A Personalized 3D-Printed Model for Preoperative Evaluation in Thyroid Surgery

Summary

Here, a new method of establishing a personalized 3D-printed model for preoperative evaluation of thyroid surgery is proposed. It is conducive to preoperative discussion, reducing the difficulty of thyroid surgery.

Abstract

The anatomic structure of the surgical area of thyroid cancer is complex. It is very important to comprehensively and carefully evaluate the tumor location and its relation with the capsule, trachea, esophagus, nerves, and blood vessels before operation. This paper introduces an innovative 3D-printed model establishment method based on computerized tomography (CT) DICOM images. We established a personalized 3D-printed model of the cervical thyroid surgery field for each patient who needed thyroid surgery to help clinicians evaluate the key points and difficulties of the surgery and select the operation methods of key parts as a basis. The results showed that this model is conducive to preoperative discussion and the formulation of operation strategies. In particular, as a result of the clear display of the recurrent laryngeal nerve and parathyroid gland locations in the thyroid operation field, injury to them can be avoided during surgery, the difficulty of thyroid surgery reduced, and the incidence of postoperative hypoparathyroidism and complications related to recurrent laryngeal nerve injury reduced too. Moreover, this 3D-printed model is intuitive and aids communication for the signing of informed consent by patients before surgery.

Introduction

Thyroid nodules are one of the most common endocrine diseases, among which thyroid cancer accounts for 14%-21%¹. The preferred treatment for thyroid cancer is surgery. However, because the thyroid gland is located in the anterior cervical area, there are important tissues and organs close to the thyroid gland in the operation area, such as the parathyroid gland, trachea, esophagus, and cervical great vessels and nerves^2,3, making the operation relatively difficult and risky. The most common surgical complications are a decrease in parathyroid function caused by parathyroid function injury or mis-resection and hoarseness caused by recurrent laryngeal nerve injury⁴. The reduction of the above-mentioned surgical complications has always been an objective for surgeons. The most common imaging method before thyroid surgery is ultrasound imaging, although its display of the parathyroid gland and nerve is very limited⁵. In addition, the variation in the position of the parathyroid gland and the recurrent laryngeal nerve in the thyroid surgery area is very high, which hinders identification^6,7. If the anatomical position of each patient can be clearly displayed to the surgeon through the model in real time during the operation, it will reduce the operational risk of thyroid surgery, reduce the incidence of complications, and improve the efficiency of thyroid surgery.

In addition, it is also challenging to thoroughly explain the surgical process to patients before surgery. Some inexperienced surgeons find it difficult to explain and convey the precise details of the operation to patients, especially because of the complexity of the thyroid gland and its surrounding structures. Each patient has their own unique anatomical structure and personal needs⁸. Therefore, a personalized 3D thyroid model based on the real anatomy of the patient can effectively help patients and clinicians. Currently, the majority of the products on the market are mass-produced based on plane diagrams. By utilizing 3D printing technology to produce a patient-specific model that reflects each patient's individual medical needs, this model can be used to evaluate the actual condition of patients with thyroid cancer and help surgeons better communicate the nature of the disease with patients.

3D printing (or additive manufacturing) is a three-dimensional construction built from a computer aided design model or digital 3D model⁹. It has been used in many medical applications, such as medical devices, anatomical models, and drug formulation¹⁰. Compared to traditional imaging, a 3D printing model is more visible and more legible. Therefore, 3D printing is increasingly being used in modern surgical procedures. Commonly used 3D-printed technologies include vat polymerization-based printing, powder-based printing, inkjet-based printing, and extrusion-based printing¹¹. In vat polymerization-based printing, a specific wavelength of light is irradiated onto a barrel of light-curing resin, which locally cures the resin one layer at a time. It has the advantages of material saving and fast printing. Powder-based printing relies on localized heating to fuse the powder material for a denser structure, but it also leads to a significant increase in printing time and cost, and is currently in limited use¹². Inkjet-based printing uses a precise spraying of droplets onto the substrate in a layer-by-layer process. This technology is the most mature and has the advantages of high material compatibility, controllable cost, and fast printing time¹³. Extrusion-based printing extrudes materials such as solutions and suspensions through nozzles. This technique utilizes cells and, therefore, has the highest soft tissue-mimicking capabilities. Due to the higher cost and bio-affinity, it is mainly used in the field of tissue engineering and less frequently in surgical organ models¹⁴.

As a result, we chose the "White Jet Process" printing technology, based on the complexity of the thyroid and its surrounding structures and the surgical schedule. This technology combines the advantages of vat polymerization-based printing and inkjet-based printing, and offers high precision, fast printing, and low cost, making it a good fit for thyroid surgery. The aim of this protocol is to make a 3D-printed thyroid cancer model, improve the prognosis of patients by providing sufficient information about the anatomical structure and variation of patients, and better inform doctors and patients about all the conditions related to the surgical process.

Protocol

This study did not need approval to perform or any sort of consent from the patients to use and publish their data, because all the data and information in this study and video were anonymized.

1. Collection of image data

1. Scan the patient's thyroid by enhanced computerized tomography (CT) to obtain the image data in DICOM format. Ensure that this process is done within 1 week before the operation and control the slice thickness so that it is ≤1 mm.

2. Processing of DICOM data

1. Import the scanned patient image data into the software (see Table of Materials) and set the appropriate threshold according to the difference in gray value between the thyroid gland and surrounding tissues or organs. As different gray values are reflections of differences in the density of different areas of the human body, set the grayscale threshold (unit: hu; on the software) to 226-1,500 to present the bone image; set the threshold to -200-226 to show the thyroid gland image. Let the software automatically identify the boxed area, or manually outline the boundary of the target area if the recognition is not satisfactory.
   NOTE: Mimics automatically select the thyroid region and use the 3D region growth technology to segment the image and calculate the 3D reconstruction. At the same time, the 3D image is optimized to reduce the roughness and the sense of steps to obtain a natural, smooth, and authentic 3D digital visualization model, which enables a more straightforward observation of the 3D model for surgeons.
2. Generate STL files from the reconstructed data model. Choose the reconstructed model in the software, click Export in the file dock, and choose STL as the exporting file format. Finally, generate the STL files successfully.

3. Medical-engineering interaction

1. Send the reconstructed 3D model preview to the doctors, who will confirm the applied requirements and anatomical structure of the 3D model and give feedback to the modeling engineer if a modification is needed. After receiving confirmations from the doctors, proceed to the production preparation stage.

4. 3D printing (Supplemental File 1)

1. Transfer the STL file data to the colorful material 3D printer and complete the parameter presets (such as printing mode, slice stroke thickness, support method, and model coloring) through the supporting 3D printing slicing software.
   1. Select the printing model according to the type of finished products (color printing models usually use White Jet Process technology, while photosensitive resin usually uses Digital Light Procession).
   2. Select slice stroke thickness parameter according to the thickness of the products (here, from 24 µm to 36 µm).
   3. Choose the support method according to the fineness of the printing model: Overall support (better protection and less damage to fine details) or Partial support (which saves materials).
   4. Select model coloring using the color palette function on the printer. Unify the arteries with red color 255 and the veins with blue color 255.
      NOTE: As other parts such as the tumor lesion are not strictly standard, surgeons can select a color according to their needs or preference.
2. Fill in hard light curing resin in the 3D printer (see Supplemental Table S1), debug the printing platform, and print using White Jet Process technology. After printing, take out the preliminary printed thyroid model.
   ​NOTE: The White Jet Process technology is based on the principle of inkjet printing, where a thin layer of photosensitive resin is printed out in one jet and then irradiated with a specific wavelength of UV light, causing a rapid polymerization reaction and curing of the photosensitive resin. This process is completed layer by layer until the print is complete.

5. Post-treatment

1. Subtract the support structure of the preliminary printed thyroid model. Grind, varnish, and cure the semi-manufactured product to obtain an individualized 1:1 isometric 3D-printed thyroid model.
   1. Subtracting support structure
      1. Wearing gloves, break apart the wrapping supports around the preliminary model and remove most of the main body of the supporting structure.
      2. Put the model into an ultrasonic cleaner with Ca(OH)₂ alkaline solution for a 15 min cleaning.
      3. Put the model into a wet sandblaster and rinse it until the rest of the support structure on the surface is washed away.
   2. Grinding
      1. Grind the model with an electric grinder, file, or grinding wheel.
   3. Varnishing
      NOTE: This process consists of spraying and manually painting.
      1. Spray the varnish into large-area color blocks on half of the surface of the model. Manually paint the small-area color blocks with varnish.
   4. Curing
      1. Put the model into a UV curing machine for 30 s of curing.
      2. Take out the model and clean it with 95% alcohol.
         NOTE: After the alcohol volatilizes completely, the production is finished.

6. Delivery

1. Package the thyroid model and complete the delivery to the surgeons before surgery.

Results

This paper presents a protocol for the construction of personalized 3D-printed models of patients' thyroids. Figure 1 shows a flow chart for establishing a personalized 3D-printed model for thyroids of patients. Figure 2 shows the personalized 3D-printed model printing device for thyroids of patients. Figure 3 shows the software interface for the establishment of a personalized 3D-printed model for thyroid patients. The interfac...

Discussion

Ultrasound may be the only preoperative imaging procedure for most patients undergoing thyroid surgery¹⁵. However, a few well-differentiated cases may suffer from advanced diseases, which invade the surrounding tissues or organs and hinder the operation¹⁶. This model may be more suitable for patients with far-advanced thyroid cancer. When the disease progresses, additional CT scanning is helpful for further diagnosis. This model is based on CT scanning, which provides more ...

Materials

Name	Company	Catalog Number	Comments
3D color printer	Zhuhai Sina 3D Technology Co	J300PLUS	Function support: automatic optimized placement, automatic model typesetting, automatic generation support, real-time layered edge cutting and printing, slice export, custom color thickness, custom placement / scaling, man hour evaluation, material consumption evaluation, print status monitoring, material remaining display, changing materials and colors, managing work queues, full / semi enclosed printing, automatic detection of model interference, layer preview, automatic pause of ink shortage, power failure to resume printing Automatic cleaning nozzle, automatic channel adaptation, ink change, automatic cleaning pipeline, follow-up laying. Range of optional materials: RGD series transparent molding materials, RGD series opaque molding materials, FLX series soft molding materials, ABS like series molding materials, high temperature resistant molding materials, Med series molding materials (first-class medical record certification), ordinary supporting materials, water-soluble supporting materials.
Mimics 21.0 software	Materialise, Belgium		DICOM data processing