Popliteal Artery Aneurysms
There is an increasing role of endovascular therapy for the management of popliteal artery aneurysmal disease.
Popliteal artery aneurysms are the most common peripheral artery aneurysms, comprising 70% to 85% of the total aneurysms in the periphery.1-3 More than 95% of peripheral artery aneurysms occur in males, and the average age of patients at presentation is 65 years. Atherosclerosis appears to be the etiology in more than 90% of cases. The true pathogenesis behind popliteal artery aneurysm formation is not known, and factors such as turbulence distal to the relative stenosis at the tendinous hiatus of the adductor magnus and repeated flexion at the knee have been postulated; this does not, however, account for the association with aneurysms in other locations or the male preponderance. Most popliteal artery aneurysms are fusiform and are bilateral in 25% to 70% of cases.1-3 Popliteal artery aneurysms are associated with abdominal aortic aneurysms in 20% to 40% of cases, but only 1% to 2% of abdominal aortic aneurysms are associated with popliteal artery aneurysms.1-3 Although the standard treatment of popliteal artery aneurysms has been open repair, there are increasing reports in the literature of endovascular management.
At the time of presentation, approximately 50% to 85% of popliteal artery aneurysms are symptomatic, and most are 3 cm to 4 cm in diameter.1-3 Traditionally, larger aneurysms have been believed to be more likely than smaller aneurysms to produce symptoms, but this has recently been questioned.4 Some studies have shown that smaller aneurysms have a higher thrombosis rate. The most common symptoms at presentation are lower-extremity ischemia and compression of adjacent structures. Twenty-five percent to 55% of patients presenting with popliteal artery aneurysms, have associated thrombosis and 6% to 25% have evidence of distal emboli.1-4 Emboli can result in occlusion of runoff vessels, which has been found to be a poor predictor of graft patency and limb salvage after reconstruction. Amputation rates of approximately 25% have been reported with thromboembolism. Rupture of popliteal artery aneurysms is uncommon, occurring in only 2% to 7% of patients, and even when rupture does occur, mortality is rare.1-3,5 However, limb-threatening ischemia associated with rupture results in a 50% to 70% amputation rate.1 Claudication is present in 4% to 40% of this patient population.1
DIAGNOSIS AND IMAGING STUDIES
Popliteal artery aneurysms are primarily diagnosed by a high index of suspicion and physical examination. They are identified as a pulsatile mass in the popliteal fossa. The initial study of choice is duplex ultrasonography. It can diagnose the aneurysm; distinguish it from other popliteal masses, such as a Baker’s cyst; accurately measure its size; and identify thrombus within the aneurysm. Figure 1 shows a thrombosed popliteal artery aneurysm as visualized by ultrasonography. Arterial flow is absent in this vessel.
The role of arteriography is not diagnostic, but rather is used in the popliteal to evaluate inflow and outflow. It can be a valuable modality in identification of anatomy, as well as possible distal embolization. Some investigators believe that this is a mandatory preoperative test because of the impact of distal outflow on long-term graft patency.1 Figure 2 is a contrast arteriogram that demonstrates a large, popliteal artery aneurysm with a normal trifurcation vessel runoff.
MRA and CTA are other potential imaging modalities. MRA is commonly performed when the patient has renal insufficiency or a contrast allergy, and it may provide an accurate assessment of the aneurysm and runoff in lieu of an arteriogram. Figure 3 is an MRA demonstrating the thrombosed popliteal artery aneurysm shown on the ultrasound (see Figure 1). Runoff is visualized through the posterior tibial and peroneal arteries. The anterior tibial artery occludes almost immediately after its origin. The left-sided popliteal artery is also aneurysmal in this patient, but this is not well demonstrated by the MRA.
Aneurysm size plays less of a role in the decision to treat popliteal artery aneurysms compared to aneurysms in other locations, such as the abdominal aorta. This is because the major morbidity from popliteal artery aneurysms is due to thromboembolism rather than rupture. Thus, symptomatic aneurysms require repair regardless of size. Other indications include aneurysms of any size that contain intramural thrombus and asymptomatic aneurysms that are >2 cm in size. Amputation rates vary from 16% to 43% in patients presenting with severe ischemia. In elective repair of these aneurysms, however, the limb loss is <1%.1-3,6
Long-term graft patency rates directly correlate with preoperative ischemic symptoms. Five-year graft patency rates for asymptomatic patients undergoing elective surgery range from 82% to 97%, but patency rates for symptomatic patients range only from 39% to 70%. Other factors that correlate with graft patency include the presence of at least two runoff vessels, the presence of a patent aneurysm, and the choice of conduit.1,7 Five-year patency rates for saphenous vein grafts are 77% to 94%, whereas patency rates for prosthetic grafts are 29% to 42%.3
Asymptomatic patients develop ischemic symptoms between 18% and 31% of the time, which accounts for the majority of the 2% to 13% of this population that eventually required amputation.1-3,8 Due to the low mortality and complication rates of operative repair in contrast to the high morbidity rate when symptoms develop, most surgeons would proceed to repair any popliteal artery aneurysm >2 cm.
In fact, small popliteal artery aneurysms have traditionally been followed with serial ultrasound. Several studies have suggested that these aneurysms have a higher rate of thromboembolism than larger aneurysms. This would suggest that the threshold for operative intervention should be lower than the current guidelines and some investigators advocate operating on all popliteal artery aneurysms.4,7
Popliteal artery aneurysms can be treated by either a medial or lateral approach. The medial approach is more commonly used. It offers the advantages of ease of access to the superficial femoral artery, popliteal artery, and the trifurcation vessels. It also allows access to the saphenous vein without changing the patient position. The posterior approach is occasionally used for smaller popliteal artery aneurysms without the need for large exposures. It has a decreased risk of associated nerve or vessel injury and shorter patient recovery times.
Popliteal artery aneurysms, especially the fusiform type, are usually repaired by bypass of the aneurysmal section of the popliteal artery and ligation of the aneurysm. Another accepted technique is endoaneurysmorrhaphy. Similar to an infrarenal abdominal aortic aneurysm repair, the aneurysm is opened and the graft is placed within it. The final widely used alternative is resection of the aneurysm and bypass. Some investigators advocate this approach for large aneurysms causing significant symptomatic compression of adjacent structures. Cumulative 10-year graft patency rates and limb salvage rates are 66% to 92% and 93% to 100%, respectively for elective procedures, and 39% to 60% and 60% to 84%, respectively in procedures performed for limb-threatening ischemia.1-3,6-9
Several studies have examined the role of preoperative thrombolytic therapy. Because the quality of the distal outflow has a strong influence on graft patency, thrombolytic therapy may restore patency to thrombosed distal vessels, thereby improving outflow and allowing a superior limb salvage result.10
With the rapid advances in endovascular technology and materials, attention has been turned to popliteal artery aneurysms. The treatment of these aneurysms with covered stents has met with mixed results. Calvet et al described one of the first series of popliteal artery aneurysm repair with covered stents.11 Between December 1993 and May 2000, a series of 25 peripheral aneurysms, mostly iliac artery, were treated with covered stents, either Cragg EndoPro (Mintec, Bahamas) or Hemobahn (W.L. Gore Associates, Inc., Flagstaff, AZ). Four of these were popliteal artery aneurysms. Midterm patency results at 30 months were poor.
In November 1998, Kudelko et al described the first placement of a Wallgraft (Boston Scientific Corporation, Natick, MA) endoprosthesis for treatment of popliteal artery aneurysms.12 The Wallgraft endoprosthesis has the advantage of a low profile, flexible design, and ease of placement, and it successfully excluded the aneurysm. At 10 months, no endoleak was visualized and the covered stent was patent. Howell et al published a series using the Wallgraft on femoral-popliteal aneurysms.13 The study described 20 aneurysms in 17 patients, 13 of whom had popliteal artery aneurysms. The study reported a 92% immediate success rate for aneurysm exclusion, the failure being a persistent endoleak that resolved in 1 month, and a 100% successful placement rate. They reported a 69% 1-year patency rate and a 92% secondary patency rate. No procedure- or device-related deaths occurred, and no limb loss occurred. Lagan et al also published a series of nine patients with popliteal artery aneurysms as part of a larger series of femoro-popliteal artery aneurysms.14 He also utilized Wallgrafts and documented a primary patency rate of 56% at 18 months and a secondary patency rate of 67%. He postulated that a good peripheral runoff with patency of at least two vessels was associated with improved graft patency.
Tielliu et al published the first prospective trial of popliteal aneurysm treatment with a self-expanding stent graft.15 Twenty-three popliteal aneurysms in 21 patients were treated with the Hemobahn stent graft over 3 years. Placement of the stent with complete exclusion of the aneurysm was 100% successful. There was a 74% patency at 15 months. Of the five occlusions, two were successfully recanalized resulting in an 87% secondary patency rate. None of the three remaining patients with persisting occlusion required amputation.
Ihlberg et al described the first placement of a covered stent for successful treatment of a ruptured popliteal artery aneurysm.16 He used a PTFE stent graft successfully on a patient with severe pulmonary disease. The patient had no complications.
Finally, Rosenthal et al describe an endovascular-assisted approach to treatment of popliteal artery aneurysms with excellent results.17,18 This method includes performing an in situ saphenous vein bypass with coil embolization of the popliteal artery aneurysm. In a study comparing 12 patients treated conventionally with 10 patients treated with an endovascular-assisted approach, a primary patency rate of 90% was found at 14 months for the assisted approach. All embolized popliteal artery aneurysms were occluded at the time of follow-up. Conventional approach yielded an 86% patency rate at 42 months, but it was associated with a 25% wound complication rate necessitating prolonged hospitalization. More research is required to validate these promising results.
New technology has allowed covered stents to be more flexible and have lower profiles. This has improved the long-term patency rates in other vascular beds and is still being determined for popliteal artery aneurysms. However, the studies show that mid-term and long-term patency rates are lower than that of traditional surgical repair. Drug-eluting stents are now available and will need to be evaluated in the peripheral arterial setting. If the success in the coronary setting translates to a similar success in the peripheral vasculature, higher patency rates can be expected. The introduction of improved antiplatelet medications, such as clopidogrel bisulfate (Plavix, Bristol-Meyers Squibb, New York, NY), has also improved primary patency rates in interventions in other vascular locations. This will also need to be further elucidated for popliteal aneurysmal disease.
The current role for the use of covered stents in popliteal artery aneurysms continues to focus on the treatment of high-risk individuals. This will change rapidly in this progressive and evolving field. However, many roadblocks still remain. The primary difficulty in evaluating the success of endovascular technology on popliteal artery aneurysms is the lack of study population. Even large academic centers may only treat a few patients per year. Oftentimes, only anecdotal evidence or trends are available and studies tend to be underpowered. Despite this fact, endovascular technology continues to show considerable promise in the future management of popliteal artery aneurysms and may well become the accepted standard of care.
Li Sheng Kong, MD, is a Vascular Fellow in the Division of Vascular Surgery, Emory University School of Medicine, Atlanta, Georgia. He holds no financial interest in any product or manufacturer mentioned herein. Dr. Kong may be reached at (404) 727-0914; email@example.com.
Karthikeshwar Kasirajan, MD, is an Assistant Professor of Surgery in the Division of Vascular Surgery, Emory University School of Medicine, Atlanta, Georgia. He holds no financial interest in any product or manufacturer mentioned herein. Dr. Kasirajan may be reached at (404) 727-8407; firstname.lastname@example.org.
Ross Milner, MD, is an Assistant Professor of Surgery in the Division of Vascular Surgery, Emory University School of Medicine, Atlanta, Georgia. He holds no financial interest in any product or manufacturer mentioned herein. Dr. Milner may be reached at (404) 727-8407; email@example.com.
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