**2. Materials and methods**

**1. Introduction**

174 3D Printing

research lab.

surgery.

[5–7].

executed plane [12].

cific anatomical structure of a patient.

One of the challenges of orthopedics and traumatology has been recreating a preoperative planning scenario that contemplates 3D space, without the need to use cadaverous specimens, and being able to reproduce it in the real world. Thanks to the advent of new technologies within the computer age, a new research model was born: "in silico" [1], which means "done by computer or via computer simulation." In addition to the phrases "in vivo" and "in vitro" of Latin, which are used in systems biology and refer to experiments done in living organisms or outside living organisms, respectively, "in silico" is translated to "in silicium," which refers to the material from which semiconductors are made, alluding to computer information storage. This creates the concept of turning a virtual computer scenario into a

One of the possibilities of in-silico research and development consists of performing virtual 3D models that faithfully represent reality. In the field of orthopedics and traumatology, this type of tool opens room for many new developments, such as a virtual 3D model based on images of computer tomography (CT) and magnetic resonance that could simulate the spe-

In orthopedics surgery, the precision of the cut when performing a particular osteotomy can have a great impact in the final surgical outcome. For example, in bone tumor resection, the osteotomy should leave free margins outside tumoral contamination but at the same time respect as much healthy bone tissue as possible. Therefore, the use of a simulation scenario to determine where and how to execute an osteotomy with the greatest precision possible would mean a clear advantage when planning and performing this type of orthopedic

In the same way that a global positioning system (GPS) can orientate a person through an unknown path, an intraoperative simulation scenario would be able to guide the path that the cutting saw must follow during a surgical procedure. The surgical planification can be done

However, virtual navigation based on images contemplates a unique point in space, therefore guiding the tip of the surgeon's instrumental through the bone surface. For this reason, it is necessary to mark the planned scheme on the patient's cortical surface and then execute it with a conventional saw under navigated guidance. This makes the level of precision and accuracy questionable, when performing a uniplanar, biplanar or multiplanar osteotomy

Many experiments have been conducted to measure the precision associated with an osteotomy, which has been virtually planned and performed under navigation. Wong et al. were one of the first to report that planned tumor resections were facilitated with the use of intraoperative navigation and that this gives clinical benefits [8, 9]. These advantages were also probed in the computer-assisted surgery of the pelvis and sacrum [10, 11]. Postoperative computer tomography (CT) images of the patient can be superimposed to the original preoperative 3D scenario, allowing digital measuring of the distance between the target plane and

in a virtual 3D model and then executed under virtual navigation [2–7].

The procedure included three distinctive stages: virtual 3D planning, printing a rapid prototyping (RP) model and image-based navigation.

## **2.1. Virtual 3D planning**

The workflow needed to obtain a 3D reconstruction of a patient's anatomical structure can be divided into three phases, which are explained below.

## *2.1.1. Image acquisition phase (computer tomography and magnetic resonance)*

Both a conventional magnetic resonance imaging (MRI) study and a multiknee computer tomography (CT) were performed for an 11-year-old patient. The patient's only symptom was recurrent knee pain, and diagnosis of metaphyseal osteosarcoma in the proximal tibia was confirmed by biopsy. The treatment chosen was transepiphyseal tumor resection and reconstruction of the defect with structural allograft bench [14].

Toshiba CT scanner (Aquilion, Japan) was used and the tomographic acquisition protocol was the following: FOV 32 cm; pixel size 0.625 mm; KV 120; 100 mAs; thickness of cut 1 mm; and height and width of image (512 × 512 pxl). Images were digitized in digital imaging and communication in medicine (DICOM) format. Siemens resonator (Avanto, Germany) was used and the digital resonance acquisition protocol was the following: FOV: 32 cm; pixel size 0.75 mm; thickness of cut 1 mm; height and width of image (256 × 256 pxl); in time T1.

#### *2.1.2. Image segmentation phase*

Once the image files are obtained, the objective is to eliminate elements that look like bone but are not bone. This process, known as image segmentation, is done by establishing a colorimetric assessment. **Figure 1** shows bone tissue represented in yellow, eliminating other elements such as cartilage, muscle, fat, skin or other elements that do not belong to the bone, such as the CT scanner lead. This procedure is performed manually by the operator and determines the final reconstruction of the bone, eliminating structures foreign to the bone tissue that can alter the anatomical form of the bone.

The bone tissue was segmented from CT scan, while the tumor tissue was segmented from MRI. Image fusion was then performed using a mold that overlaps both images in the proper place.

> tumor: one in red (unsafe margin of 3 mm) and the other in blue (safe margin of 3 mm). The tumor was colored in green while the rest of the structure corresponding to healthy tissue was

> **Figure 1.** (a) Magnetic resonance where hyperintense tumor lesion observed in the proximal region of the tibia. (b) Tomography image. The area corresponding to the tumor was painted (segmented) in green, while healthy bone was

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segmented yellow. This process is repeated in each of the cuts.

**Figure 2.** Three-dimensional in-silico planning model printed as RP model.

The printing technique consists mainly of spilling a film of 0.1 mm of dust ZP131 on the base of more dust (flat) of 2 cm of thickness. Next, a liquid adhesive is printed on that thin film leaving the shape of the first layer of the object under study (proximal tibia), which will correspond to a biplane section of 0.1 mm in thickness. This act is repeated consecutively until

Due to the fragility of the newly manufactured piece, the next step consisted of structural fixation with isocyanato, polyol and acetone. Then, both halves were bonded together with Z-Bond 101 glue, as shown in **Figure 2**. The precision of the printed piece was validated by measuring four-known distances in silico and comparing them in the RP model created.

the finished piece is obtained. In this work, the tibia was printed in two halves.

printed in white.

#### *2.1.3. 3D reconstruction and planning phase*

Once the entire volume of 2D is segmented, this volume is transformed into a 3D bone structure within a virtual scenario. By representing all three axes of space [15], this scenario implies an advance in the way of measurement previously used with CT images and 2D MRI, obtaining a virtual 3D bone that aims to reproduce reality. In this way, the bone morphology and the tumor as a structure can be obtained in a virtual space.

#### **2.2. Printing the RP model**

The rapid prototyping model was created using Z-Printer Spectrum Z-510 printer, in 1 hour and 47 min of printing. It was printed in two halves to include within it four colors that define what the tumor is and where the oncological margins are. Two planes were created near the

**Figure 1.** (a) Magnetic resonance where hyperintense tumor lesion observed in the proximal region of the tibia. (b) Tomography image. The area corresponding to the tumor was painted (segmented) in green, while healthy bone was segmented yellow. This process is repeated in each of the cuts.

**Figure 2.** Three-dimensional in-silico planning model printed as RP model.

**2.1. Virtual 3D planning**

176 3D Printing

*2.1.2. Image segmentation phase*

alter the anatomical form of the bone.

*2.1.3. 3D reconstruction and planning phase*

**2.2. Printing the RP model**

the tumor as a structure can be obtained in a virtual space.

place.

divided into three phases, which are explained below.

*2.1.1. Image acquisition phase (computer tomography and magnetic resonance)*

reconstruction of the defect with structural allograft bench [14].

The workflow needed to obtain a 3D reconstruction of a patient's anatomical structure can be

Both a conventional magnetic resonance imaging (MRI) study and a multiknee computer tomography (CT) were performed for an 11-year-old patient. The patient's only symptom was recurrent knee pain, and diagnosis of metaphyseal osteosarcoma in the proximal tibia was confirmed by biopsy. The treatment chosen was transepiphyseal tumor resection and

Toshiba CT scanner (Aquilion, Japan) was used and the tomographic acquisition protocol was the following: FOV 32 cm; pixel size 0.625 mm; KV 120; 100 mAs; thickness of cut 1 mm; and height and width of image (512 × 512 pxl). Images were digitized in digital imaging and communication in medicine (DICOM) format. Siemens resonator (Avanto, Germany) was used and the digital resonance acquisition protocol was the following: FOV: 32 cm; pixel size

Once the image files are obtained, the objective is to eliminate elements that look like bone but are not bone. This process, known as image segmentation, is done by establishing a colorimetric assessment. **Figure 1** shows bone tissue represented in yellow, eliminating other elements such as cartilage, muscle, fat, skin or other elements that do not belong to the bone, such as the CT scanner lead. This procedure is performed manually by the operator and determines the final reconstruction of the bone, eliminating structures foreign to the bone tissue that can

The bone tissue was segmented from CT scan, while the tumor tissue was segmented from MRI. Image fusion was then performed using a mold that overlaps both images in the proper

Once the entire volume of 2D is segmented, this volume is transformed into a 3D bone structure within a virtual scenario. By representing all three axes of space [15], this scenario implies an advance in the way of measurement previously used with CT images and 2D MRI, obtaining a virtual 3D bone that aims to reproduce reality. In this way, the bone morphology and

The rapid prototyping model was created using Z-Printer Spectrum Z-510 printer, in 1 hour and 47 min of printing. It was printed in two halves to include within it four colors that define what the tumor is and where the oncological margins are. Two planes were created near the

0.75 mm; thickness of cut 1 mm; height and width of image (256 × 256 pxl); in time T1.

tumor: one in red (unsafe margin of 3 mm) and the other in blue (safe margin of 3 mm). The tumor was colored in green while the rest of the structure corresponding to healthy tissue was printed in white.

The printing technique consists mainly of spilling a film of 0.1 mm of dust ZP131 on the base of more dust (flat) of 2 cm of thickness. Next, a liquid adhesive is printed on that thin film leaving the shape of the first layer of the object under study (proximal tibia), which will correspond to a biplane section of 0.1 mm in thickness. This act is repeated consecutively until the finished piece is obtained. In this work, the tibia was printed in two halves.

Due to the fragility of the newly manufactured piece, the next step consisted of structural fixation with isocyanato, polyol and acetone. Then, both halves were bonded together with Z-Bond 101 glue, as shown in **Figure 2**. The precision of the printed piece was validated by measuring four-known distances in silico and comparing them in the RP model created.

device generates a 3D coordinate system surrounding a specific area of interest, thus allowing

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After this, the model is ready to be osteotomized with navigation tools. The same tracking procedure was carried out on a cutting saw, with a 1.5 mm thick blade. In this case, an appropriate hook was used to fix the crawler to it, allowing one to see the edge of the saw and its trajectory in real time in the screen of the navigation equipment. In this way, the operator visualizes the virtual plane of the previously planned cut from the scenario in silico and

Once the navigated osteotomy was performed, the piece was opened to verify the correspon-

The segmentation of healthy and tumoral bone tissue was performed. In this way, a 3D piece was built in a virtual scenario, where surgical approaches were evaluated until a final system of osteotomies was chosen. The choice was done contemplating the oncological margins with

The preoperative planning, shown in **Figure 5**, was designed based on this 3D scenario. Two virtual osteotomy planes 3 mm thick were created. This thickness represents the cutting saw and its oscillation. The original coronal plane corresponding to the MRI at T1 was used to determine the distance of the oncological margin. This distance determines the location of the planes. In this way, we can establish a 3D cutting plane that considers the distance between

**Figure 5.** Three-dimensional preoperative planning model "in silico." Healthy bone tissue is represented in gray and tumoral tissue in green. The blue cut plane represents safe oncological margin, and the red cut plane represents unsafe

to guide the navigator by matching the 3D preoperative scheme with the RP model.

directs the saw accordingly, contemplating all three spatial axes.

colors for both distal and proximal osteotomies to the tumor.

the tumor tissue and the necessary oncological margin.

dence of the virtual planning and the RP model.

**3. Results**

oncological margin.

**3.1. Virtual 3D planning**

**Figure 3.** Real-time three-dimensional cut map displayed in the browser during cutting. The instrument colored in green corresponds to the blade of the saw. The safe margin is observed in blue.

**Figure 4.** Saw with recorder. The RP model is osteotomized with a saw, which has a recorder attached to guide the direction of the cut in a navigated manner.

#### **2.3. Image-based navigation**

Once the RP model was prepared, it was fixed to a work table with a vice. The 3D preoperative surgical plan was loaded to Stryker Navigation System II (**Figure 3**). Then, an infrared surface recorder (tracker) was firmly fixed to the RP model with two pins and a label (**Figure 4**). This device generates a 3D coordinate system surrounding a specific area of interest, thus allowing to guide the navigator by matching the 3D preoperative scheme with the RP model.

After this, the model is ready to be osteotomized with navigation tools. The same tracking procedure was carried out on a cutting saw, with a 1.5 mm thick blade. In this case, an appropriate hook was used to fix the crawler to it, allowing one to see the edge of the saw and its trajectory in real time in the screen of the navigation equipment. In this way, the operator visualizes the virtual plane of the previously planned cut from the scenario in silico and directs the saw accordingly, contemplating all three spatial axes.

Once the navigated osteotomy was performed, the piece was opened to verify the correspondence of the virtual planning and the RP model.
