aortoiliac stenting, how blood flow and stents interact
This thesis describes several subjects related to the subject of AIOD. AIOD arises from stenotic lesions that form near the aortic bifurcation, including the distal aorta and the proximal iliac arteries. The chapters in this thesis address both clinical subjects as well as subjects that combine biofluidics, medical insights and their clinical implications. The goal main goal is to form better substantiated choices in the treatment of AIOD to improve the long-term outcome.
The prevalence of PAD, of which AIOD is a subgroup, in the Netherlands in the population above 55 years old is estimation to be 19.1 %. This is based on a sub cohort of the Rotterdam study (1.). In the population of PAD patients only 6.3 presents with symptoms like IC symptoms, which in turn influence walking distance. When the walking distance drastically decreases this can become very incapacitating. When treatment is indicative a minimal invasive treatment option is often the standard of care. With this treatment option the vessels are treated from the inside (endovascular).
In the first chapter of this thesis the currently available literature on the treatment of AIOD is reviewed. The gold standard for AIOD treatment, when using endovascular techniques, is the KS method. Using this method two stents are deployed simultaneously in both iliac arteries, the stent abut or ‘kiss’ in the distal aorta, hence the name ‘kissing stents’. The techniques renders reasonable patency results with respect to invasive option of open surgery. However, after two years the patency results of the kissing stents are 15 % lower with respect to the results obtained with open surgery. Furthermore, a large diversity in the choice of treatment protocol was observed. The location of deployment of the stents (with respect to the aortic bifurcation) differs as well as the type of stent. Due to the fact that the IPD was not available we were not able to draw firm conclusions and we could not give recommendations with respect to standardization of the treatment protocol. To overcome these limitations the IPD was requested from all studies included in the review. Five authors committed to this request, supplying the data for 605 patients treated for AIOD with the KS technique. Analysis of this dataset showed no influence of stent type or other choices in the treatment protocol on the primary patency. Splitting the cohort in patients treated before and after 2005 showed that there is a significant improved in patency for patients treated after 2005.
During the following chapters we compared the geometry of different stent configurations when these are placed near the aortic bifurcation. Previous studies showed that that the geometry influences the patency. Area’s between the vessel wall and the stent (coined mismatch areas) might cause blood flow perturbations triggering a cascade of thrombus formation, neointima hyperplasia and finally restenosis or reocclusion. To resolve these mismatch area’s the CERAB technique was introduced. With this technique three instead of two stents are used, the third stent is placed in the distal aorta, creating a funnel (or cuff) to guide the blood flow in the two iliac limbs.
However, the exact differences between the geometry of the stents is not clear. Using in vitro models, making the configurations anatomy independent, we showed that the CERAB significantly decreases the mismatch area’s with respect to the KS configurations. To optimize the mismatch minimization it is of utmost importance to focus on the placement of the limbs inside the cuff. This study was also repeated in two groups of patients, treated either with the KS technique or CERAB technique. The results also showed a significant difference between the CERAB and KS technique, with respect to mismatch area. However, the differences were not as large as observed with the in vitro results. This shows that optimal deployment of the CERAB in vivo is still challenging.
Having identified the geometric differences between the KS and CERAB technique we shifted our focus to the interaction between the stent configuration and blood flow. The in vitro models were incorporated in an experimental setup to simulate blood flow in the infrarenal aorta and the iliac arteries. Using laser based particle image velocimetry, high resolution (both spatial and temporal) measurements were performed to obtain vector velocity fields near the in and out flow of the stent configurations. Results show that large areas of flow recirculation arise near the inflow of the covered kissing stent configuration during the whole cardiac cycle. These recirculation areas increase the risk of thrombosis and the wall shear stress is decreased. Low wall shear stress can cause in neoplasm the vessel wall. The bare metal stent configuration did not have recirculation at the inflow, however, between the neobifurcation and the anatomic bifurcation stasis of blood occurs. This may cause thrombus forming, as was observed bij Saker et al. (2.). In case of the CERAB configuration, no recirculation was observed near the inflow, small recirculation of blood was observed near the neobifurcation, wall shear stress levels are comparable to the control model.
Thereafter we investigated the outflow of the CERAB configuration using our in vitro setup, because clinical results showed that most graft failure is observed at the outflow, most likely due to distal stenosis. This study showed that, independently of distal stenosis severity the wall shear stress was two times lower with respect to the control situation. The low wall shear stress is probably related to the more developed outflow profile of the CERAB configuration, giving rise to a less steep velocity gradient. When introducing a significant distal stenosis, the wall shear stress is three times lower than the control situation. Thus, in spite of the favorable inflow characteristics, the outflow should be monitored closely during FU, based on both the clinical and experimental results.
Apart from investigating the local blood flow the clinical performance of the CERAB is also of importance. The last chapter of this thesis presents the 3-year follow-up results of patients treated with the CERAB technique in both the Rijnstate ziekenhuis Arnhem and the Ziekenhuis Netwerk Antwerpen. We showed that the 3-year primary patency is 82.1 %, the secondary patency is 97 %. These results are promising, given the fact that most lesions treated are TASC D lesions. In comparison to the KS technique a margin of 5.5 % in patency is gained after three years (76.6 % vs. 81.1 %). The complication rate of the CERAB technique is in the same range as the KS technique in case of AIOD treatment.
REFERENCES
(1.) Meijer WT, Hoes AW, Rutgers D, Bots ML, Hofman A, Grobbee DE. Peripheral arterial disease in the elderly: The Rotterdam Study. Arterioscler Thromb Vasc Biol. 1998;18(2):185-92.
(2.) Saker MB, Oppat WF, Kent SA, Ryu RK, Chrisman HB, Nemcek AA, et al. Early failure of aortoiliac kissing stents: histopathologic correlation. J Vasc Interv Radiol. 2000;11(3):333-6.
