Abstract
Introduction
There are various surgical interventions to manage osteoporotic vertebral compression fracture. Modular spine block (MSB) is a novel intravertebral fixator that can be assembled. This study aimed to quantitatively investigate the force distribution in vertebrae with the various structural designs and implantation methods by finite element analysis (FEA).
Methods
A three-dimensional nonlinear FEA of the L3 implanted with MSB was constructed. Different structural designs (solid vs. hollow) and implantation methods (three-layered vs. six-layered and unilateral vs. bilateral) were studied. The model was preloaded to 150 N-m before the effects of flexion, extension, torsion, and lateral bending were analyzed at the controlled ranges of motion of 20°, 15°, 8°, and 20°, respectively. The resultant intervertebral range of motion (ROM) and disk stress as well as intravertebral force distribution were analyzed at the adjacent segments.
Results
The different layers of MSB provided similar stability at the adjacent segments regarding the intervertebral ROM and disk stress. Under stress tests, the force of the solid MSB was shown to be evenly distributed within the vertebrae. The maximum stress value of the unilaterally three-layered hollow MSB was generally lower than that of the bilaterally six-layered solid MSB.
Conclusions
The MSB has little stress shielding effect on the intervertebral ROM and creates no additional loading to the adjacent disks. The surgeon can choose the appropriate numbers of MSB to fix vertebrae without worrying about poly(methyl methacrylate) extravasation, implant failure, or adjacent segment disease.
Keywords: modular spine block, finite element analysis, osteoporotic vertebral compression fracture, intravertebral fixation, adjacent segment disease
Introduction
Osteoporotic vertebral compression fractures (VCF) often lead patients to painful disabilities and functional limitations. With the rising elderly population, the number of VCF is consequently on the rise1). There are various surgical interventions to manage VCF, many of which are still under debate with regard to overall efficacy.
Open segment fixation has traditionally been proposed to treat unstable burst fractures and fragile VCF2). However, open surgery comes with the risk of excessive blood loss due to prolonged operation time. Complications such as reduced range of motion (ROM) of the spine, pseudarthrosis, adjacent segmental degeneration, or implant failure limit the indications of long segment fixation3).
Percutaneous injection of bone cement was first applied to patients with severe symptomatic vertebral hemangioma in the 1980s4). This palliative method, vertebroplasty, requires poly(methyl methacrylate) (PMMA) to augment the compressed spine. Kyphoplasty is the second-generation vertebroplasty. It can create a cavity by inflating a balloon in the vertebra. Vesselplasty, the third-generation vertebroplasty, uses a polyethylene terephthalate balloon that is left inside the vertebra to expand the compressed vertebral body and contain the bone cement at the same time5). The SpineJack system using a titanium expandable device combined with bone cement can be thought of as the fourth-generation vertebroplasty6). Vesselplasty and SpineJack cannot augment without.
The modular spine block (MSB) is a novel device that can be inserted and assembled within the vertebra. The VCF can be fixed stably by MSB, so there is no need for bone cement anymore. For treating VCF, internal fixation of implants is more optimal and physiological than augmentation with bone cement. Supplementary Figure 1 explains the implanting procedures of MSB. The previous study has validated a spinal model to evaluate the outcomes of the adjacent stresses arising from augmentation with bone cement7). The authors determined the difference in stresses on the adjacent structures after restoring the height from the different severity of the collapsed vertebral body. The results demonstrated that restoration of the initial vertebral body height can reduce the stresses on the adjacent endplates and the risk of fracture. When it comes to the treatment of fragile VCF, adjacent segment diseases (ASD) is the main challenge8). However, surgeons aimed to restore to the complete vertebral height as much as possible. The hypothesis of this study is that an intravertebral device can be fixed in the intact osteoporotic vertebra with minimal consequent adjacent segment stress. In addition, it is anticipated that unilaterally hollow MSB without endplate-to-endplate implanting brings no additional biomechanical burden on the adjacent levels. In the present study, we analyzed the biomechanical effects of a novel intravertebral integrated fixator. ROM, disk stress, and the stress distribution of finite element analysis (FEA) will be evaluated for the overall biomechanical effects of the MSB on vertebrae.
Materials and Methods
Finite element model construction of the lumbar spine
A three-dimensional degenerative finite element spine model was constructed using ANSYS 14.5 (ANSYS Inc., Canonsburg, PA, USA). The material properties of the intact model validated in our previous studies were used in the implanted (INT) model (Supplementary Table 1)9-17). The spinal model was constructed by simulating a whole lumbar vertebra with intact osteoligamentous, intervertebral discs, endplates, posterior bony elements, and seven ligaments. Ground substance and 12 double cross-linked fiber layers embedded in the ground substance included the modeled hyperelastic annulus fibrosus part of the intervertebral disc. The cancellous, posterior bone, cartilaginous endplates, and ground substance of the disk were simulated by solid elements. The nucleus pulposus and annulus fibers of the disk were simulated by fluid elements. The seven ligaments included the anterior longitudinal ligament, posterior longitudinal ligament, transverse ligament, ligamentum flavum, interspinous ligament, supraspinous ligament, and capsular ligament. The ligaments were simulated by link elements.
The degeneration of the disk and nucleus was modified to simulate the elastic modulus and Poisson's ratios of the ground substance by a nonlinear hyperelastic Mooney-Rivlin solid model and the behavior of the adjacent segments12,13). The FEA was adjusted to simulate osteoporosis by a 66% reduction in the elastic moduli of the cancellous bone, and a 33% reduction in the cortical shell, endplates, and posterior elements14).
To confirm the effectiveness of elements, a convergence test was performed and the mesh density of the INT model was modified in our previous research to test the changes in physiological ROM9,11). The results obtained with the lumbar spine model in this study were compared with those of the in vitro cadaveric tests. The ROMs under physiological motions all fell within similar ranges. This verification process has been published in previous studies15,16). The final mesh density was chosen due to a less than 0.5° (4.39%) variation in extension, a less than 0.2° (0.01%) variation in torsion, and a less than 0.1° (0.001%) variation in lateral bending. Including our finite element model, eight well-established static finite element models of the intact lumbar spine were subjected to in vitro measurements. The predictive power and the effectiveness of the intervertebral rotations, disk pressures, and facet joint forces have been published17).
Finite element analysis construction of the implants
A total of seven different finite element models were established, as shown in Fig. 1. The three-dimensional finite element models of solid and hollow MSBs were shown in Fig. 2. The MSBs are made of titanium alloy (Ti6Al4V). The Young's modulus was 110 GPa and the Poisson's ratio was 0.2818,19). The MSB cannot be dissociated separately after assembling. However, sliding between the internal members could occur along the contact faces. Therefore, the contact setting in the model between the internal MSB was set as “no separation.” The MSB was implanted into the body of the L3 spinal vertebra in various arrangements in a computerized model (Table 1). The center of the L3 spine had been hollowed out and occupied by the MSB. The motion of the MSB within the L3 spine was limited even when complete bonding was not used. This study simplified the effect of this part. It is just the same assumption of bonded contact between pedicle screws and bone in other FEAs20). The entire MSB was assumed to be fixed within the vertebral body. The interface between the MSB and the bone was simulated using bonded contact elements.
Figure 1.
Top (upper) and lateral views (lower) of the seven lumbar spine finite element models.
Figure 2.
Solid (A) and hollow MSB (B). The isometric view is shown on the left, while the lateral and top views are shown in the middle and right, respectively (unit: mm).
Table 1.
Elements and Nodes of the Seven Finite Element Models.
| Groups | MSB
structure |
Assembly
layers |
Sides | Element | Node |
|---|---|---|---|---|---|
| INT | 228,538 | 189,824 | |||
| S6-1 | Solid | 6 | Unilateral | 309,021 | 207,508 |
| S6-2 | Solid | 6 | Bilateral | 302,788 | 206,697 |
| H6-1 | Hollow | 6 | Unilateral | 448,378 | 239,798 |
| H6-2 | Hollow | 6 | Bilateral | 362,145 | 231,007 |
| H3-1 | Hollow | 3 | Unilateral | 442,537 | 239,406 |
| H3-2 | Hollow | 3 | Bilateral | 403,510 | 241,177 |
Boundary and loading condition
In our FEA, the bottom side of the L5 spine was fixed. The loading condition was evaluated using the hybrid multidirectional test method21). Besides bony structures, the muscle strength was also assessed in some studies22). However, the verification of our study was based on in vitro cadaveric experiments without assessment of the muscle strength. It was pointed out that the maximum pre-loads that can be applied to the five-segment spine specimens with dissected muscle was 150 N23). Applying more than 150 N increased significantly instability and even buckling. The pre-loads can be added more if the testing segments are reduced. Two load steps were imposed within the finite element models. During the first loading step, a perpendicular axial force of 150 N was loaded on top of the L1 spine. In the second loading step, a pure unconstrained moment was applied to ensure that the resultant ROM of the L1 to L5 spine would be 17° in flexion, 6° in extension, 9° in torsion, and 14° in left lateral bending. Under the four different moments, the ROM levels were recorded at the adjacent levels of the operated L3 (L2/L3 and L3/L4). Adjacent disk stress of the operated L3 spine (L2/L3 and L3/L4) was estimated in terms of von Mises stress. The stress levels of the MSB and the stress distribution of the vertebral body were analyzed in this study.
Results
Intervertebral ROM
Since the design of this experiment was to simulate the implantation of the MSB into the L3 spine, we focused our observations on the ROM of only the adjacent disk levels L2/L3 and L3/L4. The changes in the intervertebral ROM of the lumbar spine upon implantation of the MSB were small (less than 4%), as shown in Table 2 and Supplementary Table 2.
Table 2.
Intervertebral ROM (Unit: Degree), Intervertebral Disc Stress (Unit: kPa), and Facet Contact Force (N) of the Test Groups.
| INT | S6-1 | S6-2 | H6-1 | H6-2 | H3-1 | H3-2 | |||
|---|---|---|---|---|---|---|---|---|---|
| Flexion | L2-L3 | ROM | 3.98 | 3.98 | 3.98 | 3.98 | 3.98 | 3.98 | 3.98 |
| Disc stress | 762.07 | 761.91 | 761.93 | 762.03 | 762.03 | 762.04 | 762.03 | ||
| L3-L4 | ROM | 3.92 | 3.91 | 3.91 | 3.92 | 3.92 | 3.92 | 3.92 | |
| Disc stress | 642.80 | 642.23 | 641.99 | 642.62 | 642.65 | 642.58 | 642.65 | ||
| Extension | L2-L3 | ROM | 1.80 | 1.80 | 1.81 | 1.80 | 1.81 | 1.79 | 1.81 |
| Disc stress | 350.10 | 350.31 | 350.70 | 350.58 | 350.76 | 350.04 | 350.76 | ||
| Facet force | 34 | 34 | 34 | 34 | 34 | 33 | 34 | ||
| L3-L4 | ROM | 1.42 | 1.47 | 1.42 | 1.42 | 1.42 | 1.40 | 1.42 | |
| Disc stress | 290.11 | 289.53 | 290.18 | 290.84 | 289.07 | 288.49 | 289.07 | ||
| Facet force | 38 | 37 | 38 | 38 | 38 | 37 | 38 | ||
| Torsion | L2-L3 | ROM | 1.94 | 1.94 | 1.94 | 1.94 | 1.93 | 1.94 | 1.93 |
| Disc stress | 275.97 | 275.66 | 275.22 | 275.62 | 274.84 | 275.85 | 274.84 | ||
| Facet force | 76 | 76 | 76 | 76 | 75 | 76 | 75 | ||
| L3-L4 | ROM | 2.26 | 2.26 | 2.26 | 2.26 | 2.25 | 2.26 | 2.25 | |
| Disc stress | 299.10 | 299.15 | 299.61 | 299.49 | 299.33 | 299.52 | 299.33 | ||
| Facet force | 74 | 74 | 74 | 74 | 74 | 74 | 74 | ||
| Lateral bending | L2-L3 | ROM | 3.62 | 3.61 | 3.62 | 3.62 | 3.62 | 3.62 | 3.62 |
| Disc stress | 697.79 | 697.28 | 697.17 | 697.74 | 697.68 | 697.77 | 697.68 | ||
| Facet force | 2 | 2 | 2 | 2 | 2 | 2 | 2 | ||
| L3-L4 | ROM | 3.31 | 3.32 | 3.32 | 3.32 | 3.32 | 3.32 | 3.32 | |
| Disc stress | 642.49 | 642.94 | 642.10 | 643.38 | 642.81 | 643.89 | 643.81 | ||
| Facet force | 3 | 3 | 3 | 3 | 3 | 3 | 3 |
Intervertebral disc stress
We also focused our observations on the intervertebral disc stress of only the adjacent disk levels L2/L3 and L3/L4. The intervertebral disc stress increased by less than 1% in all four directions with respect to the original non implanted model (INT). As expected, the implantation of the MSB did not result in higher stress concentration in the adjacent intervertebral discs, as shown in Table 2 and Supplementary Table 3.
Maximum stress value on modular spine block
The specially designed U-shaped grooves at the anterior portion of the MSB that permit an elastic buckle were anticipated as weak points during compression loading. The results of this study, however, showed otherwise. The intravertebral force distribution at the anterior portion of the MSB was relatively low compared with other parts, as shown in Fig. 3. The hollow MSB design was a more ideal choice for an internal fixator compared with a solid design as it allowed packing bone grafts for better osseointegration. Even though stress would be more concentrated at the vertical pillars in the hollow design, no risk of breakdown is shown in this study.
Figure 3.
Intravertebral force distribution under flexion (A) and extension (B) in L3 (Unit: Pa).
Overall, the bilaterally implanted six-layered hollow MSB produced the highest stress values in lateral bending. It was also observed that the stress was generally higher in the six-layered models compared with the three-layered models. This difference was especially significant for torsion in the unilateral side (17.9 for H6-1 vs. 10.4 for H3-1). In terms of the difference between different structures (solid vs. hollow), the stress was generally higher in the hollow MSB than in the solid MSB. This difference was most significant for torsion in the bilateral side (28.1 for H6-2 vs. 12.4 for S6-2). Finally, with regard to the difference of laterality, the stress induced in bilateral implantation was generally higher than unilateral implantation. The differences in stress are shown in Table 3.
Table 3.
Maximum Stress Value on the MSB of the Test Groups (Unit: MPa).
| S6-1 | S6-2 | H6-1 | H6-2 | H3-1 | H3-2 | |
|---|---|---|---|---|---|---|
| Flexion | 11.8 | 7.1 | 10.1 | 11.1 | 8.24 | 9.29 |
| Extension | 9.04 | 14.5 | 13.6 | 19.7 | 9.86 | 10.8 |
| Torsion | 8.35 | 12.4 | 17.9 | 28.1 | 10.4 | 20.2 |
| Lateral bending | 14.8 | 20.2 | 6.96 | 32.9 | 6.31 | 20.6 |
Intravertebral force distribution
Flexion in L3
Under flexion analysis, stress was concentrated on the anterior-column element as well as the cortical bone. Among comparisons with regard to implant design, we showed that the top side of the sold MSB was under the greatest level of stress (Fig. 3A). This was especially significant in unilateral implantation. Stress was shown to be more evenly distributed among the implants under bilateral implantation. Within the hollow MSB, stress was more concentrated on the center pillars.
Extension in L3
Under extension analysis, stress was concentrated on the posterior-column, pedicle, and cortical bone, while low levels of stress are shown in the cancellous bone (Fig. 3B). With regard to different implant designs, the solid MSB can withstand lower stress levels than the hollow MSB. The highest levels of stress within the MSB occured at the bottom base as well as the posterior columns.
Discussion
There was a higher rate of subsequent fracture after vertebroplasty or kyphoplasty. One reason is the uneven force from cement bridging. When liquid cement is injected into the vertebral body, it is unpredictable to know the end of flow. The unequal augmentation of PMMA caused the abnormally increased stiffness and intravertebral pressure within the vertebrae. The stress is unevenly distributed to the adjacent endplate to result in fracture24). Precise assembly of the MSB can be performed through a MISS approach unilaterally or bilaterally, while keeping the endplates and disks intact and minimizing the effects on the adjacent segments. In our FEA, similar intervertebral ROM results were provided in all directions after insertion of the MSB unilaterally and bilaterally. Additionally, no more increase in disk stress and facet force were observed even under unilateral implantation of MSB at the adjacent disks in the osteoporotic models (Table 2).
Another reason for adjacent fracture after vertebroplasty or kyphoplasty is the large volume effect of the bone-cement injection. The average cement volume used was as much as 2-10 mL in vertebroplasty or kyphoplasty, but the vertebral height restoration was only less than 5 mm25). In contrast, even S6-2 was the maximum volume of MSB in the present study. It was less than 2.8 mL and far smaller in volume compared with cement bridging. On the other hand, the total height of each expanding MSB was 6.3 mm of three layers and 14.4 mm of six layers. It was similar to that of commercial implants and far less than the intact lumbar vertebral height or cement bridging both endplates6,25). Unlike the optimal technique of cementing, MSB is not necessary to augment the whole vertebral body, nor to get endplate-to-endplate filling26). Our findings suggest that the mainly vertical MSB can be sufficient to support the spine, which was consonant with the cephalic-caudal stress when standing (Table 3). The impacts on intervertebral ROM and disk stress are minimal; therefore, it is reasonable to assume that MSB is less likely to cause ASD.
As seen in our study, the maximum stress did not exceed 33 MPa after insertion of the MSB. That is far below the value of the yield strength of titanium alloy, ranging from 450 to 760 MPa, which depends on the manufacturing process27). Thus, it can be said that the MSB carries no risk of implant deformity or ultimate failure. The maximum stress value of the unilateral three-layered MSB is generally lower than that of the bilateral six-layered MSB, as shown in Table 2. One possible explanation for this result might be because the vertebrae in our FEA are designed to be solely osteoporotic, but without the compression deformity. Compared with the six-layered MSB, the three-layered MSBs are farther away from the endplates. Overall, the stress is generally higher in the bilateral, six-layered, and hollow MSB models compared with the unilateral, three-layered, and solid MSB models, respectively. In other words, large volume and hollow MSB structure shields the more stress from the bone.
A major limitation of this study is the fact that this FEA is conducted under nearly ideal conditions, which do not fully represent real clinical situations. For example, instead of establishing a model that bears the pathological properties of a compressed and collapsed vertebra, a simplified simulated osteoporotic vertebra without deformity is used (Supplementary Table 4). There is a wide variety of types of vertebral fractures, and we tried to limit the number of controlled variables. A certain level of difficulty remains in assessing the advantages and disadvantages of all the different types of diagnostic and treatment methods28). The previous results demonstrated that restoration to the initial vertebral body height can reduce the stresses on the adjacent endplates and the follow-up fractures7). The difference of the stresses on the adjacent structures after restoring the height from the different severity of the collapse. It was found that the same trend for the same percentage restoration. Surgeons just aimed to restore the vertebral height to be as complete as possible29). We used the most reliable finite element spinal model to simulate in all motions and not limited study of the few available flexion data. Moreover, this study aimed to investigate the biomechanics of intact osteoporotic vertebra fixed with MSB. So, the spinal model we analyzed was the normal health vertebra without collapse. In future studies, it will be necessary to include a vertebral compression fracture model in FEA. Another limitation lies in the lack of a comparison of the effects of the MSB and PMMA bone cement, respectively, within the vertebrae. The difference in the biomechanical effects and properties of various materials is another important issue to be discussed.
In conclusion, osteoporotic spine with intravertebral fixator is stronger than that with nothing inside. The MSB has little stress shielding effect on the intervertebral ROM and creates no additional loading to the adjacent disks. The surgeon can choose the appropriate numbers of the MSB to fix the vertebrae without worrying about PMMA extravasation, implants failure or ASD.
Conflicts of Interest: The authors declare that there are no relevant conflicts of interest.
Sources of Funding: None.
Author Contributions: Yi-You Huang designed the study. Jui-Yang Hsieh, Shao-Ming Chuang, and Chen-Sheng Chen participated in the conception, acquisition, analysis and interpretation of the data, drafting, and final preparation of the manuscript. Jyh-Horng Wang and Po-Quang Chen revised the manuscript. All authors read and approved the final manuscript.
Ethical Approval: It is not necessary for finite element analysis to ethical approval.
Informed Consent: Informed consent for publication was obtained from all the participants in this study.
Supplementary Materials
Brief implanting procedures of MSB (from the left to the right): transpedicular insertion of the first MSB member, first MSB member placement, second MSB member assembly, and third MSB member assembly.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Supplementary Materials
Brief implanting procedures of MSB (from the left to the right): transpedicular insertion of the first MSB member, first MSB member placement, second MSB member assembly, and third MSB member assembly.