Comparison of the Optimal Design of Spinal Hybrid Elastic Rod for Dynamic Stabilization: A Finite Element Analysis

Round 1

Reviewer 1 Report

In this study, the biomechanical effects of different ratios of spinal hybrid elastic (SHE) rods were investigated using finite element analysis. This paper is unique and worthy of publication.

In order to characterize the SHE rods, a comparison with the commonly used material (cobalt-chromium composite) rods is necessary; ROM, disc stress, and facet joint contact forces were slightly increased at the adjacent L2-L3 level compared to intact lumbar spine model, but how are these compare to the commonly used material?

Author Response

• Thanks for your kind comments. The changes to our manuscript were highlighted by using the dark red text. Our responses are as follows. The details are in the Discussion in the revised manuscript.
• PEEK (polyether ether ketone) is a low elastic modulus, similar to that of bone (3.6 GPa), and less rigid than titanium (115 GPa) [22]. PEEK rod is nonrigid and has recently been used to stabilize the spine dynamically. PEEK rods closely approximated the physiologic load sharing [23]. However, the hard surface, durability and fatigue fracture of PEEK rods should be concerned. They were associated with initiating scratching at the rod-screw interface and needed larger diameter locked in specific pedicle screws [24]. Compared to PEEK rod, the outer PCU shell is scratch resistant. The SHE rod system is a 5.5mm diameter for universal pedicle screws. However, it is still necessary to compare the two materials in further biomechanical research.

Author Response File: Author Response.pdf

Reviewer 2 Report

 

Model was not described clearly enough (geometrical structure, properties of bone tissue as well as other biological materials).  Boundary and loading conditions section is not clear. The results, in a qualitative sense, are trivial,  because the effect of applying two rods attached to the intact spine - no fractures and other diseases – is easy to predict. However, the presented values ​​are difficult to assess due to deficiencies in the description of the model (geometry, material parameters) and boundary conditions.

Author Response

• Thanks for your kind comments. The changes to our manuscript were highlighted by using the dark red text. Our responses are as follows. The details are in the Materials and Methods in the revised manuscript.
• 2.1. Finite element model construction of the lumbar spine
• A three-dimensional, nonlinear finite element model of the human lumbar spine was created using the commercial software ANSYS 14.5 (ANSYS Inc., Canonsburg, PA, USA). The material properties of the intact lumbar spine model (INT) have been investigated in previous studies [14-17]. The geometrical structure of our INT model was constructed from the computer tomographic images of the lumbar spine in a 19-year-old male. Each coronal plane slice was obtained by the 3-mm interval and enlarged to identify the different regions of the tissues. The edge of the spinal disc and the geometry of the nucleus were measured and referred to Panagiotacopulos et al. 's study [18]. The 30-50% of the total disc area was defined as the nucleus, and the rest was assumed as the disc annulus. The range of motion (ROM) of the INT was compared with that of cadaveric specimens in in vitro testing. The reliability and similar stiffness were validated [19]. The present mesh density was selected based on the convergence test and the model validated in our previ-ous studies [14, 16]. Overall, the discrepancy between the in vitro tests and our finite ele-ment simulation was within one standard deviation. The model also contained interver-tebral discs, endplates, posterior bony elements, and seven ligaments. These intervertebral discs were composed of a ground substance, hyperelastic annulus fibrosus, and incom-pressible nucleus pulposus with 12 double-cross-linked fiber layers embedded in the ground substance (Table 1).

• 2.2. Boundary and loading conditions
• All models were constrained at the bottom of the L5 vertebrae. For INT, two load steps were imposed within the finite element models. During the first loading step, a perpen-dicular axial force of 150 N was loaded to the top of the L1 spine. In the second load step, a pure unconstrained moment was applied to ensure that the resultant ROM of the L1 to L5 spine was equal for all motions. The displacement control angle was determined using the minimally incremental force method. A load was applied with flexion 24°, extension 12.6°, torsion 18.8° and lateral bending 24.8°. The boundary load is the maximum load. Applying a constant ROM has been proven to be applicable in predicting adjacent seg-ment effects after spinal implantation [16]. The resultant intervertebral ROM and stress of the intervertebral disc and facet joint contact forces were analyzed. Distortion energy the-ory was applied to the intervertebral discs. The von Mises stress of each model was ob-tained after applying torque in each direction of the model.

• Table 1. Material properties used in the finite element model.

Material		Young’s modulus (MPa)	Poisson’s ratio	Area (mm2)	Element type
Bone	Cortical	Ex=11300, Gx=3800 Ey=11300, Gy=5400 Ez=22000, Gz=5400	Vxy=0.484 Vyz=0.203 Vxz=0.203		Solid 185
	Cancellous	Ex=140, Gx=48.3 Ey=140, Gy=48.3 Ez=200, Gz=49.3
	Posterior bone	3500	0.25
	Cartilaginous endplates	24	0.4
Disc	Nucleus pulposus	1666.7			Fluid 80
	Ground substance	C10=0.42 C1=0.105			Solid 185
	Annulus fibers Outermost Second Third Innermost	550 495 412.5 357.5		0.76 0.5928 0.4712 0.3572	Link 10
Facet joint					Contact 174
					Target 170
Interface (Implants/ bone)					Contact 174
					Target 170
Ligaments	Anterior longitudinal	7.8		24	Link 10
	Posterior longitudinal	10		14.4
	Transverse	10		3.6
	Flavum	15		40
	Interspinous	10		26
	Supraspinous	8		23
	Capsular	7.5		30

 

Author Response File: Author Response.pdf

Reviewer 3 Report

I would like to thank the authors for allowing me to review manuscript on a particularly intersting topic.

Authors studied a rather complex medico-surgical issue that is the balance between the need of movment and the need of stability of spine arthrodesis.

Concerning, the introduction, th think authors should discribe more extensively the literature on dynamic system and better explain why their study is important and needed.

The method is unclear and intelligible for readers unaccustomed to this type of model and should be much more educational for readers.

Similarly, the number of models used seems to me much too low to have a statistical value of the results.

Similarly, the tables are difficult to understand because they lack explanatory legends and units of measurement.

The discussion would also benefit from getting closer to the clinical indications and the existing literature on the interest but also the pitfalls of this type of surgery.

I therefore think that the authors should work on the whole manuscript to clarify it so that it is better understood by all. We also need to increase the number of experimental evaluations.

Author Response

• Thanks for the reviewer’s comments and recommendations. Basically, the FE analysis was different from the statistical analysis.
• Statistical science is to predict the possibility of some variables and analyze its trend. Therefore, it needs to have more samples to predict any possibility under different variables and examine its p-value under confidence region.
• However, the FE analysis is computer science. The FE method is a numerical method for solving problems of engineering and mathematical physics. Typical problem areas of interest in engineering and mathematical physics that are solvable by use of the FE method include biomechanical analysis. Such as space engineering or in-vivo biomechanics, they are difficult to collect many specimens to undergo experiment, especially statistical analysis.  As a result, it needs to rely on analytical solutions given by a mathematical expression that yields the values of the desired unknown quantities at any location in a body. Simply speaking, the FE analysis is like a supercomputer to “calculate” biomechanical change. The FE model is only one sample rather than many samples collected for statistical analysis. As a result, there was no FE analysis to perform statistical analysis in all FE studies.

Author Response File: Author Response.pdf

Reviewer 4 Report

This paper investigated some biomechanical effects of SHE rod in different ratios FEA. Except for the discussion/conclusion parts, the paper's logic and presentation in general are clear with scientific soundness. I suggest the authors re-construct these two parts by putting partial contents in discussion into the conclusion. Several citations especially in the discussion part are ambiguous which could mislead readers. 

 

Author Response

• Thanks for your kind comments. The changes to our manuscript were highlighted by using the dark red text. Our responses are as follows. The details are in the Discussion ＆ 5. Conclusions in the revised manuscript.
• 4. Discussion
• The SHE rod dynamic stabilization system is intended to balance the stability and motion of the spine after surgery. In this study, the overall trend indicated that ROM, disc stress, and facet force decreased moderately in the implanted L3-L4 levels, and increased mildly in the adjacent L2-L3 levels. Nevertheless, the stress shielding effect on adjacent levels is mild. The present results demonstrate that the SHE rod can provide good spinal stability and avoid high loading at the adjacent levels.
•     PEEK (polyether ether ketone) is a low elastic modulus, similar to that of bone (3.6 GPa), and less rigid than titanium (115 GPa) [21]. PEEK rod is nonrigid and has recently been used to stabilize the spine dynamically. PEEK rods closely approximated the physiologic load sharing [22]. However, the hard surface, durability and fatigue fracture of PEEK rods should be concerned. They were associated with initiating scratching at the rod-screw interface and needed larger diameter locked in specific pedicle screws [23]. Compared to PEEK rod, the outer PCU shell is scratch resistant. The SHE rod system is a 5.5mm diameter for universal pedicle screws. However, it is still necessary to compare the two materials in further biomechanical research.
•     Nitinol is an elastic shape-memory alloy that minimizes stress shielding, which has even been shown to trend toward superior fatigue resistance compared with titanium [24]. However, nitinol is not a popular material in orthopedic applications because of its poor corrosion resistance and anti-wear properties [25]. To this end, we introduced a PCU polymer as an insulator. The outer PCU shell enveloped the inner Nitinol stick, providing perfect insulation against the titanium screw heads and nuts. In the present FEA, the NS stress increased markedly in the upper and lower portions. This phenomenon is directly related to the connection between the screw head and nut. Nevertheless, the NS stress was always lower than the yield strength of nitinol (816MPa) [26]. NS in the SHE system has a low potential implant failure rate. Even in a worst-case scenario of the thinnest PS of the SHE rod system (Nt45) after rod fracture, the biomechanical effects still afford nearly sufficient spine support and gentle adjacent segment stress [27].
•     The spacer in the Dynesys system is made of PCU, which is the same material as PS [19]. The stabilization of the pretension provided by the spacer and cord in Dynesys differs from that in the SHE rod system. In addition, PCU is less rigid than nitinol. The outer PS stress decreased under the same load owing to the stress shielding by the inner NS. As such, we observed no obvious trend in PS stress as the NS diameter was changed. In most of the present models, the PS stress decreased. The maximum PS stress was still much lower than the mean tensile strength of the PCU (30MPa) [13]. Though Dynesys were some different from materials in the SHE rod system, they both have the same intended use [28]. The SHE rod system provides no inferior spinal support compared to Dynesys system.
•     No other single clinical material can exhibit biphasic rigidity and flexibility. There-fore, we propose the hybrid use of semi-rigid and flexible materials. The semi-rigid nitinol is used for the inner stick, and the flexible PCU is used for the outer shell. A commercially available SHE rod was devised based on the results of the precise ratio of the hybrid used in this study. Considering the industrial technologies, the PCU shell is too thin in Nt45 to manufacture. The optimal NS diameter/PS thickness ratio of the SHE rod system should be 3.5/2.0 mm due to the lower stress concentrations for the durability of the implant.
•     This study had some limitations. All spinal models were healthy with no pathological properties or defects. Degeneration or fractures are common in patients, and posterior decompression is often performed following spinal fixation. As such, the models we applied did not exactly coincide with common clinical situations. The interface between the pedicle screw and PS was simulated to be bonded, as was that between the PS and NS. The model presents an idealized fabrication of the SHE rod.
• 5. Conclusions
• The SHE rod system is a 5.5mm diameter for universal pedicle screws. The inner Ni-tinol is elastic to minimize stress shielding. The outer PCU shell is insulated and scratch resistant. The SHE rod system provides sufficient spinal support and increases gentle ad-jacent segment stress. Considering the durability of the implant, the optimal NS diameter/PS thickness ratio of the SHE rod system is 3.5/2.0 mm.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Looking at the newer version of the article, I see that some necessary information has been added. In this way, it is more acceptable that the first version from a technical point of view. 

Reviewer 3 Report

The authors made efforts to clarify their work. Thank you.