Optimizing Cement Distribution in the Fractured Area for Osteoporotic Vertebral Compression Fractures Through Biomechanical Effects Analysis Based on a Three Dimentional Lumbar Finite Element Modeling

doi:10.21203/rs.3.rs-100325/v1

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Optimizing Cement Distribution in the Fractured Area for Osteoporotic Vertebral Compression Fractures Through Biomechanical Effects Analysis Based on a Three Dimentional Lumbar Finite Element Modeling

https://doi.org/10.21203/rs.3.rs-100325/v1

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Background: While cement distributes sufficiently in the fractured area and relatively symmetrically around the fractured area, three types of cement mass location in the vertebral body are commonly seen when performing bipedicular percutaneous vertebral augmentation (PVA) for osteoporotic vertebral compression fractures (OVCFs), including anterolateral (AL), anteromedial (AM) and posterolateral (PL). However, little is known about differences of biomechanical behaviors among these three types of cement distribution so far. The present study aimed to investigate biomechanical effects of AL, AM and PL in the fractured area on OVCFs.

Methods: Three dimentional finite element methods were utilized to construct OVCF model and simulate AL, AM and PL in the fractured area for OVCFs treated with PVA. Distributions and magnitudes of von Mises stress in cortical and cancellous bone and maximum displacement of the four models were compared.

Results: Compared with the OVCF model, Distribution of von Mises stress in cortical bone was unchanged while that in cancellous bone was transferred to be concentrated symmetrically at cancellous bone surrounding cement after PVA. Maximum displacement and maximum von Mises stress in cortical bone in AL decreased the most significantly, while AM created the lowest maximum von Mises stress in cancellous bone.

Conclusions: Cement distribution between AL and AM may balance stress in cortical and cancellous bone, better restoring vertebral strength, meanwhile, providing sufficient vertebral stability.

Orthopedic Surgery

Osteoporotic vertebral compression fracture

Percutaneous vertebral augmentation

Cement distribution

Finite element analysis

Fractured area

Osteoporotic vertebral compression fractures (OVCFs) are very common in the elderly, with an estimated 1.4 million new fractures occurring every year worldwide [1]. Until recently, symptomatic OVCFs were treated commonly with conservative methods including bed rest, analgesics, braces, and physical therapy. However, percutaneous vertebral augmentation (PVA), such as percutaneous vertebraplasty and percutaneous kyphoplasty, has been introduced as an alternative treatment option [2, 3]. Biomechanical studies showed significant increases in the stiffness and strength of individual fractured vertebra after PVA [4, 5]. Apart from rapid pain relief, another immediate effect of PVA was an increase of anterior vertebral height, which reduced kyphosis in patients [6–8]. The realigned spinal column and regained height in the augmented vertebra may decrease pulmonary and gastro-intestinal complications and early morbidity related to compression fractures [9].

Our previous biomechanical study demonstrated that various cement distributions relative to the fractured area could produce different impacts on biomechanical behaviors of OVCFs treated with PVA [10]. Biomechanical behaviors of sufficient versus insufficient cement distributions in the fractured area and symmetrical versus asymmetrical cement distributions around the fractured area were compared. Insufficient cement distribution in the fractured area led to significant increase in maximum displacement of augmented vertebra, which might explain unrelieved pain after PVA in treatment of symptomatic OVCFs. While insufficient cement distribution in the fractured area increased maximum von Mises stress in cortical and cancellous bone of augmented vertebra significantly, asymmetrical cement distribution around the fractured area increased maximum von Mises stress in cancellous bone significantly, both of which might be the reason for recollapse of augmented vertebra. These findings were verified by related clinical studies [11, 12].

While cement distributes sufficiently in the fractured area and relatively symmetrically around the fractured area, three types of cement mass location in the vertebral body are commonly seen according to our clinical practice when performing bipedicular PVA for OVCFs, including anterolateral (AL), anteromedial (AM) and posterolateral (PL). To the best of our knowledge, however, few biomechanical studies exist that show the differences of biomechanical behaviors among these three types of cement distribution so far. In order to further optimize cement distribution in the fractured area for OVCFs treated with PVA, this study was performed to investigate biomechanical effects of AL, AM and PL in the fractured area on OVCF on the basis of our previous study [10].

Development of normal L1-L5 finite element (FE) model

A normal three-dimensional digital anatomical FE model of L1-L5 was created based on CT images of a L1-L5 motion segment without injury or radiographic evidence of degeneration. The image data were from a 26-year-old male who requested CT scan for health examination at our hospital. He was explained about the research purpose and signed the consent form. Slice thickness was 0.625 mm. The slice images were preserved in a computer and then imported to Mimics 19.0 software (Materialise Inc., Leuven, Belgium) for generation of the 3-dimensional geometries of L1-L5 vertebra. Then, a smoothing process was performed in Geomagic Studio 2013 software (Geomagic Inc., Research Triangle Park, NC, USA) to remove spikes and holes on the surface of the vertebral geometries. The geometries of intervertebral discs which were difficult to separate from the CT images, were created using SolidWorks 2017 sofeware (SolidWorks Inp., Concord, MA, USA). Each vertebra consisted of a cancellous core surrounded by a cortical shell layer with thickness of 0.5 mm [13]. Cartilaginous endplates were simulated with thickness of 0.5 mm at both ends of each vertebra [14]. The nucleus pulpous occupied about 43% of the total disc [15]. Facet joints were modeled with a cartilage layer with thickness of 0.2 mm and a gap of about 0.6 mm [16]. Element types of cancellous bone, posterior bony element, cartilaginous endplates, annulus and nucleus pulpous were defined as solid elements. Cortical bone and facet joint cartilage were defined as shell elements. In order to mimic collagen fibers of annulus fibrosus, four layers of collagen fibers were embeded radially in the annular ground substances. For each layer two bundles of fibers were modeled with orientation of about ± 30 ° with respect to the horizontal plane using truss elements [17]. A fiber cross-sectional area of 0.1mm² was assumed [18]. Ligaments were modeled with truss elements. The assigned material properties were assumed to be homogeneous, linear, and isotropic and truss elements were tension-only. Tied constraints were used to ensure disc and ligament attachments to the vertebra and prevent any relative movement during the simulations. Frictionless Surface-based, finite sliding contact was defined for facet joint cartilages [19]. Material properties were taken from the literatures and listed in Table 1 [18, 20, 21]. Abaqus 6.12 (Abaqus, Inc., Providence, RI, USA) was used for numerical analysis. A mesh convergence study was conducted and considered to be convergent when the prediction results obtained by two consecutive mesh resolutions had differences smaller than 5% [22, 23]. The final mesh consisted of 8042 solid elements, 177037 shell elements, 3840 truss elements and a total of 285469 nodes (Fig. 1).

Table 1

Material properties of different structures in the finite element models
Component name	Young’s modulus (MPa)	Poisson’s ratio	Cross-section area (mm²⁾
Cortical bone	12000 (osteoporotic: 8040)	0.3	/
Cancellous bone	100 (osteoporotic: 34)	0.2	/
Posterior structure	3500 (osteoporotic: 2345)	0.25	/
Cartilaginous endplate	23.8 (osteoporotic: 15.9)	0.4	/
Nucleus pulposus	1	0.495	/
Annulus: ground substance	4.2	0.45	/
Annulus: fiber	25	0.3	0.1
Facet joint Cartilage	23.8	0.4	-
ALL	20	0.3	40
PLL	70	0.3	20
LF	50	0.3	40
CL	50	0.3	30
ISL	20	0.3	40
SSL	28	0.3	30
ITL	28	0.3	10
Cement	3000	0.4	/
All = anterior longitudinal ligament; PLL = posterior longitudinal ligament; LF = ligamantum flavum; CL = capsular ligament; ISL = interspinous ligament; SSL = supraspinal ligament; ITL = intertransverse ligament.

Model Validation Method

To validate the normal FE model, rang of motion (ROM) and intradiscal pressure (IDP) were the parameters chosen for validation. For ROM, the inferior endplate of L5 vertebra was fixed in all degrees of freedom and Loads were applied in L1 superior endplate. Pure moment of 7.5 N m in flexion/extension, left/right lateral bending, and left/right axial rotation were independently applied. The ROMs in each spinal level were recorded. For IDP, pure compressive forces of 300N and 1000N were independently applied between the L4 and L5 levels, the IDPs in L4-L5 disc were recorded. The simulation results predicted by the FE model were compared with corresponding experimental data reported in the literatures [24, 25].

Development Of Osteoporotic L1-l5 Fe Model

Because OVCF is investigated, model of an osteoporotic L1-L5 was built. According to methods reported by Polikeit et al [21], a model with osteoporosis was defined as follows. The elastic moduli of all bony structures were reduced, by 66% for the cancellous bone, and by 33% for the cortical shell, the endplates, and the posterior elements. The other structures were left unchanged. Material properties were listed in Table 1.

Simulation Of Ovcf

Referring to simulation method reported by Chiang et al. [26], the model (Pre-augmented) was constructed with the following steps to simulate OVCF on L3. A cleft was horizontally penetrated into the vertebral body by 20 mm through the center of the anterior cortical shell. The size of the cleft was approximately 20 mm, 30 mm and 2 mm in depth, width and height, respectively (Fig. 2).

Simulation Of Cement Augmentation

For bone cement material, PMMA was applied because it represents the most common clinically used material. Its material property was listed in Table 1 [21]. Two cement cylinders with the same volume were vertically implanted around the fractured area symmetrically to mimic bipedicular PVA with sufficient cement distribution in the fractured area and symmetrical cement distribution around the fractured area. The volume of each cement cylinder was approximately 3 ml. For simulation of AL, both of cement cylinders were embedded in the anterior column and in proximity to the lateral border of the vertebral body. Subsequently, the two cement cylinders were relocated medially adjacent to midline of the vertebral body to mimic AM. Being different from AM, the two cement cylinders of AL were relocated posteriorly with partial cement cylinders locating in middle column to simulate PL. Eventually, we got four different models for test, including the fractured model before PVA and three augmented models (Fig. 2).

Loads And Boundary Conditions

A pure moment of 7.5 N m was applied independently for flexion/extension, left/right lateral bending, and left/right axial rotation. The inferior endplate of L5 vertebra was fixed in all degrees of freedom and Loads were applied in L1 superior endplate. Abaqus 6.12 (Abaqus, Inc., Providence, RI, USA) was used for numerical analysis. The magnitudes and distributions of von Mises stress in cortical and cancellous bone and maximum displacement of L3 vertebral body were recorded. Von Mises stress has been proposed as a parameter of failure criteria for bone [21, 27] and maximum displacement a parameter of stability [28].

Validation of the normal L1-L5 FE model

Under 7.5 N m pure moments, the ROMs of flexion, extension, lateral bendings and axial rotations in each spinal level predicted by the FE model were within the ranges reported by Panjiabi et al. (Fig. 3a-d)[24]. L4-L5 IDPs under compressive forces of 300N and 1000N were in agreement with the results of the in vitro study (Fig. 3e) [25].

Distributions and magnitudes of von Mises stress in cortical bone of L3 in OVCF models before and after PVA

The results of distributions of von Mises stress under flexion in cortical bone of L3 in OVCF models before and after PVA were shown in Fig. 4a. Compared with distribution before PVA, it was unchanged after PVA, which was still concentrated at posterior unfractured area. Similar results could be seen in extension, lateral bendings and axial rotations.

Regarding magnitude of maximum von Mises stress in cortical bone of L3, it decreased significantly after PVA under all loading conditions with that in AL decreasing the most significantly. Comparing AM with PL, it was lower in AM under flexion, extension and axial rotations while it was lower in PL under lateral bendings (Fig. 5a).

Distributions and magnitudes of von Mises stress in cancellous bone of L3 in OVCF models before and after PVA

The results of distributions of von Mises stress under flexion in cancellous bone of L3 for OVCF models before and after PVA were shown in Fig. 4b, and demonstrated that von Mises stress was concentrated at posterior unfractured area before PVA, but transferred to be concentrated at cancellous bone surrounding bone cement after PVA. However, differences were not detected among augmented models, in which von Mises stress was concentrated below and above fractured area symmetrically. Similar results could be seen in extension, lateral bendings and axial rotations.

Regarding magnitude of maximum von Mises stress in the cancellous bone of L3, it increased after PVA under flexion and lateral bendings while decreased under extension (with exception of PL) and axial rotations. It was lowest in AM under all the loading conditions among AL, AM and PL. Comparing AL with PL, it was lower in AL under flexion and extension while it was lower in PL under lateral bendings, the differences under axial rotations were not obvious (Fig. 5b).

Maximum displacement of L3 for OVCF models before and after PVA

Regarding maximum displacement of L3, it decreased significantly after PVA under all loading conditions with that in AL decreasing the most significantly. Comparing AM with PL, it was lower in AM under flexion, extension and axial rotations while it was lower in PL under lateral bendings (Fig. 5c).

This finite element model was developed on the data collected from a spinal CT scan of a healthy lumbar spine and biomechanical material properties reflecting the pathological characteristics of vertebral osteoporosis. A validated five-vertebra segment was constructed, but not individual vertebra, because a five-vertebra segment model with intervertebral discs and facet joints might not only highly simulated the motion and load transfer of lumbar spine when OVCF was simulated in the middle vertebra, but also avoid loading positions and boundary conditions being directly connected to the target vertebra of L3, which may have an influence on biomechanical behaviors of L3 [29]. The validation test proved that the constructed three-dimensional finite element model could accurately simulate physiological activity and load transfer at the lumbar spine and, therefore, could be a valuable tool for later research. Although stress measurements of cortical and cancellous bone were not validated experimentally, this could not be an influential factor in our study because the parameters in this study were compared in terms of relative differences among fractured models before and after PVA with different distributing patterns of cement.

The anterior cortex of compressed vertebra is usually injured and not continuous before union. In the current study, we attempted to re-create a compression fracture model where a cleft was horizontally penetrated into L3 vertebral body through the center of the anterior cortical shell. This failure model may be attributed to the type A1.2 fracture, that is, the wedge impaction fracture [30]. For bipedicular augmentation, two vertically orientated cement cylinders were implanted in the vertebral body. Similar methods to simulate PVA had been reported by Polikeit et al.[31] and Liebschner et al.[32], and they also agreed that this shape was comparable with the cement distribution seen on radiographs of treated patients. We acknowledge that using cylinders to simulate PVA is done for ease of computer simulation, and not a direct reflection of the reality of PVA. Because the simulation of PVA in the current study can be easily created through finite element analysis method and ensure the repeatability of the study. We are aware that different shapes of cement might occur in the vertebral bodies. Thus, preliminary experiments, in which another shape of cement (cement cake) with equivalent volume was used to simulate PVA, had been done to analyze how cement shape affect findings in our previous study. We found that different stress and displacement can be produced using cement cakes to simulate PVA, but the same conclusion can be drawn using cement cakes to simulate PVA like cement cylinders. In addition, because our previous study demonstrated that sufficient cement distribution in the fractured area and symmetrical cement distribution around the fractured area may provide better structural support, as compared with insufficient and asymmetrical cement distribution [10]. In this study, the two cement cylinders were implanted around the fractured area symmetrically to mimic bipedicular PVA with sufficient cement distribution in the fractured area and symmetrical cement distribution around the fractured area. Using this simulation method may relatively accurately reflect different biomechanical effects of AL, AM and PL in the fractured area on OVCFs.

Our current study is focus on distributions and magnitudes of von Mises stress in cortical and cancellous bone and maximum displacement of L3 for OVCF models before and after PVA. Distribution of von Mises stress in cortical bone was unchanged while that in cancellous bone was transferred to be concentrated symmetrically at cancellous bone surrounding cement after PVA, which is consistent with our previous study [10] and a finite element analysis conducted by Polikeit et al. [21]. This is likely due to the fact that increase in vertebral stability of OVCF after PVA depends on interdigitation between cement and cancellous bone. In addition, it is known that back pain caused by OVCFs is mostly likely to be related to periosteal nerves aggravated by micromotion at the fractured area and recollapse is due to reduced strength of OVCFs. Thus, stabilization of the fractured area and restoration of vertebral strength are the most acceptable mechanisms for pain relief and preventing recollapse after PVA in treatment of OVCFs. Interestingly, we discovered that maximum displacement of L3 decreased significantly after PVA under all loading conditions with that in AL decreasing the most significantly. It indicates that vertebral stability can be better restored by AL. Furthermore, maximum von Mises stress in cortical bone of L3 had the same trend as maximum displacement of L3, which implys that AL can better restore vertebral strength in terms of stress in cortical bone. However, AM created the lowest maximum von Mises stress in cancellous bone of L3, which suggests that AM can better restore vertebral strength in terms of stress in cancellous bone. Given that both cortical and cancellous bone contribute to vertebral strength, we believe that cement distribution between AL and AM can balance stress in cortical and cancellous bone, better restoring vertebral strength. Meanwhile, since difference of maximum displacement of L3 between AL and AM was not obvious, sufficient vertebral stability can be provided with cement distribution between AL and AM.

Some limitations of this study need to be mentioned. Firstly, according to our previous study, location of fractured area of OVCFs could be classified into 3 types including superior, middle, and inferior [11]. Some OVCFs involve one part while others may involve two or three parts. The current model only focused on OVCFs with middle fractured area. Secondly, wedge-like vertebra simulating compression fracture was not created. Only the idealized status of anterior vertebral height correction was simulated in this study, however, PVA only partially restore the vertebral shape in most cases. Third, simulation of cement augmentation with cement cylinders is not a direct reflection of the reality of PVA. Above shortcomings may limit the application of the results to real world clinical scenarios. Clinical study evaluating the findings from this study would be expected in the future.

While cement distributes sufficiently in the fractured area and relatively symmetrically around the fractured area, various cement mass location in the vertebral body including AL, AM and PL have differences in restoring vertebral stability and strength of OVCFs due to the fact that different stress and displacement of corresponding augmented models were produced. Cement distribution between AL and AM may balance stress in cortical and cancellous bone, better restoring vertebral strength, meanwhile, providing sufficient vertebral stability.

OVCF

Osteoporotic vertebral compression fracture; PVA:Percutaneous vertebral augmentation; AL:Anterolateral; AM:Anteromedial; PL:Posterolateral; FE:Finite element; ROM:Rang of motion; IDP:Intradiscal pressure

Acknowledgements

Not applicable

Authors’ contributions

YLQ, JXB and LGL contributed to conceiving and designing the study. YLQ, WR and HXC performed the experiment. YLQ, LZ, FYH and ZJ analyzed the data. YLQ play the main role in writing the article. All authors read and approved the final manuscript.

Funding

The research was supported by Medical Science and Technology Foundation of Guangdong Province (NO.A2017024) and Traditional Chinese Medicine Bureau of Guangdong Province (NO.20182030).

Availability of data and materials

All data and materials are contained within the manuscript.

Ethics approval and consent to participate

This study was granted an exemption from requiring ethics approval by the ethics board of The First Affiliated Hospital of Guangzhou University of Chinese Medicine. Written consent was obtained from the participant.

Consent for publication

Not applicable.

Competing interests

The authors declare no conflict of interest.

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Optimizing Cement Distribution in the Fractured Area for Osteoporotic Vertebral Compression Fractures Through Biomechanical Effects Analysis Based on a Three Dimentional Lumbar Finite Element Modeling

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Version 1

Abstract

Figures

Introduction

Methods

Development of normal L1-L5 finite element (FE) model

Model Validation Method

Development Of Osteoporotic L1-l5 Fe Model

Simulation Of Ovcf

Simulation Of Cement Augmentation

Loads And Boundary Conditions

Results

Validation of the normal L1-L5 FE model

Discussion

Conclusions

Abbreviations

Declarations

References

Status:

Version 1