3D-printed aortic stenosis model with fragile and crushable calcifications for off-the-job training and surgical simulation

Surgical simulation devices can be helpful and cost-effective adjuncts to on-the-job training. In this tutorial we present our method for creating an aortic stenosis model with realistically fragile and crushable calcifications, using modern 3D-printing techniques.  The model can be used for training and surgical simulation and is an effective aid to learning for young cardiovascular surgeons.   

3D-printing technology has found a number of applications in the treatment of various cardiovascular diseases [1-5]. 3D-printed models are especially useful for anatomical assessment, because they enable us to observe, touch, feel, and understand 'dimensional' characteristics or irregularities in anatomy through our sensory system. However, 3D printing has not yet been used to simulate functional characteristics of diseases, which can be crucial to the proper comprehension of the pathology in question. 

In order to help our trainees better understand the relatively common disorder of aortic stenosis (AS), we constructed a 3D model with fragile and crushable calcifications - a functional replica of calcification.

To create our model we used computed tomography (CT) images of a 71-year-old woman before aortic valve replacement for her AS. 

The shapes of soft tissues and hard calcifications were extracted from the preoperative CT images. The 3D data were sent to a multi-material 3D printer that was able to use both a rubber-like material and a hard engineering plastic. The artificial calcification was made to have a crushable structure, specially created by piling 0.75-mm particles triangularly, in order to accurately simulate real calcifications. 

Our 3D model is used by young cardiovascular surgeons for training in valve resection and decalcification. Because the simulated calcification is fragile and can be crushed with surgical instruments, delicate techniques are needed to remove it. An ultrasonic decalcification device can also be used, as in a real operation. The 3D model effectively mimics the touch and feel of the calcification found in aortic stenosis. 

We were also able to use the model for an endovascular simulation using a balloon to simulate transcatheter aortic valve replacement (TAVR). In the simulation described here, the calcifications and valve leaflets were shown to be pressed into the Valsalva sinus without obstructing the coronary ostium.
 

We used computed tomography (CT) images of a 71-year-old woman after aortic valve replacement for her AS.

Discussion
Calcifications and their fragility are a key challenge for the surgeon in the treatment of AS. In some patients there are huge clumps of calcium, whereas in others there are sheets of calcium involving the valve and aortic wall; calcium deposits may also be friable and sand-like. 

The feeling of our calcification was similar to that of soft calcification in a human body. Different structures and materials will need to be used  to simulate different types of calcification. For instance, our calcification could be made tougher by enlarging the particle size to 0.76, 0.77, or 0.78 mm and keeping the grid interval within 0.75 mm.

The expanding balloon catheter simulation was an important part of the training procedure, but we feel there are some limitations when applying this simulation to real surgical situations. Every patient is different. If the calcification stiffness of the patient could be measured noninvasively by an imaging examination, such as CT or echocardiography, we could then  replicate the patient's valve preoperatively to simulate expandability of the valve and its outflow tract for TAVR. Although the calcifications kept their shapes during this endovascular simulation, a wider calcified lesion might be cracked by the ballooning stress.

Mitral annular calcification (MAC) is another calcium-based lesion. In mitral valve replacement, MAC can lead to harmful complications, such as left ventricular rupture. We believe our 3D AS model with fragile, crushable calcifications is a useful  adjunct to off-the-job training and it also offers safe simulation of pathology for individual patients in a clinical setting (Figure 1).

New idea
Our original aim was to design artificial calcification. We created a special structure by piling 0.75-mm particles triangularly to make gaps between the particles (Figure 2A, B). This structure made it possible to  change solid calcification into a fragile and crushable substance , even though it was made of a hard, tough engineering material. 
We used 3D graphics software (Meshmixer version 3.3.15, Autodesk Inc., San Rafael, CA, USA) to design and optimize the structure, using its functionality to make a pattern out of the solid calcification data with 0.75-mm spheres in a hexagonal grid.

We used Objet500 Connex3 printer (Stratasys Ltd., Eden Prairie, MN, USA) to create 3D AS models, because the printer was able to use both an engineering plastic (VeroWhitePlus) for the calcification, and a rubber-like resin (Agilus30) for the other soft tissues simultaneously. The final 3D polygon data in Standard Triangulated Language format was sent to the 3D printer of a printing service company.

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The authors would like to acknowledge technical support with 3D polygon data and printers from Japanese Society of Cardiovascular 3D-modeling for Medical 3D Print (J3D, Osaka, Japan), L&L Co., Ltd. (Osaka, Japan) and Yasojima Proceed Co., Ltd. (Osaka, Japan). The authors also gratefully acknowledge assistance from the expert radiographers of Kansai Rosai Hospital and Sakurabashi Watanabe Hospital.

This work was supported by a research grant from the Japan Organization of Occupational Health and Safety [15C083g to Takashi Shirakawa]. This is a cooperative study with Japanese Society of Cardiovascular 3D-modeling for Medical 3D Print (J3D), Osaka, Japan.

Authors
Takashi Shirakawa^{a, b}, Masao Yoshitatsu^a, Yasushi Koyama^c, Hiroki Mizoguchi^a, Koichi Toda^b, and Yoshiki Sawa^b

Author affiliations
^aDepartment of Cardiovascular Surgery, Kansai Rosai Hospital, Amagasaki, Japan
^bDepartment of Cardiovascular Surgery, Osaka University Graduate School of Medicine, Suita, Japan
^cDepartment of Diagnostic Radiology and Cardiology, Sakurabashi Watanabe Hospital, Osaka, Japan

Corresponding author
Takashi Shirakawa
Department of Cardiovascular Surgery, Kansai Rosai Hospital
3-1-69 Inabaso, Amagasaki, Hyogo, 660-8511, Japan

Phone: +81-6-6416-1221
Email: tkshirakawa@gmail.com