Introduction

With the advances in the endoscopic technologies, great achievements have been obtained in the surgery of paranasal sinus; however, the complicated paranasal sinus surgery remains a big challenge. Because of anatomical variation, adjacency with important structures and technical limitations, a surgery involving paranasal sinus is still considered a complicated operation, which requires sufficient anatomical knowledge of paranasal sinus and continuous surgical training [1]. The anatomy of paranasal sinus and endonasal skull base surgery is a complicated three-dimensional (3D) spatial structure, which may be difficult spatial imagination for beginners that lack anatomical awareness, depending on CT scans and textbook alone, and a cognitive mistake may result in irreversible surgical failure.

The 3D printing technology allows the transformation of sectional anatomical CT images into more realistic and visualized three-dimensional physical objects, which makes the learning about paranasal sinus anatomy more direct and simpler. We, therefore, aimed to create a 3D printed paranasal sinus–skull base model, and a page-turning design was employed to display the internal air cells of paranasal sinus and the neighboring spatial structures, so as to more easily understand the building-blocks concept of the air cells surrounding the frontal recess, the classification of ostiomeatal complex and frontal cells, and the anatomical structure of Haller cells, making abstract concepts concrete.

Materials and methods

Construction of a 3D printed model

CT scanning was performed on a 128-slice spiral CT scanner (GE Medical Systems; Waukesha, WI, USA) with 1.250 mm in slice thickness and 512 µm × 512 µm in resolution, and the data pertaining to CT scan of paranasal sinus and skull base were captured from a healthy individual. All CT data were processed by interpolation with the imaging workstation into the original data in a DICOM format, and then directly loaded into the software Mimics version 19.0 (Materialise; Leuven, Belgium). Then, the bone tissues were separated from the paranasal sinus and skull base regions using Hounsfield unit thresholds, and the segmented bone was stored as a Mimics object (mask). Some soft tissues such as nasal concha and mucous tissues required manual editing with a MaskEdit component. The processed mask was subjected to 3D reconstruction and mild fairing optimization and the optimized model was incised along the median line. To display the 3D printed model of the right-side paranasal sinus, the right-side paranasal sinus model was segmented into several flake-like models with unequal thickness in the coronal direction, and the neighboring flake-like models were connected via the rotating shaft structures. The combined paranasal sinus–skull base model can be displayed page by page like a book. The created model was stored as a 3D printed file in a STL format.

The 3D model was printed with a FDM+ desktop 3D printer (RACO; Xiameng, China), which has a layer resolution of 0.02 mm, a positioning precision of 0.02 mm, X, Y and Z-axis resolution of 0.6 µm, a nozzle diameter of 0.2 mm, and polylactic acid (PLA) as materials. The best advantage of this printer is easy removal of the support structure of the 3D model after printing.

The 3D printing software Cura version 15.02 (Ultimaker B.V.; Geldermalsen, The Netherlands) was employed, and the model was printed with a layer thickness of 1.2 mm and a printing speed of 50 mm/s.

Assessment of the 3D printed model

A total of nine senior doctors participated in the assessment of the 3D printed model relative to CT findings. The participants required identification of frontal sinus, ethmoid sinus, maxillary antrum, sphenoid sinus and its opening, frontal recess, uncinate process, nasolacrimal duct, internal carotid artery, optic canal, superior nasal concha, middle nasal concha and sphenopalatine foramen on CT images, and assessed the simulation and accuracy of the 3D printed model based on the anatomical structure profiles as revealed by CT images, and its advantages and educational values. The survey instrument employed a seven-point Likert scale.

Results

The 3D printed paranasal sinus–skull base model overall seemed like a realistic anatomical specimen, which had strong visual and stereoscopic sensation (Fig. 1). If the page was turned back one by one, the direction of frontal sinus and its opening, frontal recess, direction of nasolacrimal duct, agger nasi cells, attachment point of the uncinate process, ethmoid bulla, plica semilunaris between uncinate process and ethmoid bulla, ethmoidal infundibulum, cribriform plate and roof of ethmoid, maxillary antrum and its opening, anterior and superior ethmoidal air cells separated by the horizontal and vertical plates of middle nasal concha, Haller cells, superior nasal concha, sphenoethmoidal recess, sphenopalatine foramen, pterygopalatine fossa and boundary of inferior pterygoid fossa, sphenoid sinus and its opening, and direction of optic canal and internal carotid artery were clearly identified in sequence, and the 3D spatial location and neighboring relationships of these anatomical structures were seen with naked eyes (Fig. 2). The surface of the model had no apparent rough sense, which may be attributed to high precision of the printer. However, anatomical details cannot be identified, such as sutura, and the thickness of the bone model seemed greater than the real bone. The printing of the 3D model consumed approximately 76 h and 200 g materials, with a material cost of approximately 3 dollars.

Fig. 1
figure 1

Half of the 3D printed paranasal sinus–skull base model. The model has strong 3D stereoscopic sensation and is designed in a page-turning pattern. The anatomical details of the inner part can be displayed slice by slice

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Fig. 2
figure 2

The 3D printed model is cut into nine slices with unequal thickness. With the opening of each slice, the inner anatomical structure can be seen, while CT images may not identify the slice that completely corresponds to the model, which may result in minor deviation. In addition, some soft tissues of the inferior nasal concha are removed during the 3D modeling, which look thinner as compared to that on CT images. FS frontal sinus, ANC agger nasi cells, ND nasolacrimal duct, UACOEB upper air-cell of ethmoid bulla, EB ethmoid bulla, UP uncinate process, HPMNC horizontal plates of middle nasal concha, VPMNC vertical plates of middle nasal concha, MA maxillary antrum, SNC superior nasal concha, HC Haller cells, SS sphenoid sinus, SR Sphenoethmoidal recess, SSO sphenoid sinus opening, SF sphenopalatine foramen, PC pterygoid canal, FR foramen rotundum

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The anatomical structure profile of the 3D printed model was similar to that of CT image (Table 1). Compared with CT scanning, the 3D printed model was considered to provide more 3D spatial visual information, and be of great value in medical education, pre-surgical auxiliary examinations of complicated paranasal sinus surgery and pre-surgical education (Table 2).

Table 1 Comparison of the similarity of the anatomical structure between the 3D printing model and CT scans
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Table 2 Advantages and educational value of the 3D printing model
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Discussion

As a revolutionary technology, 3D printing is currently one of the most popular techniques. With rapid developments, 3D printing has been applied in the field of medical sciences more and more widely, including the use for medical research and manufacturing models for medical education [2, 3]. In otorhinolaryngology, previous studies focused on the manufacture of temporal bone simulation models and the application of 3D printing in simulated ear surgery [4,5,6,7,8]; however, there is little knowledge pertaining to the application of 3D printing in the research of paranasal sinus. The paranasal sinus and skull base have fine structures, with complicated inner spatial structures, and there is currently a lack of teaching models. It is, therefore, difficult for beginners and young physicians to learn according to textbooks and CT images or anatomical atlas. In the present study, a life-like 3D printed paranasal sinus–skull base anatomical model was generated. This model not only displayed large anatomical structures, such as maxillary antrum, sphenoid sinus and frontal sinus, but also successfully precisely restored fine structures of small air cells like ethmoid sinus. In addition, the spatial relationships of paranasal sinus and skull base were seen, including the boundary of pterygopalatine fossa, the direction of optic canal and internal carotid artery, and their corresponding locations in the sphenoid sinus wall. The model created in this study was designed in a page-turning pattern, and the model was sliced in a coronal position, with the neighboring slices connected via a rotating shaft. With the slice-to-slice opening of the model, the anatomical details of inner paranasal sinus and the neighboring spatial–structural relationships were displayed. The model seemed like a textbook pertaining to the anatomy of paranasal sinus and skull base, and browsing the spatial 3D structure of inner paranasal sinus looked like opening a book.

In this study, the model was printed with a FDM+ desktop 3D printer with a nozzle diameter of 0.2 mm and a layer thickness of 1.2 mm, which had satisfactory printing precision and surface effects. In addition, the model was recognized by all participants; however, completion of the 3D printing required 76 h. Although the use of a larger nozzle (0.4 mm in diameter), and an increase in the layer thickness and printing speed may shorten the time of printing, the surface effect of the printed model may be low, with emergence of obvious laminated printing textures seen. Since the biological fitting accuracy of a model mainly correlates with the slice thickness and resolution of original CT data, the CT image data of paranasal sinus was captured with 1.250 mm in slice thickness and 512 µm × 512 µm in resolution, which met the requirements of 3D printing. If a 625 µm slice thickness is assigned for CT scanning, a model with a higher biological fitting accuracy may be printed.

In the current study, all participants thought that the printed model could satisfactorily restore the anatomical structural features of paranasal sinus and skull base, which had a high educational value and reference value for surgeries. Compared with CT imaging, however, the 3D printed model had a poor ability to display local details, failed to differentiate between soft tissues and bone tissues, and failed to display sutura; however, the model can provide more visualized 3D spatial structural information, notably the spatial inter-correlations of some important structures. This facilitates better understanding of the structural features and surgical concept of paranasal sinus, helps surgeons know the risk of complicated paranasal sinus and skull base surgeries more clearly, and is beneficial to shorten the duration of surgery and reduce intraoperative bleeding. In addition, this may contribute to better pre-surgical communication with patients. It is postulated that the 3D printed model may become a common examination like CT scan, and serve as another important supplementary examination in addition to CT scan in the future.

This study has some limitations. In this small-scale investigation, all participants were sampled from a single center, which may contribute to favorable results. Further multi-center, large-scale studies are required to overcome these problems.

Conclusion

The 3D printed paranasal sinus–skull base model, designed in a page-turning pattern, is able to restore the internal anatomical characteristics and spatial–structural relationship of paranasal sinus, and is cheap and easy to perform, which may be extensively applied in anatomical studies of paranasal sinus, resident training, pre-surgical education and surgical planning.