**3. Results**

**Figure 4.** Saw with recorder. The RP model is osteotomized with a saw, which has a recorder attached to guide the

**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.

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

direction of the cut in a navigated manner.

**2.3. Image-based navigation**

178 3D Printing

#### **3.1. Virtual 3D planning**

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 colors for both distal and proximal osteotomies to the tumor.

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 the tumor tissue and the necessary oncological margin.

**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 oncological margin.

**3.3. Image-based navigation**

**4. Discussion**

its corresponding oncological margin planes.

blue osteotomy plane, which corresponds to the safe margin.

cases the unsafe margin, colored in red, maintained all its integrity.

proximal tibia presenting osteosarcoma, for comparison.

The surgeons were able to reproduce the preoperative plan on the RP model by using a conventional saw under intraoperative navigation guidance. The procedure was validated by opening the piece and confirming its correlation with the previous biplanar osteotomy planned in silico. This is represented as the distance between each cut edge of the piece and

**Figure 7.** (a) Osteotomized RP model. (b) Pathological anatomy of tibial osteosarcoma. The similarity between the pathological anatomy and our RP model in colors can be noted, as well as the accuracy of the cut coinciding with the

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The distal plane showed correspondence with the safe margin limit of the planned plane, not reaching the red plane (unsafe margin) at any point. The proximal plane obtained was parallel to the planned plane but showed a translation of 1 mm toward the healthy tissue side. In both

In **Figure 7(a)**, we can see both halves of the RP model after osteotomies were performed, identifying clearly where the margin was greater than planned for the proximal plane and how the distal plane was correctly executed. In **Figure 7(b)**, we can see the real specimen of

Although the combination of preoperative planning with image-based navigation has already been used in other areas such as maxillofacial surgery [16], spine surgery [17] and cardiac surgery [18], among others, no work could be found in which RP models were cut with saw as part of the training for surgical navigation. As a matter of fact, one technical question that was present before doing this work was whether the RP model could withstand the oscillation of an orthopedic saw without being destroyed. In this experience of only two cases, we probed that the behavior of the material maintained its structure until the end of the experiment.

**Figure 6.** Validation distances.


**Table 1.** Measurement of distances to validate the RP model's precision.

#### **3.2. Printing the RP model**

By printing the bone in two halves, we were able to create a solid structure that corresponded in size and shape to what was observed in our virtual scenario.

The validation of said procedure was carried out by comparing the measurement of four known distances in silico to their measurement in the RP model (**Figure 6**). These differences are stated in **Table 1**.

The average relative error, considering the virtual measurement as true measurement, is of 0.9469%. The level of error is considered tolerable for this application; therefore, it can be concluded that the prototyping method shows good reproduction of patient's structure in real scale.

**Figure 7.** (a) Osteotomized RP model. (b) Pathological anatomy of tibial osteosarcoma. The similarity between the pathological anatomy and our RP model in colors can be noted, as well as the accuracy of the cut coinciding with the blue osteotomy plane, which corresponds to the safe margin.

#### **3.3. Image-based navigation**

The surgeons were able to reproduce the preoperative plan on the RP model by using a conventional saw under intraoperative navigation guidance. The procedure was validated by opening the piece and confirming its correlation with the previous biplanar osteotomy planned in silico. This is represented as the distance between each cut edge of the piece and its corresponding oncological margin planes.

The distal plane showed correspondence with the safe margin limit of the planned plane, not reaching the red plane (unsafe margin) at any point. The proximal plane obtained was parallel to the planned plane but showed a translation of 1 mm toward the healthy tissue side. In both cases the unsafe margin, colored in red, maintained all its integrity.

In **Figure 7(a)**, we can see both halves of the RP model after osteotomies were performed, identifying clearly where the margin was greater than planned for the proximal plane and how the distal plane was correctly executed. In **Figure 7(b)**, we can see the real specimen of proximal tibia presenting osteosarcoma, for comparison.
