very rare and may occur as a complication of inflammatory disorders or trauma. The natural history shows that popliteal aneurysms expand by approximately 10%/year and larger aneurysms enlarge more rapidly than smaller ones; aneurysms < 2 cm increased 1.5 mm/yr, and 2–3 cm grew by 3 mm/yr (Pittathankal).

Guvendik once called PAA “the sinister harbinger of sudden catastrophe”⁴ because of its unpredictability, significant and highly cumulative risk of complications, chronic embolization, thrombosis, and acute ischemic complications.^1–7 PAA progress to complications within 5 years in almost 70% of cases and can result in limb loss (30–40%).¹⁷ Depending on the diameter of the aneurysm, 18% of the patients have local pain, swelling, or severe pulsation. In addition, compression of the popliteal vein may lead to distal edema and venous thrombosis.¹⁸ Further, distal embolization and occlusion is present in 29%, which may result in claudication, rest pain, or gangrene. The major complication is the critical ischemia, which is present in 21%, with a high risk for amputation in 50%.¹⁹ The rupture is very rare at 2%.²⁰ In addition, several observational studies of asymptomatic aneurysms suggest that the risk of complications increase with the size and presence of mural thrombus. It is greatest in aneurysms larger than 2 cm and ranges from 24% at 1 year, to 35% at 4 years, to 74% at 5 years.^24–26

There is a general consensus that symptomatic popliteal aneurysms should be repaired irrespective of size. And given that the severity of outcomes associated with symptomatic popliteal aneurysms include a risk of major amputation in 50%,^2,3,28 asymptomatic PAA larger than 2 cm in diameter should be treated too, particularly if they contain significant mural thrombus.^27–31

The standard surgical treatment is still the vein interposition. In the most favorable series, the 5-year patency rates were over 95% for asymptomatic aneurysmsand around 70% in patients with symptoms. Surgical mortality was low at 1–5%, but morbidity rates as high as 40% were reported. Ligation and bypass does not prevent blood flow in the aneurysm and may lead to expansion and complications. When feasible, an excision should be performed.^32–36

Concerning the high morbidity of standard surgical procedure, transfemoral endoluminal graft stenting is an alternative approach to conventional operative repair.^6,21

The aim of this study was to determine the midterm durability and anatomic formation that is feasible to repair with this technique and the need for reintervention after stent-graft placement.

Methods

The study is based on a single-center prospective observation analysis in patients in our institution who underwent endovascular repair of symptomatic PAA with Viabahn from January 1999 through December 2003. Patients were subjected to a strict follow-up protocol that required a clinical and duplex ultrasound evaluation at 3, 6, 9 and 12 months and spiral computed tomography (CT) after the first and second year. Additional investigations were obtained whenever indicated.

Selection criteria:

Inclusion criteria used in patient selection for endovascular treatment of popliteal aneurysms were:

1. aneurysm length less than 10 cm;

2. aneurysm diameter less than 5 cm;

3. original distal vessel diameter of min (6 mm);

4. sufficient run off with two open tibial arteries.

Thrombosed aneurysms and aneurysms with an essential side branch, which might be occluded by graft, have not been included in this study.

Imaging. Preoperative imaging includes an initial, noninvasive duplex ultrasound to determine the extent of aneurysm, detect the thrombus and assess runoff. CT-angiography and 3-D reconstruction was performed for case planning to assess landing zones and distance measurement.

Antiplatelet therapy. All of our patients were placed on Antiplatelet monotherapy with clopidogrel. This rationale was based on the Antiplatelet Trialists Collaboration, which demonstrated a 32% reduction in graft occlusion in patients who received Antiplatelet agents.³⁷ The Clopidogrel versus Aspirin in Patients at Risk of Ischemic Events (CAPRIE) trial demonstrated an increased benefit of clopidogrel over aspirin.³⁸ However, the data were not specific for lower limb lesions. In a univariate analysis, the Groningen group found that the additional use of clopidogrel was the only factor that predicted the success of a popliteal stent graft.³⁹

Stent-graft procedure. All procedures were performed by endovascularly-trained vascular surgeons in an operating room. Spinal anesthesia was used in most cases. We preferred to use antegrade ipsilateral femoral access through a small groin incision.

The commercially available Viabahn endograft system was used in all cases, considering it is the only covered stent graft that shows significant flexibility and provides enough radial force for use in popliteal arteries. Depending on the graft diameter, they were delivered through an 8–10 Fr sheath. For final planning, we per-formed an intraoperative angiography and fixed the definitive position, size, and length of the stent-graft. The device is relatively intolerant to oversizing; therefore, precise diameter measurements and balloon postdilatation were important for full expansion and apposition of the device to vessel wall.

The diameter of the stent-graft was calculated from the largest diameter of proximal neck, with an oversizing factor of maximal 10%. In our patients, the size ranged from 7–10 mm, in 60% (16 patients) we used an 8-mm stent-graft. The landing zone should have a seal of 2–3 cm above the aneurysm and 1–2 cm below. In 3 cases we had to use 2 devices because of the difference in diameter at the proximal and distal neck of the aneurysm. We overlapped 3 cm in cases, that more than one stent was required. Because of the particular stent-deployment mechanism, we used a stiff wire for the final placement.

Definitions

A successful procedure was defined as a completed endovascular procedure that demonstrated a primary exclusion. During follow up, the need for intervention due to endoleak, occlusion, stenosis, dislocation, and reduction of the maximum aneurysm diameter were recorded.

Results

We repaired 27 symptomatic PAA from 1999–2003 in 24 patients. There were 22 males and 2 females. The mean age was 58 years. The mean diameter was 2.9 cm, with a range from 1.9–5.3 cm. The mean length was 4.6 cm with a range from 3–9.1 cm. The mean follow up was 48 months. Based on the “intent-to-treat analysis”, the technical success rate was 100%. We had 2 occlusions, one after 3 months. However, after a thrombectomy and changing the antiplatet therapy, this reconstruction has stayed open for 46 months, the other case is repaired by a femoropopliteal vein bypass. We had 2 central leaks that were repaired endovascularly; the mean reduction of the original diameter of the aneurysm was 40%. In addition, we did not have any dislocation or graft fracture, nor observed any “in-stentstenosis” or “edge phenomena” at any time.

Discussion

Transfemoral endoluminal graft stenting is an alternative approach to conventional operative repair of the popliteal aneurysm. In our experience, the endovascular procedure with Viabahn is safe, effective, and less complicated to perform. The exclusion and technical success rate is high in cases with a limited lesion, a short length (< 10 cm), aneurysms with a diameter < 5 cm, original vessel diameters > 6 mm, and a sufficient runoff with 2 open tibial arteries. The patency rate in midterm follow up is comparable to open surgical repair. Due to the absence of “ingraft stenosis”, we expect a high long-term patency rate.

To achieve high technical and clinical success rate using endovascular procedures, it is important to regard pathogenesis, localization, and morphology of the lesion and to take notice of the particular technical aspects of the Viabahn device.

With regard to morphology, it is necessary to respect collateral circulation as far as possible. Severe calcification in the landing zone could cause substantial technical problems. Therefore, preoperative diagnostics should include angiography, CT scan, and a duplex ultrasound to evaluate all these components.

Characteristics of the popliteal artery and Viabahn device. The biomechanical forces that influence the popliteal artery during movement are unique. Morphological and dynamic changes correspond with the patient’s age.

In patients under 30 years of age, minor arterial curves were noted proximal to adductor hiatus in the flexed position. In patients over 45 years, one or more curves were located distally to the adductor hiatus. In patients over 60 years, the curves did not disappear during knee extension.⁴⁰

Diaz and colleagues investigated the morphological changes during knee flexion and described the presence of “hinge points” in the popliteal artery.⁴¹

The Viabahn is a self-expanding graft with an ultra-thin expanded PTFE coating on the luminal surface. It shows significant flexibility and is relatively kink-resistant, even with considerable radial forces. It adjusts more to the anatomy of the aneurysm than straightening the distance. In our opinion, Viabahn is the only available covered stent that could be indicated for use in the popliteal artery.

Limitations of current stent-grafts. Depending on localization, curves, and diameter of the aneurysm, the device can shorten significantly and may also land in an aneurysm sac or bow into it. The misplacement of a stent may make it necessary to use several grafts. To avoid this, we performed an angiogram and made the final decision for the graft length and position intraoperative. The average abbreviation in the preoperative measurement and definitive excluded distance was approximately 10% in our study. This should be considered when choosing the length of the prosthesis. Further, we released the grafts over a stiff wire to achieve exact placement. Another issue is that a tapered stent graft is still not available to compensate for size changes from the femoral artery to the belowknee popliteal artery. Currently, the only technical solution is overlapping different diameter grafts. To avoid wrinkling, we did not oversize the prosthesis more than 10%.

Decisive for the choice of the diameter is the peripheral vessel diameter; this is measured in CT scan. In case of a big difference between central and peripheral vessel diameter, we prefer to overlap two endografts outside the flexion zone, which is usually possible with the available endografts.

With improvement of endovascular technique and the development of a more flexible, tapered and heparinized device, the limitations of this technique will be solved.

Conclusion

There is a general consensus that symptomatic popliteal aneurysms should be repaired, irrespective of size. Several studies have shown that medical therapy has no effect on the enlargement of PAA, and ischemic complications cannot be prevented by watchful waiting.¹⁰

For acute limb ischemia resulting from a thrombosed popliteal aneurysm, thrombolysis, or even emergency surgery, are not safe options. The severity of outcomes includes a risk of major amputation of up to 50%.^25,27,28 Moreover, there is a sharp contrast between the rates of complications with urgent rather than elective treatment.¹⁰

So, the question is not whether PAA should be treated:. “Popliteal aneurysm is safe to leave.”⁴² Asymptomatic PAA larger than 2 cm in diameter should be treated too, particularly if they contain significant mural thrombus.^19–24

Because of the biomechanical particularity of the popliteal artery and rareness of PAA, it is the last holy bastion of conventional open vascular surgery. There is no doubt that open surgical exclusion has been standard therapy, with excellent long-term patency. We should, however, recognize that one of the main arguments against early elective operative therapy is the high morbidity rate of the excision procedure.

Endovascular popliteal aneurysm repair is technically feasible and safe, with less morbidity. A selected cohort of patients with limited lesions, suitable anatomy, and appropriate medical therapy (clopidogrel), achieved patency rates in 4-year follow up, similar to those who had open surgery.

In summary, endovascular treatment is an emerging technique with favorable results. Consequently, it should be considered on a par with open procedures.