In-House Virtual Surgery Planning and 3D Printing for Head and Neck Surgery with Free Software: Our Workflow

Abstract

Study Design: This study explores the workflow of in-house computer-aided design/manufacturing (CAD/CAM) in head and neck oncologic surgery, focusing on 3D printing of biomodels and cutting guides. Objective: We aim to describe a validated workflow for an in-house 3D Printing Department within a level III University Hospital of the Spanish Public Health System using free software. Methods: Our study outlines a cost-effective and time-efficient workflow utilizing free software for 3D printing. We assess the feasibility of establishing an in-house department compared to outsourcing to the biomedical industry. Results: The study demonstrates that creating an in-house 3D Printing Department in a hospital setting is a viable option. We discuss the advantages, including cost savings and reduced lead times, as well as the limitations of this approach. Conclusions: In conclusion, establishing an in-house 3D Printing Department has the potential to significantly streamline complex head and neck oncologic surgery procedures. This approach can enhance accessibility to advanced CAD/CAM, making them more efficient and cost-effective within the healthcare system.

Introduction

Computer-aided design/computer-aided manufacturing (CAD/CAM) for head and neck oncological surgery by printing biomodels, cutting guides, and patient-specific implants (PSI) facilitates to make these highly complex surgical procedures more predictable. Despite this, this technology is not without drawbacks, as the increased costs and delivery times when planned with the biomedical industry are one of the reasons why it is usually reserved for the most complex cases [1,2,3,4]. For this reason, the possibility of creating a 3D Printing Department inside the hospital has been recently discussed, performing the whole process in-house(IH), using either commercial software [5,6], free software [7,8,9,10], or software developed at the hospital [11]. These alternatives attempt to maintain the benefits of CAD/CAM while reducing its disadvantages [6,7,8,9,10,11,12,13,14]. The aim of this paper is to present the In-House 3D Department (IH3D) validated workflow with free software in a tertiary level university hospital of the Spanish National Health System.

Workflow (Segmentation, Modeling, Printing, Post-Processing, Validation, and Sterilization)

At our IH3D, we have the capacity to manufacture customized biomodels and surgical guides, but not to manufacture PSI (for cost and cost-effectiveness reasons). The workflow consists of 6 steps to obtain a validated and sterilized surgical guide or biomodel, ready to be used at the operating room. In our workflow, the surgeon is in charge of the Virtual Surgical Planning (VSP); the engineers are in charge of the printing, post-processing, and validation; and the Sterilization Department performs its part under the guidelines and supervision of the IH3D engineers according to their standardized protocol, to ensure that the final product complies with the quality standards in force. Regarding time frames, the VSP is usually done within 2 to 4 h, depending on the complexity of the case and the surgeon’s experience with the software. Our engineers have the validated, sterilized, and packaged models available in approximately 1–2 working days. Therefore, the entire planning and printing process in our IH3D setting is reduced to less than 72 h.

The validation of this free software and workflow took place within the approval process for a Custom Medical Device Manufacturing License by Product Family submitted to the Ministry of Health of the Community of Madrid, Spain. Additionally, ISO 13485 Certification was obtained through an external audit conducted by Aenor [15,16,17,18,19,20,21].

Segmentation

The segmentation process is performed with the opensource 3D Slicer software [22]. Segmentation consists in converting images obtained from a radiological in DICOM format (.dcm), such as a Computerized Tomography (CT), Magnetic Resonance Imaging (MRI), or Positron Emission Tomography (PET), into a three-dimensional object in Stereolithography (STL or .stl) format. Imaging tests need to be acquired in a high resolution with slices up to 1.25 mm maximum [23]. The process consists of the following:

• Open the 3D Slicer software.
• Drag the folder with the DICOM format images to the program or load it from the “File” section or from the upper toolbar by clicking on “Data”. It is important that no special characters or accents are included in the source folder, as this may cause errors in the loading of the DICOM images.
• Load the patient’s DICOMs that are the subject of the planning in the “DICOM” module.
• Orient the multiplanar reconstructions in the “Transforms” module.
• Select the anatomical structures to be highlighted in the “Volumes” module.
• Generate a preview of the three-dimensional mesh in “Volume Rendering”.
• Adjust the voxels size to be isotropic and define the anatomical area of interest to be segmented with “Crop Volume”.
• Perform the segmentation of the selected volume with the tools of the “Segment Editor” module. The most used tools of this module are the “Threshold” to quickly segment structures based on their Hounsfield Units (HU), and the “Grow from Seeds” from which the “Paint” tool is used to mark areas of interest in one color and areas of no interest in another, identifying HU subgroups to segment structures such as a mandibular tumor, which is different in HU from the rest of the surrounding tissues. With the “Margin” tool, a mesh can be generated that will correspond to the margins we wish to give to the segmented tumor, which are normally 10 to 15 mm. It is important to check the correct segmentation of the structures in the multiplanar reconstructions, according to the previous diagnosis made in the accredited software of the hospital’s own diagnostic imaging machines.
• Through the “Segmentations” module, export the generated mesh to “Models”.
• Once the mesh has been exported to “Models”, it is possible to save the generated object in .stl format. To do this, we must go to “SAVE”, modify the default format of our object from .vtk (Visualization Toolkit) to .stl, and save it in the desired folder.

Modeling

The modeling process will be performed with the freesource software Meshmixer© from Autodesk© [24]. This process involves modifying a three-dimensional object in .stl format to create biomodels or cutting guides. The process is divided into two parts: Virtual Surgical Planning and 3D Design. It consists of the following phases:

Virtual Surgical Planning:

• Open the Meshmixer program.
• Load the STL(s) by dragging them on the start screen from the source folder or with the “Import” option from the start screen, in the left vertical toolbar or in “File”, in the upper left corner. Sometimes if more than one object is imported at the same time and they should be aligned from their segmentation, they may appear separated from each other, to solve this you must import all objects at the same time from the folder with the “Import” option.
• Align the model from the “Edit” module by selecting the “Align” option with all the selected objects of interest.
• Define the front view in the upper right square “Set Current view as Front”. Performing this step together with step 3 facilitates the incorporation of future elements since it positions our object in the center of the three-dimensional space.
• Analyze possible defects in our STL with the “Inspector” option in the “Analysis” module and refine the model with the “Select” module using its functions available in “Edit” and “Deform”. Perform at the end of this process again “Inspector” to ensure that there are no errors and fill those artificially created with “Flat Fill” or “Smooth Fill” as appropriate.
• To create a biomodel (e.g., an orbit), it would be sufficient to generate a solid object with the “Make Solid” function from the “Edit” module, and visually verify its conformation and perform “Inspector” again. If you have had to mirror the contralateral side, you must perform the “Make Solid” after having used the “Mirror” and “Transform” functions, the latter allows us to move our object in space to orient it in its new arrangement as a contralateral orbit.

3D Design:

• To design the surgical guides, we will need the “Select” and “Edit” modules. For example, in the case of a mandibular tumor with fibula free flap reconstruction, we will transfer two planes, one on each side of the tumor, for the resection in the location previously decided in the diagnostic phase with the accredited software. These planes will define the angulation of the resection to make it match with the angulation of the fibula flap cut. Therefore, once the planes have been placed on the solid mandible with the “Transform” function, we will use the “Create Pivot” function, both in the “Edit” module and we will place a pivot in the center of the plane by selecting “Snap to Group Center”. We will place the solid fibula with the “Transform” function in the desired position, duplicating the fibula with “Duplicate” according to the fragments we need, and we will include new planes in the intersections between the fragments (Figure 1).
• For the mandibular resection guide, we will use the “Select” module and paint the area of interest, it is recommended to have support of some of the components of the guide on unequivocal anatomical landmarks such as the mandibular angle. Next, we select the edges and perform “Smooth Boundary” to soften them, select everything again and perform “Offset” of 0.5 mm to have a certain margin of error in the adaptation of the guide. We make “Flip Normals” and a new “Offset” but this time of—3 mm so that the guide has 3 mm thickness, the number is negative because when we make “Flip Normals” previously the face is oriented in the opposite direction and in this way we have it oriented back to us. After performing again “Smooth Boundary” on the edges and having both edges selected, we perform the “Join” function to close the mesh. With the mesh closed we will make an “Extrude” of 3 mm selecting the edges around the cutting plane to improve the contact surface with the saw. From the “Meshmix” module in the “Letters” section, we drag on the guide, a P or a D, for example, to mark the fragments of the guide as proximal or distal and orient it more easily. Then, we will do “Make Solid” and we will only have to do “Plane Cut” oriented on the Pivots of the cutting planes to remove the part of the guide that we do not want, creating a flange type resection guide. To make the holes to fix the guide we will use cylinders that we will drag to the guide from the “Meshmix” module that we find in the “Primitives” section. The diameter of the cylinders must match the diameter of the screws used to fix the guide. To perform this substraction, we will need a Boolean operation and it does not usually give error if it is done when the cylinders and the guide are converted into solid. After placing the prisms through the guide with “Transform”, we select first the guide and then the cylinders in the “Edit” module to perform the “Boolean Difference” function and subtract the cylinders from the guides. In addition, we recommend adding with the “Add Tube” function from the “Edit” module a tube of 3 mm radius by selecting “Spline (Outside)”, with the combination “Boolean” and with an appropriate “Target Len”, which allows to better place the most distal fragment of the guide or those fragments of the same with worse anatomical reference, it is convenient to add it before making the solid object so that when “Make Solid” is performed, the generated tube is included (Figure 2).
• For the fibula cutting guide, we will have to perform the same steps, but each guide will be designed on the fibula placed in its final position on the mandible, obtaining as many guides as fragments of fibula we design, obtaining guides separated from each other, being the surgeon who intraoperatively decides the distance between the fragments if there is more than one. We will use “Plane Cut” oriented on the Pivots in the same way as in the case of the resection guide. Another option (for both guides) is to design boxes with grooves to set the direction of the saw cut (slot type resection guide). The fibula guide can be designed (when it has more than one fragment) as a set moving the cutting planes to a fibula that will be used as working fibula out of its position in the mandible, although we agree with what has been published that it only increases the design time not adding more precision; therefore, we prefer the flange type resection guide directly over the final position of the fibula fragments [7] (Figure 3).
• For the biomodel of the resection on which we can adapt a stock reconstruction plate, we will only have to obtain with “Plane Cut” on the Pivots of the planes of both the mandibular resection and the intersections of the fibula, obtaining on one side the distal and proximal mandibular fragments, as well as the fibula fragment or fragments. These fragments, previously being already solid, we elect them at the same time and we join them in the same object from the “Edit” module with the “Combine” function. Then we will make a new “Make Solid” of the set, since normally the result of the operation causes the fragments to merge since there is no real distance separating them. For the “Make Solid” functions, we recommend selecting “Accurate” in the “Solid Type” option and usually, a “Solid Accuracy” and “Mesh Density” higher than 200 is not necessary to obtain a good definition and density.
• If a positioning guide of the fibula fragments with respect to the remnant mandible is required, a “mandible guiding tray” can be designed according to the design described by Arce et al. [25]
• It should be analyzed one last time with “Inspector” to repair possible defects of our guide or biomodel prior to export.
• Export the generated surgical guide in Binary STL format from the “Export” module.

Validation

To finalize the design process of both the biomodel and the surgical guide, it is convenient to use another freesource software such as Meshlab [26], which will allow us to define the type of mesh and the analysis of its topology, to avoid the generation of errors in the lamination program. It is convenient to define the type of mesh we are going to work with and its topological characteristics. This type of mesh is a closed mesh, also known as a “manifold”, which perfectly covers the surface of the object following Euler’s equation: C + V = A + 2, where C is the number of faces, V is the number of vertices, and A is the number of edges in the mesh. In practice, we often encounter meshes generated by professional 3D modeling software with anomalies in the topology that cause them not to close. These anomalies can be a problem if they are not taken into account to the point of having printing problems once the mesh is sent in .stl format to the lamination program.

The Meshlab software will first allow us to define whether our mesh is two-manifold or non-manifold, for this purpose:

• We will import the mesh to the workspace. The first thing that the application will ask us is if we want to unify duplicated vertices, to which we select yes.
• Select the menu “Filters”, the module “quality measure and computations” and the option “compute topological measures”: at this point, we can determine the number of vertices, edges, and faces, if the mesh has unreferenced vertices, if it has 0 holes, its genus, the number of connected components, and the most important thing is the type of mesh: twomanifold or non-manifold. For example, the program indicates in a pop-up window the information about a two-manifold mesh: V: 216 E: 642 F: 428, Unreferenced Vertices 0, Boundary Edges 0, Mesh is composed by 2 connected components, Mesh has 0 holes, Genus is 1, Mesh is two-manifold.
• Our objective is that the mesh to be studied is twomanifold, so all the points of the example must be fulfilled, since the mesh is closed, has no holes, no duplicity of vertices, and no referencing. In this case, the mesh will be ready for printing and will have no errors in the lamination program.
• In the case that the mesh is non-manifold, we see, for example, how the program would indicate in a pop-up window the information: V: 7933 E: 23796 F: 15852, Unreferenced Vertices 0, Boundary Edges 44, Mesh is composed by 4 connected component(s), Mesh has 4 non-two-manifold edges and 12 faces are incident on these edges, Mesh has 5 non-two-manifold vertexes and 26 faces are incident on these vertices, Genus is undefined (non-two-manifold mesh), Mesh has an undefined number of holes (non-two-manifold mesh). The process is complicated by the fact that it will be necessary to analyze on the mesh all the open edges, duplicated faces, and edges used by more than two triangles, until a good printable two-manifold mesh is obtained.

MeshLab also allows comparing by superimposition the structured light scan of the printed model with the .stl sent to the lamination program. In our IH3D, the comparison between the planned mandibular model and the printed one has been performed, and the differences between the two are less than a millimeter, which ensures the accuracy of our VSP and serves as a validation method.

Printing and Post-Processing

The 3 printers we use in our 3D Department are two Fused Deposition Modeling (FDM) printers and one Sterelitography (SLA) printer. For FDM, we have an Ultimaker S5 and an Ultimaker S3 (Ultimaker, Utrecht, The Netherlands). From SLA, we have the FormLabs 3BL, with its respective curing and washing machine the Form Wash and Form Cure, both in L size (Formlabs, Somerville, MA, USA). Ultimaker S5 and Formlabs 3BL printers are ISO 13485 certified [15]. Both companies provide their free printing software to transform the Binary STL files into the printable file called G-code (or .GECODE).

Our FDM technology has a lower cost both for acquisition and maintenance. This is at the cost of lower resolution and less post-processing time, being sufficient with the removal of the supports generated during printing, either manually or by thinning them in the case of using soluble support filament. In the context of our IH3D, the material we mainly use is ABS Medical Smartfil as it is a USP (United States Pharmacopeia) Class VI material and meets the requirements of ISO 10993-1, so it is considered biocompatible [20,27]. Ultimaker has its free-source 3D printing software, Cura (Ultimaker, Utrecht, The Netherlands). Cura analyzes the model in Binary STL to correct possible errors and automatically designs the supports that the model needs to be printed, generating a real-time simulation of the printing process. We configure Cura to print with a layer height of 0.2 mm, obtaining a good finish of the model and at the same time reducing the printing time. We use this technology mainly for printing biomodels because it is cheaper and has a sufficient printing resolution, which makes it ideal for printing parts that require more printing material.

The SLA technology uses a liquid photopolymer for manufacturing, this rendering a higher resolution, at the cost of an increase in price, the need to purchase additional post-processing machines and more expensive corrective and preventive maintenance. After printing, the washing process is carried out in the Form Wash with 99% isopropyl alcohol; the washing time depends on the type of photopolymer and varies between 10–20 min. Once the model has been manufactured and washed, it requires a selective curing in the FormCure to obtain the final mechanical properties. This process is performed using ultraviolet light with a wavelength λ = 405 nm. The curing parameters are time(t) = 30 min at Temperature(T) = 70 °C. After curing, the substrates are finally removed. The liquid photopolymer we use for the impression is Surgical Guide Resin which is also USP Class VI and meets the requirements of ISO 10993-1 [20,27]. PreForm (Formlabs, Somerville, MA, USA) is the Formlabs printing software which is also free, its function is the same as Cura, but it is used for Formlabs printers. We set up Preform with a print resolution of 0.1 mm. Because of its higher resolution, but also because of its higher cost, we usually use it for printing surgical guides, as they require more precision and a smaller amount of impression material.

Sterilization

We use different sterilization methods depending on whether our model or guide is printed with FDM or SLA technology. FDM technology uses thermoplastic filaments, so special attention must be paid to the softening temperature according to ASTM E2092 [28], since those filaments that have this temperature below 60 °C cannot be sterilized by any physical procedure such as steam. For this reason, our FDM models are either not sterilized, since if a reconstruction plate or an orbital mesh has to be premodeled on a biomodel, this is normally done before surgery; or if you want to have the biomodel in the operating room or a surgical guide is performed, with another chemical procedure, either hydrogen peroxide in plasma phase (state between liquid and gas) works with a T = 45 °C and a t = 45 min or Formaldehyde T = 60 °C and a t = 5 h or T = 50 °C and a t = 3 h; to avoid incidents after the process, it is advisable to carry out a compatibility study of the filaments used in order to determine which is the optimal procedure according to the type of filament.

For SLA printed models, the sterilization process can be performed by a physical process such as steam, but our experience tells us that the use of a chemical process such as hydrogen peroxide will guarantee an uneventful sterilization process [8,10]. The advantages of steam are that it is a fast, inexpensive, and non-toxic method. On the other hand, the high temperature can have undesirable effects on the structure of the material. The latter we have been able to verify in some models in SLA, in the weakest sections of the model or producing flaking on the external surface of the same.

After sterilization, all our models are numbered, labeled, and packaged according to the recommendations of Annex I and XIII of Regulation 2017/745 [17].

Discussion

In this article, we show our IH3D workflow in a Public Hospital of the Spanish National Health System. There is evidence to support the use of CAD/CAM for head and neck surgeries in general [4] and for mandibular reconstruction in particular [1]. It has been demonstrated that ischemia times, operative times, and even hospital stay are reduced, with no differences in the rate of complications [1,4]. In addition, it improves the accuracy of the reconstruction, the esthetic result, the dental rehabilitation rate, and increases the chances for condyle sparing resection [4].

Currently, we do not have the possibility to print PSI in our IH3D. Publications comparing CAD/CAM head and neck patients with PSI vs those with conventional plates bent on biomodel present varied results. Lee et al. studied these differences with intraoperative hand-bent plates, reporting significant data on reduced operative time and reduced hospital stay in favor of the PSI group. Despite this, if they did not bend the plate intraoperatively, there might not be differences between groups. In addition, the groups are not homogeneous with respect to the number of tracheostomized patients, and this, together with a longer operative time, could explain the differences in hospital stay [2]. Richard Su’s group at Queen Mary Hospital in Hong Kong have published several papers on their protocol for performing IH PSI with commercial software. The design is performed IH as well as the printing of guides and models, the PSIs are printed and post-processed by engineers at another facility [5]. According to their results, there are statistically significant differences in absolute precision in favor of PSI, but no differences were found in physiological deviations, surgical time, hospital stay, bone union, or occlusal level. They argue that despite being more accurate with PSI, there is no knowledge of the Minimal Clinically Important Difference (MCID), so the results cannot be over-interpreted [3,29]. Lor et al., in their article on CAD/CAM for midface fractures, find PSI too expensive, arguing that it is not economically realistic for some institutions to produce them [12].

In spite of the advantages of CAD/CAM, it has been widely published about its disadvantages, mainly the increase of costs and lead times, which are even higher if a PSI is requested [4,6,7,8,10,11,14,30,31]. To try to mitigate these drawbacks, the idea of an in-house 3D Printing Department with commercial software in hospital centers arises, both for the printing of guides and biomodels, and even PSI [5,6]. The saving method consists in the reduction of surgical times offered by CAD/CAM, forcing to have a large volume and needing to include other related departments to make this technology profitable in the context of a IH3D [12,13,14]. Thus, this option reduces lead times better than costs [6,12,32].

This has been the justification for some groups to experiment with different free software to try to bring the advantages of CAD/CAM to a larger number of patients, reducing costs and delivery times in the context of IH3D [7,8,9,10]. Bosc et al. present disease-free and recurrence-free survival results similar to those described in other series [8]. Also, similar levels of accuracy to those reported with commercial CAD/CAM, without the need for intraoperative modifications, or modifying the surgery due to the manufacturing protocol of the guides [8,10]. In conclusion, the sole uncertainty regarding its use has revolved around concerns related to the current legislative context. To secure both the Custom Medical Device License and ISO 13485 Certification, we have undergone adjustments in our workflow, workspace, printers, and materials for biomodel and guide production to align with the requirements of MDR 2017/ 745, MDR 2017/746, MDCG 2021-24, MDCG 2021-3, MDCG 2019-11, ISO 13485, ISO 10993, and the European Machinery Directive 2006/42/EC. Specifically, in validating free software, we have ensured that the products generated through them in real-world applications align with expectations, with tolerance intervals less than a millimeter, as previously explained. This validation process is applicable to both free and commercial software, essential for obtaining the Customized Medical Devices Manufacturing License from national authorities and ISO 13485 Certification through an external audit [15,16,17,18,19,20,21].

Thus, in our center, we have opted for the implementation since 2020 of an IH3D with free software for IH CAD/CAM planning, producing surgical guides, and biomodels for more than 50 patients/year between facial trauma and head and neck oncology, reserving for the industry the most complex cases and those requiring a PSI.

Conclusions

It is possible to develop an in-house 3D Printing Department with the free software option in any hospital with a low budget, making the advantages of 3D technology now available to all our patients.