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. 2022 Dec 20;48(22):E382–E388. doi: 10.1097/BRS.0000000000004553

Screw Pullout Strength After Pedicle Screw Reposition: A Finite Element Analysis

Yu-Xing Ye a,b, Da-Geng Huang a,b,✉, Ding-Jun Hao a,b, Jia-Yuan Liu a,b, Jia-Jia Ji a,b, Jin-Niu Guo a,b
PMCID: PMC10602223  PMID: 36541576

Abstract

Research design.

Finite element analysis based on computed tomography images from the lumbar spine.

Objective.

Determined the pullout strength of unsatisfactorily placed screws and repositioned screws after unsatisfactory place in lumbar spine surgery.

Background.

Pedicle screws are widely used to stabilize the spinal vertebral body. Unsatisfactory screws could lead to surgical complications, and may need to be repositioned. Screw removal and reposition, however, may decrease pullout strength.

Methods.

We conducted a three-dimensional finite element analysis based on high-resolution computed tomography images from a 39-year-old healthy woman. Pullout strength was determined with the screw placed in different orientations at the same entry point (as selected by the Magerl method), as well as after removal and reposition. The material properties of the vertebral body and the screw were simulated by using grayscale values and verified data, respectively. A load along the screw axis was applied to the end of the screw to simulate the pullout.

Results.

The pullout strength was 1840.0 N with the Magerl method. For unsatisfactorily placed screws, the pullout strength was 1500.8 N at 20% overlap, 1609.6 N at 40% overlap, 1628.9 N at 60% overlap, and 1734.7 N at 80% overlap with the hypothetical screw path of the Magerl method. For repositioned screws, the pullout strength was 1763.6 N, with 20% overlap, 1728.3 N at 40% overlap, 1544.0 N at 60% overlap, and 1491.1 N at 80% overlap, with the original path. Comparison of repositioned screw with unsatisfactorily placed screw showed 14.04% decrease in pullout strength at 80% overlap, 5.21% decrease at 60% overlap, 7.37% increase at 40% overlap, and 17.51% increase at 20% overlap, with the screw path of the Magerl method.

Conclusions.

Removal and reposition increased the pullout strength at 20% and 40% overlap, but decreased the pullout strength at 60% and 80% overlap. For clinical translation, we recommend removal and reposition of the screw when the overlap is in the range of 20% to 40% or less. In vitro specimen studies are needed to verify these preliminary findings.

Key words: finite element analysis, spine, pedicle screw, pullout strength, reposition

Pedicle screw fixation system is widely used in spinal surgery to correct deformity and to maintain the stability of the spine until solid fusion.1 Device-related complications include screw fracture, screw loosening, connector slippage, and screw pullout.2–4 Unsatisfactory screws placement could lead to surgical complications (e.g. nerve injury), and may need to be repositioned. In a previous study by Kang et al 5 screws were inserted into the bilateral pedicles of each vertebral body in 31 fresh human thoracic vertebrae and the torque of each insertion was measured. The screws were then removed completely and repositioned along the original screw paths. A tensile load was applied to the screw tail to simulate pullout. Removed screws and then repositioned along the original screw paths decreased the torque on screw reinsertion but did not affect the pullout strength. Goda et al 6 harvested 54 fresh specimens of vertebral body. They inserted the pedicle screw along the correct axis on the one side, broke through the pedicle or the endplate on the other side, and then removed the wrong screw and repositioned it along the correct axis, applied a tensile load at each screw tail to compare the pullout strength. Found pedicle screw reposition could decrease the pullout strength. Placing screws in different directions could also influence the pullout strength, in a previous study, Patel et al 7 compared the pullout strength of cortical and cancellous screws inserted in materials of different densities at angles of 0°, 10°, 20°, 30°, and 40° to the horizontal. Found that the maximal pullout strength was obtained at an angle of 0°. And Robert et al 8 used a bone model, with the screw angle gradually increased from 0° to 10° to 20°, the screw pullout strength gradually decreased, but no further decrease was found when the angle was further changed. We conducted a literature review and failed to identify in-depth studies of pullout strength of the repositioned screws. Most of the previous experiments have studied the influence of screw insertion direction on the pullout strength and the influence of screw reposition after screw damaging the vertebral body on the pullout strength, but no one had studied the influence of the overlap volume of screw paths on the pullout strength.

We conducted a three-dimensional finite element analysis (FEA) to simulate the removal and repositioning of pedicle screws. Pullout strength was determined under three conditions: (1) Magerl group: screw placement with the standard Magerl method; (2) unsatisfactory group: unsatisfactory screw placement with 80%, 60%, 40%, and 20% volume overlap with the hypothetical Magerl method screw paths; (3) repositioned group: screw removal and then repositioned by the Magerl method, with 80%, 60%, 40%, and 20% volume overlap to the original screw path. All the screw entry points were the same as the Magerl method, and the overlap of the screw paths was limited to the sagittal plane for easy of calculation.

MATERIALS AND METHODS

Finite Element Models

All simulation work was performed using the commercial FEA software ANSYS 15.0 (ANSYS Inc., Cannonsburg, PA). The model used for the FEA was based on the human lumbar spine (L4) of a 39-year-old healthy female volunteer and constructed using high-resolution computed tomography (CT) image data (2×64 row dual-source CT; Siemens Medical Systems GmbH, Germany). The two-dimensional CT images were imported into Mimics 20.0 software (Materialise, Leuven, Belgium) to generate three-dimensional geometries of the vertebrae. And then, we created a fine tetrahedral unit mesh model using the Geomagic Wrap 2021 software (3D Systems, Rock Hill, SC, USA). The three-dimensional model of the pedicle screw was created in SOLIDWORKS 2020 (Dassault Systemes, Paris, France). The parameters of the screw used in the FEA were previously validated by Van et al 9 The screw was a commercially available Ti-6Al-4V cylindrical pedicle screw (Fradis Medical, Salouel, France). Geometric parameters of the screw are shown in Supplemental Digital Content Table 1, http://links.lww.com/BRS/B970.

Material properties of the vertebrae in the FEA were identical to that in the previous experiments,10 with the formula: Density=1×HU, E (Elastic modulus)=7.136×Density−172.3, i.e. the assignment was based on the grayscale values of the vertebrae CT images. Poisson ratio was 0.3 and 0.2 for cortical and cancellous bones, respectively. Material properties of the screw was assumed to be homogeneous cobalt-chromium alloy, with Young modulus of 220 GPa, and Poisson ratio of 0.33.11 The mechanical behavior of both the screw and vertebrae was considered isotropic and linear. Informed consent was obtained before data collection and use, and this study was approved by the ethics committee of hospitals.

Three conditions were simulated in FEA (Figure 1): (1) Magerl group: screw placement with the standard Magerl method; (2) unsatisfactory group: unsatisfactory screw placement with 80%, 60%, 40%, and 20% volume overlap with the hypothetical Magerl method screw paths; (3) repositioned group: screw removal and then repositioned by the Magerl method, with 80%, 60%, 40%, and 20% volume overlap to the original screw path.

Figure 1.

Figure 1

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Finite element models. A, Magerl method of screw placement. B–E, 80%, 60%, 40%, and 20% overlap with the hypothetical screw path. F–I, 80%, 60%, 40%, and 20% overlap with the unsatisfactory screw path.

Constraints and Loading Conditions

A fixed constraint was applied to a circular projection surface of 13-mm radius on the upper and lower surfaces of the vertebral body to maintain stability during screw pullout. The pullout strength was defined as the first force peak.12,13 Edge-to-edge contact is further achieved by using a penalty method at the junction of the screw and vertebral body contact to form a contact pair between the matched volume and the housing unit.14 The friction coefficient of the screw and vertebral bone contact surface was set to 0.2 to maintain the sensitivity of the screw pullout force to changes in the friction coefficient below 10%, as previously described.15–17 As the screw tensile load could not be defined in terms of pullout speed in the experiment, an axial displacement load was applied to the end of the screw to simulate screw pullout.18 (Figure 2)

Figure 2.

Figure 2

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Constraints and loads.

Mesh Convergence Test

The finite element (FE) model (L4) was tested for mesh convergence. Three mesh resolutions were generated consecutively (in the order of mesh 1, mesh 2, and mesh 3) for this FE model. mesh 1 had the least number of elements and nodes among the three mesh resolutions. mesh 2 and mesh 3 had approximately twice the numbers of elements and nodes than the previous mesh resolution. The number of elements and nodes for each mesh resolution were shown in Supplemental Digital Content Table 2, http://links.lww.com/BRS/B971.

The three mesh resolutions were tested under the vertical downward compression of 600 N to simulate the weight of the human body. The von Mises stress was calculated and compared for different parts of the FE model. Jones et al 19 and Ayturk et al 20also showed that the mesh can be considered convergent when the difference in prediction results between two consecutive mesh resolutions was <5%. The purpose of mesh convergence test is to find the appropriate mesh resolution that has a sufficiently large number of elements to ensure the accuracy of the simulations. It is also required that the total number of elements for the mesh resolution should be as small as possible to save the simulation time.21

FE Model Validation

The FE model of vertebrae was tested for validation. Generally speak, the greater number and diversity of comparisons between the model and the experimental data, the less likely that the model predictions are flawed.22 First, the screw pullout was simulated on the FE model with convergence analysis to obtain preliminary pullout strength and compared with the experimental data previously reported in the literature.7,23,24 Second, simulation results in this study were also compared with the results of existing well-validated FE models reported by Matsukawa et al.25

RESULTS

Mesh Convergence Test

The predicted results show that the largest difference of predicted von Mises stress between mesh 1 and mesh 3 occurred in cortical bone, which was 12.76%. The posterior bony elements had the lowest sensitivity to mesh resolutions because the difference of von Mises stress between mesh 1 and mesh 3 was 6.94% and the difference between mesh 2 and mesh 3 was 1.76%. The difference of von Mises stress between mesh 2 and mesh 3 were <5% in all parts of the model. And mesh 2 was considered as stress convergence (Figure 3).

Figure 3.

Figure 3

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The predicted von Mises stress difference in percentage between mesh 1 and mesh 3 and between mesh 2 and mesh 3 in different parts with vertical downward compression. FE indicates finite element.

FE Model Validation

To increase the reliability of the model validation, the simulated loading protocol was the same as the in vitro experiment. The pullout strength of simulation was compared with those of previous in vitro studies by Krishnan et al,23 Amaritsakul et al,24 and Patel et al 7 Furthermore, the results of simulation were compared with those studied by Matsukawa et al 25 (Figure 4).

Figure 4.

Figure 4

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The comparison of pullout strength between the present finite element model and previous studies for validation.

The Pullout Strength

The pullout strength of the screw placed by the Magerl method was 1840.0 N, and the displacement of the screw reached 0.5 mm in 25 seconds (Figure 5A) (Supplemental Digital Content Table 3, http://links.lww.com/BRS/B972). For unsatisfactorily placed screws (Supplemental Digital Content Table 4, http://links.lww.com/BRS/B973), the maximal pullout strength was 1734.7 N at 80% overlap in volume between the screw path and the hypothetical Magerl method screw path (Figure 5B). The minimal pullout strength occurred at 20% overlap with 1500.8 N (Figure 5E). It was 1628.9 N at 60% overlap (Figure 5C) and 1609.6 N at 40% overlap (Figure 5D). All the unsatisfactorily placed screws took 25 seconds (Supplemental Digital Content Table 5, http://links.lww.com/BRS/B974) to reach the maximal pullout force while achieving a displacement of 0.5 mm (Supplemental Digital Content Table 6, http://links.lww.com/BRS/B975). For repositioned screws, the maximal pullout strength was 1763.6 N at 20% overlap in volume between the screw path and the original screw path, while the screw underwent a displacement of 0.28 mm in 14 seconds (Figure 5I). The minimal pullout strength occurred at 80% overlap with 1491.1 N, while the screw was displaced by 0.3 mm in 15s (Figure 5F). It was 1544.0 N at 60% overlap (Figure 5G) and 1728.3 N at 40% overlap, both reached the maximum pullout force at 15 seconds, accompanied by a displacement of 0.3 mm (Figure 5H).

Figure 5.

Figure 5

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Force-time curve; A, Magerl method of screw placement. B–E, 80%, 60%, 40%, and 20% overlap with the hypothetical screw path. F–I, 80%, 60%, 40%, and 20% overlap with the unsatisfactory screw path. The red rectangle shows the time and displacement to reach the maximal pullout force.

The Von Mises Stress of Screw

The maximal von Mises stress of screw in Magerl group was 1211.3 MPa (Supplemental Digital Content Table 3, http://links.lww.com/BRS/B972). The maximal von Mises stress of unsatisfactory group was 826.79 MPa at 80% overlap, 508.63 MPa at 60% overlap, 398.27 MPa at 40% overlap, and 293.19 MPa at 20% overlap. For repositioned screws, the maximal von Mises stress was 284.66 MPa at 80% overlap, 322.08 MPa at 60% overlap, 510.23 MPa at 40% overlap, and 524.48 MPa at 20% overlap (Supplemental Digital Content Table 7, http://links.lww.com/BRS/B987). And the von Mises stress of all screws was mainly concentrated in the tail of the screw (Supplemental Digital Content Figure 6, http://links.lww.com/BRS/B977).

The Von Mises Stress of Vertebrae

The maximal von Mises stress in Magerl group was 7.67 MPa. For unsatisfactory group, the maximal von Mises stress was 7.45 MPa at 80% overlap, 7.39 MPa at 60% overlap, 7.14 MPa at 40% overlap, and 7.07 MPa at 20% overlap. The maximal von Mises stress of repositioned group was 6.13 MPa at 80% overlap, 6.24 MPa at 60% overlap, 6.77 MPa at 40% overlap, and 7.01 MPa at 20% overlap (Supplemental Digital Content Table 8, http://links.lww.com/BRS/B978). And the von Mises stress of vertebrae was relatively dispersed, high stress areas were mainly concentrated in the place around the contact part of the screw tail with the vertebrae (Supplemental Digital Content Figure 7, http://links.lww.com/BRS/B979).

The Strain of Vertebrae and Screw

In our experiment, no strain occurred in the screw, whereas the strain occurred in the vertebrae during screw pullout. The area where the strain occurred was around the screw, especially the part in contact with the threads (Supplemental Digital Content Figure 8, http://links.lww.com/BRS/B980).

DISCUSSION

The subject of this experiment is to determine the pullout strength of unsatisfactorily placed screws and repositioned screws after unsatisfactorily placed screws in lumbar spine surgery. Previous studies have been more extreme, using screws to destroy the vertebrae, reposition it, and then compare the pullout strength,5,6 which is easy, but has low accuracy. There is no in-depth study of pullout strength of repositioned screws by the overlap volume of the screw path.

To make the experimental results more precise and closer to the actual situation, the boundary conditions, as well as the application sites and directions of displacement loads, were strictly controlled in the process of setup the FE model. The current study showed the maximal screw pullout strength (1840.0 N) was achieved when the screw was placed by the Magerl method, which is consistent with previous studies,7,8,23–27 that the maximal pullout strength was obtained with 0° insertion in sagittal plane. The minimal screw pullout strength (1491.1 N) was achieved when the overlap between the path of unsatisfactorily placed screw and the hypothetical path by the Magerl method was 80%. In addition, the pullout strength of both the unsatisfactory and repositioned group was less than that placed by correct method, which is also consistent with the previous study.6,28

For unsatisfactorily placed screws, the pullout strength in sequence from smallest to largest was 20%, 40%, 60%, and 80% overlap. The smaller the overlapping volume, the greater the angle between the screw and the horizontal direction, and the lower the pullout strength. These results were largely consistent with the results of previous studies.7,8 However, the gaps in pullout strength between the two adjacent operating conditions was greatly different. There was an important reason for this result: the bone mineral density (BMD) in the vertebrae is heterogeneous10,29 and there is a significant positive correlation between BMD and pullout strength.9 When the screw passed through the areas of high BMD, it produced a larger pullout strength.

For repositioned screws, the pullout strength in sequence from smallest to largest, was 80%, 60%, 40%, and 20% overlap. The most important reason for this result was the difference in the degree of engagement between the screw and the bone,6,30,31 the more the screw engaged with the bone, the larger the pullout strength. The screw path left when the unsatisfactory screw was removed resulted in less contact between the repositioned screw and the bone, which further led to a lower pullout strength of the screw, as well as decreased the time and displacement required to achieve the maximal pullout force.12,13

Comparison of repositioned group with unsatisfactory group showed 14.04% decrease in pullout strength at 80% overlap, 5.21% decrease at 60% overlap, 7.37% increase at 40% overlap, and 17.51% increase at 20% overlap. On the basis of these results, we recommend removal and reposition of the screw when the overlap between the path of unsatisfactorily placed screw and the hypothetical path with the Magerl method was 20% and 40%, rather than 60% or 80%.

The von Mises stress of all screws was mainly concentrated in the tail of the screw (Supplemental Digital Content Figure 6, http://links.lww.com/BRS/B977), which was the same as that found previously,32–34 and this resulted in higher stress in the corresponding regions of the vertebrae as well (Supplemental Digital Content Figure 7, http://links.lww.com/BRS/B979).

It has been described in the previous studies that the strain occurred mainly around the screws, and the main reasons were the pressure of the thread on the bone in contact with it destroyed the bone around the screw and the stiffness of the vertebrae was weaker than that of the screw14,17,35; the results of current study were coincided with these findings (Supplemental Digital Content Figure 8, http://links.lww.com/BRS/B980). At the same time, the strain on the vertebrae was mainly in the vicinity of the screw (i.e. in the cylinder around the screw), suggesting that it may not be necessary to model the entire vertebral body structure.9,36

Although several studies considered the vertebrae to be linear materials in the FEA found that they are nonlinear materials in reality.37,38 Another shortcoming of the study was the profile contour of the vertebrae, BMD, the dimensions of the pedicle, the thickness of the vertebral plate, and the length of the transverse process and spinous process were based on the high-resolution CT images of a single adult healthy woman, and might not be representative of the patient population.39,40 Third, only the axial pullout was simulated, and the screw actually experienced loads in many directions, like bending loads up and down.13,41 A last limitation of the study was that the results based on the FEA must be verified by in vitro experiments in the future to be truly convincing.

CONCLUSIONS

In this study, we developed a simple FE model (L4) of the lumbar spine. We found that reposition of malpositioned screws will increase pullout strength when the overlap between the path of the unsatisfactorily placed screw and the hypothetical path with the Magerl method was between 20% and 40%, rather than 60% or 80%. Therefore, in the clinical setting, we recommend removal and reposition of malpositioned pedicle screws when overlap is between 20% and 40% or less. The results must be replicated in cadaveric and animal specimens before further investigation in the clinical realm can be supported.

Key Points:

  • The pullout strength of both the unsatisfactory and repositioned group was less than that placed by correct method.

  • Removal and reposition increased the pullout strength at 20% and 40% overlap, but decreased the pullout strength at 60% and 80% overlap.

  • The von Mises stress of all screws was mainly concentrated in the tail of the screw.

  • The von Mises stress of vertebrae was relatively uniform, high stress areas were mainly concentrated in the place around the contact part of the screw tail with the vertebrae.

  • The area where the strain occurred was around the screws, especially the part in contact with the thread.

Supplementary Material

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ACKNOWLEDGMENTS

The authors thank the volunteers, families, researchers, and clinical staff for the significant contribution to this study.

Footnotes

D.-G.H. contributed to the study design, operation, and is the corresponding author. Material preparation, data collection, and analysis were performed by Y.-X.Y. and J.-Y.L., J.-J.J., and J.-N.G. participated in the literature search. D.-J.H. prepared the figures and tables. The first draft of the manuscript was written by Y.-X.Y.

The authors report no conflicts of interest.

Supplemental Digital Content is available for this article. Direct URL citations appear in the printed text and are provided in the HTML and PDF versions of this article on the journal's website, www.spinejournal.com.

Contributor Information

Yu-Xing Ye, Email: 1454041164@qq.com.

Da-Geng Huang, Email: 172070617@qq.comn.

Ding-Jun Hao, Email: haodingjun758@yeah.net.

Jia-Yuan Liu, Email: Liujiayuan96@126.com.

Jia-Jia Ji, Email: Jijiajia23@163.com.

Jin-Niu Guo, Email: Guojinniu764@126.com.

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