New operations for truncus arteriosus repair using partial heart transplantation.
Central Message.
Three-dimensional printed heart models are used to contribute new operations for truncus arteriosus repair using partial heart transplantation.
Heart valve replacement has poor outcomes in infants because state-of-the-art homografts do not grow or self-repair. For homograft replacement of semilunar valves in the systemic position, the in-hospital mortality is 40%1 and structural deterioration occurs within months. In the pulmonary position, structural deterioration mandates replacement after an average of 5 years.2 For homograft replacement of truncal valves,3 the infant mortality is 50% to 75%.4, 5, 6, 7 Therefore, there is an urgent clinical need for growing and self-repairing heart valve implants.
We developed a new approach for delivering growing and self-repairing heart valve implants that is based on transplantation. This approach is called “partial heart transplantation” because only the part of the heart containing the heart valve is transplanted. The rationale for partial heart transplantation is that neonates with orthotopic heart transplants reach adult size without the need for reintervention8 because transplanted hearts grow9 and failure of the transplanted semilunar valves is exceedingly rare.10, 11, 12, 13 Partial heart transplantation uses living homografts. Living homografts differ from conventional homografts in 3 important aspects (Table E1). First, living homografts are tissue typed. Second, living homograft ischemia times are minimized. Third, partial heart transplant recipients receive immunosuppression. These differences keep the living homografts viable, allowing them to grow and self-repair. Therefore, immunologically quiescent partial heart transplants might last a lifetime.
Partial heart transplantation is particularly suitable for neonatal truncal valve replacement because Ross pulmonary auto-transplantation is not possible. However, the surgical design space for partial heart transplantation in truncus arteriosus remains unexplored. We hypothesized that 3-dimensional (3D) printed heart models can be used to design new operations for truncus arteriosus repair using partial heart transplantation.
Materials and Methods
The 3D printed models (Figure E1) of structurally normal hearts and hearts with truncus arteriosus were purchased from the Hospital for Sick Children in Toronto. Briefly, electrocardiographically gated computed tomography scans were used to acquire imaging data that were postprocessed for 3D modeling using threshold-based segmentation and computer-aided design processes.14, 15, 16 The models were then printed using a PolyJet Photopolymer (TangoPlus, Stratasys Ltd) that emulates cardiac tissue and is validated for use in surgical simulation.14 Operations for partial heart transplantation were developed in multiple iterations to identify and refine critical surgical steps (Figure 1).
Figure E1.
Computer-aided design and 3D printed high-fidelity polymer models of a structurally normal heart and a heart with truncus arteriosus.
Figure 1.
For the donor operation, a structurally normal heart model was dissected to excise the cardiac outflow tract en bloc. For the recipient operation, a heart model with truncus arteriosus was used. The coronary buttons were harvested, and the truncal root was excised. The ventricular septal defect can be closed with a patch or donor ventricular septal tissue. The donor graft and coronary buttons were then implanted.
Results
Operation 1 involves transplantation of the cardiac outflow tract en bloc. The donor heart is procured in the usual fashion. On the back table, the living homograft containing both the aortic and pulmonary roots is dissected (Figure 2). The recipient ascending aorta is divided, and the truncal valve is interrogated to confirm that it is not repairable. The donor living homograft (Figure E2) is then used to replace the truncal valve.
Figure 2.
Diagrammatic representation of operation 1 (top row) and operation 3 (bottom row).
Figure E2.
En bloc dissection of a piglet cardiac outflow tract for operation 1.
Operation 2 involves separate transplantation of the aortic and pulmonary roots. The donor living homografts are separately dissected like conventional homografts.17,18 The implant operation resembles a double root replacement with conventional homografts.3,19
Operation 3 involves subcoronary implantation of the aortic valve. The donor living homografts are dissected as for operation 2. The donor aortic valve sinuses of Valsalva are resected, leaving just the heart valve. The recipient operation involves excision of the dysfunctional truncus valve and implantation of the living homograft in a subcoronary position (Figure 2). Continuity of the right ventricle and pulmonary arteries is then established using the pulmonary living homograft.
Operation 4 involves preservation of the native truncal valve and use of a living homograft as a growing right ventricle to pulmonary artery conduit using standard surgical techniques.
Discussion
Three-dimensional printed models have been used for surgical planning,20, 21, 22 surgical training,16 morphology teaching, and patient education.14 We use 3D printed models to develop new operations for truncus arteriosus repair using partial heart transplantation. The key advantage of this approach is that high-fidelity 3D printed models of hearts with truncus arteriosus are readily available. In contrast, access to human specimens with truncus arteriosus for surgical research is limited and there are no large animal models for truncus arteriosus. Therefore, only donor operations can be evaluated using natural tissue (Figure E2). The major challenge in using 3D printed models is that commercially available materials suboptimally simulate natural tissue elastic properties and strength.16 As a result, the 3D printed tissues do not stretch well and poorly hold fine Prolene sutures.
The proposed operations have distinct indications. Operations 1 and 2 are new treatment options for neonates with unrepairable truncal valve dysfunction. Operation 3 avoids coronary reimplantation but is only suitable for children with a sufficiently large native truncal root that accommodates subcoronary implantation of the living homograft valve. Operation 4 offers a growing right ventricle to pulmonary artery conduit for neonates who do not require truncal valve replacement. Additionally it is possible to repair the truncal valve using living homograft valve cusp tissue (Operation 5), but the available 3D models are not suitable for simulating surgery of the truncal valve itself.
These operations have important advantages over orthotopic heart transplants. First, orthotopic heart transplants invariably fail from ventricular dysfunction over time,8 whereas partial heart transplants do not include the ventricle. Of note, semilunar valve dysfunction of orthotopic heart transplants is exceedingly rare,11,12 and the semilunar valves are spared in fulminant rejection of orthotopic heart transplants.13 Second, the donor pool for living homografts is larger than for orthotopic heart transplants because hearts with low ventricular function and slow progression to donation after cardiac death are viable sources.23 Third, stopping immunosuppression would simply turn the living homograft into a conventional homograft.24
Conclusions
We used 3D printing to design new operations for truncus arteriosus repair using partial heart transplantation. To our knowledge, this is the first application of 3D printing to design new operations.
Footnotes
This research was supported by grants to T.K.R. from the American Association for Thoracic Surgery, the Brett Boyer Foundation, the Saving Tiny Hearts Society, the Emerson Rose Heart Foundation, the South Carolina Clinical & Translational Research Institute (NIH/NCATS UL1 TR001450), NIH/NHLBI R41HL169059-01 and Philanthropy by Senator Paul Campbell.
Disclosures: The authors reported no conflicts of interest.
The Journal policy requires editors and reviewers to disclose conflicts of interest and to decline handling or reviewing manuscripts for which they may have a conflict of interest. The editors and reviewers of this article have no conflicts of interest.
Appendix E1
Table E1.
Comparison of orthotopic heart transplants, living homografts, and homografts
| Orthotopic heart transplant | Living homograft | Homograft | |
|---|---|---|---|
| Graft | Heart | Valve | Valve |
| Tissue matching | Yes | Yes | No |
| Ischemia minimized | Yes | Yes | No |
| Immunosuppression | Yes | Yes | No |
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