Construction of the intact model
In the present study, a healthy adult female volunteer without any history of lumbar diseases was selected and the data of her CT scans (AQUIRRON 64, Toshiba, Japan) with thickness of 0.625 mm per slice was provided by the Department of Radiology at The First Affiliated Hospital of Guangzhou University of Chinese Medicine. The tomography images were stored in Digital Imaging and Communications in the Medicine (DICOM) format.
The collected raw data in DICOM format were imported into Mimics research 19.0 (Materialize, Leuven, Belgium) for three-dimensional reconstruction. Subsequently, the 3D model generated by Mimics was imported into Geomagic Studio 2013 (3D Systems Corporation, South Carolina, USA), and the spikes and the features were deleted, smoothing was performed with a polygon mesh, and the triangles were made uniform in size. Then, a patch was generated by the following tools: Construct Patches, Grid and Fit Surfaces. The smoothed model was saved and imported into SolidWorks 2017CAD (SolidWorks Corporation, Concord, MA, USA). Cancellous bone, cortical bone, annulus fibrosus, nucleus pulposus, endplate, and articular cartilage parts were created in the Parts Interface window. The nucleus pulposus accounted for approximately 44% of the disc volume[9], the thickness of the cortical bone was approximately 0.5 mm[9], and the cartilaginous endplates were modeled to be approximately 1 mm thick[10, 11]. The initial gap between the articulating surfaces was based on computed tomography images and was approximately 0.3-0.6 mm. The above parts were integrated into an intact lumbar model.
Three-dimensional scanning models of pedicle screw
A 3D scanner (Solutionix Rexcan CS+ 3D scanner, SolutioniX, Korea) was provided by the Department of Digital Orthopedics and Biomechanics Laboratory at Guangzhou University of Chinese Medicine. The apparatus used image registration and 3D matching technology to create a point cloud of the geometric surface by instrument scanning, and then a 3D computer-aided design model was formed. The specific operation steps are as follows: first, the developer evenly sprayed the surface of the fenestrated pedicle screw; after that, Ezscan 2017 software was used to scan the fixed screws automatically; after finishing the scanning, the redundant parts of the 3D models were deleted using the lasso tool, and the file was saved in STL format.
The 3D models generated by the scanner were imported into Geomagic Studio 2013 and SolidWorks 2017CAD for further processing. Finally, the models with realistic geometry were used for the assembly of surgical models. The length and outer diameter of the pedicle screws (DePuy Synthes, California, USA) were 50 and 6.0 mm, respectively (Figure 1).
The model of bone cement
A patient who was undergone fenestrated pedicle screw with cement-augmented was randomly selected. The cement model was constructed by using the postoperative lumbar CT data through above software. The volume of bone cement was approximately 1.73cm3 and distributed in a lump pattern. Then, the cement model with 1.73ml PMMA was scaled to 1 ml and 2.5 ml.
Construction of instrument models with different volumes of PMMA
Based on the forms of real rod and cage, the models of rod and cage were constructed in the Parts Interface window of SolidWorks 2017CAD. The outer diameter of the rod was 5.5mm. The length and height of the cage were 24 and 12 mm. Subsequently, unilateral transforaminal lumbar interbody fusion (TLIF) was performed in right to remove the facet joint, articular cartilage, part of the annulus fibrosus, cartilaginous endplate, and nucleus. The cage, screws, cement, and rod were assembled with the lumbar spine model to construct six operative models, separately. The cage is located in the right of center of the intervertebral space. To decrease variation caused by various cage/screw locations and increase the consistency and reproducibility of the numerical and experimental results, we have used a consistent location for all models in our work (Fig. 2 and Fig. 3).
Material properties, boundary and loading conditions
The mesh model generated in SolidWorks 2017CAD was imported into ANSYS Workbench 17.0 (ANSYS, Ltd., Canonsburg, Pennsylvania, USA ), and previous literature was referenced to set the cortical bone (osteoporosis), cancellous bone (osteoporosis), articular cartilage, endplates, annulus fibrosus, nucleus pulposus, cages, bone cement, and internal fixation (Table 1) [10, 12, 13]. The ligaments were simulated using spring elements that were only stressed by pulling force [14]. The contact type between the models was defined in the connection, where the facet joint contact type was “frictional” and the frictional coefficient was 0.2; the remaining contact types were set to be the “bonded” mode[11]. In order to enhance the accuracy of calculation, the type and size of mesh in the models are controlled: the mesh type is set as tetrahedron mesh, the size of articular cartilage mesh is 0.5mm, and the remaining objects are 2.0mm. Finally, the boundary and loading conditions of the five models were set[10, 13]: with all degrees of freedom of the sacrum were constrained throughout the whole analysis, a 150-N vertical axial preload was imposed on the superior surface of L3 and a 10-N/m moment was applied on the L3 superior surface along the radial direction to simulate 6 different physiological motions: flexion, extension, left lateral bending, right lateral bending, left rotation and right rotation. The ROM and the disc stress at L3-4 and inferior articular process stress at L3 were analyzed and compared to investigate the biomechanical stability of adjacent segment with different volumes of PMMA.