In a 2008 report from the Boston Scientific EPI: A Carotid Stenting Trial for High-Risk Surgical Patients (BEACH) study (Iyer et al. 2008), a multicenter prospective trial of 480 carotid artery stenosis patients at high risk for CEA in which CWS was used, the procedure success rate was 98.3%, and the failure rate was 1.7%. While the report did not provide detailed reasons for the failures, they may have been due to a high degree of tortuosity, as in the present study. Although stent placement difficulty occurred in 2 lesions (6.5%) in the present study, a different stent was eventually used in 1 of these lesions, so the stent placement failure rate was 3.75%. Both of these lesions had pronounced curvatures with tortuosity ≤90°.
A study by Lin et al. (2005) used tortuosity as an indicator of degree of curvature in CAS based on the angle of a central line running through the lumens of the CCA and the ICA, wherein an angle <30° was classified as mild, an angle of 30-60° was regarded as moderate, and an angle >60° was deemed severe tortuosity. The study found that moderate and severe tortuosities were significantly more prevalent in patients aged ≥80 years, suggesting that the frequency of severe carotid tortuosity is strongly influenced by patient characteristics. Similarly, 1 of the 2 unsuccessful stent placement patients in the present study was ≥ 80 years old (83 years old). However, while Lin et al.’s classification is an indicator of ICA curvature, it cannot be used as an indicator of curvature in CWS placement, because if the stenosed region is not located close to the inflection point, the stent can be placed without having to extend the curvature.
Care needs to be exercised when placing closed-cell stents because, unlike open-cell stents, they undergo shortening after placement and cause the stented blood vessel to straighten out. The manufacturer, Boston Scientific, therefore recommends placing the closed-cell CWS via its distal portion at least 1 cm from the distal end of the stenosed region. Delayed CWS shortening reportedly causes restenosis, because when a CWS is placed in a tortuous lesion, it causes straightening and shortening of the blood vessel, which in turn causes the stent to move caudally such that, particularly when there is a considerable difference between the lumen diameters of the ICA and CCA, shortening is likely to occur in the CCA, which has the bigger lumen diameter (Yoon et al. 2009). Takayama et al. (2011) argued that, in light of this delayed shortening, “longer is better” when selecting the length of a CWS. Since shortening must be taken into account when placing a CWS, in the present study, stent placement failure was defined as the inability to place the stent at least 0.5 of a vertebral body from the distal end of the stenosis. Both of the 2 unsuccessfully stented lesions were type B and had tortuosity ≤90°. Shortening occurred immediately after placement in both of these lesions, with the stent migrating to the distal part of the curve in 1 lesion and downwards to the proximal part of the curve in the other lesion. We think that this occurred because the inflection point was adjacent to the stenosis, so that placing the CWS caused the curve of the blood vessel to extend (straighten), thus preventing the stent from remaining in place.
The limitations of the present study were its small cohort size and retrospective design. Moreover, since the carotid arteries are fixed within the carotid duct, the blood vessels are less likely to expand as a result of CWS placement in the case of high lesions. The distance from the stenosed region to the entrance of the carotid duct may therefore be an influential factor, but this has yet to be investigated. We do believe, however, that evaluating the location of stenosed regions and the extent of tortuosity prior to surgery is a useful way to identify potential stent placement difficulties.
