The study of anatomy is an important part of medical education. Furthermore, with a sound knowledge of anatomy, we can design new surgical fixation techniques. A good example of spinal surgery is the pedicle screw fixation technique, which was rarely used before 1907 [1]. However, as more anatomic features of this technique were described, spine surgeons began to attempt the technique, and it has since become widely used in thoracic, lumbar, and sacral regions [2, 3].

To study the morphology of the human spine and new spinal fixation methods, scientists require cadaveric specimens, which are dependent on donation. However, in most countries, the number of people willing to donate their body is low [4]. Indeed, some religions and faiths discourage people from donating their body, and as a result, access to cadaveric specimens is very limited in some countries.

Without sufficient numbers of cadaveric specimens, some researchers learn new techniques only by studying radiographic images [5–8]. Computed tomographic analysis may be sufficient for morphometric study, but the use of cadaveric specimens is preferable and more reliable [9–11]. Moreover, cadaveric research is necessary for some studies.

3D printing is a process for making a 3D-printed model of almost any shape from a 3D digital model or other electronic data source [12]. The 3D printing methods include selective laser melting, laser sintering, fused deposition modeling, stereolithography, laminated object manufacturing and fused filament fabrication [13, 14]. Many materials are used in these different 3D printing techniques; e.g. thermoplastics (PLA and ABS) are commonly used for the fuse filament fabrication technique, where as titanium alloys and cobalt chrome alloys are used for the selective laser melting technique. Researchers can choose different materials according to the 3D printing technique to be used and the properties, cost and color of materials that they prefer [12, 15, 16].

We previously reconstructed a 3D digital spinal model from CT scan data, and it has been shown that this 3D digital model is morphologically accurate [17]. Accordingly, an accurate 3D model printed from a 3D-reconstructed digital model would allow for studying the morphological features of the 3D printed model, which may solve the problem of scarce cadaveric specimens.

This study was performed following the Declaration of Helsinki principles and was approved by the Institutional Review Board (IRB) of The Second Affiliated Hospital of Wenzhou Medical University. Written informed consent was obtained from all participants.

Forty-five computed tomography (CT) scans of cervical, thoracic or lumbar spines of patients (mean age 42.5±7.7 years (range 31–54 years))were obtained using the Star PACS system (INFINITT, Seoul, South Korea) of our hospital. The included cervical, thoracic and lumbar spines lacked spinal disease, as shown by CT scans for health examination or because the patients had presented with oral, anterior neck, cardiac, pulmonary or abdominal diseases. Patients with any spinal abnormality, such as fracture, scoliosis or tumor, were excluded.

We measured the following parameters using the Star PACS System, which was proven to achieve accurate measurements in previous studies [8, 18]:

C1 (Fig 1): Width diameter; Anteroposterior diameter; Width of vertebral canal; Anteroposterior diameter of vertebral canal; Width of anterior tubercle; Height of anterior tubercle; Width of posterior tubercle; Height of posterior tubercle.

C2 (Fig 2): Max Anteroposterior diameter; Max left-right diameter; Anteroposterior diameter of vertebral body; Width of vertebral canal; Anteroposterior diameter of vertebral canal; Frontal height of axis (including odontoid process).

C3-L5 (Fig 3): Width of vertebral body; Anteroposterior diameter of vertebral body; Left height of vertebral body; Right height of vertebral body; Width of vertebral canal; Anteroposterior diameter of vertebral canal; Width of right pedicle; Height of right pedicle; Width of left pedicle; Height of left pedicle. Because the pedicles of the cervical spine are very small, Width of right pedicle, Height of right pedicle, Width of left pedicle, and Height of left pedicle were not measured at C3–C7.

The CT scan data were then imported in DICOM format into Mimics software v10.01 (Materialise, Leuven, Belgium)for 3D reconstruction. The threshold value was set at “Bone (CT)”, “226-Max”, which is optimal for bone reconstruction. After the 3D digital images were calculated and reconstructed, we removed the bone, which was not needed, and every vertebra was then separated. The data were then saved in STL format and imported into Cura software. After a 3D digital model was formed, we saved it in Gcode format and exported it to a3D printer (3D ORTHO Waston Med Inc. Changzhou, Jiangsu, China) to print the objects. The scale was set at 1:1, and PLA (Polylactic acid:(C₃H₄O₂)_n), with a molecular weight of 5000–700000 according to the product instruction, was used as the print material. After the 3D-printed models were obtained, the above-mentioned parameters were measured again.

Forty-four parameters of the cervical spine, 120 parameters of the thoracic spine, and 50 parameters of the lumbar spine were measured.

For C1, the respective values of WD, APD, WVC, APDVC, WAT, HAT, WPT, and HPT were 76.29±4.43, 43.23±2.37, 28.84±1.87, 27.90±1.73, 7.66±0.98, 12.22±0.81, 7.74±1.08, and 11.76±1.27 m min the radiographic images and 76.33±4.20, 43.15±2.37, 28.68±1.69, 28.01±1.75, 7.53±1.18, 12.13±1.12, 7.88±1.06, and11.63±1.29 m min the 3D-printed models (Table 1). For C2, the respective values of MAPD, MLRD, APDVD, WVC,APDVC, and FHA were 44.69±1.86, 52.46±1.99, 13.24±1.32, 22.66±1.12, 16.23±1.65, and 37.45±2.74 m min the radiographic images and 44.73±1.91, 52.54±2.02,13.36±1.40, 22.51±0.96, 16.39±1.58, and 37.63±2.68 m min the 3D-printed models (Table 2). For C3–C7, the WVD, APDVD, WVC, APDVC, RHVD, and LHVD parameters of C3 were21.86±1.33, 16.35±1.14, 22.72±1.30, 13.18±0.97,13.85±1.15, and 13.91±1.08 mm, respectively, in the radiographic images, and most of them gradually increased by C7. A similar trend was found in the 3D models (Table 3).

The respective WVD, APDVD, LHVD, RHVD, WVC, APDVC, WRP, HRP, WLP, HLPT parameters of T1 were 31.36±1.41, 18.88±1.24, 19.50±0.87, 19.40±1.13, 21.44±1.16, 14.50±0.61, 9.35±0.83, 10.51±0.68, 9.48±0.55, and 10.33±0.85 mm in the radiographic images, and most of them gradually increased by L5 (46.44±2.64, 32.43±2.03, 13.41±1.39, 13.12±1.31, 26.27±1.46, 26.36±1.44, 32.13±2.22, 17.73±1.96, 16.91±1.60, and 17.24±1.97 mm, respectively). We found that the 3D printed model data also showed similar trends (Table 4 and Table 5).

All paired t-test values comparing the radiographic image and 3D-printed model data of all parameters were >0.05 (C1: Table 1; C2: Table 2; C3–C7: Table 3; T1–T12: Table 4; L1–L5: Table 5). Therefore, Intraclass Correlation Coefficient (ICC) analysis was used to assess the correlation between the data from radiographic images and 3D-printed models.

Furthermore, 88.6% of all parameters of the cervical spine (Tables 1, 2 and 3), 90% of all parameters of the thoracic spine (Table 4), and 94%ofall parameters of the lumbar spine (Table 5) were >0.800. The other ICC values were <0.800 and >0.600, and none were <0.600.

Surgeons are constantly exploring novel internal or external fixation techniques to improve healthcare for patients. When they identify “new ideas”, surgeons should ideally test their feasibility using cadaveric models. Unfortunately, the rate of body donation for use in research is very low [4]. Furthermore, in most developing countries, particularly those with strong religious beliefs or without higher education, the rate of donation is lower than in developed countries [19, 20].

In China, universities and medical schools have been faced with an ongoing shortage of cadavers for education and research because of aspects of the Chinese culture [21];the donation rate has also been found to be low in Greece [20]. As a result, for most surgeons, it is difficult to obtain sufficient cadaveric specimens on which to test their “new ideas”.

In addition, most cadaveric specimens are stored in formalin, a storage technique that will change the shape of the bone if the specimens are stored for long periods. This is unacceptable for experiments that require accurate data.

To allow surgeons to test the feasibility of newly developed fixation techniques, we must provide accurate spine bone models. With the development of 3D digital reconstruction, it is now possible to test new fixation techniques on 3D digital images. Puchwein et al [17]studied the morphometry of the odontoid peg and its impact on ventral screws (one screw or two screws) using3D digital images; similar methods could resolve the problem caused by a lack of available cadavers. Unfortunately, digital images sometimes do not provide sufficiently accurate data. We previously imitated trans-pedicle, trans-disc oblique lumbar interbody fixation using 3D digital images. For L1/2, L2/3 and L3/4 screws, the data from 3D digital images and cadavers were similar, but the data were different for L4/5 and L5/S1 screws because the screw angles were blocked by iliac bone and by part of the L5 inferior articular process [22].

3D printing techniques can use 3D digital images to print 3Dmodels, creating the possibility of printing a morphologically accurate 3Dmodel. In our study, 214 parameters from C1 to L5were measured. The method for measuring data using radiographic images was a little different from that using the 3D-printed model because the largest width values of the vertebral body and pedicle were not in the same image; therefore, systematic error could not be avoided. To minimize this error as much as possible, we chose the best matched images for measurements, and forty-five spines were included to minimize individual error. The results showed no significant difference between the data from radiographic images and from 3D-printed models. Our results showed that 88.6% of the ICC values for the parameters of the cervical spine, 90% of the ICC values for the parameters of the thoracic spine, and 94%of the ICC values for the parameters of the lumbar spine were >0.800. These results prove the strong resemblance between data from radiographic images and 3D-printed models. The mean age of the patients from which the 45 CT scans were takenwas42.5±7.7 years (range from 31 to 54 years old), and all of the patients were adults with normal spinal structure. To decrease the error as much as possible, patients with spinal diseases were excluded.

The printed material (PLA) is not very expensive, costing approximately$150 per kilogram ($0.15 per gram). One atlas or axis model is approximately 5–10 grams, whereas other cervical through lumbar vertebrae models range from10 to 35 grams. This cost will be reduced as the method becomes more widely used.

In this study, we provide a protocol for printing accurate 3D spinal models that can be used by surgeons and researchers. This 3D-printed model is inexpensive and can easily be obtained for spinal fixation research.