Nearly one quarter of newborns born with congenital heart disease need to undergo heart surgery or other procedures within their first year of life. Pediatric heart surgery is especially difficult to plan because congenital heart disease encompasses many rare deformations not seen in adults, and there few resources to model them. Realistic physical models of heart valves can help surgeons by allowing them to examine and familiarize themselves with the anatomy of a rare defected valve prior to a scheduled surgery. A surgeon can try several valve repair approaches on the model, and plan the surgery accordingly. The shape of structures can be accurately modeled using direct 3D printing, but currently available soft printing materials may still be too rigid to imitate human tissue. We propose a method for designing 3D-printable molds, which can be filled with a highly elastic material to create flexible and tear-resistant heart valve models.
3D Slicer software is used for designing the 3D-printable mold. Step 1: Segmentation. Segment the heart valve from a 3D image using Segment Editor module. Step 2: Mold design. Create a rectangular prism segment surrounding the valve using Scissors effect. Separate the rectangular prism into two segments to create the top and bottom mold pieces, following the shape of the heart valve. Add air tunnels in the top mold segment to allow air bubbles to escape when filling the mold. Export the top and bottom mold segments as models and save as STL files. Step 3: Mold printing. 3D print the mold models using standard ABS printing plastic. Step 4: Physical model creation. Fill the mold with a highly elastic material such as silicone or PVC and allow to set. Remove the valve model from the mold and trim any excess material. This method was followed to create silicone models of 3 pediatric congenitally abnormal valves.
For simulating heart valve tissue, we created a silicone mixture using Dragon Skin® silicone (Smooth On, Inc., Macungie, PA, USA). Six cardiac surgeons evaluated the heart valve models, practiced suturing on them, and filled out a questionnaire assessing the models. All six surgeons considered the silicone models to be useful for surgical planning. Four out of six surgeons found the silicone material has a realistic flexibility, and cuts and holds sutures well without tearing. For very thin or detailed structures such as heart valve leaflets, manually separating the mold into two pieces took 2-5 hours. To speed up the process, we wrote a module in 3D Slicer to automate mold creation for heart valves. This method can be adapted to create flexible models of various other structures such as liver and brain tumours, and can be automated for a specific structure. Conclusion. Custom molds can be designed in the open source 3D Slicer software to create patient-specific models of heart valves and other anatomical structures, for use in surgical training and planning. Dragon Skin® silicone is well suited to mimicking soft tissue due to its flexibility and resistance to tearing.