We present a case of a 52-year-old female patient (BMI 29.5 kg/m\(^{2})\) who underwent laparoscopic colorectal resection (T3N1M0) for colonic adenocarcinoma with adjuvant chemotherapy. In follow-up computed tomography (CT) 2 years after primary resection a single metachronous metastasis has been found. The patient has been submitted to laparoscopic right hemihepatectomy. Prior to surgery a 3D model of the patient’s liver has been printed using the technique described below.

The process of model development consists of four major phases: object segmentation, 3D model computer processing in common view, slicing and 3D printing, finishing and assembly with silicone curing (Fig. 1).

Fig. 1 Workflow schema as described in the text. The key element of developing an approach was to process 3D models in a common view; this included dividing large virtual models into smaller, printable parts that were ready to assemble shortly after printing Full size image

Object segmentation

Raw contrast-enhanced, venous phase CT images (152 slices in one image series; scanned with Optima CT660, GE Healthcare; image size 512\(\,\times \,\)512 px; slice thickness 2.50 mm) were saved in a DICOM format. The images were then imported into open-source Horos software. The segmentation of anatomical structures was performed in a semi-automatic approach with pre-built functions available in the software. After using a region-growing, threshold-based algorithm, regions of interest (ROIs) were manually evaluated and corrected. Parameters for segmentation algorithms were chosen manually. Following segmentation, a 3D surface rendering was performed to verify segmented structures, and the desired models were exported as mesh-type, stereolithography (STL) files.

Mesh models were finalized using two free-to-use programs: Meshmixer (Autodesk, Inc., San Rafael, CA, USA) and Blender (Blender Foundation, Amsterdam, Netherlands; open-source software). Processes that were performed using the software included the following: (1) the evaluation and removal of artifacts, i.e., overlapping vessel walls; (2) generating the model manifold (watertight); (3) adding thickness to the walls, which is a necessity in FDM printing; and (4) dividing the models into parts to fit the 3D printer dimensions and to prepare for silicone curing.

3D model processing in a common view

It is necessary to create multiple parts of one object when using this technique (for example, the liver parenchyma) to assemble the model, cast the silicone inside, and demount external parts afterward. Thus, during 3D processing in a common view (parallelly in the same coordinate system), multiple STL ready-to-print parts are created from one initial file, which are later printed and constructed into a model.

Division was conducted in Blender and Meshmixer software programs; both allowed the execution of the same actions, and the choice was based on the user’s preference. Most importantly, vessels were split by a plane cut, with the plane located in the area in which they entered the liver parenchyma (Fig. 2e–g; hepatic vein separated from the inferior vena cava, Fig. 2h). To enable the connection of previously divided parts of the vascular tree, a cylinder was created at the side of the vessel fragment (Fig. 2e), and a matching opening (Fig. 2h) was formed at the other side using Boolean operations. Of note, both a plane cut and Boolean operations are basic functions in 3D modeling that are available in almost every designated software, and their use is straightforward. Those Boolean-based structures protect the correct location of every element after assembly.

Boolean difference operations were also executed on the external parts, and vessel parts that passed through the structure were subtracted from the external parts. This resulted in openings (see Fig. 2a–d) that allowed for assembly in later stages.

Fig. 2 Physical parts of the 3D-printed liver model: 4 parts of the liver parenchyma (a–d) 3 parts of the hepatic veins (e–g), and the inferior vena cava (h). The portal vein (i) and the tumor with a connector as an assembly supporting element (j) are shown. Visible openings for the assembly of vessels on liver contour parts (a–d) can be seen; visible Boolean-based openings (h) and matching cylinder-shaped holders on vascular parts (e, g) are also shown. A 20 cm ruler is provided for scale below the models. Parts in the photos have undergone PLA postprocessing Full size image

Liver contour-defining parts were then designated; in this case, they were divided into 4 segments. Planes of division passed through areas of vessels entering the parenchyma. This type of plane cut allowed the subsequent dismounting of the outer parts (Fig. 2a–d) to expose the final silicone model, which was crucial for the success of this technique.

The tumor, since it is usually not connected to any of the modeled vessels, must be attached to other available structures. A connector, shaped as an elliptic cylinder, was modeled and virtually unified with the tumor (Fig. 2j). Although the shape makes a visible difference, the supporting part can be painted after printing to avoid ambiguity of this solution.

It is necessary to divide the structure into smaller parts for simplicity or due to limitations of the 3D printer; slicing the element with a plane cut in a designated location is recommended. Both portions can later be reconnected using adhesive. In the present model, the middle and left hepatic vein were divided as described (Fig. 2f–g).

Slicing and printing

Prepared models were sliced using open-source Cura (Ultimaker, Geldermalsen, Netherlands) software and printed using colored PLA with a desktop FDM Ultimaker 2+ 3D printer.

The total printing time can range from 60 to 100 h, depending on the size of the model, number of parts, printing accuracy, and type of printer. In this case, printing required approximately 72 h and was executed in 6 print jobs, due to interchanging material and build plate dimensions.

Finishing PLA parts

The printed parts were subjected to a postprocessing stage to maximize the smoothness of the silicone surface. This is a necessary step that prevents cloudiness of the casted silicone parenchyma. First, all of the PLA parts of the liver parenchyma (inner sides) were sanded with 100–300 grit sandpaper. Sanded parts were washed with water and dried for several minutes. This was followed by coating with XTC-3D (Smooth-On, Inc., Macungie, PA, USA), a self-leveling resin. Every part was covered in a thin layer and left to cure for approximately 3 h until the resin dried. Both steps (sanding and coating) were repeated once to ensure that the surface was sufficiently smooth for silicone casting.

If not processed, PLA liver parenchyma parts would appear significantly cloudy at the silicone surface, which would obscure all of the elements inside and render the model useless.

The inner elements (vessels and tumor) were coated with a thin layer of resin for additional cosmetic benefit only, since they do not affect model transparency.

Assembly and silicone casting

The finished physical models (Fig. 2) were assembled as follows:

1. Multipart structures were glued together according to the assembly simulation (Fig. 3) using a common cyanoacrylate-based adhesive (known as Super glue\(^{\textregistered }\)). Orifices and matching cylinder-shaped filling parts were prepared using Boolean-based functions; this preserves the original location, rotation, and proportions of the entire structure after assembly.

Fig. 3 Assembly simulation. All of the parts in the 3D modeling software are shown in a common view. The goal is to determine the order of bonding with cyanoacrylate adhesive. In this study, the bonding order was as follows: a the two liver parenchyma parts; b the portal vein; c the inferior vena cava connected to the right hepatic vein and the tumor; d the third parenchyma part; e the left and middle hepatic vein; and f the fourth and final liver parenchyma part. This exact order was replicated during the assembly of the actual physical model Full size image

2. After bonding all of the parts with cyanoacrylate adhesive, the model was additionally protected with insulating tape and plasticine in the connecting areas between parts. This step prevents silicone leakage during the silicone casting phase, and since it does not affect the model structure, it is safe to use (after this step the model appeared as shown in Fig. 4).

Fig. 4 Assembly phase. All parts were connected with cyanoacrylate adhesive and secured with insulating tape and plasticine to prevent unevenness between connecting surfaces of the outer (red) parts and leaking of silicone (a, b). An opening was drilled at the top of liver model (c, d) to insert a funnel for the addition of silicone Full size image

Fig. 5 Photographs around fully complete liver model Full size image