Three-dimensional (3D) printing has evolved significantly since it was first developed in the 1980s, with improvements in computing power and the development of consumer 3D printers leading to more widespread use. There has been an explosion in the use of 3D printing technology in medicine. We present a brief overview of 3D printing technology and a review of the current and future uses in the field of congenital heart disease (CHD).

3D Printing Process

The typical process for generating a 3D printed model is fairly standard. Using a 3D imaging dataset, typically from CT or MR data, the areas of interest for printing are highlighted, a process called segmentation. While there are some software programs available that can do this in a semi-automated fashion, it is still mostly done manually. Once a segmented model is generated, it needs to be cleaned to fix any errors made during the segmentation process and to ensure that the model is actually capable of being physically printed. The cleaned model is then sent to the 3D printer for printing; depending on the size and complexity of the model, this can take anywhere from 1 to 24 hours or more. There are several different printer technologies available, included fused deposition modelling, stereolithography and selective laser sintering, and there is a wide range in the costs for the printers and materials and various advantages and disadvantages for each.

Current Uses

3D printed models have been shown to be useful in facilitating communication between physicians and patients as well as enhancing medical education for a variety of learners. The use of patient-specific 3D models during consultation with adolescent CHD patients improved their understanding of their condition and may improve follow-up during the transition from adolescence to adulthood.¹ Models have been used as an adjunct to didactic teaching about CHD for nurses, medical students and resident physicians. Most studies have relied on subjective data to identify an improvement in learner understanding and confidence.^2-6 Objective data in the form of pre- and post-lecture board-style tests suggest that 3D models were a useful addition to lectures on vascular rings.⁷

There have been an increasing number of studies showing the utility of 3D printed models in clinical decision making and interventional planning.^8-12 CHD is often associated with complex and unique spatial relationships that can be difficult to grasp from 2D imaging, such as CT, MRI or echocardiography. Double outlet right ventricle is a common lesion for 3D modeling due to the complex geometry between the ventricular septal defect (VSD) and the outflow tracts which can impact the ability to perform a biventricular repair. 3D models have been used to understand the relationship between the VSD and the left ventricular outflow tract for baffling and have been shown to be associated with shorter surgical time, presumably as a result of better preoperative understanding of the anatomy.^8,12,13 Interventional catheterization procedures have also been facilitated by 3D models, including atrial septal device closure in the setting of septal rim deficiency, angioplasty of a pulmonary venous baffle in a patient with the Mustard repair of transposition of the great arteries and device closure of a mitral valve leaflet perforation due to bacterial endocarditis.^10,14,15 These procedures were simulated using 3D models, allowing practice of complicated procedures and ensuring proper device selection before attempting intervention.

Adults with CHD often have complex post-operative anatomy as well as multiple prior surgeries that make re-intervention more challenging and higher risk. Many are also at risk for developing heart failure and could benefit from mechanical cardiac support or heart transplantation, but their anatomy and risk factors may preclude these. 3D printed models can help identify the best approach for implanting mechanical support devices as well as making decisions regarding appropriate donor harvesting for transplantation.^16,17 This pre-procedural information may have the added benefit of shorter surgical times as well.¹⁶

Novel Techniques

Currently, most 3D printed models are generated from single imaging datasets (CT or MR). Newer techniques, described as "hybrid" 3D printing, allow users to integrate imaging data from CT, MR and 3D echocardiography to maximize the benefits from each modality to optimize the clinical data obtained from the models.¹⁸ This technique may allow for generation of more accurate models for surgical planning, particularly surgical valve interventions, but is still quite complicated and time-consuming. There has also been early work utilizing datasets from newer imaging modalities, such as rotational angiography during cardiac catheterization, which appears promising as another source of accurate imaging data for model generation.¹⁹ Work has also been done to try to develop more lifelike 3D printed models for simulation by incorporating multiple materials into one model, such as soft, rubbery material for vessels or valves and harder materials to simulate calcifications.²⁰

An important tool that has been developed is the National Institutes of Health's 3D Heart Library (https://3dprint.nih.gov/collections/heart-library), an online, free access library of digital 3D models that can be downloaded and printed.²¹ The digital models are generated from MRI data from actual patients and submitted by participating institutions. This work is important for standardizing imaging, segmentation and model generation techniques to ensure consistent accuracy, which will be particularly important as 3D printed models become incorporated into routine clinical care.

Current Limitations

There are significant technological limitations that need to be overcome for the field to continue to advance. Several authors have noted that currently there is no consistent, systematic method for segmentation of imaging data to generate digital models for printing.^22,23 Segmentation is a complex, time consuming process without a good semi- or fully automated option. In addition, each step of segmentation and model preparation introduces potential for errors or inaccuracies that could lead to incorrect surgical planning or novel device development.^20,24 Development of semi-automated cardiac segmentation protocols that can be used universally, regardless of the source data, will be a major area for improving 3D printing implementation to speed up the process of virtual and physical model generation. Machine learning has been identified as a potential tool to overcome many of these issues.^22,25 Other issues include a lack of materials that simulate actual cardiac structures and an inability to easily show the dynamic structural changes that occur throughout the cardiac cycle.²³ The ability to produce sterile models that could actually be held and manipulated by surgeons during a procedure could allow for more direct interaction during procedures and potentially provide different views to aid in the intervention.⁸ Other major impediments to adoption that will also need to be overcome are high start-up and maintenance costs and long print times.

The Future

Perhaps the most exciting future use of 3D printing in medicine is bioprinting. Bioprinting of cellular material is a developing method for creating patient specific tissue engineered implants. Tissue engineering has existed for some time, but with 3D printing, more lifelike scaffolds for cells to migrate onto can be created. Currently there is no ideal printer, but industry has taken notice and is working to develop better printers that can provide optimal resolution and speed with low costs and high viability of the printed cells. There is also an increasing amount of work trying to print cells directly into the structures of interest.²⁶ One group has developed a technique using a patient-specific 3D printed mandrel as a guide for creating a cell-free tissue graft that can be surgically implanted. Within 6 months, the underlying scaffold can be almost completely resorbed and replaced with natural tissue that essentially behaves like the normal surrounding vessel.²⁷ The potential to develop vascular grafts that can become replaced with a patient's own tissue is a "holy grail" of congenital heart surgery that could markedly reduce the need for re-operations to account for somatic growth. The same group has also begun work on using their cell-free tissue graft technique to develop personalized Fontan conduits that can be embedded with the patient's own stem cells during the surgical procedure.²⁸ 4D bioprinting is the newest evolution in which printing materials are designed to respond to specific stimuli (such as heat) to alter their shape. This could be a method to allow for growth of implanted structures, such as stents, or eventually bioprinted bones and organs.²⁹

3D printing will play a major role in the future care of children and adults with CHD. It can allow for the ultimate "precision medicine" by tailoring implantable prostheses and interventional devices to the specific needs of each patient's complex congenital cardiac anatomy. Future improvements in the software and hardware and ease of model generation will lead to further adoption and integration into routine clinical care.

References

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Keywords: Heart Defects, Congenital, Bioprinting, Printing, Biomedical Technology, Models, Biological, Computers, Heart Septal Defects, Atrial, Echocardiography, Diagnostic Imaging

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