Pedicle Screws

The intact lumbar spine model was modified to simulate instrumented LLIF with different types of internal fixation. In each group, ASD was assumed to occur at the segment cranial to the upper instrumentation (L3/4). Successful bone graft fusion with LLIF + BPS was simulated at L4/5. The ASD segment for each group underwent a) LLIF + posterior extension of BPS, b) PLIF + posterior extension of BPS, c) LLIF + lateral screw, and d) stand-alone LLIF. In the ASD model, nuclear pulposus and lateral annulus fibrosis resection procedures were performed at the L4/5 segment, and subsequent insertion of a lateral cage was performed with BPS fixation. At the L3/4 segment, models a, c, and d underwent typical L3/4 LLIF surgery with or without additional fixation. In PLIF model b, laminectomy, nuclear pulposus and posterior annulus fibrosis resection were performed at L3/4, with posterior cage and BPS fixation (Figs. 1 and 2). The rest of the L2–5 element components were preserved.
Fig. 2

ASD segment (L3/4) for each group was underwent: a LLIF + posterior extension of bilateral pedicle screw, b PLIF + posterior extension of bilateral pedicle screw, c LLIF + lateral screw, d Stand-alone LLIF

The lateral-inserted cage (48 mm length, 22 mm width, 9 mm height) was box-shaped, with an 8-degree incline between the superior and inferior surfaces (DePuy Synthes Spine, Inc., Raynham, MA). A posterior-inserted cage (23 mm length, 10 mm width, 9 mm height) was placed in the PLIF model (DePuy Synthes Spine, Inc., Raynham, MA). Two kinds of cages were centred on the middle sagittal plane in the disc space. Three simulated constructs were adopted for internal fixation except model d (Fig. 2). The internal fixation and cage implants were reconstructed in Solidworks CAD software and fitted closely to the vertebral and endplate structure. In these ASD models, the diameter of the pedicle screws was 6.0 mm, and the lengths of the screws were set to reach the anterior or lateral cortex of the vertebral body. All screws were fixed to the vertebral bodies without allowing relative motion, which were assigned the contact surfaces to be tied in ABAQUS software. The rods connecting the screws were selected for lofting and reconstruction to ensure the exact fit. Pedicle screws and rods were defined using a “Tie” constraint at the interfaces. A finite sliding algorithm with a coefficient of friction of 0.2 was defined between the cage and endplate to allow for any small relative displacements between the two contacting surfaces. Titanium alloy (E = 110 GPa) and polyetheretherketone (E = 3.6 GPa) material properties were defined for the posterior/lateral configuration and interbody cages [28 (link)].

Interbody fusion was completed using the XLIF technique as described by Ozgur et al. [3 (link)]. Briefly, the patient was placed in the lateral decubitus position with the hip at the level of the break in the operating table. The chest and hip areas were secured to the table with tape. A previous report has suggested that the patient’s position is critical, and there was a slight learning curve for surgeons when performing the surgery involving insertion of the “downside” PPS with the patient in the lateral decubitus position [8 (link)]. For example, positioning the patient too far from the edge of the bed can limit the surgeon’s ability to place the hand low enough to medialize the downside pedicle screws. However, this is less of a concern when performing a lateral PPS procedure using the Viper Prime^TM.
Once the position was decided, the XLIF was performed according to the previous method [3 (link)]. This facilitates access to the largest number of disc spaces with a relatively small incision. Blunt dissection was then used to access the disc spaces under fluoroscopic guidance. After removal of the disc material with a rongeur, a Cobb elevator was advanced gently under fluoroscopy guidance along the endplates to release the contralateral annulus. Cage size trials were followed by additional disc curettage and rasping of the endplates. All cages were inserted using two containment sliders to protect the endplates and to keep the graft material inside the cage. For all patients, the side-to-side cage size was decided according to the width of the endplates at that level based on intraoperative fluoroscopic guidance, and titanium cages with a standard 18 mm width were used. The maximum distraction achieved during discectomy using the trial inserts provided guidance as to the height of the cage. The choice of these XLIF cages (CoRoent XL; NuVasive Inc., San Diego, CA, USA) was decided by the surgeon. Cage lengths ranged from 45 to 55 mm, and heights from 8 to 12 mm.
The lateral PPS technique using the Viper Prime^TM instrument is shown in Fig. 2. Following the XLIF, patients in the P group were turned to the prone position and then prepared again and draped. Bilateral PPS surgery was then performed with the patient in the prone position. Patients in the L group remained in the lateral decubitus position for PPS fixation. An image in the AP view was taken to mark the lateral radiographic borders of the pedicles for screw placement. Using a lateral view, the center of each pedicle was identified and marked. A small incision 2–3 cm lateral to the lateral radiographic borders of each pedicle was made for percutaneous exposure, and the stylets were then docked at the junction of the transverse process and the superior articular process. The stylets were then inserted with a hammer to hold the spot within the pedicles. After the stylets were inserted into the pedicle inner rim, an image in the AP view was taken to confirm in the lateral view that the posterior body wall had been reached. At that point, the C-arm was then brought to a lateral position to maximize the working space for screw placement. After all screws had been inserted, a rod was passed percutaneously and secured to the screw heads using setscrews.
Fig. 2

a Intraoperative images demonstrating placement of the PPS using the Viper Prime^TM with the patient in the lateral decubitus position. Marking the location of the incision. b Upside PPS placement. c Downside PPS placement

In this study, we used 3D-FEA software (Mechanical Finder [MF], version 10.0, Extended Edition, RCCM Co. Ltd., Tokyo, Japan). We analyzed computed tomography (CT) data obtained from a 64-year-old woman with a bone mineral density of 0.717 g/cm². CT was performed at 0.625-mm intervals from the cervical spine to the pelvis, and the CT data were transferred to MF. The ethics committee of our institute approved the use of this patient’s CT data (Approval No. 1748). We created 3D-FEA bone models from the first thoracic vertebra (T1) to the pelvis by extracting bone contour lines using MF. The models consisted of tetrahedral elements with a length of 1.4 mm.
We derived the mass density of the bone ρ (g/cm³) from the CT value (Hounsfield Unit, HU) and calculated the non-homogeneous Young's modulus distribution based on Keyak's formula [3 (link), 4 (link)] to determine the material properties of the finite elements. The Young's modulus E (MPa) is expressed as indicated in Formula 1, as follows: $$E=\left\{\begin{array}{c}0.001\left(\rho ,=,0\right)\\ 33900{\rho }^{2.20}\left(0,<,\rho ,\le ,0.27\right)\\ 5307\rho +469\left(0.27,<,\rho ,<,0.6\right)\\ 10200{\rho }^{2.01}\left(0.6,\le ,\rho \right)\end{array}\right)$$
Figures 1A and B show the diagrams of element decomposition and non-homogeneous Young's modulus distribution, respectively. Table 1 shows the Young's modulus and Poisson's ratio values for the vertebral body and intervertebral discs.Fig. 1

Element segmentation diagram of multi-vertebrae and Young’s modulus distribution diagram. A FE models of spinal fusion. B Heterogeneous distribution of Young's modulus, E. FE, finite element; E, Young’s modulus (MPa)

Table 1

Young's modulus and Poisson's ratio of each element

	Young’s modulus	Poisson’s ratio	Element type
Cortical bone	Determined by Formula 1	0.4	Tetrahedral
Cancellous bone	Determined by Formula 1	0.4	Tetrahedral
Disc	7.5 MPa	0.4	Tetrahedral

We obtained imaging data for the implants used in the actual surgery via micro-CT, which included a pedicle screw (PS; screw), an S2-alar-iliac screw (S2AI), and a transverse hook (TH; hook). Computer-aided design (CAD) data for these implants were created based on the CT data. The CAD software SOLIDWORKS^® (Concord, MA, USA) was used to create the implant models. The diameter of the screw analyzed in the current study was 5.5 mm, with a length ranging from 35 to 40 mm. The diameter and length of the S2AI screw were 8.5 and 90 mm, respectively. The diameter of the rod was 6.0 mm. After creating the implant models, we imported the bone and implant models into MF and created the FE models of long spinal fusion by combining these data. All screws and rods were fixed. The contact condition between implant and bone was set as bonding contact.