Introduction
Popliteal artery aneurysms (PAAs) occur at an incidence of 7.39 per 100,000 people, accounting for 80% of peripheral artery aneurysms. Typically asymptomatic, they are most commonly found incidentally on peripheral arterial disease screening examinations or when screening is based on the known presence of an aortic aneurysm, which confers a 12%–40% chance of popliteal artery aneurysm occurrence. PAAs may occasionally cause symptoms related to compression of surrounding structures (nerve palsy or leg swelling), but more frequently the symptoms are directly correlated with arterial occlusion in the form of emboli (i.e., blue toe syndrome) or acute limb ischemia from aneurysm thrombosis. Repair is recommended for PAAs ≥2 cm maximum diameter or when symptoms are present. Consideration should also be given to prophylactic repair for smaller aneurysms without symptoms when there is evidence of significant thrombus burden with prior embolization (poor run-off).
Traditionally, PAAs have been treated with open surgical techniques. Repair options include interposition reconstruction with excision of the aneurysmal segment or bypass with proximal and distal ligation of the aneurysm. Both medial and posterior approaches are well described, with the use of autologous vein preferred, but prosthetic conduits perform relatively well compared with its use in occlusive disease. Elective open surgical repair for PAAs is frequently reported, with greater than 80% cumulative patency at 3 years.
Despite the high patency rates for open repair, endovascular stent grafts provide an alternative to open PAA repair, with a greater number of patients undergoing endovascular repair than ever before. Early reports of endovascular PAA repair utilized the Wallgraft (Boston Scientific, Minneapolis, Minnesota), a self-expanding covered stent based on the Wallstent platform. At the time, it was the only available self-expanding covered stent, but it has limitations given its stainless steel base, which gives it limited flexibility and allows for a fair amount of lengthening and foreshortening depending upon the vessel diameter in which it is deployed. With the introduction of newer nitinol-based endoprosthesis, namely the Gore Viabahn (Gore Medical, Flagstaff, Arizona), came the potential of increased flexibility and easier deployment without the concerns of foreshortening, thus opening the door for more widespread use of the technique.
Endovascular repair of popliteal aneurysms has been reported with patency greater than 70% up to 4 years. Secondary interventions are most common for loss of patency, with in-stent stenosis resulting from kinking or in-folding of grafts. Additional reintervention may be required for extension of aneurysmal disease with subsequent endoleaks.
Evaluation for endovascular versus open repair of popliteal aneurysms is based on a variety of factors, including medical risk for anesthesia, baseline functional capacity, patient lifestyle, vascular anatomic characteristics, and acuity of presentation. In general, emergency repairs are a result of thrombotic events and thus are most frequently treated by open repair in our practice. This is not a mandate, of course, because catheter-directed therapies for management of acute thrombosis are being more frequently used and can allow more expeditious clot removal even for Rutherford IIb patients, but this has been our general experience to date. Patient concerns regarding recovery time from surgery may lead toward endovascular repair for highly functional patients needing to return to work quickly or to help care for loved ones. Similarly, older individuals and those with more medical comorbidities are more likely to be preferred for endovascular repair, but this will be balanced with objective evaluation of endovascular candidacy. Although a near infinite subjective discussion can be had regarding personal or patient preferences for when to perform open versus endovascular repair, a more appropriate first question concerns simple anatomic adequacy for repair, which will be discussed in greater detail later.
Specific Concerns
Recognizing the increased interest in an endovascular first approach to treatment, it is important to realize that there is a particular set of challenges for PAA repair. During preoperative planning, we focus first on the anatomic challenges, including length of the distal seal zone and status of the tibial run-off for assessing candidacy for endovascular repair. The necessary landing zone for adequate seal is not well established for popliteal aneurysms. Most would agree 1–2 cm should be adequate, but this can be a challenge for the distal landing zone in particular. There is great variability in the origin of the anterior tibial artery, with many patients having a high origin, which compromises the length of a distal landing zone. If necessary, the anterior tibial artery can be sacrificed and the length extended to the tibioperoneal trunk for a landing zone. This is not without a cost, of course, because it decreases the run-off from three to two vessels and the tibioperoneal trunk is typically a smaller caliber vessel than the popliteal artery, which makes the size match between the distal and proximal landing zones more challenging.
Run-off scoring is an important data-point when comparing patency of lower extremity interventions for occlusive disease. The same is true when treating popliteal aneurysms. Patients with compromised tibial outflow have consistently worse outcomes compared with patients with three-vessel run-off. The outflow may or may not be able to be modified with endovascular techniques, and this must be taken into account when deciding upon open versus endovascular repair. As a general practice, we have not routinely treated tibial occlusive disease pre-emptively with a goal of improving run-off score because the general patency for tibial interventions remains somewhat hindered. This is a consideration that warrants future evaluation because below-the-knee intervention skill sets and tools continue to improve.
When selecting stent sizes, we must recognize that there are no commercially available stent grafts specifically designed for PAA treatment. That said, there can be sizing difficulties when attempting to achieve seal. As with most aneurysmal pathology, 10%–20% oversizing is recommended. An average normal below-knee popliteal artery is 4–6 mm and an average normal proximal superficial femoral artery is 5–7 mm. In the setting of aneurysmal disease, it is not uncommon to have generalized arteriomegaly or borderline aneurysmal below-knee popliteal artery segments. While a traditional open bypass can overcome this with a direct anastomosis, the placement of a self-expanding, oversized stent in an already dilated below-knee popliteal artery raises concerns akin to treating large infrarenal aortic necks. With a size discrepancy between the seal zones proximally and distally, it is best to deploy the smallest stent size first so that a larger stent can be landed inside of this to avoid endoleaks. Another difference between more common lower extremity stenting performed for occlusive disease is the overlap between stents. For occlusive disease, overlap is generally minimized to avoid over-stiffening, with subsequent concern for stent fracture; but, for treating popliteal aneurysm, we need adequate overlap to ensure seal in the same way as when treating aortic aneurysms. This is especially true in very large aneurysms with minimal thrombus, which carries the risk of stent separation if insufficient overlap is used. While there is no well-defined guideline, we extrapolate from aortic aneurysm repairs and aim for at least 2–3 cm of overlap between devices to minimize this risk.
As with all aneurysms, successful repair is dependent upon proper planning and sizing of the vessel. A challenge that frequently arises is the presence of metal artifact on CT scans from prior orthopedic procedures (knee replacements, hip replacements, etc.). This can create too much artifact to evaluate the popliteal artery properly, in particular preoperatively. Additionally, patients with renal insufficiency may not be able to obtain CT angiograms. Angiography cannot reliably identify normal versus aneurysmal vessel because it is effectively a lumenogram. To overcome this obstacle, the best preoperative tool is ultrasound or CTA. Preoperative imaging can be used to identify segments of normal versus aneurysmal artery quite reliably and to measure the diameter of the landing zone for stent sizing ( Fig. 34.1 ). Intra-operatively, intravascular ultrasound can be used for this identification, with the additional advantage of providing both the diameter and the location of the healthy vessel for landing.
