How to Create and 3D Print a Model of the Skull and Orbit for Craniomaxillofacial Surgeons

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This video article offers concise step-by-step instructions for surgeons interested in printing their own three-dimensional models for use in the treatment of cranio-maxillo-facial pathologies.

Abstract

Three-dimensional (3D) anatomical models are used in many ways in cranio-maxillo-facial (CMF) surgery, including being used to press-fit plates, mold splints, and for student teaching. Their use has many advantages, including the possibility of lowering operative time and allowing for more precise reconstructions with personalized plates, meshes, and splints. This can now be done in-house to speed up model availability for trauma surgery as well. Three-dimensional printers and software are quickly evolving—printers now are easily accessible, and the models are inexpensive to print. However, for a surgeon with no IT training, 3D printing even a simple anatomic model may be a challenge. The purpose of this article is to offer simple, step-by-step video tutorials demonstrating the process of extracting a CMF model from a patient CT scan, doing basic manipulation to the model, and then printing it in-house with a prosumer grade 3D printer. It is our hope that this user-friendly article will allow more surgeons and scientists to use 3D printing and its advantages.

1. Introduction

The printing of three-dimensional (3D) models for cranio-maxillo-facial (CMF) surgery is growing in popularity, in part due to their multiple uses (press-fitting plates and meshes, fabricating occlusal splints) [1,2,3] and their many advantages (lowering operating time, increasing accuracy of reconstruction, etc.) [4,5]. The technology behind 3D printing has also become much more user friendly, 3D printers are now affordable and the models themselves are inexpensive to create [1,2,6,7,8]. However, there remains a steep learning curve for surgeons with no IT training.

In this article, we showcase three videos demonstrating a concise step-by-step process for extracting a bony model from a CT scan, applying basic modifications, and 3D printing it in-house. It is our hope that this will make 3D printing more accessible to CMF surgeons worldwide.

2. Requirements

The following software are required, which are available free of charge for non-commercial use: 3D Slicer [9] v4.11 (Slicer community, http://www.slicer.org accessed on 1 May 2022), Autodesk MeshMixer v3.5.474 (Autodesk, San Rafael, CA, USA), and Ultimaker Cura v4.13.1 (Utrecht, Utrecht, The Netherlands) for sending files to filament printers. The 3D Slicer is widely known in the medical community, Autodesk Meshmixer is used extensively in dentistry [10], and Ultimaker Cura is a well-known open-source slicer upon which many other slicers are built.

3. Steps

Having access to a 3D printer is only a small part of the printing process. Creating the patient model to print is currently the limiting factor for most surgeons and scientists. Figure 1 illustrates the overall process of creating a 3D bone model of the skull or orbit from imaging, manipulating, and sending it to the printer.

3.1. Skull Model Creation from a CT Scan Using 3D Slicer

The first part of video 1 [https://youtu.be/v2AgYAjd1qM] demonstrates how to create a simple skull model from a CT scan and export it for use in a 3D software. This process of isolating and selecting a structure (i.e., the skull) is called segmentation. Once the structure is segmented, the three-dimensional model can be displayed and saved.

In the second part of video 1 [https://youtu.be/v2AgYAjd1qM], additional information is provided to allow advanced users to further refine the selection of thin bone as can be found in the orbit. An advanced selection tool is explained as well as a technique to smooth out the remaining defects.

3.2. Model Manipulation Using Autodesk Meshmixer

In video 2 [https://youtu.be/VNA-uCBkADE], the Autodesk Meshmixer is used to correct errors in the generated skull model. Virtual Surgical Planning (VSP) is performed at this step. Here, basic manipulations of the model such as cutting off unnecessary parts and removing loose bone parts are performed. Generally, it is useful to remove unnecessary anatomic regions in order to reduce printing time [1,4,11]. More complex VSP cases can be outsourced to most companies offering CMF plates and screws [1,2].

3.3. Preparing the Model for Printing and Sending It to the Printer (Slicing) Using Ultimaker Cura

Video 3 [https://youtu.be/50n2agH7_2M] demonstrates the slicing and printing process using Ultimaker Cura. To send a 3D model to a filament or resin printer, it needs to be opened in a “slicer” software (not to be confused with 3D Slicer). This software is usually supplied by the printer manufacturer. This software automatically adds supports for printing, configures printing parameters, and sends the necessary instructions to the printer to print the model.

If the model is required in the operating room, it can be wrapped in a sterile plastic bag. Some resin printing materials are certified for sterilization and do not require sterile bag wrapping for usage in the operating field.

The two most common types of 3D printers are filament and resin printers. Filament printers come at a wide range of prices and work by depositing thin layers of plastic in order to form a shape. Generally speaking, the technology used for filament printers is more mature. A variety of plastic filaments exist to create models of different colors, weight, texture, and biomechanical properties. PLA filaments have been shown to the be adequate, reliable, and inexpensive for use in CMF model printing [1,2]. One can expect a relatively high printing reliability [1] with automatically defined settings on prosumer grade printers. An adequate printer usually costs between USD 3000 and 10,000. Although cheaper printers exist, the considerable amount of time required for the assembly and manual calibration of the printer, to find by trial-and-error optimal printing parameters, as well as their high print failure rate make them less than ideal for busy clinicians starting with 3D printing.

At this time, resin printing also appears to be a very promising technology as it often allows for faster printing of complex CMF models. However, resin printing is still a very messy process. It involves manipulating liquid resin, cleaning models with alcohol, and curing the model with UV light. Sometimes, models also need to be “cooked” at a specific temperature to fully cure. This takes a considerable amount of time for a clinician, as well as a large working space with adequate ventilation and protective equipment.

4. Applications

Three-dimensional imaging alone is useful for diagnosis and surgical planning. Three-dimensional printed models such as this one have many additional benefits. They permit a better 3D visualization of defects/fractures than can be seen by coronal, sagittal, and axial cuts on a CT scan. Additionally, these models are useful in the teaching of fellows, residents, and medical students. Once the basics are mastered, more advanced steps allow for broad applications. For example, for a simple unilateral orbital fracture, the contralateral (intact) side can be isolated, mirrored, and printed. Since the models are sized to scale, plates can be pre-bent and sterilized for use [1], decreasing surgical time [4].

5. Limitations of In-House 3D Printing

This method demonstrates the printing of an anatomical model of the pathology. To perform more complex prints, an additional step of virtual reduction and/or osteotomies is necessary. The Autodesk Meshmixer allows for certain basic manipulations, but more complicated virtual reductions and bone movements are often outsourced to software engineers who have access to more powerful software. The VSP models can then be sent back over the internet for printing locally [1,2]. The approval of local regulations, such as FDA [12] or Health Canada, also needs to be taken into account before using in-house 3D printed models for patient care.

6. Conclusions

This article presents a simple and accessible method for in-house 3D printing. We use videos to demonstrate three easy steps to 3D print an orbit. First, we show how to extract images from a CT scan to a slicer software. Then, we demonstrate some simple modifications by virtual surgical planning (VSP). Finally, we show how to export and print a model. As the technology behind VSP and 3D printing evolves and gets more user friendly, it will gain even further uptake by clinicians and scientists.