Since we were using the CT-scans that the LUMC cardiologists provided us with, we had a model of the heart that was more or less anatomically correct. However, the valves were missing from this model. The scan had a resolution with voxels of approximately 1 by 1 millimeter. Because the cusps of the valves and the heart strings are thinner than that, they didn’t show up on the CT-scan. Therefore we had to model them ourselves.
The aortic and pulmonary valves look quite similar. They are both semilunar valves and consist of three leaflets that are shaped like a half full moon, hence the name. These were fairly simple to model in Rhino. We did a test print to see what material combination we could or should use and to show in Leiden. We got some useful feedback. On the first try the connections of the cusps with the artery were located in a single plane, while in reality the leaflets are attached to the arteries in semi-circles. This was quite easily fixed so then the aortic and pulmonary valve could be placed inside the heart. Because the aorta and the pulmonary artery always roughly have the shape of a circle, this model can be used in almost any heart.
The tricuspid and mitral valve were more of a challenge. They are also quite similar, except for one major difference: the tricuspid valve has three cusps and the mitral has two. Therefore the modeling principles could be used for both, but they did result in two separate models. What made it difficult to model were the very thin heart strings that protrude from the edges of the leaflets and connect them with the papillary muscles that are attached to the walls of the ventricles. Every heart is shaped a little differently, especially ones with a cardiac defect. For that reason it was vital that the valve would be easy to adjust and the heart strings could be deformed and their ends dragged to connect to different places. At the same time we had to make sure the cusps and strings were thick enough to print properly. Our test piece showed that the strings and the leaflets required a minimal thickness of 1 millimeter and probably more to increase the durability. Anatomically this would not be entirely correct, but it would still look small in relation to the rest of the heart and show how the connection is made. In Rhino we modeled the leaflets as one piece, because in a real heart they are all connected as well. The strings were created separately and made adjustable with Grasshopper, which we also used to automatically combine all the parts into a closed polysurface so that the valve would be printable.
In the end we had three separate models. One that can be used as the aortic and pulmonary valve, one tricuspid valve and one mitral valve. The semilunar valve can be placed in the heart model by simply scaling and rotating it. The ventricular valves can be put in the right position by deforming the cusps and dragging the heart strings to the right place.