Creation of Patient-Specific Silicone Cardiac Models with Applications in Pre-surgical Plans and Hands-on Training

Complex congenital heart disease is a difficult 3D problem. Existing 3D printing technologies fail to closely replicate myocardial tissue properties. This protocol creates models for high-fidelity surgical training.

This technique cost effectively creates patient-specific 3D models from their imaging. Modeling the internal and external anatomy of their organs allows us to intimately know and care for our patients. Creating these models in a material that closely represents anatomical tissue allows this technique to apply to any procedure trainer that includes anatomy.

To begin, open the myocardial case model STL file in a CAD program. In Magics, import the STL files generated through import part two. In the project management window, select the transparent option of model rendering.

Use the left and right mouse buttons to control the rotation and panning tool for adjusting the view of the myocardial base by pointing the apex of the heart down and the aorta arc horizontal. To trim the excess myocardial case material from the model, go to tools, select cut and indicate polyline, then select points of interest and click apply. Similarly, cut the myocardial case into multiple pieces.

Go to cut, select indicate polyline, then select points of interest for making a horizontal cut through the aorta that divides the myocardial case into a lower half containing the apex and upper half, as well as for making two vertical cuts along the widest section of the myocardial case's lower half and upper half. And at last, click apply. To ensure proper alignment during assembly, using a prop generation tool and a Boolean subtraction tool with a clearance value of 0.25 millimeters, create matching props and prop cavities and add them to the myocardial case pieces by clicking add props.

Indicate the position on the model and then click apply. In one of the upper half pieces of the myocardial case, create a silicone fill hole of a diameter of one centimeter and verify hole placement with an SME. Select check diagnostics for checking diagnostics on all case pieces individually to find errors like inverted normal, bad edges, bad contours, near-bad edges, planar holes, or shells in the myocardial case pieces.

If an error is detected, click auto-resolve to repair it. And alternatively, errors can be fixed manually or by using a fixing tool or wizard. If errors cannot be fixed using the alternate option, fix the errors using a shrink wrap tool.

Click on fix, select shrink wrap, and follow the dialog to adjust the shrink wrap sample interval and gap fill values as necessary for correcting the errors on the specific piece without altering the physiology upon SME review. Then save or export the individual myocardial case pieces as STL files. Open the myocardial case and blood pool models in the appropriate slicer software to produce 3D printing files.

Add 3D print supports to all pieces manually or use an automatic support generation tool provided in the software if available. Slice the models to generate G-code for use on the 3D printer with the parameters described in the text manuscript and save or export the G-code. Upload the printing file to the 3D printer.

After ensuring that the correct filament is loaded onto the 3D printer, start printing. When the printing is completed, use the needle nose pliers and tweezers to remove all support material from the printed pieces. Assemble the myocardial case pieces around the blood pool mold, ensuring all pieces fit together tightly.

If the myocardial case cannot fit around the blood pool, make small adjustments to the case mold pieces using a handhold rotary sanding tool to remove material. Next, smooth the surface of the myocardial case using acetone vapor. To do so, line the bottom and sides of a container with paper towels that will not be affected by acetone.

Pour the acetone on the bottom paper towel and allow it to diffuse up the paper towels to the side of the container, but not form a pool on the bottom. Place a piece of aluminum foil in the container to cover the bottom paper towel. Orient the myocardial case pieces onto the aluminum foil so that the faces desired to be smooth are vertical, ensuring that the pieces do not touch one another or the paper towels on the walls of the container.

Cover the container with aluminum foil and allow the myocardial case pieces to remain undisturbed in the container until up to 80%of the desired surface finish is achieved. Once the surface is smooth, carefully remove the myocardial case pieces from the container, touching only the outer surfaces, and allow the pieces to completely degas in a well-ventilated area for about 30 minutes or until the pieces become completely dry and hard. Measure the volume of the myocardium segmentation using CAD software to determine the amount of silicone needed.

Thoroughly agitate part A and part B of the silicone. If color is desired on the model, add pigment and mix all parts and pigment thoroughly, then place a thoroughly mixed silicone into a vacuum chamber for two to three minutes at 29 inches of mercury. When this silicone mixture is completely degassed, remove it from the chamber and submerge the blood pool in the silicone mixture to ensure that all voids and cavities in the blood pool get filled with silicone and the blood pool is coated thoroughly.

In a well-ventilated area, thoroughly spray all pieces of the myocardial case with an easy release product. Assemble the lower half of the myocardial case around the apex of the blood pool. In case of leakage, seal the leak using clamps, hot glue, or clay.

Next, pour silicone into the space between the blood pool and case wall until the assembled myocardial mold pieces are filled. After assembling the remaining pieces of the myocardial case, secure the case pieces tightly using rubber bands and clamps as necessary. When the silicone is solidified, remove the silicone heart from the myocardial case and trim off any silicone seams created from the space between the case pieces or the fill hole.

After identifying all the blood vessels with open ends on the silicone model, trim any silicone covering the vessels to expose the ABS blood pool inside. Submerge the silicone heart in acetone and ABS will begin to soften 10 to 15 minutes after acetone submersion. Once ABS is softened, remove large chunks of ABS with tweezers to increase the speed of the ABS dissolving process.

When most of the ABS blood pool has dissolved, rinse the silicone heart with clean acetone two to three times to remove all the ABS from the silicone. Remove the cardiac model from the acetone and allow the remaining acetone to evaporate from the model in a well-ventilated area. For evaluating the accuracy and functionality of the silicone model, two incisions were made, one in the anterior left ventricle and the other in the posterior right ventricle to visualize the inner anatomy.

The expected ventricular septal defect was present as was the appropriate interior structures and a GORE-TEX patch was sewn into the model by a congenital heart surgeon to correct the ventricular septal defect. Designing the mold is crucial. Make sure that you cut the mold into as many pieces as you need to fit around the blood pool.

This technique unlocks the creation of intricate patient-specific silicone models. We are no longer constrained by 3D printing material limitations. Rather, we are empowered to meet any demand for complex surgical trainers.