3D Printing of Intracranial Aneurysms Using Fused Deposition Modeling Offers Highly Accurate Replications

Discussion

The current analysis shows that aneurysm models produced with FDM offer a high level of accuracy. The method is applicable over a wide range of aneurysm geometries and sizes. After surface finishing, the models are rendered impermeable to water and can be easily incorporated into vascular flow models; the incorporation is facilitated by the digital addition of tube connectors. The resulting setup can be used to familiarize operators with a specific aneurysm geometry or to study the behavior of vascular implants in a given anatomy. Vessel segments <1 mm were not reliably reproduced or were obstructed; this problem may be related to the printer and building material used and the surface-finishing procedure. It may represent a limitation of the FDM technique for manufacturing very small vascular branches and aneurysms. Currently available printers offer layer thicknesses below the 254-μm-layer thickness used in our project, which may allow manufacturing of even small vascular branches that were not patent with our current approach, but the current lower range is approximately 100 μm (eg, 127 μm on the Fortus printer; Stratasys, Eden Prairie, Minnesota). This range may be a drawback of FDM compared with other manufacturing methods, such as stereolithography and material jetting, which offer spatial resolutions of approximately 25 μm. Another limitation of FDM is the relatively slow build speed due to the inertia of the plotting heads and the required frequent changes in direction of the plotting heads.¹⁰

Advantages of FDM include the wide availability and relatively low cost of printers and building materials. One of the most interesting roles for additive manufacturing in interventional neuroradiology is the idea of in-hospital, on-demand production of aneurysm models directly available to physicians involved in aneurysm treatment. Our data show that FDM is principally suitable for this purpose but needs to be further compared with other additive manufacturing techniques for aneurysm model production.

Previously described neurovascular models have commonly used silicone; these models are typically produced by using a dissolvable core and casting liquid silicone around it.^3,5 For patient-specific models, the core can be produced as a solid replica of the vessel lumen by direct additive manufacturing or by producing a mold that is then used to obtain wax replicas of the vessel lumen, which can be subsequently coated with silicone to produce the final model.^5,7,8 This multistep manufacturing process has been reported to last approximately 2 weeks,⁴ whereas direct manufacturing of hollow models by using FDM and surface finishing in our study took approximately 3–6 days. If surface finishing could be avoided, possibly with new building materials, production speed may be further increased. The continuing development of additive manufacturing machines and technologies, such as printing a single object with different building materials,¹¹ will further expand the tools available for producing vascular models and necessitates continuous evaluation of different printers and techniques.

Not all aneurysm geometries could be successfully produced. Particularly small vessel segments with an inner diameter of <1 mm were prone to short-segmental occlusions (Fig 3). Due to the layer-by-layer manufacturing process, vessel segments that run diagonally to the x–y plane of the model are particularly susceptible to these occlusions, while those running parallel to the z-axis should be less affected. Due to the complex geometry of the aneurysm models, occlusions can be difficult to predict. A different potential cause of interior occlusions or alteration of the interior surface is residue after surface finishing. Imperfections of the inner surface can impede wire and catheter movement and promote turbulent flow, which could affect hemodynamic measurements performed in the models compared with the real endothelial surface.

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Fig 3.

Small vascular branch occlusions. Multiplanar reformation from nonenhanced 3D rotational angiography of model ID F (A) shows a short-segment occlusion (arrow) of the lumen. A similar, somewhat longer occlusion was encountered in model ID H (arrow in B).

Another limitation of the current models is the use of a semitranslucent building material. This currently restricts their use to an environment with fluoroscopic capabilities because catheter movement can barely be directly observed through the material; thus, the use of camera equipment instead of fluoroscopy is not currently feasible. However, transparent building materials for FDM are available commercially, and their use for aneurysm models should be further assessed. Other additive manufacturing methods, such as stereolithography, may offer an improved transparency and higher spatial resolution compared with FDM due to differences in optical properties of the resulting models.¹⁰ Another limitation of the presented approach is the rigidity of the models. This means they will not realistically respond to catheter or coil forces, limiting the degree of realism when simulating procedures such as coil embolization. A growing understanding of the forces that occur inside the aneurysm and catheters during endovascular procedures¹² and the mechanical properties of the vessel wall⁵ will help define the required properties for producing elastic aneurysm models with 3D printing.

Several critical parts of neurointerventional procedures depend heavily on the elasticity of the anatomic structures, such as the dislocation of catheters or coils, rupture of the aneurysm, and movement of vessels in the access path. Accurate replication of these properties will greatly increase the degree of realism. Elastic building materials are available for different additive manufacturing techniques including FDM, and their properties should be assessed. A further limitation of our study is its retrospective design with manual selection of different aneurysm morphologies. However, this approach allowed us to assess the feasibility of the technique by using a relatively small sample.

The clinical goal for the development of vascular models is improvement of patient safety by helping physicians acquire and maintain the necessary interventional skills. Apart from using physical models, neurointerventional procedures can also be practiced by using commercially available virtual reality simulators, which generate angiography-like images in a computer simulation controlled by specialized catheters and wires. The advantages of this approach are its great flexibility, broad range of available procedures, and practically limitless repeatability.¹³ However, haptic feedback is limited, and the behavior of catheters, coils, and other implants is simulated; these features severely affect the degree of realism, particularly in critical situations such as dislocation of implants and catheters. Furthermore, each device has to be specifically integrated into the simulation; this step limits the available interventional tools. In this regard, physical vascular models offer the advantage of using real catheter materials with realistic haptic feedback, which allows detailed studies of catheter and implant behavior. To our knowledge, no study exists directly comparing the effects of training for neurointerventional procedures with virtual reality simulators versus vascular models. Further assessment of the way these training environments help develop and maintain procedural skills and evaluation of their impact on procedural duration and complications seem highly promising targets for improving patient safety.

Possibly the most exciting prospect for patient-individual models is the idea of individualized treatment planning. If it could be shown that coils and other implants behave similarly in a 3D model compared with reality, an entire neurointerventional procedure could be performed in vitro to help plan the actual operation. The principal feasibility of this technique has recently been demonstrated by using a patient-specific silicone model.⁴ This approach may offer exciting advances with regard to patient safety. For inclusion into clinical practice, the speed of production and the cost of the models will be highly relevant, which may argue in favor of additive manufacturing over silicone-based model manufacturing. Furthermore, the ability to repetitively “treat” an individual aneurysm model or treat several models of the same aneurysm in parallel with different therapeutic approaches could improve our understanding of the suitability of the plethora of currently available aneurysm treatment modalities and may help in standardizing indications. An increased use of vascular models may also reduce the need for using animals as part of neurointerventional training courses and for a variety of research endeavors.