Development of a Low-Cost Biomodel of the Subaxial Cervical Spine and its Application in the Training of Instrumentation Techniques

Abstract

Introduction

The training of spine surgeons requires an extensive learning curve, especially in instrumentation techniques. Cadaveric training, while valuable, is expensive and involves ethical and legal challenges. In this context, the use of biomodels emerges as an alternative for anatomical study and training of decompression techniques and spinal instrumentation.

Objectives

The primary objective is to describe a method for creating a low-cost biomodel of the subaxial cervical spine. The secondary objective is to compile the main posterior instrumentation techniques with a step-by-step guide for training on the developed biomodel.

Materials and Methods

A biomodel of the subaxial cervical spine was created based on computed tomography (CT) scan using a 3D printer. The chosen material was acrylonitrile butadiene styrene (ABS). The techniques for making the biomodel were detailed.

Results

The subaxial cervical spine biomodel is a cost-effective and reproducible alternative for training in decompression and instrumentation techniques. Surgeons in training can use the described biomodel to aid in surgical training and planning.

Conclusion

The use of biomodels such as the one described can transform spine surgeons' training by assisting in surgical training and planning. Further studies are needed to confirm the effectiveness of biomodels, particularly in complex spinal surgeries.

Resumo

Introdução

A formação de cirurgiões de coluna exige uma curva de aprendizado extensa, especialmente nas técnicas de instrumentação. O treinamento com cadáveres, além de dispendioso, envolve desafios éticos e legais. Nesse contexto, o uso de biomodelos surge como uma alternativa para estudo anatômico e treinamento de técnicas de descompressão e instrumentação da coluna.

Objetivos

O objetivo primário é descrever um método para a criação de um biomodelo da coluna cervical subaxial de baixo custo. O objetivo secundário é descrever um compilado das principais técnicas de instrumentação via posterior com um guia passo a passo para treinamento delas no biomodelo desenvolvido.

Materiais e Métodos

Um biomodelo da coluna cervical subaxial foi criado baseado em imagens de tomografia computadorizada (TC) utilizando uma impressora 3D. O material escolhido foi acrilonitrila butadieno estireno (ABS, na sigla em inglês). As técnicas para a criação do biomodelo foram detalhadas.

Resultados

O biomodelo da coluna cervical subaxial é uma alternativa econômica e reproduzível para o treinamento em técnicas de descompressão e instrumentação. Cirurgiões em formação podem utilizar o biomodelo descrito para auxiliar no treinamento e planejamento cirúrgico.

Conclusão

O uso de biomodelos como o descrito pode transformar a formação dos cirurgiões de coluna, auxiliando no treinamento e planejamento cirúrgico. Estudos adicionais são necessários para confirmar a efetividade dos biomodelos, especialmente em cirurgias espinhais complexas.

Keywords

biomodel - cervical spine - subaxial - instrumentation

Palavras-chave

biomodelo - coluna cervical - subaxial - instrumentação

Introduction

Cervical spine pathologies are challenging and require a high level of training for surgical success. The training of spine surgeons ideally involves laboratory training, but the use of cadavers is limited due to high costs and ethical and legal issues.

The process of biomodeling and its application in spine surgery was initially described by D'Urso et al. in 1999.[1] Biomodels accurately reproduce biological structures from computed tomography (CT) images that are reconstructed with synthetic materials.[2] They have several applications in the medical field, including patient education, training of health professionals, anatomical study, and preoperative planning.[3] [4]

Some studies have demonstrated that spinal biomodels help with preoperative planning and can lead to better surgical success rates. Preoperative use of biomodels can lead to changes in the decision-making process regarding the choice of osteosynthetic materials and can reduce operating time, especially in complex cases, such as deformity procedures.[2] The development of biomodels enables the reliable reproduction of spinal anatomy, and they have been increasingly used in practice for training and preoperative planning in selected cases.

Despite the increasingly frequent use of biomodels, few studies detail their manufacturing process and production costs. Furthermore, studies detailing cervical spine instrumentation techniques in biomodels are scarce in the literature. Herein, we detail the fabrication process of a subaxial spine biomodel, its fabrication cost, and applications in training instrumentation and decompression techniques.

Objectives

The present study aims to describe the technique for developing three-dimensional (3D) printed models of the subaxial cervical spine, specifically designed for training in decompression and instrumentation techniques. As a secondary objective, we provide a step-by-step guide for the main posterior instrumentation techniques, including lateral mass and pedicle screw placement. The integration of both the model and the guide will facilitate effective learning and skill acquisition.

Materials and Methods

A biomodel of the subaxial cervical spine was created, and the details of the manufacturing process are described below.

To create the biomodel, CT scans from the 3D Slicer software library were used. The DICOM files generated by the CT scans were reconstructed using 3D Slicer software version 4.0. This is an open-access DICOM visualization platform that allows the creation of 3D anatomical models for visualization and printing. The Threshold tool in this software was used to isolate a specific intensity range from the CT scan, which was later reconstructed into a 3D model.

For additional processing of the biomodel, the Islands and Smoothing tools were utilized. The Islands tool allows the removal of small reconstruction artifacts, while the Smoothing tool improves the surface quality, making it more homogeneous.

After reconstruction, the files were processed in Meshmixer (Autodesk), an open-access computer-aided design (CAD) software platform that allows manipulation and editing of STL files in a virtual space. During this process, a base was added to the model to facilitate fixation, optimizing its use in surgical simulations ([Fig. 1]).

Fig. 1 Lateral view of the biomodel supported on its base (A). Axial view of the biomodel and its base (B).

Slicing (preparing the biomodel for printing) was performed using Creality Print software, and the printer used was the Elite K1 Max.

The filament chosen to perform the printing was acrylonitrile butadiene styrene (ABS), as it is the commercial filament that most closely matches the properties of bone.[5] The settings used for printing were obtained from the study by Pradhan et al.[6] The thickness of the impression wall (Shell) corresponds to the thickness of the cortical layer of the bone (1.1 mm), while the infill corresponds to the density of the cancellous bone (25%). The 3D honeycomb fill pattern was used because its spatial configuration most closely resembles cortical bone.[7]

The speed settings and table and nozzle temperatures for ABS printing were adjusted according to the manufacturer's recommendations. Thus, the table temperature was set to 100°C and the nozzle temperature to 240°C. The default print speed was set to 60 mm/s. It is important to highlight that these parameters need to be adjusted for each device, according to the manufacturer's guidelines, to guarantee ideal printing quality, as well as the integrity of the biomodel.

A simulator for instrumentation of the low cervical spine was also developed. It aims to present the available space in the operative field to simulate access to only the spinal elements visible during surgery ([Fig. 2]). This procedure was inspired by the Spine Box project, developed for training lumbar spine instrumentation.[8]

Fig. 2 Fixation platform for surgical simulation (A). Photo of the platform with the biomodel inside (B).

Results

A biomodel that reliably reproduces the characteristics of the subaxial cervical spine was developed. With the biomodel, it is possible to practice subaxial cervical spine instrumentation techniques, including the placement of lateral mass screws and pedicle screws. The poor positioning of the material can be checked, and the surgical technique can be adjusted by the surgeon in training ([Fig. 3]). Decompression techniques, such as laminectomy and foraminotomy, can also be performed on the biomodel, as well as laminoplasty training.

Fig. 3 Side view of the biomodel demonstrating a screw transfixing the transverse foramen, indicating the improper placement of the instrument.

Also noteworthy are the properties of the biomodel, which enable verification of screw positioning without the need for radioscopy or tomography. Additionally, the model is characterized by its ease of fabrication, high reproducibility, and low production cost. The estimated material cost for producing the biomodel is ∼ R$ 10 (∼ USD 1.74 in the current exchange rate), while the complete simulator can be assembled for ∼ R$ 50 (USD 8.70 in the current exchange rate). Both the biomodel and the simulator are freely available for download on the GrabCAD platform at the following link: [https://grabcad.com/library/development-of-a-biomodel-of-the-subaxial-cervical-spine-and-its-application-in-the-training-of-instrumentation-techniques-1.].

The biomodel and its simulator box are licensed under the Creative Commons CC BY-NC license (Development of a biomodel of the subaxial cervical spine and its application in training instrumentation techniques, copyright 2025 by Dr. Pedro Ivo Palhares Monteiro). Download, reproduction, and noncommercial use are permitted free of charge.

We suggest positioning the biomodel as shown in [Fig. 2], fixed to the table with a type-C clamp. The surgeon's position should be similar to that in the operating room, lateral to the craniocaudal direction, operating on the contralateral side. Training in the various subaxial cervical spine fixation techniques should be carried out according to [Figs. 4] [5] [6] [7] [8] [9] [10], which detail the steps for each technique. After the end of the simulation, the biomodel can be removed from the fixation box and its configuration allows the surgeon to check the instrumentation, observing the correct placement of the instrument or any imperfections, as shown in [Fig. 3].

Fig. 4 Riew technique. The entry point into the lateral mass (white circle), is 1 mm inferior and medial to the midpoint (A1). Lateral tilt in posterior view towards the superolateral quadrant of the lateral mass (white arrow) (A2). Lateral view demonstrating the angulation of the screw in the lateral mass (B). Axial view demonstrating lateral angulation of the screw (white arrow) (C).

Fig. 5 Nazarian technique. Entry point into the lateral mass, 3 mm inferior to the upper limit of the facet and 5 mm medial to the lateral limit of the lateral mass (white circle) (A). 10-degree lateral tilt (white arrow) (A2). Lateral view demonstrating the perpendicular inclination to the surface of the lateral mass (white arrow) (B). Axial view demonstrating the trajectory of the screw (black arrow) (C).

Fig. 6 Magerl technique. Entry point (white circle), 1 mm superior and medial to the midpoint of the lateral mass (A1). Side inclination of 20 to 30° (A2). Lateral view, demonstrating the trajectory parallel to the articular facets (white arrow) (B). Axial view of the screw trajectory (white arrow) (C).

Fig. 7 Takayasu technique (transfacet fixation technique). Entry point at the midpoint at the junction of the lower and middle third (white circle) (A1). Lateral angulation (white arrow) (A2). Lateral view of the sagittal angulation of 60 to 80° (white arrow) (B). Axial view of the screw trajectory (white arrow) (C).

Fig. 8 Abumi technique or funnel technique. The entry point is slightly lateral to the midpoint of the lateral mass and close to the lower limit of the articular facet of the cranial vertebra (white circle) (A). The craniocaudal inclination is ideally identified using an image intensifier (white arrow) (B). Axial view demonstrating possible medial inclination angles, ranging from 25 to 45° (C).

Fig. 9 Hacker Technique. Entry point, as lateral as possible, inferior to the facet of the superior vertebra (A). The craniocaudal inclination is identified using an image intensifier (B). The medial inclination angle is similar to the contralateral lamina angle, generally ∼ 50° (C).

Fig. 10 Sliding technique. The target vertebral lateral mass is divided into three equal parts by longitudinal lines, and the ideal entry point is chosen at a point on the lateral longitudinal line and slightly below the inferior edge of the facet joint. B: Using the point described in Figure A, drilling is performed and the "pedicle probe" is inserted; the procedure is halted upon encountering resistance at the cortex of the medial edge of the pedicle ([Fig. 1b]). To overcome this resistance, the probe is rotated 180 degrees, causing its tip to curve outward ([Fig. 2b]). Subsequently, by reducing the medial angle, the probe avoids the medial cortex and can continue advancing towards the vertebral body. At this point, the cortex of the medial edge of the pedicle acts as a sliding guide, facilitating the safe insertion of the probe ([Fig. 3b]). Upon reaching a depth of 15 mm, with the probe's tip reaching the vertebral body, it should be rotated another 180° so that its tip curves inward ([Fig. 4b]). Adjusting the medial angle, the pedicle probe is then continuously inserted until reaching a depth of 20 mm ([Fig. 5b]). To expand the path, the probe is rotated 360° twice ([Fig. 6b]). Following these steps, the pedicle walls are checked, tapped, and the screw is passed through.

The main techniques used for instrumentation of the subaxial cervical spine are described below.

Subaxial Cervical Spine Instrumentation Techniques

Several instrumentation techniques of the subaxial cervical spine have been described. Lateral mass fixation, preferred in our department and most centers, has lower complication rates compared with pedicle screw instrumentation.[9] However, transpedicular fixation demonstrates biomechanical superiority. Therefore, in selected cases such as revision surgeries, osteoporosis, and tumors, the use of transpedicular fixation becomes necessary.[10]

The Riew technique[11] is routinely used in the instrumentation of the lateral mass, being safe and easy to learn. One of its advantages is that it does not depend on memorizing angles ([Fig. 4]). The Nazarian technique[12] is a good option when the incision does not allow, at the C6 level, the angulation for using the Riew technique ([Fig. 5]). The techniques of Takayasu and Magerl (Moon, 2020) have been implemented as rescue techniques in cases of lateral mass fractures ([Figs. 6] [7]). Generally, screws 14 to 16 mm long are used to perform these techniques.

More complex techniques involve the placement of transpedicular instrumentation, which has narrower indications and involves a greater risk of vertebral artery injury or spinal canal violation. The Abumi technique is one of the techniques used to perform transpedicular fixation. Associated with this technique, the funnel technique can be performed. To perform the procedure, at the entry point specified by Abumi, a high-speed diamond-tipped drill is used to drill a funnel-shaped channel. After removing the external surface of the articular mass and creating the channel until the entry of the pedicle, flexibility is obtained to adjust the insertion angle, reduce lateral inclination, and facilitate screw placement. By enlarging and deepening the funnel-shaped canals with curettes, the medial cortex of the posterior part of the pedicle and the pedicle cavity are visualized, making visualization of the pedicle safer, in addition to preventing its violation. After preparing the insertion canal, a specially designed pedicle probe is used, followed by the tap, and, finally, screws are inserted into the pedicle with the aid of a lateral image intensifier to confirm the direction and depth of insertion ([Fig. 8]).[13]

Other options are the Hacker technique ([Fig. 9]) and the Sliding technique ([Fig. 10]). Their limitation is the impossibility of being performed in deformities and conditions that greatly alter the shape of the vertebrae. In this way, the biomodel assists in training techniques, enabling the first steps in the instrumentation of the cervical spine.

Discussion

Three-dimensional printing is a promising innovation, and the first research involving this technology dates to the 1970s. D'Urso et al. were the first to develop a spinal biomodel. The authors studied five complex cases of spinal deformity and enabled more assertive surgical planning.[1] Since then, the use of spinal biomodels has been increasingly studied, and their application is promising in the surgical field.

The literature has demonstrated that training with biomodels increases the surgeon's accuracy in spinal instrumentation, reducing the risk of hardware-related complications. With technological development, biomodels tend to be increasingly realistic and can help in the preoperative planning of complex surgeries, such as tumors and deformities.[2] [14]

Izatt et al. studied the applicability of biomodels in complex spinal surgery. Biomodels were manufactured for 26 patients, including 21 deformity and 5 tumor cases. The surgeons stated that anatomical details were better visible on the biomodels than on other imaging studies in 65% of cases, and exclusively visible on the biomodel in 11% of cases. Changes in the decision regarding the choice of osteosynthetic materials were seen in 52% of cases, and the operative time was reduced by a mean of 8% in tumor patients and 22% in deformity cases. Their study highlights the applicability of biomodels in complex spinal patients.[2]

Other studies have shown the benefits and extensive applicability of spinal biomodels. Parr et al. highlighted the utility of biomodels in spine surgery, including a reduction in fluoroscopic events, reduction of complications, planning of device sizes, and patient education.[14] Araújo Júnior et al. conducted an experimental study demonstrating the applicability of biomodels for surgical laminoplasty training and were the first to describe the use of this technology for surgical technique training on the cervical spine. The authors showed that laminoplasty training is feasible and reproducible with the use of biomodels.[3]

Our proposed biomodel provides a detailed and anatomical study of the subaxial cervical spine, with a focus on posterior approaches. With this biomodel, it is possible to train various instrumentation and decompression techniques. Furthermore, it is low-cost and can be easily replicated. Biomodels can be used for training in challenging cases, which require simulation before the procedure itself, through the reconstruction of the personalized anatomy according to the settings previously described in the methodology.

Some of the advantages of biomodels include low cost, replicability, and anatomical accuracy. However, the production of biomodels presents some challenges. It is necessary to have appropriate software and a quality 3D printer. In addition, new technologies and materials must be developed to faithfully reproduce human bone consistency and ligament structures.

The surgeon in training must master all the techniques described in the present study, since each one has its indications, and the greater the knowledge, the greater the surgeon's repertoire. Mastering the different techniques will influence the quality and surgical results.

Limitations of the Study

Our study has some limitations that should be acknowledged. We describe the creation of a biomodel of only the subaxial cervical spine; the upper cervical spine was not included in our model. Furthermore, we describe a model with normal anatomy, and personalized biomodels should be created for patients with anatomical variations, tumors, or spinal deformities. Future studies are necessary to validate the use of biomodels and better understand their applicability.

Conclusion

The creation and implementation of a personalized biomodel for training subaxial cervical spine instrumentation techniques, together with the description of the techniques, represents an innovative approach to rapid learning of cervical spine instrumentation techniques. Biomodels enable surgeons unfamiliar with these techniques to take the first steps in the long learning curve of cervical spine instrumentation. This technology can significantly contribute to the training of surgeons by providing a more accessible, ethical, and efficient way to acquire skills and knowledge essential for surgical success.

Conflict of Interests

The authors have no conflict of interests to declare.

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Publication History

Received: 04 November 2024

Accepted: 09 September 2025

Article published online:
29 December 2025