Does Sagittal Location of the Fracture Region of Vertebral Body Affect the Interfixation Instrument Biomechanics or Surgical Strategy in Thoracolumbar Fracture Treatment: a Finite Element Analysis

doi:10.21203/rs.3.rs-2954498/v1

Download PDF

Research Article

Does Sagittal Location of the Fracture Region of Vertebral Body Affect the Interfixation Instrument Biomechanics or Surgical Strategy in Thoracolumbar Fracture Treatment: a Finite Element Analysis

https://doi.org/10.21203/rs.3.rs-2954498/v1

This work is licensed under a CC BY 4.0 License

Version 1

posted

You are reading this latest preprint version

Background: Posterior internal fixation is the main method used for the treatment of thoracolumbar fractures. Fractures often occur in the upper 1/3 of the vertebral body. However, they can also occur in the middle or lower 1/3 of the vertebral body. At present, there is no report discussing the potential effects of sagittal location on instrument biomechanics or surgical strategy. The objects of this study was to investigate the effect of the sagittal location of the fracture region of the vertebral body on the biomechanics of the internal fixation system and surgical strategy.

Methods: A finite element model of the T11-L3 thoracolumbar segment was established based on a healthy person’s CT scan. Different sagittal fracture location finite element models were created by resection of the upper 1/3, middle 1/3, and lower 1/3 of the L1 vertebral body. Three surgical strategies were utilized in this study, namely proximal 1 level and distal 1 level (P1-D1), proximal 2 level and distal 1 level(P2-D1), and proximal 1 level and distal 2 levels (P1-D2). Nine fixation finite element models were created by combining fracture location and fixation strategies. Range of motion, von Mises stress, and stress distribution were analyzed to evaluate the effects on the instrument biomechanics and the selection of surgical strategy.

Results: In all three different fixation strategies, the maximum von Mises stress location on the screw did not change with the sagittal location of the fracture site; nevertheless, the maximum von Mises stress differed. The maximum rod stress was located at the fracture site, with its value and location changed slightly. In the same fixation strategy, a limited effect of sagittal location on the range of motion was observed. P2D1 resulted in a shorter range of motion and lower nail stress for all sagittal locations of the fracture compared with the other strategies; however, rod stress was similar between strategies.

Conclusions: The sagittal location of a fracture may affect the stress and distribution, but does not influence the selection of surgical strategy.

thoracolumbar fracture

sagittal location

instrument

biomechanics

surgical strategy.

The thoracolumbar spine is the transition region from the fixed thoracic spine to the flexible lumbar. It is the most common area for spinal fractures, accounting for 90% of cases ¹, among which greater than 20% of cases were burst fractures ². Fracture fragments can invade the spinal canal and potentially cause spinal cord injury. The main focus in the treatment of thoracolumbar fractures is spinal stability. Burst fractures involve the anterior and middle columns, which are considered unstable ³. Internal fixation is the most important surgical option for the treatment of unstable thoracolumbar fractures. Internal fixation could be divided into anterior, posterior, and combined anterior and posterior operations. Posterior transpedicular internal fixation is the most commonly used method. Short-segment, limited long-segment, and long-segment fixation has been previously reported in the literature ^4–6. These techniques have resulted in good clinical outcomes; however, instrument looseness and breakage can occur, with an incidence of up to 16% ⁷.

Finite element (FE) analysis offers the advantages of good repeatability and cost-effectiveness. Thus, it is widely used in research on spinal fractures for internal instrument development, mechanical testing, and spine fracture biomechanical research ^{8, 9}. Numerous studies have compared the maximum stress and distribution of different fixation methods. However, not all internal instruments have the same fracture location as predicted in clinical practice.

Studies revealed that the location and size of upper endplate injury in the coronal plane affect internal fixation and vertebral body stress. Wang and Hu found that in cases with 4/5 endplate fractures, internal fixation should not be removed after surgery ¹⁰. In patients with spinal tumors, the location and size could also affect spinal biomechanics ¹¹. Most thoracolumbar fractures involve the upper 1/3 of the vertebral body; however, the middle or lower 1/3 may occasionally be involved ¹². Therefore, we hypothesized that the sagittal distribution of fractures may affect the internal fixation max stress and its location.

In this study, three types of A3 fracture models were established by resecting the upper, middle, and lower 1/3 of the L1 vertebral body, respectively. Three internal fixation strategies were used, namely, one proximal and distal segment (P1D1), two proximal and one distal segment (P2D1), and one proximal and two distal segments (P1D2). The objectives of this study were to 1) investigate the effects of fracture sagittal distribution on internal fixation biomechanics, and 2) determine its potential influence on the selection of internal fixation strategy.

A 25-year-old male volunteer without spinal trauma, cancer, or a history of surgery was involved in the study with written informed consent. The study was approved by the Ethics Committee of Fu Yang Hospital (Anhui, China).

Computed tomography images of T10-L4 were obtained using a Brilliance 256 CT scanner (Philips Brilliance iCT256, Eindhoven, Netherlands). A three-dimensional model of T11-L3 was established with Mimics 21.0(Materialise, Leuven, Belgium). The cortical bone and cancellous bone were based on geometry using the “Threshold” and “Regional Growth” tools. The mesh structure was prepared using the preprocessing software Geomagic Studio 12.0 (Geomagic, Cary, NC, USA). Notably, the thickness of the cortical bone and endplate was 1 mm and 0.5 mm, respectively.

UGNX12.0 (Dassault Systèmes, S.A, Paris, France) was used to construct the intervertebral disc. The nucleus pulposus and annular fibers were constructed separately. Additionally, the volume ratio of the annulus fibrosus to the nucleus pulposus was set to 6:4.

A baseline three-dimensional FE model of a healthy T11-L3 was created first. The vertebral body, endplate, nucleus pulposus, and annulus fibrosus were treated as bonded. Moreover, frictionless contact was used to simulate the sliding contact between articular cartilages. The model was assumed to be homogeneous, isotropic, and linearly elastic. The material properties used are presented in Table 1 (Alizadeh et al., 2013, Kim and Kim, 2010, Park et al., 2015, Wang et al., 2018). The ligaments (i.e., the anterior longitudinal ligament, posterior longitudinal ligament, ligamentum flavum, capsular ligament, and interspinous ligament) were constructed as nonlinear spring elements in ANSYS Workbench (Ansys, Pittsburgh, PA, USA), and the material properties were shown in Table 2 (Rohlmann et al., 2006).

Table 1

Material properties assumed for different components of the finite element (FE) model
Spinal site	Young’s modulus (MPa)	Poisson’s ratio
Vertebra
Cortical	12,000	0.3
Cancellous bone	100	0.2
Endplate	23.8	0.4
Cartilage	11	0.4
Intervertebral disc
Nucleus pulposus	1	0.49
Annulus fibrosis	4.2	0.4
Pedicle screws and rods	110,000	0.3

Table 2

Material properties assumed for different components of ligaments
Ligament	Rigidity	Strain ε (%)	Rigidity	Strain ε (%)	Rigidity	Strain ε (%)
Anterior	347	0–12.2	787	12.2–20.3	1,864	20.3
Posterior	29.5	0–11.1	61.7	11.1–23	236	23
Capsular	36	0–25	159	25–30	384	30
Intertransverse	0.3	0–18.2	1.8	18.2–23.3	10.7	23.3
Flavum	7.7	0–5.9	9.6	5.9–4.9	58.2	49
Supraspinal	2.5	0–20	5.3	20–25	34	25
Interspinal	1.4	0–13.9	1.5	13.9–20	14.7	20

Model creation for different sagittal fracture distribution

The fractured vertebra model was created using SolidWorks(Simulia, USA). The upper, middle, and lower 1/3 of L1 were resected, and the posterior structure was maintained to establish an unstable type A3 thoracolumbar fracture (according to the AO spinal fracture classification) (Fig. 1).

Creation of the pedicle and screw and rod models

Pedicle screws (6 mm×50 mm for lumbar; 5.5 mm×45 mm for thoracic vertebra) and rods (5.5 mm) were modeled using SolidWorks. The screws were inserted into the vertebra and connected with rods (Fig. 1). Bonded contact is used between the screw and the vertebra, as well as between the screw and the rod. The mesh size was set to 1 mm for each screw and rod, and the union included a total of 42,923 elements and 152,473 nodes.

Models of different surgical strategies

The models included upper 1/3 fracture, (U-P1D1, U-P2D1, U-P1D2), middle 1/3 fracture, (M-P1D1, M -P2D1, M -P1D2), and lower 1/3 fracture (L-P1D1, L -P2D1, L-P1D2) (Fig. 2). Nine models were created in total.

Load and boundary

The load and boundary conditions were based on research published by Wang et al. ¹³. The L3 vertebra body was fixed. A force of 400 N was applied to all the models. Movement in coronal, sagittal, and transverse planes were evaluated, including extension, lateral flexion, and rotation motions. The flexion and lateral flexion moments were assumed to be 7.5 N*M, while the axial rotation moment was assumed to be 5.5 N*m.

Assessment indices

The range of motion (ROM) of T12-L2 was assessed in the nine FE models under six loading conditions. Data for the maximum von Mises stress and location were also collected and analyzed.

Validation of the intact T11-L3 FE model

Following the creation of the normal T11-L3 FE model, data on movement induced by 7.5 N*M in flexion and 5.5 N*M in rotation were collected. The ROM values of the T11-L3 segment were as follows: flexion 6.36°; extension 8.12°; left bending 9.9°; right bending 7.85°; left rotation 4.61°; and right rotation 3.78°. The ROM results were comparable with those reported by Alizadeh et al. (2013), Disch et al. (2007), and Schmoelz et al. (2010) (Fig. 3).

ROM of the T12-L2 in nine FE models

In all models, the maximum ROM was obtained at the flexion motion. The ROM values are shown in Table 3 and Fig. 4. In the same fracture location, P2D1 fixation had the shortest ROM versus the other methods. Moreover, under the same fixation model, the model of the upper 1/3 fracture had the shortest ROM. These findings indicated that lower fracture locations were associated with larger ROM.

Table 3

Range of motion (ROM) of the finite element models of different fixation strategies (°)
Fracture location	Upper 1/3 fracture			Middle 1/3 fracture			Lower 1/3 fracture
Model	P1D1	P2D1	P1D2	P1D1	P2D1	P1D2	P1D1	P2D1	P1D2
Motion
Flexion	11.63	9.93	10.07	12.85	9.17	9.59	12.85	10.83	11.00
Extension	4.77	4.60	4.26	4.75	4.57	4.40	4.75	4.57	4.40
Left bending	1.35	0.74	0.63	0.82	0.70	0.65	1.72	0.70	0.63
Right bending	1.32	0.53	0.62	1.87	0.70	0.64	1.00	0.68	0.64
Left rotation	2.20	1.70	1.85	1.90	1.80	1.90	2.17	1.79	1.92
Right rotation	2.20	1.86	1.80	1.79	1.80	1.91	2.19	1.79	1.93

Maximum von Mises stress on the screws and rods

Data on the maximum von Mises stress on pedicle screws are presented in Fig. 4. The maximum stress, location, and motion are shown in Fig. 5. The maximum Von Mises stress on screws was obtained at the flexion motion in all models, and P2D1 was associated with the lowest screw stress compared with the other techniques. However, similar stress on rods was observed in all models.

Only slight differences in stress on screws and rods were noted between different fracture locations (Fig. 6).

To investigate the influence of sagittal location of vertebral body fracture on internal fixation system stress and the selection of surgical internal fixation strategy, 9 finite element models with three types of sagittal and three surgical strategies were developed in this study. The simulation results showed that the sagittal location of the fracture does not affect the surgical strategy choice but can affect the internal fixation stress.

Spinal fractures often occur in the thoracolumbar segment, and T12 and L1 are the most frequently involved sites (e.g., L1 accounts for 63% of cases) ¹⁴. Thus, we selected L1 to establish the FE model. There are numerous fixation methods for thoracolumbar fracture. In addition to P1D1, P2D1, and P1D2 included in this study, there are also P2D2, and P1D1 with injured vertebral nailing, etc. However, nailing may not apply to the treatment of a burst fracture combined with a pedicle fracture. Furthermore, it is difficult to establish the FE model because the screws are exposed in upper 1/3 fractures. It is thought that P2D2 does not significantly reduce stress on screws compared with P2D1 and P1D2. P2D1 involves the fixation of more segments, thereby resulting in loss of movement. P2D2 is not recommended as the first choice for the treatment of thoracolumbar fractures ^{15, 16}. Therefore, in this study, we selected the P1D1, P2D1, and P1D2 fixation techniques.

Multiple posterior internal fixation surgical techniques have been widely used in clinical practice, with significant effects in the treatment of thoracolumbar fractures. However, controversy remains regarding the selection of surgical strategies ¹⁷. In recent years, studies have analyzed the biomechanical stability of fracture regions using different internal fixation models. The majority of these studies modeled the lower 1/3 of the vertebral body resection. Nevertheless, fractures can occur in the upper, middle, and lower regions ^{16, 18, 19}. Additionally, L1 is the most common site of thoracolumbar fractures. Therefore, T11-L3 was chosen to establish a fracture model of upper, middle, and lower 1 / 3 fractures of L1. Our results show that the sagittal distribution of fractures influences the ROM. In all models, the largest ROM was obtained at the flexion motion. This finding is consistent with those noted in previous studies ^{4, 15, 19}. Under the flexion motion, fractures in the middle and upper 1/3 are associated with the shortest and longest ROM, respectively. For example, in the P1D1 fixation model, the ROM values for the upper, middle, and lower 1/3 were 11.63°, 9.93°, and 10.07°, respectively. The probable cause is that fractures located in the middle 1/3 have more uniform internal fixation stress is more uniform and greater stability. The pedicle plays an important role in the stability of the spine. When the fracture is located in the upper 1/3, the pedicle is involved, thereby increasing the ROM.

The biomechanical stability of fixation models is related to the extent and location of fixation ²⁰. This study showed no significant difference in the ROM of extension, lateral bending, and axial rotation between the P2D1 and P1D2 models at the three fracture levels. The ROM values for both models were lower than those recorded for the P1D1 model at all six states of motion. This implies that six-screw fixation in the fracture area could provide more spinal stability than short-segment fixation. Theoretically, the addition of the fixation segment provides additional fixation points for fracture reduction and kyphosis correction. This is consistent with the conclusions of previous studies ^{21, 22}. In all models, the maximum screw stress was obtained under flexion motion. The distance between the fracture position and the screw affects the instrument stress ²³. Our findings showed that, under the same type of internal fixation, the fracture location altered (reduced or increased) the stress on the screw but did not affect the maximum stress location (Fig. 4, 6).

It has been documented that stress concentration occurs in adjacent segments under short-segment fixation. This may cause looseness and breakage due to the increased bearing stress of the internal fixation ²¹. The results of this study further indicated that an increase in the number of screws can reduce the average stress, thus reducing the risk of screw breakage ²². In the three fracture distributions analyzed in this study, the maximum stress of P1D2 and P2D1 was significantly lower than that of P1D1. This result was comparable with those reported in previous studies ²⁴. Hence, P2D1 was considered more appropriate in this setting than P1D2 ¹⁶. However, they did not consider the sagittal location variation of the fractures. The current study showed that P2D1 has good stability in different sagittal distribution models and small internal fixation stress. The results indicate that the sagittal location of the fracture does not affect the choice of surgical strategy.

The fracture site is the most unstable part. The maximum stress on the rod is mostly located at the fracture site ⁹. The present study yielded similar results. The site of maximum stress on the rod shifted downward in parallel with the location of the fracture, and the stress value changed accordingly. For the three fixation strategies, the maximum stress on the instrument in the upper and lower 1/3 fracture models was similar. In the middle 1/3 fracture model, P2D1 and P1D1 were associated with similar stress. The stress noted for these techniques was higher than that recorded for P1D1. These findings could guide physicians regarding the clinical management of type A3 fractures.

This study had several limitations. Firstly, only the A3 fracture models were established in this study and the boundary conditions in terms of the complex segmental motion of the human spine in the thoracolumbar segment were simplified Thus, models with actual ROM and with other fracture subtypes should be warranted in the future. Secondly, the screw thread size should be considered for more realistic screw stress analyses in future studies. In addition, using a nonlinear material constitutive model is necessary to study the localized failure of internal fixation systems. However, the stress trends for the different procedures observed in this study are comparable to previous studies. Finally, although there are numerous posterior internal fixation methods used to treat fractures, only three techniques were modeled in this study. Further investigations should be performed to evaluate more biomechanical properties of other models concerning other posterior internal fixation methods.

In this study, we investigated the effect of fracture sagittal location on internal fixation stress and the selection of surgical strategies by establishing an effective FE model. The results showed that the sagittal location of fractures did not affect the choice of surgical strategies; however, it affected the magnitude of stress and distribution of the internal fixation system.

Conflict of Interest

The authors declare no conflict of interest.

Funding

This study was supported by the Key Project of Natural Science of Bengbu Medical College (grant numbers: BYKY2019226ZD, BYKY2019227ZD)

Author Contributions

All authors listed have made a substantial, direct, and intellectual contribution to the work, and approved it for publication.

Rosenthal BD, Boody BS and Jenkins TJ, et al. Thoracolumbar Burst Fractures. Clin Spine Surg 2018; 31(4): 143–151.
Shahrestani S, Ballatori AM and Chen XT, et al. Identifying risks factors in thoracolumbar anterior fusion surgery through predictive analytics in a nationally representative inpatient sample. Eur Spine J 2022; 31(3): 669–677.
Petersilge CA and Emery SE. Thoracolumbar burst fracture: evaluating stability. Semin Ultrasound CT MR 1996; 17(2): 105–113.
Girardo M, Masse A and Risitano S, et al. Long versus Short Segment Instrumentation in Osteoporotic Thoracolumbar Vertebral Fracture. Asian Spine J 2021; 15(4): 424–430.
El BH, M AA and Saleh AK, et al. Short-Segment Fixation of Thoracolumbar Fractures with Incorporated Screws at the Level of Fracture. Orthop Surg 2020; 12(1): 170–176.
Liang C, Liu B and Zhang W, et al. Clinical Effects of Posterior Limited Long-Segment Pedicle Instrumentation for the Treatment of Thoracolumbar Fractures. J Invest Surg 2020; 33(1): 25–30.
Mu S, Wang J and Gong S. Mechanical Analysis of Posterior Pedicle Screw System Placement and Internal Fixation in the Treatment of Lumbar Fractures. Comput Math Methods Med 2022; 2022(6497754.
Guo H, Li J and Gao Y, et al. A Finite Element Study on the Treatment of Thoracolumbar Fracture with a New Spinal Fixation System. Biomed Res Int 2021; 2021(1–9.
Wong C, Hu H and Tsai C, et al. Comparison of Posterior Fixation Strategies for Thoracolumbar Burst Fracture: A Finite Element Study. Journal of Biomechanical Engineering 2021; 143(7).
Wang P and Hu X. Biomechanical finite element analysis of superior endplate collapse after thoracolumbar fracture surgery. Ann Transl Med 2020; 8(12): 753.
Galbusera F, Qian Z and Casaroli G, et al. The Role of the Size and Location of the Tumors and of the Vertebral Anatomy in Determining the Structural Stability of the Metastatically Involved Spine: a Finite Element Study. Transl Oncol 2018; 11(3): 639–646.
Guo LX and Li WJ. A biomechanical investigation of thoracolumbar burst fracture under vertical impact loads using finite element method. Clin Biomech (Bristol, Avon) 2019; 68(29–36.
Wang W, Pei B and Pei Y, et al. Biomechanical effects of posterior pedicle fixation techniques on the adjacent segment for the treatment of thoracolumbar burst fractures: a biomechanical analysis. Comput Methods Biomech Biomed Engin 2019; 22(13): 1083–1092.
Krismann O, Rauschmann M and Sellei R, et al. Operative results of AO A3 and A4 Traumatic Injuries in the thoracic and lumbar spine. A multicentre surveillance study of 4230 patients from the German Spine Registry (DWG-Register). J Neurosurg Sci 2022.
Basaran R, Efendioglu M and Kaksi M, et al. Finite Element Analysis of Short- Versus Long-Segment Posterior Fixation for Thoracolumbar Burst Fracture. World Neurosurg 2019; 128(e1109-e1117.
Wong CE, Hu HT and Tsai CH, et al. Comparison of Posterior Fixation Strategies for Thoracolumbar Burst Fracture: A Finite Element Study. J Biomech Eng 2021; 143(7).
Cahueque M, Cobar A and Zuniga C, et al. Management of burst fractures in the thoracolumbar spine. J Orthop 2016; 13(4): 278–281.
Li C, Zhou Y and Wang H, et al. Treatment of unstable thoracolumbar fractures through short segment pedicle screw fixation techniques using pedicle fixation at the level of the fracture: a finite element analysis. Plos One 2014; 9(6): e99156.
Su Y, Wang X and Ren D, et al. A finite element study on posterior short segment fixation combined with unilateral fixation using pedicle screws for stable thoracolumbar fracture. Medicine (Baltimore) 2018; 97(34): e12046.
Wang H, Mo Z and Han J, et al. Extent and location of fixation affects the biomechanical stability of short- or long-segment pedicle screw technique with screwing of fractured vertebra for the treatment of thoracolumbar burst fractures: An observational study using finite element analysis. Medicine (Baltimore) 2018; 97(26): e11244.
Jindal R, Jasani V and Sandal D, et al. Current status of short segment fixation in thoracolumbar spine injuries. J Clin Orthop Trauma 2020; 11(5): 770–777.
Xu M, Yang J and Lieberman I, et al. Stress distribution in vertebral bone and pedicle screw and screw-bone load transfers among various fixation methods for lumbar spine surgical alignment: A finite element study. Med Eng Phys 2019; 63(26–32.
Zhang T, Wang Y and Zhang P, et al. Different fixation pattern for thoracolumbar fracture of ankylosing spondylitis: A finite element analysis. Plos One 2021; 16(4): e250009.
Wu Y, Chen CH and Tsuang FY, et al. The stability of long-segment and short-segment fixation for treating severe burst fractures at the thoracolumbar junction in osteoporotic bone: A finite element analysis. Plos One 2019; 14(2): e211676.

No competing interests reported.

Download PDF

Version 1

posted

You are reading this latest preprint version

Does Sagittal Location of the Fracture Region of Vertebral Body Affect the Interfixation Instrument Biomechanics or Surgical Strategy in Thoracolumbar Fracture Treatment: a Finite Element Analysis

Status:

Version 1

Abstract

Figures

Introduction

Materials and Methods

Model creation for different sagittal fracture distribution

Creation of the pedicle and screw and rod models

Models of different surgical strategies

Load and boundary

Assessment indices

Results

Validation of the intact T11-L3 FE model

ROM of the T12-L2 in nine FE models

Maximum von Mises stress on the screws and rods

Discussion

Conclusion

Declarations

References

Additional Declarations

Status:

Version 1