The left atrial appendage (LAA) is a small, finger-like structure in the heart's left atrium. Despite its small size, the LAA has been shown to affect cardiovascular health significantly. One of the challenges associated with the LAA is that it is its location, an area of the heart with poor blood flow, which can lead to blood stagnation. Patients with atrial fibrillation often experience insufficient contraction of the left atrium, predisposing the LAA morphology to hemostasis and thrombus formation, resulting in an increased risk of cardioembolic events. To prevent these pathologies, oral anticoagulation therapy is typically used as the primary treatment option for patients. However, not all patients are eligible for long-term oral anticoagulation therapy, which can cause complications such as bleeding. These circumstances led to the development of alternative treatment options, such as percutaneous occlusion devices. However, several drawbacks remain. Peri-implant leakage and device-related thrombosis are common complications in LAA closure procedures. Efforts have been made to reduce these risks, but interpatient heterogeneity remains challenging. Ongoing research aims to develop better treatment options for patients with atrial fibrillation and other cardiovascular conditions. One area of innovation is additive manufacturing (AM), also known as 3D printing, which can improve the accuracy of the selection of LAA closure devices. AM allows for the creation of complex and precise structures with high levels of customization, making it an attractive tool for personalized medicine. AM can generate personalized LAA used in medical practice device simulations, reducing the risk of device-anatomy mismatch and improving the procedure's success rate. Additionally, numerical simulation techniques are being employed to model the behavior of LAA, allowing researchers to optimize device performance and minimize the risk of complications.

This study presents a structural numerical approach for analyzing the optimal material for 3D printing a personalized LAA through finite element analysis (FEA). A 3D model of an LAA was obtained from an actual patient's computerized tomography (CT) scan and subsequently modeled numerically. The material selection for computational analysis is crucial to mimic human tissue's mechanical behavior accurately. When designing a cardiovascular training model, the chosen material for the LAA model must withstand the LAA pressures of device implantation. Higher radial resistance often correlates with higher tensile resistance in materials. Mechanical tensile tests were performed to evaluate the radial resistance of multiple materials. Thermoplastic polyurethane (TPU) material exhibited significant deformation during the tests, reaching 40% without breaking, which indicates its potential as a suitable material for replicating the biological tissue of the LAA. The ADINA® software was employed to perform the finite element analysis (FEA). The study yielded a maximum displacement of 0.4 millimeters for the LAA model, demonstrating a close resemblance to a real LAA in FA condition. Therefore, it was possible to conclude that TPU is a potential material to produce the LAA model for pre-procedural occlusion planning. By reducing the risk of complications associated with percutaneous occlusion devices, these innovations can improve the outcomes and quality of life for patients with AF and other cardiovascular conditions.

The range of materials available for 3D printing has been expanding rapidly in recent years. However, for very specific requirements, such as an anatomical model, there is not always a suitable material available. In particular, the design of gradients in elasticity, color, or surface properties is not truly represented by pure materials. Material jetting allows 3D printing of multiple materials simultaneously, resulting in composite materials with new properties. This study investigated and compared the mechanical properties of pure and composite materials and the possibilities of predicting composite properties by knowing the proportions of the pure materials used.

Samples of commercially available materials (VeroClear, RGD8530, Stratasys Ltd., Minnesota, USA) in their pure and mixed matrix inclusion forms were produced using material jetting (Connex3, Stratasys Ltd., Minnesota, USA). The composites had 1mm³ unit cells, including a cubic inclusion with a volume fraction of f_inc=10%, 30% or 45% RGD8530 in a VeroClear matrix. They were mechanically characterized by uniaxial tensile testing according to ISO 527 for Young’s modulus and Poisson’s ratio. In order to find and validate a method for predicting the properties of the multimaterials, multimaterial homogenization and finite element (FE) modelling were evaluated and compared with the measurement results. Inclusion size and geometry were characterized by optical coherence tomography (OCT) and digital microscopy.

The materials had Young's moduli ranging from 800MPa to 2.5GPa. Multimaterial composites were never as stiff as the primary materials' volume weighted average (26.5±2.7% softer for 45% inclusion volume). OCT scans revealed deviations from the digital design, more specifically a rounding of the inclusion edge, as well as blurred material interfaces within the polyjetted layers.

Models that assume ideal interface conditions, such as multimaterial homogenization or conventional FE simulation, are not capable of producing the measured Young’s moduli. A functional simulation model to predict the uniaxial Young’s modulus, can be established using FE simulation and considering a contact stiffness of F_A=2.2TN/m³ and an inclusion edge radius of r=220, as seen in the OCT data.

In conclusion, matrix-inclusion composites have a non-trivial behavior of elastic properties, which needs an adapted model to predict. The established FE model incorporates the interface stiffness between the individual materials used and the geometric deviations from the digital design that occur during the 3D printing process. This enables more complex parts to be produced using less primary material by predicting, designing and 3D printing structures with precisely defined mechanical properties and gradients that are required to mimic biological structures.

Background

Three-dimensional (3D) digital and additively manufactured models are increasingly used for pre-operative planning, especially for orthopaedics applications. However, the size of the model’s minimal detectable features in clinical CT imaging has not been determined, hence it remains unknown which features might remain undetected. Furthermore, it is crucial to identify potential error sources during CT imaging and image processing, quantify their effect on the 3D model accuracy, and develop an optimized workflow to allow for the reliable use of 3D models in the clinical routine. This study aimed to investigate the minimal detectable bone feature size in CT images and corresponding digital 3D models and to determine the errors attributed to different CT technologies, scanners, scan protocols (clinical versus high dosage for improved model quality), segmentation algorithms, as well as specimen orientation and consequently quantify the resulting geometrical deviations on a defined bone fracture model.

Materials and Methods

Incisions in the diaphyseal radii with 200 and 400 μm width, and bone lamellae (bony displacements) with 100, 200, 300, and 400 μm width were generated in twenty paired forearm anatomic specimens (age: 78 ± 8 years (5 male and female, each)). Additionally, a throughout osteotomy was performed in the diaphysis, held in place with additively manufactured guides to simulate a complete 100 μm wide fracture. Specimens were scanned with different CT scanners and corresponding digital 3D models were created. The effects of CT technologies/scanners, specimen positionings, scan and segmentation protocols, and image post-processing settings on feature detectability were assessed. Furthermore, the intra-and inter-operator variabilities were assessed for the segmentation threshold and 3D model accuracy. Three-dimensional reconstructions of surface scans of the physical specimens were used as ground truth to compute the specific geometry deviation.

Results

In CT images, fracture gaps of 100 μm, and bone lamellae of 300 μm and 400 μm were identified at a rate of 80 to 100%, respectively, independent of the investigated settings. In contrast, only 400 μm incisions and bony displacements were visible in digital 3D CT models. Hereby, the detection rate was independent of the scan settings but dependent on the selected CT technology. image segmentation and post-processing algorithms. Intra- and inter-operator variability were fair to excellent for 3D model accuracy (intra-class correlation of 0.43 to 0.92) and mean 3D deviation was < 0.16 mm on average for all operators, using a simple global segmentation threshold and minimal post-processing.

Conclusion

This first systematic investigation of the effect of multiple variables affecting 3D model accuracy demonstrated that sub-voxel imaging resolution was achieved for all variables. Thus, state-of-the-art CT imaging allows for the detection of bone features down to 100 μm. Corresponding digital 3D models still enable the identification of 400 μm features but require verification with the original CT image series.

Acknowledgements

This work has been partially supported by the Austrian Research Promotion Agency with the project “Additive Manufacturing for Medical Research, M3dRES (nr: 858060).

Hallux Valgus, also called a bunion, is a deformation of the metatarsophalangeal joint, more common in adults, with an estimated prevalence of 23% in the 18-65 age group and 35.7% over 65 years. There is a 3:1 relation for females, tending to increase with age [1]. Several reasons contribute to this type of deformity associated with tight shoes and high heels [2]. One of the theories is based on the idea that a continuous overload leads to deformation of the first metatarsal, with stretching of the capsule. The resulting imbalance of the first metatarsal phalangeal joint leads to increased deformity. The condition worsens, forming a bulge on the medial side [3].

The corrective treatment of the Hallux Valgus deformity usually involves surgery, with several surgical procedures reported. However, the Chevron Osteotomy is the procedure usually followed in this type of treatment, consisting of translating a distal portion containing the head of the first metatarsal, produced by the cut in the sagittal plane of a 60º angle of the distal apex. The resulting central exostosis is excised, with subsequent plication of the medial capsule [4]. This surgical procedure is performed manually, without support systems, depending on the result of the surgeon's experience.

This work proposes a new fixation device that allows the blade's positioning, stabilization, and guidance to guarantee precision in the cutting procedures in the Chevron Osteotomy. The development of the device involved the identification of the criteria for positioning the blade and necessary positional adjustments to ensure the best alignment and stabilization of the cutting blade to perform the Osteotomy. The methodology starts with the geometry of foot bones, obtained from a computed tomography scan using the Mimics Innovation Suite^Ò software. Based on the geometry, a modular support device was conceived and 3D modelled (with Solidworks^Ò software) to be anchored in two Kirschen wires to be applied, one in the first metatarsal and the other in the first proximal phalanx.

The device was prototyped, and a protocol for the experimental tests was defined. Artificial bones were produced from the geometry of the bones, considering the cortical and trabecular components. The tests were conducted in the laboratory with an experimental simulation of the Chevron Osteotomy, varying the positional adjustments associated with the technique. The obtained results demonstrate the device's effectiveness with well-defined cuts.

References

[1] Nix S., Smith M., & Vicenzino B. Prevalence of hallux valgus in the general population: a systematic review and meta-analysis. Journal of foot and ankle research, 3, 21. https://doi.org/10.1186/1757-1146-3-21, 2010.

[2] Sharma J, Arora A, Gupta S. Disorders of the toe - Hallux valgus. Textbook of Orthopaedics & Trauma, Vol. 4. Jaypee Brothers Publishers, 2019.

[3] Lerat J-L. Cheville-Pied: L’Hallux Valgus. Traumatologie Orthopedie. Centre Hospitalier Lyon-Sud, Service de Chirurgie Orthopedique et de Medecine du sport, Cap. 6, 2011.

[4] McKean J., Park J. Hallux Valgus. Lineage Medical, Inc. https://www.orthobullets.com/foot-and-ankle/7008/hallux- valgus, janeiro 2023.