Dimensional accuracy of medical models of the skull produced by three-dimensional printing technology by advanced morphometric analysis

Sharmila Aristotle; Shantanu Patil; Saikarthik Jayakumar

doi:10.4103/jasi.jasi_202_21

ORIGINAL ARTICLE

Year : 2022 | Volume : 71 | Issue : 3 | Page : 186-190

Dimensional accuracy of medical models of the skull produced by three-dimensional printing technology by advanced morphometric analysis

Sharmila Aristotle¹, Shantanu Patil², Saikarthik Jayakumar³
¹ Department of Anatomy, SRM Medical College Hospital and Research Center, Kattankulathur, Tamil Nadu, India
² Department of Translational Medicine, SRM Medical College Hospital and Research Center, Kattankulathur, Tamil Nadu, India
³ Department of Basic Medical Science; Department of Medical Education, College of Dentistry, Majmaah University, Al Zulfi, Riyadh, Saudi Arabia

Date of Submission	07-Dec-2021
Date of Decision	30-Jun-2022
Date of Acceptance	26-Jul-2022
Date of Web Publication	20-Sep-2022

Correspondence Address:
Dr. Sharmila Aristotle
Department of Anatomy, SRM Medical College Hospital and Research Center, Kattankulathur - 603 203, Tamil Nadu
India

Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jasi.jasi_202_21

Abstract

Introduction: Three-dimensional (3D) printing creates a design of an object using software, and the process involves by converting the digital files with a 3D data using the computer-aided design into a physical model. The aim of the study was to investigate the accuracy of human printed 3D skull models from computed tomography (CT) scan data via a desktop 3D printer, which uses fused deposition modeling (FDM) technology. Material and Methods: Human anatomical cadaver skulls were CT scanned in 128-slice CT scanner with a slice thickness of 0.625 mm. The obtained digital imaging and communications in medicine files were converted to 3D standard tessellation language (STL) format by using MIMICS v10.0 software (Materialise, Leuven, Belgium) program. The 3D skull model was printed using a Creatbot DX desktop 3D FDM printer. The skull model was fabricated using polylactic acid filament with the nozzle diameter of 0.4 mm and the resolution of the machine was maintained at 0.05 mm. The accuracy was estimated by comparing the morphometric parameters measured in the 3D-printed skull with that of cadaver skull and with CT images to ensure high accuracy of the printed skull. Fourteen morphometric parameters were measured in base and cranial fossa of the skull based on its surgical importance. Results: Analysis of measurements by inferential statistical analysis of variance for all three groups showed that the 3D skull models were highly accurate. Reliability was established by interobserver correlation for measurements on cadaver skull and the 3D skulls. Dimensional error was calculated, which showed that the errors between three groups were minimal and the skulls were highly reproducible. Discussion and Conclusion: The current research concludes that a 3D desktop printer using FDM technology can be used to obtain accurate and reliable anatomical models with negligible dimensional error.

Keywords: Computed tomography scan, fused deposition modeling, morphometry, skulls, three-dimensional printing

How to cite this article:
Aristotle S, Patil S, Jayakumar S. Dimensional accuracy of medical models of the skull produced by three-dimensional printing technology by advanced morphometric analysis. J Anat Soc India 2022;71:186-90

How to cite this URL:
Aristotle S, Patil S, Jayakumar S. Dimensional accuracy of medical models of the skull produced by three-dimensional printing technology by advanced morphometric analysis. J Anat Soc India [serial online] 2022 [cited 2023 Mar 28];71:186-90. Available from: https://www.jasi.org.in/text.asp?2022/71/3/186/356493

Introduction

Additive manufacturing technology refers to three-dimensional (3D) printing technology that produces 3D objects, which are digitally defined by computer-aided design.^[1] This 3D printing technology has now extended its hands representing a big opportunity in the medical field helping the health care workers. This technology is gaining much importance in the field of medicine recently to supplement human donor organs, to preplan surgical procedures, to produce cost-effective surgical tools, and to improve the lives of those reliant on prosthetic limbs.^[2],[3] Inexpensive 3D-printed models can also be used as teaching aid-simulating surgical models, which are very useful to impart surgical skills especially for oromaxillary and neurosurgical procedures that involve complex anatomy. They are also used as educative aids to explain the area involved for patients before surgery.^[4]

The major advantage of this technology involves manufacturing models that can be patient specific, and it can be customized for transplant, implants, and prosthesis.^[5] The 3D printing applications in the coronavirus disease 2019 outbreak help in the development of various stable and compatible masks such as N95, facial shields, coronavirus specimen collection apparatus, valves used for ventilators, fabricated 3D-printed pills, and many more applications.^[6]

Despite significant and wide advances in medicine involving 3D printing, notable scientific and regulatory challenges remain, demanding the accuracy, reproducibility, and biocompatibility for the usage of 3D-printed models on large scales. Currently, there are no gold standard measurements for the accuracy of medical models. With all these developing technologies, performing manual segmentation of the medical images and the process of conversion of the digital imaging and communications in medicine (DICOM) to standard tessellation language (STL) format demands detailed knowledge of anatomy to avoid a potential source of error.^[7] Several parameters such as width of the fused ceramic/polymer filament, layer thickness, placement and direction of the target area, printing speed, and raster angle are required for achieving the desired quality of the product.^[8]

Hence, the study was carried out for validating the accuracy of the medical models of the skulls by standardizing the measurements taken from the original cadaver skulls. This study uses fused deposition technique (FDM) for creating the models since this technique is widely used, easily available, and produces cost-effective models.

Material and Methods

The study was approved by the Scientific and the Institutional Ethical Committee with ethical clearance no.: 1162/IEC/2017 of SRM Institute of Science and Technology, Kattankulathur, Chennai.

Study design

Analytical cross-sectional study.

Fabrication of three-dimensional skull

Twenty cadaver skulls with intact measurable areas were procured from the Department of Anatomy, SRM Medical College Hospital and Research Center. The skulls were CT scanned in a 128-slice computed tomography (CT) scanner with a slice thickness of 0.625 mm. The DICOM files were then processed and converted into Standard Tessellation Language (STL) files using the MIMICS v10.0 software (Materialise, Leuven, Belgium). The generated STL files of the 3D model were further cleaned up by CreatWare v 6.4.6, (CreatBot, Zhengzhou city, China), a customized GUI software, and the skull was printed. The material used was polylactic acid (PLA) using the fused deposition modeling (FDM) method.

Fourteen parameters were selected in the posterior cranial base and the middle cranial fossa of the skull. These skeletal landmarks used are highly relevant, clinically based on the literature. The 14 measurements were taken in all three groups of 20 cadaver skulls, 20 CT images, and 20 fused deposition skulls.

The parameter measurements taken in the base of the skull were (1) occipital condyle length for the right and left sides; (2) occipital condyle width for the right and left sides; (3) anterior intercondylar distance; (4) posterior intercondylar distance; (5) anteroposterior diameter of foramen magnum; and (6) transverse diameter of foramen magnum [Figure 1].

Figure 1: The measurements taken in the base of skull. 1: Occipital condyle length; 2: Occipital condyle width; 3: Anterior intercondylar distance; 4: Posterior intercondylar distance; 5: Anteroposterior diameter of foramen magnum; 6: Transverse diameter of Foramen magnum

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The measurements were taken in the cranial fossa: (1) sella turcica; (2) clivus; (3) distance between anterior clinoid processes (ACP); (4) distance between posterior clinoid processes; (5) distance between foramen ovale and apex of the petrous part of the temporal bone on the right side; and (6) distance between foramen ovale and apex of the petrous part of the temporal bone on the left side [Figure 2].

Figure 2: The measurements taken in the cranial fossa. 1: Sella turcica; 2: Clivus; 3: Distance between ACP; 4: Distance between posterior clinoid processes; 5: Distance between foramen ovale and apex of the petrous part of the temporal bone on the right side; 6: Distance between foramen ovale and apex of the petrous part of the temporal bone on the left side. ACP: Anterior clinoid processes

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For cadaver skull, and 3 D-printed skulls measurements

High-resolution digital photographs of the cadaver skulls and 3D-printed skull models were taken using a digital camera Nikon D 5600 digital SLR camera. Both the cadaver skulls and printed skulls were calibrated by placing in the same position under a regular grid sheet by drawing a straight line measuring 10 μm as known measurement.

The measurements were taken using the digital image analysis program known as Digimizer Software version 4.3.0. The morphometric measurements obtained in the Digimizer programmer window were taken into database. The mean and standard deviation were obtained for all variables measured.

For computed tomography image measurements

The raw data collected were reformatted in the axial, coronal, and sagittal plane, and then 3D images were created using the Radiant software version 5.5.0. Distance for linear measurement on the CT determined with the internal digital caliper of the workstation was recorded. All the measurements were taken by two independent observers at different intervals for ensuring reliability.

Results

Statistical analysis was done using the statistical software package SPSS Statistics for Windows, Version 19.0 Armonk, New York: IBM Corporation and a probability level of P < 0.05 was considered statistically significant.

The results are tabulated in [Table 1] and [Table 2]. Comparison of means across three groups using analysis of variance showed that there was no statistically significant difference (P > 0.05) between the means scores of all three groups across all 14 parameters.

Table 1: Measurements in base of the skull for all the three groups

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Table 2: Measurements in cranial fossa of all the three groups

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Analysis of the data demonstrated that the morphometric measurement values obtained from 3D print were similar with those obtained from CT scan and cadaver skull, thus proving that the 3D-printed skulls models are accurate replicas of the original skull. To assess the reliability of the independent observers, intraclass correlation coefficient was calculated independently for the cadaver skull, CT, and 3D-printed skull. A high degree of reliability was found between the three groups measured between the two observers [Supplementary Table 1]. To analyze the dimension errors, absolute and relative difference was calculated by comparing the measurements of 3D models with the measurements of cadaver skull and CT measurements. The dimension error measured in terms of absolute and relative difference is summarized in [Supplementary Table 2]. It is pertinent to point out that the errors between three groups were minimal. The parameters OC-T-R; OC-AP-L; AIC, PIC, FM-AP, Sella turcica, PCP, FO-APT-R, FO-APT-L has least errors, when compared between the 3D skull model and cadaver skull. Whereas, these parameters OC-AP-R; OC-T-L; FM-T-R; clivus and ACP measurements got least error when compared with the CT measurements. [Figure 3] and [Figure 4] show the absolute and relative error difference of 3D-printed skull in comparison with dry skull (blue bar) and CT skull (orange bar) measurements.

Figure 3: : The absolute difference of 3D-printed skull in comparison with Dry skull(blue bar) and CT skull(orange bar) measurements . CT: Computed tomography, 3D: Three dimensional. OC: Occipital condyles, FM: Foramen magnum, AP: Anteroposterior, T: Transverse diameter, R: Right, L: Left, AIC: Anterior intercondylar distance, PIC: Posterior intercondylar distance, ACP: Anterior clinoid process, PCP: Posterior clinoid process, FO: Foramen ovale, APT: Apex of petrous part of temporal bone

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Figure 4: The relative difference of 3D-printed skull in comparison with dry skull(blue bar) and CT skull(orange bar) measurements. CT: Computed tomography, 3D: Three dimensional, OC: Occipital condyles, FM: Foramen magnum, AP: Anteroposterior, T: Transverse diameter, R: Right, L: Left, AIC: Anterior intercondylar distance, PIC: Posterior intercondylar distance, ACP: Anterior clinoid process, PCP: Posterior clinoid process, FO: Foramen ovale, APT: Apex of petrous part of temporal bone

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Discussion

Different printing technologies can lead to different manufacturing errors; inaccuracy in any form in worst case may harm the patient and may lead to fatal errors.^[9] This study evaluates the accuracy and reliability of FDM skulls, which can be used for clinical practice. Threes groups were compared here to ensure the accuracy of the printed models. Moreover, the morphometric parameters chosen for measurements are highly relevant surgically and begin used in various craniovertebral junction surgeries, transoral, and trans-sphenoidal surgeries. The parameters, which are chosen for measurements to determine the accuracy of the fabricated skulls, are not reported in the literature to the best of our knowledge.

Nizam et al. studied the dimensional accuracy of four adult human skull models produced by rapid prototyping technology using stereolithography apparatus; eight linear measurements were measured on each of the original skull and the 3D-printed skull model using an electronic digital caliper and found that the skull models were accurate but with high standard deviation.^[10] El-Katatny et al. determined the accuracy of skull and mandibular models using FDM and noted an outstanding accuracy using FDM process, which coincides with our study for accuracy with FDM skulls.^[11] Huotilainen et al. who compared physical medical skull models fabricated at three different institutes from an identical DICOM data set showed a large variation in dimensions of the physical skull models.^[7]

Inaccuracy in dimensional values of 3D-printed anatomical mandibular models using five different printing technology, which can be utilized for clinical purposes, was determined by Msallem et al.^[9] However, Ibrahim et al. mandibular models showed greater dimensional accuracy details, which can reproduce more accurately in selective laser sintering (SLS) than polyjet and 3D-printing.^[12]

Therapeutic methods of craniofacial bone repair were studied in a systematic review of about 43 relevant clinical trials and case series including human and animal studies of about 81 patients for short- and long-term effectiveness of 3D printing strategies for skull bone repair by Maroulakos et al.^[13]

Accuracy of skull models for craniofacial skeleton repair by SL technology produces accurate results; however, midface parameters in the maxilla were prone to more error than those of other regions of skulls due to thin bony walls.^[14] For structures, demanding higher accuracy improvement can be achieved by utilizing smaller pixel resolution in the CT images. SLA models can be utilized in any complex surgeries for preoperative surgical planning.^[15]

Brouwers et al. validated the 3D-printed anatomical models using two PLA printers. Nine anatomic specimens were chosen including three pelvis, three hands, and three feet. The replicated models were compared with that of original specimens. They found that the printed models were accurate in both printers.^[16] Brown et al. used digital light processing and polyjet printing techniques for determining the accuracy of the scaffolds in dental models and reported that both printers produced clinically acceptable models with high accuracy.^[17] However, high accuracy was noted in SLS printer with a similar study done for determining the accuracy in 10-tooth replica models. Armin Andreas Sokolowski et al. and Ibrahim et al.'s mandibular models also showed high validated anatomical details, using SLS and polyjet when compared to 3D-printing.^[12],[18]

Various scholars have determined the accuracy of the fabricated models based on various software used for printing and even the accuracy was identified for the printing direction used. Gendviliene et al. assessed the morphology and dimensional accuracy of 3D-printed models using different synthetic materials used for printing, with a similar study of Anthony Tahayeri et al.^[19],[20] Antonino Lo Giudice et al. evaluated the accuracy of the printed mandible in four different types of software for the semiautomatic segmentation and compared with manual segmentation.^[21] Zhang et al. studied the influence of the 3D printing technique and different printing layer thicknesses on dental models.^[22]

An inaccurate 3D-printed model can result in inappropriate surgical outcome, leading to severe consequences for both the patient and surgeons. However, accuracy and reliability are currently mentioned in only a small number of literature, and errors can occur during any time of the process like imaging, segmentation, postprocessing, and 3D printing steps. Accuracy has to be verified and validated in areas involving thin bones, small foramina, and some acute bony tubercles and landmarks of the skull.^[23] A mean variation of ±0.5% for the size of 100 mm is considered as a negligible error and the accuracy of the 3D model is acceptable for clinical purposes.^[10]

Conclusion

To produce an accurate and error-free biomodel, the factors determining the accuracy should be kept in mind before printing. The major component of human error of accuracy is the nature of the anatomical structure of interest, defining the exact location of the anatomical landmarks placed to be captured by the CT scanner. This study measured various clinically relevant parameters and noted that the 3D-printed models are accurate and reliable.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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