**1. Introduction**

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 research lab.

A common approach to measure surgical precision is to use the resected specimen, which contains the tumor, obtained from surgery. Possible methods include the histological evaluation of the specimen, which determines the distance between the cut edge and the tumor by microscopy imaging measurement [12]. However, this does not allow comparison with the planned osteotomy, as it only reports the effective oncological margins. Another method consists of CT scanning the surgical specimen and adding it to the preoperative 3D scenario as another 3D piece [13]. The specimen piece location is manually matched against the original bone structure by an operator, obtaining the best possible image registration. The distance

Three-Dimensional Printing and Navigation in Bone Tumor Resection

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The possibility of measuring the surgical precision obtained after performing an osteotomy is a key factor to allow continuous improvement in the field of orthopedic surgery. This is necessary for evaluating the effect of new instruments and surgical technologies, new surgical techniques, or any development that needs to be tested. Moreover, surgeons could experience a better learning curve for navigation systems if they could study the results they obtain.

In this context, the technological advancement provided by 3D printing represents an interesting possibility. 3D printers have become very popular and it is common to find them in clinical research environments. The concept of rapid prototyping (RP) allows to create very specific models based on computer-assisted designs (CAD). In this way, a virtual scenario can be reconstructed for visualizing the bone, the tumor and the planes of osteotomies and then print those structures, giving the surgeon the possibility to have in his hands a 3D model, in real scale, that faithfully represents the patient's situation. As described below, we can use this prototype model to simulate a surgery. The principles of navigation based on images (combining magnetic resonance images [magnetic resonance imaging MRI] with CT studies) can be applied to the RP model [6–8, 10]. In this way, a correspondence between the real structure of the RP model and its 3D reconstruction is obtained in the navigation computer. Finally, surgeons can carry out the reproduction of a virtual plan in a prototyped bone.

**1.** if it is possible to create a 3D model in a virtual scenario based on CT scans and MRI, which can simulate the morphology of the bone structure, the tumor and the oncological

**2.** if it is possible to print a 3D prototype (in this case, a model of proximal tibia) in real scale, which is representative of an oncological patient. This includes contemplating the healthy

**3.** if it is possible to use this prototype to test osteotomies under navigated guidance based

The procedure included three distinctive stages: virtual 3D planning, printing a rapid proto-

margins, obtaining a virtual three-dimensional preoperative planning;

bone tissue, the tumor tissue and the oncological margins;

between the target plane and the executed plane can then be measured virtually.

The main objectives of this chapter are to evaluate:

on images.

**2. Materials and methods**

typing (RP) model and image-based navigation.

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 specific anatomical structure of a patient.

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 surgery.

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

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 [5–7].

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 executed plane [12].

A common approach to measure surgical precision is to use the resected specimen, which contains the tumor, obtained from surgery. Possible methods include the histological evaluation of the specimen, which determines the distance between the cut edge and the tumor by microscopy imaging measurement [12]. However, this does not allow comparison with the planned osteotomy, as it only reports the effective oncological margins. Another method consists of CT scanning the surgical specimen and adding it to the preoperative 3D scenario as another 3D piece [13]. The specimen piece location is manually matched against the original bone structure by an operator, obtaining the best possible image registration. The distance between the target plane and the executed plane can then be measured virtually.

The possibility of measuring the surgical precision obtained after performing an osteotomy is a key factor to allow continuous improvement in the field of orthopedic surgery. This is necessary for evaluating the effect of new instruments and surgical technologies, new surgical techniques, or any development that needs to be tested. Moreover, surgeons could experience a better learning curve for navigation systems if they could study the results they obtain.

In this context, the technological advancement provided by 3D printing represents an interesting possibility. 3D printers have become very popular and it is common to find them in clinical research environments. The concept of rapid prototyping (RP) allows to create very specific models based on computer-assisted designs (CAD). In this way, a virtual scenario can be reconstructed for visualizing the bone, the tumor and the planes of osteotomies and then print those structures, giving the surgeon the possibility to have in his hands a 3D model, in real scale, that faithfully represents the patient's situation. As described below, we can use this prototype model to simulate a surgery. The principles of navigation based on images (combining magnetic resonance images [magnetic resonance imaging MRI] with CT studies) can be applied to the RP model [6–8, 10]. In this way, a correspondence between the real structure of the RP model and its 3D reconstruction is obtained in the navigation computer. Finally, surgeons can carry out the reproduction of a virtual plan in a prototyped bone.

The main objectives of this chapter are to evaluate:

