To the Editor: In 1996, Parodi et al[1] introduced the technique of fenestrated endovascular aneurysm repair (f-EVAR) for treatment of pararenal abdominal aortic aneurysms. They use physician modified stent grafts (PMSGs) for fenestration, which allows for stent grafts to gain sufficient proximal sealing zones and maintain the blood perfusion of visceral arteries.[2] Stent grafts used in the f-EVAR technique developed from PMSGs to Company Modified Devices (CMDs) and off-the-shelf devices.[3] To improve the precision of PMSGs and shorten the time of stent graft modification, various techniques that assist in the process of fenestration have been researched and developed, including 3-dimensional (3D) printing technique, the Mixed Reality (MR) technique, and the 3D parametric surface planar topological guide plate. This article describes the current status and future development of these techniques.

Currently, there are two kinds of CMDs: Zenith Fenestration (ZEFN) stent grafts produced by Cook and Anaconda fenestration stent grafts produced by Treumo [Supplementary Figure 1, https://links.lww.com/CM9/B943].[4,5] In a single-center study of 100 consecutive patients treated with the ZEFN stent grafts from 2012 to 2017, the success rate was 98%, and the perioperative mortality rate within 30 days was 2%.[6] Another multicenter study reported the perioperative and follow-up outcomes of 127 patients who were treated with Anaconda fenestration endograft for complex abdominal aneurysms. The success rate was 87%, and the perioperative mortality rate within 30 days was 4%.[7] Generally, both of the aforementioned CMDs have good feasibility and safety in treating complex abdominal aortic aneurysms.

Off-the-shelf devices mainly include three kinds of fenestration stent grafts, produced by the p-Branch (Cook, USA), Ventana (Endologix, USA), and WeFlow-JAAA stent systems (Weiqiang, Hangzhou, China) [Supplementary Figure 1, https://links.lww.com/CM9/B943]. Wu et al[8] systematically reviewed studies related to the treatment of juxtarenal abdominal aortic aneurysms using the p-Branch stent and found an acceptable initial postoperative success rate (87%), and the rates of early and mid-term postoperative reintervention that should also not be ignored were 10% and 30%, respectively. Studies on anatomical adaptation of off-the-shelf devices have suggested that nearly 40% of juxtarenal and pararenal abdominal aorta aneurysms do not meet the anatomic criteria for endovascular repair using either of p-Branch or Ventana, which justifies the need for additional designs.[9] The WeFlow-JAAA stent system is still in clinical trials.[10]

A single center retrospective study involving 82 patients treated with PMSGs or CMDs between 2010 and 2016 indicated that the main difference between PMSGs and CMDs is mainly related to the surgical index and reintervention rate. No significant differences in perioperative complications, type I or type III endoleak, or survival were observed between PMSGs and CMD.[11] Another study enrolled fifty-four patients with acute complex aortic aneurysms treated with PMSGs reported a 16.7% in-hospital mortality and 73.2% 2-year survival. The CMDs typically require 6 to 12 weeks to manufacture which cannot be applied in emergency treatment.[12] Overall, PMSGs are better suited for situations such as emergency surgery.[13]

The key point of the stent fenestration technique is the accuracy of the fenestration position. A variety of assistive techniques have been developed for physicians to modify stent grafts accurately and intuitively. Currently, the most widespread and acknowledged assistive fenestration technique in clinical practice is 3D-printed model-assisted stent fenestration. Other physician-modified stent-assisted techniques include MR-assisted stent fenestration and 3D parametric surface planar topology guide plate-assisted stent fenestration.

The first study of 3D-printed model-assisted stent fenestration for EVAR was published by Winder et al.[14] A hollow transparent model of an abdominal aortic aneurysm neck was printed to evaluate the applicability of Cook Zenith stent grafts in patients with short aneurysm necks. Early 3D-printed models are solid and focus on the aneurysm structure; then, hollow models were created with orifices of visceral arteries, enabling surgical simulations to be performed.[15–17] Early the material of 3D-printed models was rigid, which cannot simulate the interaction between the stent and the aortic wall perfectly; then the flexible model was developed that can better guide fenestration.[18] The existing technique of 3D printing model-assisted aortic stent fenestration is relatively mature and has been used in clinical practice with a good effect [Supplementary Figure 2, https://links.lww.com/CM9/B943].[19,20]

The MR technique has been used in orthopedics, plastic, cardiovascular, and many other medical fields.[21,22] Use of MR to complete aortic stent fenestration was first reported by Jiang et al.[23] By employing the original Digital Imaging and Communications in Medicine data of computed tomography angiography (CTA), the authors rebuilt the stent’s 3D visualization model from the patient’s thoracic aortic artery. During the process of stent modification, 3D holographic images of the modified stent were presented in MR glasses worn by the operators through image fusion and spatial positioning. The real stent was fenestrated accurately based on the MR images. Reconstruction of the left common carotid artery and the left subclavian artery was performed, and the patient had no endoleaks or other complications at the 1-year postoperative follow-up.

Fu et al[24] first reported the application of 3D parametric surface planar topographic guide plates in the process of abdominal aortic stent fenestration. Seventeen patients with complex aortic lesions involving important branches with 6 underwent urgent surgery and 11 elective surgery were included. They extracted patients’ abdominal aortic aneurysm model and then designed a planar guide plate based on patients’ CTA images. Intraoperatively, modification of stent grafts was performed under the guidance of the planar guide. The time from accessing CTA images to preparing and sterilizing the guide plate was within 1 h. In above study, one patient died perioperatively; two cases of endoleaks (type Ic and IIIc endoleaks) were found during follow-up. Initial success of 3D parametric surface planar topographic guide plates in abdominal aortic stent fenestration was achieved, but the long-term outcome needs more research.

3D-printed model-assisted aortic fenestration is relatively mature in clinical applications, with a wide range of applications and proven results. However, rigid models cannot simulate the interaction between the arterial wall and the aortic stent well, and deformation of the flexible model may introduce greater errors.[18,25] The direction of further development is to make the model more realistically simulate the stent-vessel interaction. The 3D printing model can also be used in patient, trainee education, and presurgical planning. The MR and 3D parametric surface planar topology guide techniques have advantages in treating emergency patients and algorithmic simulation of stent-vessel interactions is possible. Those will help to improve the accuracy of fenestration. Use of MR can further reduce the financial burden of the treatment process. The stent and hologram created by MR can match freely and adjust the position of the stent compared with the 3D-printed model, which can further enhance the precision of stent modification. The MR technique has the widest range of applications and the best prospects for future development. Guide plates with the lowest technical threshold and fastest fabrication time can be better adapted to emergencies.

All three techniques are currently effective in accomplishing aortic stent fenestration, and all have the potential for further development. More research is needed in the future to compare and validate the accuracy of these three techniques.

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