ANALYSIS ON THE EFFECT OF HALF ANGLE ON THE  DISPLACEMENT OF PEDICLE SCREW DURING AXIAL  PULL-OUT TEST IN CANCELLOUS BONE USING 2D  AXISYMMETRIC FE MODEL

Pedicle screw fixations are commonly used in the treatment of spinal pathologies. For effective treatment, stable anchorage between the screw and bone is necessary. In this study, the influence of proximal and distal half angle of the screw, on the displacement of fixation and stress transfer are simulated using a 2D axisymmetric finite element model. A parametric study was performed by varying the proximal half-angle between 0° and 60° in steps of 10° and the distal half angles are considered as 30° and 40°. The material properties and boundary conditions are applied based on previous studies. Frictional contact is considered between the bone and screw. Results show that, displacement of fixation is observed to be minimum at a proximal half angle of 0° and maximum at an angle of 60°. High stress concentration is observed in first few threads with highest maximum von Mises stress at an angle of 60°. High stress transfer was obtained for proximal half-angles of 40° and 50°. It is observed that, this method might aid to develop better pedicle screws for treatment of Scoliosis.

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Influence of pedicle screw thread width and recovery time after surgery on fixation strength

Current Directions in Biomedical Engineering ◽

10.1515/cdbme-2021-2192 ◽

2021 ◽

Vol 7 (2) ◽

pp. 751-754

Author(s):

Harikrishna Makaram ◽

Ramakrishnan Swaminathan

Keyword(s):

Cortical Bone ◽

Pedicle Screw ◽

Stress Shielding ◽

Stress Transfer ◽

Fixation Strength ◽

Screw Thread ◽

Fe Model ◽

Contact Conditions ◽

Transfer Parameter ◽

Pull Out

Abstract Introduction: Pedicle screw fixation systems are widely used for treatment of various spinal pathologies, including spinal stenosis, scoliosis, spinal deformities and fractures. Stress shielding is considered to be a major factor contributing to insufficient fixation strength, leading to screw loosening. In this study, the influence of pedicle screw thread width on the displacement of pedicle screw and stress transfer is analyzed using 2-Dimensional axisymmetric finite element (FE) model. Methods: FE model consisting of cancellous and cortical bone, along with pedicle screw is developed for this study. The pedicle screw thread width is varied between 0.1 mm and 0.6 mm in steps of 0.1 mm, while the other geometric parameters, including screw half-angle, pitch, diameter, and length are kept constant. Three different contact conditions between screw and bone, such as frictionless, frictional, and bonded are considered to simulate hours, days, and months after surgery, respectively. The material properties and boundary conditions are applied based on previous studies. An axial force of 80 N is applied on the screw head to simulate axial pull-out test. Results: Similar patterns of stress distribution are observed for all screw models, with high stress concentration above the first thread. The highest displacement in screw is observed shortly after surgery, while the highest displacement in cancellous and cortical bone is observed few days and months after the surgery, respectively. The average von Mises stress in screw decreases with increase in thread width for all contact conditions. In few hours/days after the surgery, stress transfer parameter increases with increase in thread width, up to a thread width of 0.5 mm and then decreases. The changes in stress transfer parameter are negligible few months after the surgery. Conclusion: This study highlights the influence of thread width on displacement and stress transferred to the bone, at different durations after the surgery. It is observed that a thread width of 0.5 mm exhibits the highest stress transfer, leading to reduced stress shielding and improved bone remodeling. It appears that this study might aid in developing better pedicle screws for the treatment of various spinal pathologies.

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A Novel Technique for Measuring Pedicle Screw Forces In Situ

ASME 2008 Summer Bioengineering Conference, Parts A and B ◽

10.1115/sbc2008-192219 ◽

2008 ◽

Cited By ~ 1

Author(s):

Samuel Q. Tia ◽

Jennifer M. Buckley ◽

Thuc-Quyen Nguyen ◽

Jeffrey C. Lotz ◽

Shane Burch

Keyword(s):

Pedicle Screw ◽

Pedicle Screws ◽

Clinical Failure ◽

Strain Gages ◽

Destructive Testing ◽

Novel Technique ◽

Pull Out ◽

Pull Out Strength

Long posterior fusion constructs in the lumbar spine cause substantial posteriorly directed loading of the supporting pedicle screws, particularly during patient bending activities. Although there are numerous documented accounts of clinical failure at the pedicle screw-bone interface [1,2], the in situ pull-out strength of pedicle screws in long surgical constructs has not been characterized. Previous biomechanical studies have quantified pedicle screw pull-out force in cadaveric models through destructive testing or in nondestructive cases, through the use of custom-machined pedicle screws instrumented with strain gages [3–6]. However, these techniques involve altering screw geometry and may fail to properly simulate in vivo mechanical loading conditions. The goal of this study was to develop and validate a sensor system for measuring pedicle screw pull-out forces in long posterior constructs in situ during multi-segmental cadaveric testing.

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Correlation between the Computed Tomography Values of the Screw Path and Pedicle Screw Pullout Strength: An Experimental Study in Porcine Vertebrae

Asian Spine Journal ◽

10.31616/asj.2019.0170 ◽

2020 ◽

Vol 14 (3) ◽

pp. 265-272

Author(s):

Atsushi Ikeura ◽

Taketoshi Kushida ◽

Kenichi Oe ◽

Yoshihisa Kotani ◽

Muneharu Ando ◽

...

Keyword(s):

Computed Tomography ◽

Bone Mineral Density ◽

Pedicle Screw ◽

Bone Mineral ◽

Lumbar Vertebrae ◽

Pedicle Screws ◽

Mineral Density ◽

Screw Insertion ◽

Pull Out ◽

Pull Out Strength

Study Design: Biomechanical study.Purpose: To assess the correlation between the computed tomography (CT) values of the pedicle screw path and screw pull-out strength.Overview of Literature: The correlation between pedicle screw pull-out strength and bone mineral density has been well established. In addition, several reports have demonstrated a correlation between bone mineral density and CT values. However, no previous biomechanical studies investigated the correlation between CT values and pedicle screw pull-out strength.Methods: Sixty fresh-frozen lumbar vertebrae from 6-month-old pigs were used. Before screw insertion, the CT values of the screw path were obtained for each sample. Specimens were then randomly divided into three equal groups. Each group had one of three pedicle screws inserted: 4.0-mm LEGACY (4.0-LEG), 4.5-mm LEGACY (4.5-LEG), or 4.5-mm SOLERA (4.5-SOL) (all from Medtronic Sofamor Danek Inc., Memphis, TN, USA). Each screw had a consistent 30-mm thread length. Axial pull-out testing was performed at a rate of 1.0 mm/min. Correlations between the CT values and pedicle screw pull-out strength were evaluated using Pearson’s correlation coefficient analysis.Results: The correlation coefficients between the CT values of the screw path and pedicle screw pull-out strength for the 4.0-LEG, 4.5-LEG, and 4.5-SOL groups were 0.836 (p <0.001), 0.780 (p <0.001), and 0.873 (p <0.001), respectively. Greater CT values were associated with greater screw pull-out strength.Conclusions: The CT values of the screw path were strongly positively correlated with pedicle screw pull-out strength, regardless of the screw type and diameter, suggesting that the CT values could be clinically useful for predicting pedicle screw pull-out strength.

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Evaluation of the Effect of Fixation Angle between Polyaxial Pedicle Screw Head and Rod on the Failure of Screw-Rod Connection

Applied Bionics and Biomechanics ◽

10.1155/2015/150649 ◽

2015 ◽

Vol 2015 ◽

pp. 1-9 ◽

Cited By ~ 6

Author(s):

Engin Çetin ◽

Mustafa Özkaya ◽

Ümit Özgür Güler ◽

Emre Acaroğlu ◽

Teyfik Demir

Keyword(s):

Mechanical Properties ◽

Pedicle Screw ◽

Pedicle Screws ◽

Maximum Load ◽

Test Results ◽

Screw Head ◽

Pull Out ◽

Flexion Moment ◽

Polyaxial Screws ◽

Different Diameters

Introduction.Polyaxial screws had been only tested according to the ASTM standards (when they were perpendicularly positioned to the rod). In this study, effects of the pedicle screws angled fixation to the rod on the mechanical properties of fixation were investigated.Materials and Method.30 vertically fixed screws and 30 screws fixed with angle were used in the study. Screws were used in three different diameters which were 6.5 mm, 7.0 mm, and 7.5 mm, in equal numbers. Axial pull-out and flexion moment tests were performed. Test results compared with each other using appropriate statistical methods.Results.In pull-out test, vertically fixed screws, in 6.5 mm and 7.0 mm diameter, had significantly higher maximum load values than angled fixed screws with the same diameters (P<0.01). Additionally, vertically fixed screws, in all diameters, had significantly greater stiffness according to corresponding size fixed with angle (P<0.005).Conclusion.Fixing the pedicle screw to the rod with angle significantly decreased the pull-out stiffness in all diameters. Similarly, pedicle screw instrumentation fixed with angle decreased the minimum sagittal angle between the rod and the screw in all diameters for flexion moment test but the differences were not significant.

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A Computational Approximation to Model Variation in Cancellous Bone Screw Pull-Out

Journal of Medical Devices ◽

10.1115/1.4032868 ◽

2016 ◽

Vol 10 (2) ◽

Author(s):

A. Piper ◽

C. J. Brown

Keyword(s):

Cancellous Bone ◽

Geometric Model ◽

Reaction Force ◽

Bone Geometry ◽

Design Parameters ◽

Bone Screw ◽

Proximal Half ◽

Pull Out ◽

Half Angle ◽

Experimental Findings

Cancellous bone screws are used to achieve good pull-out characteristics when connected to cancellous bone. This study examines some screw characteristics, including pitch and inner diameter, using a model of cancellous bone with a range of bone apparent densities (ADs). This was achieved using bone geometry based on microCT-scanned cancellous bone and converted into a geometric model using mimics® software. The finite element (FE) models were produced in ansys®. The calculated reaction force for pull-out of 0.2 mm shows the influence of design parameters. Change in the proximal half angle increased the stiffness by about 15% in line with the experimental findings of others. An increase in pull-out reaction force with an increase in bone AD was also observed. However, when a particular screw geometry in lower AD bone was modeled and then rotated through 180 deg on plan, a significant reduction in reaction force was noted. Further models with screws of similar geometry in the same location showed similar reductions in reaction force and hence pull-out stiffness. Examination of the geometry of the bone/screw interface indicates that in certain positions there is little cancellous bone to support the implant—leading to low pull-out reaction forces, which is very difficult to predict. The study also examined the effect of increasing the bone stiffness adjacent to the implant, and concluded that, even in bone of low AD, increases in pull-out stiffness might be achieved.

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Comparison of Fatigue Behaviors of Spinal Implants Under Physiological Spinal Loads: A Finite Element Pilot Study

Volume 1: 37th Computers and Information in Engineering Conference ◽

10.1115/detc2017-67783 ◽

2017 ◽

Author(s):

Ming Xu ◽

James Yang ◽

Isador H. Lieberman ◽

Ram Haddas

Keyword(s):

Pedicle Screw ◽

Axial Rotation ◽

Von Mises Stress ◽

Fe Model ◽

Fusion Surgery ◽

Spinal Loads ◽

Spinal Implants ◽

Exact Geometry ◽

Von Mises ◽

Biomechanical Tests

The fusion surgery is a standard treatment for scoliosis. Fatigue-related failure is one common cause for the fusion surgery implant. Due to the high cost of revision surgery, it is of clinical value to study the fatigue behaviors of the spinal implants under physiological spinal loads. In the literature, biomechanical tests and finite element (FE) methods have been used to study the fatigue of the spinal implants. Compared with biomechanical tests, FE analysis has the advantage of low cost and high efficiency. Due to the high computational cost, no FE study has been modeled the exact geometry of the pedicle screw (including the thread) in the screw-bone connection within the multi-level spine FE model. This study introduced a feasible FE-based method to predict the fatigue behaviors of the spinal implants with exact geometry of pedicle screw. One previously-validated FE spine model was utilized to provide physiological spinal loads and was bilaterally fused with pedicle screws and rods at L3-L4 spine levels. The exact geometry of the pedicle screw was simulated in this study for accurate stress prediction. The fused spine FE model was subjected to six loading directions (flexion/extension, left/right lateral bending, and left/right axial rotation). For each loading direction, a pure bending moment of 10 Nm was tested. First, FE analysis was performed for one loading cycle. Range of motion, maximum von Mises stress values of the spinal implants were recorded and compared for the six tested loading conditions. Then, based on the stress/strain history of the spinal implants for one loading cycle provided by the FE simulation, fatigue life cycles of the spinal implants were calculated using strain-based Smith-Watson-Topper equation. Flexion produced the largest range of motion at the adjacent level. Axial rotation produced the largest von Mises stress in the spinal implants. Except for lateral bending, the von Mises stress predicted in the screws fused at the superior vertebra was larger than that in the screws fused at inferior vertebra. The method introduced in this study will be used to study different screw fixation methods in the future work.

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Partial Threading of Pedicle Screws in a Standard Construct Increases Fatigue Life: A Biomechanical Analysis

Applied Sciences ◽

10.3390/app11041503 ◽

2021 ◽

Vol 11 (4) ◽

pp. 1503

Author(s):

Fon-Yih Tsuang ◽

Chia-Hsien Chen ◽

Lien-Chen Wu ◽

Yi-Jie Kuo ◽

Yueh-Ying Hsieh ◽

...

Keyword(s):

Finite Element ◽

Fatigue Life ◽

Pedicle Screw ◽

Pedicle Screws ◽

Biomechanical Analysis ◽

Von Mises Stress ◽

Shaft Length ◽

Element Analysis ◽

Screw Design ◽

Screw Length

This study proposed a pedicle screw design where the proximal 1/3 of the screw is unthreaded to improve fixation in posterior spinal surgery. This design was also expected to reduce the incidence of mechanical failure often observed when an unsupported screw length is exposed outside the vertebra in deformed or degenerated segments. The aim of this study was to evaluate the fatigue life of the novel pedicle screw design using finite element analysis and mechanical testing in a synthetic spinal construct in accordance with American Society for Testing and Materials (ASTM) F1717. The following setups were evaluated: (i) pedicle screw fully inserted into the test block (EXP-FT-01 and EXP-PU-01; full thread (FT), proximal unthread (PU)) and (ii) pedicle screw inserted but leaving an exposed shaft length of 7.6 mm (EXP-FT-02 and EXP-PU-02). Corresponding finite element models FEM-FT-01, FEM-FT-02, FEM-PU-01, and FEM-PU-02 were also constructed and subjected to the same loading conditions as the experimental groups. The results showed that under a 220 N axial load, the EXP-PU-01 group survived the full 5 million cycles, the EXP-PU-02 group failed at 4.4 million cycles on average, and both EXP-FT-01 and EXP-FT-02 groups failed after less than 1.0 million cycles on average, while the fatigue strength of the EXP-FT-02 group was the lowest at 170 N. The EXP-FT-01 and EXP-FT-02 constructs failed through fracture of the pedicle screw, but a rod fractured in the EXP-PU-02 group. In comparison to the FEM-FT-01 model, the maximum von Mises stress on the pedicle screw in the FEM-PU-01 and FEM-PU-02 models decreased by −43% and −27%, respectively. In conclusion, this study showed that having the proximal 1/3 of the pedicle screw unthreaded can reduce the risk of screw fatigue failure when used in deformed or degenerated segments.

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A biomechanical study of a novel biomimetic hip implant.

10.32920/ryerson.14647719 ◽

2021 ◽

Author(s):

Shaheen Khurshid

Keyword(s):

Three Dimensional ◽

Stress Shielding ◽

Stress Transfer ◽

Biomechanical Study ◽

Von Mises Stress ◽

Fe Model ◽

Polyamide 12 ◽

Hip Stems ◽

Von Mises ◽

Three Dimensional Finite Element

A three dimensional finite element (FE) model of a novel carbon fibre polyamide 12 composite hip stem was used to compare with two commerically available (Exeter and Omnifit) hip stems to minimize stress shielding and bone resorption. A virtual axial load of 3000N was applied to the FE model which replicated the experimental study. Strain and stress distributions were computed and compared with experimental results. Experimentally, three hip stems had their distal portions rigidly mounted and had strain gauges placed along the surface at 3 medial and 3 lateral locations. From the FE analysis, the von mises stress range for the composite hip stem was 200% and 45% lower than that in the Omnifit and Exeter implants, respectively. The aggregate average difference between FE and experimental microstrains for four proximal strain gauge locations were 7.5% (composite), 11.5% (Exeter), 14.6% (Omnifit), and the composite hip stem's stiffness (1982N/mm) was lower than the metallic hip stem stiffnesses (Exter, 2460N/mm; Omnifit, 2543 N/mm). This study showed considerable improvement in stress transfer to bone tissue.

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Comparative analysis of clinical factors associated with pedicle screw pull-out during or immediately after surgery between intraoperative cone-beam computed tomography and postoperative computed tomography

10.21203/rs.3.rs-52538/v5 ◽

2021 ◽

Author(s):

Satoshi Sumiya ◽

Kazuyuki Fukushima ◽

Yoshiro Kurosa ◽

Takashi Hirai ◽

Hiroyuki Inose ◽

...

Keyword(s):

Risk Factors ◽

Computed Tomography ◽

Pedicle Screw ◽

Odds Ratio ◽

Pedicle Screws ◽

Cone Beam ◽

Clinical Factors ◽

Factors Associated ◽

Pull Out ◽

History Of

Abstract Background No studies to date have elucidated the clinical factors associated with pedicle screw pull-out during or immediately after surgery. The aim of this study was to assess the frequency of pedicle screw pull-out by comparing intraoperative scans obtained using cone-beam computed tomography (CBCT) with postoperative scans obtained using computed tomography (CT). We also sought to determine the incidence of pedicle screw pull-out and identify relevant risk factors. Methods This was a retrospective analysis of prospectively collected data for 742 pedicle screws placed in 76 consecutive patients who underwent at least triple-level posterior fixation for thoracic or lumbar spinal injury, spinal metastasis, or pyogenic spondylitis between April 2014 and July 2020. Pedicle screw pull-out distance in the axial and sagittal planes was compared between CT scans obtained 2 days postoperatively and CBCT images acquired intraoperatively. Risk factors associated with pedicle screw pull-out were investigated by multivariate logistic regression analysis. Results Pedicle screw pull-out was seen with 58 pedicle screws (7.8%) in 26 patients (34.2%). There were significant differences in age, number of fused segments, frequency of diffuse idiopathic skeletal hyperostosis (DISH), and medical history of osteoporosis for pedicle screw pull-out. Risk factors for pedicle screw pull-out were older age (odds ratio 1.07, 95% confidence interval 1.02–1.130) and a diagnosis of DISH (odds ratio 3.35, 95% confidence interval 1.12–10.00). Several cases suggest that use of connecting rods was an important factor in intraoperative pedicle screw pull-out. Conclusions Our findings suggest that age, number of fused segments, presence of DISH, and medical history of osteoporosis are risk factors for pedicle screw pull-out, with the greatest being older age and DISH.

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Comparative analysis of clinical factors associated with pedicle screw pull-out during or immediately after surgery between intraoperative cone-beam computed tomography and postoperative computed tomography

10.21203/rs.3.rs-52538/v3 ◽

2020 ◽

Author(s):

Satoshi Sumiya ◽

Kazuyuki Fukushima ◽

Yoshiro Kurosa ◽

Takashi Hirai ◽

Hiroyuki Inose ◽

...

Keyword(s):

Risk Factors ◽

Computed Tomography ◽

Pedicle Screw ◽

Odds Ratio ◽

Pedicle Screws ◽

Cone Beam ◽

Clinical Factors ◽

Factors Associated ◽

Pull Out ◽

History Of

Abstract Background No studies to date have elucidated the clinical factors associated with pedicle screw pull-out during or immediately after surgery. The aim of this study was to assess the frequency of pedicle screw pull-out by comparing intraoperative scans obtained using cone-beam computed tomography (CBCT) with postoperative scans obtained using computed tomography (CT). We also sought to determine the incidence of pedicle screw pull-out and identify relevant risk factors.Methods This was a retrospective analysis of prospectively collected data for 742 pedicle screws placed in 76 consecutive patients who underwent at least triple-level posterior fixation for thoracic or lumbar spinal injury, spinal metastasis, or pyogenic spondylitis between April 2014 and July 2020. Pedicle screw pull-out distance in the axial and sagittal planes was compared between CT scans obtained 2 days postoperatively and CBCT images acquired intraoperatively. Risk factors associated with pedicle screw pull-out were investigated by multivariate logistic regression analysis.Results Pedicle screw pull-out was seen with 58 pedicle screws (7.8%) in 26 patients (34.2%). There were significant differences in age, number of fused segments, frequency of diffuse idiopathic skeletal hyperostosis (DISH), and medical history of osteoporosis for pedicle screw pull-out. Risk factors for pedicle screw pull-out were older age (odds ratio 1.07, 95% confidence interval 1.02–1.130) and a diagnosis of DISH (odds ratio 3.35, 95% confidence interval, 1.12–10.00). Several cases suggest that use of connecting rods was an important factor in intraoperative pedicle screw pull-out.Conclusions Our findings suggest that age, number of fused segments, the presence of DISH, and medical history of osteoporosis are risk factors for pedicle screw pull-out, with the greatest being older age and DISH.

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