Spinal screws
Monoaxial pedicle screws (Catalog no. CMS05135, Kyocera Medical Corporation, Kyoto, Japan) measuring 45 mm in length, with an outer threaded diameter of 5.5 mm, an inner threaded diameter of 3.8 mm (start point), and of 4.6 mm (end point), made of titanium alloy (Ti-6Al-4 V[ELI], American Society for Testing and Materials [ASTM] F136) were used in this study. To measure the ISQ value with the Osstell ISQ® system (Osstell Integration Diagnostics, Gothenburg, Sweden), two neodymium magnets (Magfine Corporation, Miyagi, Japan) were attached to the head of the pedicle screw as previously reported [19].
Spinal dissection and treatment before the experiment
For the experimental use of fresh non-frozen human cadavers, written informed consent was obtained from each donor according to the ethical guidelines of our institution (approval number: 20070026). Dissection was performed as soon as possible post-mortem. All experiments were approved by the Ethics Committee (approval number: 20150385). Nine human thoracolumbar spinal sections (Th8–L5) were used in this study (mean donor age, 76 years; age range, 72–86 years; seven female and two male cadavers). The intervertebral disks and ligaments were dissected from the vertebrae. Vertebrae that showed spinal fusion (n = 4) or were damaged during preparation (n = 6) were excluded. The remaining vertebrae were visually inspected and diagnosed using micro-CT images by two spinal surgeons (11 and 27 years of experience), and vertebrae showing fractures or spinal metastases were excluded (n = 3). The specimens were individually wrapped in Ziploc polyethylene bags (S.C. Johnson & Son, Incorporated, Racine, WI) for micro-CT imaging [17].
Micro-CT and MSCT imaging before pedicle screw insertion
After the scan of a BMD phantom (RATOC, Tokyo, Japan), the vertebrae were scanned using a micro-CT system (R_mCT2 FX, Rigaku Corporation, Tokyo, Japan) with the following settings: x-ray voltage, 90 kVp; tube current, 160 μA; exposure time, 120 s, continuous (non-stepping) rotation. For the BMD phantom and each vertebra, a stack of 400 cross-sectional slices, corresponding to a total height of 57 mm, was reconstructed with a slice-to-slice distance of 1 pixel (144 μm). The scan data of the BMD phantom were used to convert the CT number (Hounsfield units) to BMD (mg/cm3).
After micro-CT, MSCT was performed. The vertebrae were placed in an in-house rounded rectangle-shaped tank (Fig. 1a, b) that simulated the human body, and MSCT imaging was performed with a BMD phantom (B-MAS200, Fujirebio, Tokyo, Japan) (Fig. 1c, d) using an MSCT system (Discovery CT750, GE Healthcare, Chicago, IL) using a standard protocol (for the phantom, 120 kVp, 250 mA, voxel size 683 × 683 × 5,000 μm, field of view 350 × 350 mm; for the vertebra: 120 kVp, 250 mA, voxel size 293 × 293 × 625 μm, field of view 150 × 150 mm) (Fig. 1d). The scan data of the BMD phantom were used to convert the CT number to BMD (mg/cm3) [25].
Fixation strength experiment
Pedicle screw insertion
Pedicle screw insertion was performed by a spinal surgeon. A pilot hole was drilled in the vertebra using an awl and a 20-mm probe without a tap. The pedicle screw was inserted 40 mm.
Insertion torque measurement
The digital torque gauge HTGA-5 N (Imada Company Limited, Aichi, Japan) was used to measure the insertion torque (peak torque [19, 26, 27]) at 40-mm insertion. The specifications of this torque gauge were as follows: accuracy ± 0.5% (full scale) ± 1 digit and sampling rate 2,000 data/s. The insertion torque (Newton meter, N m) is a moment force, and it increases progressively as the screw advances in the vertebra. This insertion torque peaks before the screw head comes in contact with the vertebra, which is defined as peak torque. This torque is experienced as the test force of the pedicle screw by the surgeon (Fig. 2a) [19, 26].
Resonance frequency analysis
It was conducted using a specific device (Osstell Integration Diagnostics, Gothenburg, Sweden) without screw contact after completion of the pedicle screw insertion as reported previously (Fig. 2b) [19]. Materials were not held by a fixture during measurement and were placed on a normal laboratory table instead. The pedicle screw was vibrated with a micromagnetic wave, which generated inertial forces owing to the mass of the magnets in a plane perpendicular to the axis of the screw.
Micro-CT imaging after pedicle screw insertion
Micro-CT imaging was performed again for the vertebrae with the pedicle screws using the same protocol as that before screw insertion (Fig. 3b).
Pull-out force measurement
The pull-out force measurement was performed according to ASTM-F543-07 testing standards [28]. The vertebrae were placed on a specially fabricated fixture with a self-position adjustment function to ensure vertical pull-out alignment. Then, they were held in the appropriate position on a base plate. The maximum pull-out force was measured using AG-IS 10 kN (Shimadzu Corporation, Kyoto, Japan; testing speed 5 mm/min) [28]. Strength was continuously recorded in 0.1-mm increments until its peak (Fig. 2c).
Volume BMD and micro-architectural parameters of the vertebrae
Imaging analysis was performed using dedicated software (TRI/3D-BON, RATOC, Tokyo, Japan) to calculate volume BMD and the micro-architectural parameters of the vertebrae. First, 3D volume position adjustment between pre-insertion (both micro-CT and MSCT) (Fig. 3a) and post-insertion (micro-CT) (Fig. 3b, c) images was performed, and the volume of the cancellous bone where the pedicle screw was present was extracted (Fig. 3d, e). The cutoff value was 120 mg/cm3. The cortex was extracted before calculating the 3D volume position adjustment. Volume BMD and micro-architectural parameters were calculated. The following micro-architectural parameters were assessed: bone volume fraction (bone volume/total volume: BV/TV, %), bone surface density (bone surface/total volume (BS/TV), mm−1), trabecular thickness (mm), trabecular separation (mm), trabecular number (mm−1), structure model index (SMI), and number of nodes per volume (NNd/TV, mm−3) [17, 29,30,31]. SMI is a measure for the relative number of rod- and plate-like trabecular bone structures, with values ranging from 0 (ideal plate-like structure) to 3 (ideal rod-like structure), values in between representing a mixture of plates and rods [32]. In recent years, SMI has frequently been used to investigate the relationship between CT and implant fixation strength in research [17, 32]. NNd/TV, unlike SMI, indicated the complexity of trabecular bone structure from the viewpoint of the number of bonding points of the trabecular beam structure.
Statistical analysis
The three measures (peak torque, pull-out force, and ISQ value) and parameters of both micro-CT and MSCT were obtained. First, Spearman’s rank moment correlation coefficient (ρ) with Bonferroni correction was used to evaluate the relationship among the three measures. The significance level was set at p = 0.0166. Second, the same analysis was performed to evaluate the relationship between the three measures and the parameters of both micro-CT and MSCT. The significance level was set at p = 0.00625. All statistical analyses were performed using SPSS Statistics software version 24 (International Business Machines Corporation, Armonk, NY).