Below-the-Knee Intervention: Promises and Reality

Occlusive disease affecting the below-the-knee (BTK) arteries is a common and often serious manifestation of atherosclerosis. It tends to carry a number of significant implications, but causing claudication (in the absence of more proximal disease) is not one of them! So here’s the first important message: claudication is not an appropriate indication for treatment of BTK disease alone. Critical limb ischemia (CLI) is the only acceptable indication for intervention in this setting. Surgical bypass grafting has long been considered the “gold standard” (or “standard of care”) for BTK revascularization. Conversely, percutaneous catheter-based intervention (PTA) has failed to gain the popularity and acceptance it enjoys in other territories. In fact, I would venture to say that the majority of vascular surgeons continue to believe that BTK revascularization remains firmly within the “sacred domain” of surgical treatment. I can think of three reasons (or groups of reasons) that might explain this state of affairs:

1. The fact that distal bypass surgery can and does produce excellent and durable results when using a good-quality autogenous saphenous vein graft, and in the hands of skilled vascular surgeons who are experienced with this type of reconstruction.

2. Interventional shortcomings in the form of a lack of focus on the BTK territory, of which physicians and industry are equally culpable. The result has been, historically, that only sporadic single-center reports and very few substantial series of treated patients with sufficient follow-up have been published. With one possible exception (the BASIL trial1), level-1 evidence is notorious for its absence. The industry’s failure to develop dedicated BTK systems — that take into account the unique needs and characteristics of the BTK arterial morphology and disease — has compounded these shortcomings. It should be self-evident by now that tibial and peroneal arteries are more than just “straight coronaries without motion”.

3. Myths and misconceptions, only loosely based on fact, that are particularly prevalent in the vascular surgical community. And interestingly, on the “flipside”, some of the same ardent defenders of such views fail to place appropriate emphasis on the frequently significant “costs” of limb-salvage bypass surgery in terms of morbidity and mortality, wound healing complications, prolonged recovery, and difficulties in achieving a return to pre-operative baseline functional status for many patients.

The above considerations notwithstanding, limb-threatening BTK disease does represent a significant challenge vis-à-vis endovascular therapy. The extensive nature of the occlusive arterial lesions, the frequent presence of heavy calcification, and small vessel diameters all contribute to such difficulties. Nonetheless, it is abundantly clear today that many such patients can be treated percutaneously, either as a first-line preferred approach, or as the initial treatment strategy that may need to be followed by surgical revascularization in the face of subsequent failure of PTA. Defining “success” (and even “failure”) after BTK revascularization is another important matter worthy of thoughtful consideration. Unlike endovascular treatment of more proximal lesions (such as in the SFA or iliac arteries) where claudication relief and long-term patency are paramount, the ultimate goal of BTK intervention (whether surgical or endovascular) is to preserve limbs and promote healing — with sustained patency at the intervention site becoming less crucial. Measurable indicators of arterial patency (such as direct duplex imaging, and segmental Doppler pressure indices) should perhaps be included in the follow-up evaluation scheme for the sake of completeness, but they rank second in importance and relevance to the above-mentioned clinical endpoints.

Endovascular Intervention for BTK Arterial Disease

As previously stated, interventional efforts have been reported only sporadically over the past 20 years, especially the last 10. It is difficult to draw firm conclusions from available data, with the exception of a recent publication describing results in a randomized clinical trial that compared bypass surgery versus endovascular therapy for patients presenting with CLI from infra-inguinal disease and judged to be candidates for either form of treatment. The authors found comparable outcomes in terms of amputation-free survival, but with increased costs for surgery-allocated patients in the short term.¹ While balloon angioplasty continues to be the common denominator to — essentially — all forms of BTK intervention, there are several other technical and device options (Table 5). Stents have been used only infrequently in infra-popliteal arteries, with PTA bail-out representing the most common indication. As stated, we do not currently have a dedicated BTK stent, but it would be safe to predict the availability of such devices in the near future. A self-expanding (Sx) nitinol platform is likely to emerge as the winner, but balloon-expandable (Bx) stents may retain a role for treatment of focal calcified lesions. Drug-eluting stents (DES) and bio-absorbable devices have been tried and reported, but the amount and quality of available data do not allow for meaningful conclusions or guidelines at this time. It is important to recognize that unlike surgical bypass therapy where even the most extensive occlusions can be treated (provided one can demonstrate reconstitution of at least one target distal vessel at the ankle or beyond), endovascular BTK revascularization is generally reserved for less extensive forms of the disease, i.e., focal stenotic lesions and relatively short occlusions. The presence of heavy, circumferential calcification throughout the BTK territory represents an important limitation to all forms of revascularization, but even more so for endovascular therapy. Hemodialysis patients with end-stage renal disease represent a particular challenge, with dismal outcomes after both surgical and interventional treatment in a large proportion of such cases.

Overview and a Future Perspective

It is undeniable that endovascular therapy has emerged as an important treatment tool for many patients with BTK disease and CLI. But from the outset we should be prepared to discard useless and ill-conceived opinions and “agendas” on either extreme: A. that BTK disease is and should remain surgical sacred territory and no interventional effort should ever be attempted for they are always doomed to failure; and B. that percutaneous approaches are now possible and likely successful for most if not all patients with CLI, regardless of disease extent or severity. As it is often the case, the most reasonable and beneficial approach lies somewhere between those two, but with an emphasis on gradually increasing capabilities on the interventional side, and appropriate assessment of the pros and cons of surgical bypasses — including the significant costs many patients face after the operation. Contemporary vascular surgeons, mainly those with endovascular skills and an ongoing interventional practice, are rapidly evolving in the manner they approach patients with BTK disease, reserving an increasingly important place for endovascular therapy. Needless to say, many interventional cardiologists look upon the BTK territory as an opportunity for catheter-based therapy and are beginning to perform it. We can be sure to see rapid growth in this area in the future, propelled both by an evolving mindset and understanding of the disease and vascular morphology, as well as the development and availability of BTK-dedicated interventional systems.