Finite element analysis of posterior anatomical locking plate for distal tibia

Transcription

1 Chinese Journal of Tissue Engineering Research December 18, 2017 Vol.21, No.35 Finite element analysis of posterior anatomical locking plate for distal tibia Wang Hai-yan, Xu Gui-cun, Cai Yong-qiang, Li Zhi-jun, Zhang Shao-jie, Gao Shang, Wang Xing, Li Xiao-he ( Department of natomy, School of asic Medicine, Inner Mongolia Medical University, Hohhot , Inner Mongolia utonomous Region, China; Department of Emergency Surgery, the ffiliated Hospital of Inner Mongolia Medical University, Hohhot , Inner Mongolia utonomous Region, China) Cite this article: Wang HY, Xu GC, Cai YQ, Li ZJ, Zhang SJ, Gao S, Wang X, Li XH. Finite element analysis of posterior anatomical locking plate for distal tibia. Zhongguo Zuzhi Gongcheng Yanjiu. 2017;21(35): doi: /j.issn orcid: (Xu Gui-cun) bstract CKGROUND: The posterior anatomical locking plate has obtained excellent fixing performance in the treatment of complex fractures of the distal tibia; however, there are many drawbacks such as nail and plate broken, and the related rules are never reported. OJECTIVE: To establish a finite element model of posterior anatomical locking plate for distal tibia and to provide references for its design and improvement. METHODS: The imaging data of a male patient aged 34 years old (body mass: 68 kg) with fracture on the left distal tibia were imported into Mimics16.01 software, and the proposed reconstruction parts were determined based on the default threshold of the software. The files were imported into the nsys11.0 software, the distal tibial surface was fixed, and a force of 340 N was vertically loaded on the supine surface of the horizontal section of the large tibial shaft. RESULTS ND CONCLUSION: There was completely connected with the stepped surface of nut and screw thread portion in locking steel plate. The strain on the plate depended on the stress values. The posterior anatomical locking plate for distal tibia was more favorable for the fixation at the fracture site and could reduce the fixed plate and screws loosening, both of which could affect the fixed efficacy. Subject headings: Tibia; Finite Element nalysis; Tissue Engineering Funding: the Overseas Science and Technology Project of Inner Mongolia utonomous Region in 2016; the Science and Technology Project of Inner Mongolia utonomous Region in 2016 Wang Hai-yan, Master, ssociate professor, Master s supervisor, Department of natomy, School of asic Medicine, Inner Mongolia Medical University, Hohhot , Inner Mongolia utonomous Region, China Corresponding author: Xu Gui-cun, Master, Chief physician, Department of Emergency Surgery, the ffiliated Hospital of Inner Mongolia Medical University, Hohhot , Inner Mongolia utonomous Region, China INTRODUCTION The posterior anatomical locking plate has achieved satisfactory fixed performance in the treatment of complex fractures of the distal tibia; however, nail and plate broken and poor bone fusion usually occur [1-2]. Therefore, how to avoid the problems is an urgent issue [3-6]. Complex irregular bodies can be transformed into simple discrete elements by finite element analysis, in which, the stress of the whole structure and apparatus is analyzed by calculating the stress of each unit to guide the clinical practice. It not only provides the detailed stress data of the fracture site and the internal implant system [7-9], but also can evaluate the effects of different surgical methods and treatment methods on the function and mechanical properties of distal tibia [10-14]. esides, it can be used to mechanically evaluate and test the manufacture and clinical application of the fixing apparatus, and therefore to further optimize the design [15-19]. In the current study, the stress distribution of posterior tibial anatomical locking plate and non-locking plate was measured and compared by finite element analysis software, thereby providing a digital physical model for the improvement of the fixed instruments. SUJECTS ND METHODS Design Finite element analysis. Time and setting This study was conducted from June to September 2016, at the Digital Medical Research Center of Inner Mongolia Medical University, China. Subjects One patient (male, 34 years old, and body mass: 68 kg) with fracture on the left distal tibia was enrolled in this study. This study was approved by the Ethics Committee of Inner Mongolia Medical University (No ), and the written consent was obtained from the patient prior to the study. Materials The posterior tibial anatomical locking internal plant system ( holes, right, 80 mm) produced by Double Engine Medical Material Co., Ltd. was used (Figure 1). Corresponding author: Li Xiao-he, M.D., Professor, Department of natomy, School of asic Medicine, Inner Mongolia Medical University, Hohhot , Inner Mongolia utonomous Region, China ccepted: ISSN CN /R CODEN: ZLKHH 5691

2 Methods The finite element model establishment of distal tibia locking plate and screw Parametric modeling of the posterior tibial anatomical locking internal plant system was performed by the Pro/Engineer3.0 mechanical modeling software (Parametric Technology Corp., Shanghai, China) and the files were output and stored in IGES format (Figure 1). The CT scan data were imported into the Mimics16.01 software (Materialise, Shanghai, China), and the proposed reconstruction parts were determined based on the default threshold of the software. The fixation system for the posterior tibial anatomical locking plate was established on Pro/Engineer3.0 and the reconstructed distal tibial fracture model was imported into Mimics True-up was performed in the software based on the locking plate fixation principle; self-tapping locking screws were installed into all the screw holes in group, and then the trued-up system was divided by surface mesh and body mesh, exported and saved in STL format. The true-up and fixation were performed for non-locking plate in group ; the files were imported into the nsys11.0 software (NSYS Inc., Canonsburg, P, US) to establish a finite element model and simulate the stress distribution of the fixation system in a normal human body in the standing position. The distal tibial surface was fixed, and a force of 340 N was vertically loaded on the supine surface of the horizontal section of the large tibial shaft. The stress distribution in the screw holes and screw tails in the steel plate was analyzed. Configuration of the computer hardware: sus 43EI243SV-SL laptop (CPU: Intel core i52430m, CPU frequency: 2.4 GHz, memory: 2 G, DDR3, MHz, Windows XP operating system). Software: Mimics16.01 (Materialise, Shanghai, China), Geomagic Studio v9.0 (Parametric Technology Crop., Shanghai, China); Pro/Engineer3.0 (Parametric Technology Corp., Shanghai, China), nsys11.0 (nsys Inc., Canonsburg, P, US). Establishment of distal tibial fracture model The CT scan data were imported into the Mimics16.01 to reconstruct the structure by setting the threshold. Images of different structures were realized by different masks, and the noise and redundant structures were erased through planar editing and stereoscopic editing. Finally, each mask was reconstructed by three-dimensional calculation to clearly visualize the model of distal tibial fracture. The files were saved in IGE format and modified in the reverse engineering software Geomagic Studio v9.0 by manual and automatic methods to repair the damaged surface and body, thus ensuring that the boundary of the reconstructed structure was smooth, clear and real (Figure 2). The establishment of finite element models of posterior anatomical locking plate for distal tibia The three-dimensional model of distal tibial fracture was 5692 imported into Mimics16.01, and the established locking plate and screw fixation system was imported using the MedCD of Mimics16.01; then the three parts were trued up using the true-up module of the software (Figure 3). The imported nsys surface mesh files in the LIS format were transformed into body mesh files using nsys11.0, and were then imported into the Mimics16.01 for material distribution. The Heinz gray value of each unit of the body mesh was calculated based on the scanning image data using the FE function of Mimics16.01, and then the corresponding materials were defined respectively, according to different gray scale range. The finite element model (Figure 1) of each material was established based on its assignment (referring to previous literatures [5-6], Table 1). Loading and calculation (1) The lower surface of distal tibia model was fixed, and a load of 340 N was applied continuously and on the upper surface of the tibia, and evenly distributed on each node of the surface. There were two types of load: static vertical load, a load of 340 N applied on the upper surface of tibia; and rotating loading, a load of 340 N applied on the upper surface of tibia with 3 N m torque. Stress and strain measurement The stress and strain were expressed by qualitative and quantitative methods. The former was represented by distribution cloud figure, and the latter was calculated as follows: (1) The mean VonMises stress and deformation of different sections of the steel plate and screws were calculated (stresses of 15 point at each section level of the steel plate was measured; the screw was divided into three sections, and stresses of 15 evenly distributed point were measured on the cylindrical surface) (Figure 3). Main outcome measures The stress values on different parts of the posterior anatomical locking plate were measured by nsys software. The stress and stain values at different positions were measured. Statistical analysis SPSS11.0 software (SPSS Inc., Chicago, IL, US) was used for statistical analysis, and the data are expressed as mean±sd. Paired t test was used to compare the stresses of steel plate and screw hole circumference, and three sections of the screw were analyzed by repeated measures variance. RESULTS The finite element models of posterior anatomical locking plate for distal tibia finite element model of posterior anatomical locking plate fixation for distal tibia was successfully established using three-dimensional reconstruction software Mimics16.01 and finite element software nsys11.0. There were totally units and nodes, and the established finite element model showed good geometrical similarity with the real structure. P.O. ox 10002, Shenyang

3 Note: () Physical map of the internal plant system; () three-dimensional model of the internal plant system of posterior tibial anatomical locking plate. C D Figure 1 The physical map and three-dimensional model of the internal plant system of posterior tibial anatomical locking plate ( holes, right, 80 mm) Figure 2 The reconstruction of distal tibia model Note: () Coronal plane; () horizontal plane; (C) sagittal plane; (D) three-dimensional model. Figure 3 Regional distribution of fixed steel plate Note: was the top, C was the bottom, and was the midpoint of the plate. Note: () Stress; () strain. Left: non-locking, right: locking Note: () Stress; () strain. Up: non-locking, down: locking Figure 5 Comparison of the stress and strain values between non-locking and locking groups in rotating state Table 2 Comparison of the stress and strain values in different sections when the steel plate was upright (x±s, n=15) Section Unlocked Locked t P 3.23± ± ± ± C 29.56± ± ± ± ± ± Note: C were the sections in Figure 3, respectively. Verification of the finite element model of posterior anatomical locking plate for distal tibia The model in this research was established based on the verified method by Shih et al [8], and therefore this model was qualified. The stress and strain comparison of the finite element models of posterior anatomical locking plate for distal tibia With the increasing of section (From to C), the stress was increasing under upright + rotating state or upright, Figure 4 Comparison of the stress and strain values between non-locking and locking groups in upright state Table 1 The material characteristic constant and element type of the fixation model of posterior anatomical locking plate for distal tibia Site Element type Young s modulus (MPa) Cortical bones 8-node solid Cancellous bones 8-node solid Fixation Solid Poisson s ratio (µ) Table 3 Comparison of the stress and strain values in different sections when the steel plate was rotating (x±s, n=15) Section Unlocked Locked t P 0.04± ± ± ± C 29.32± ± ± ± ± ± Note: C were the sections in Figure 3, respectively. there was a positive correlation between stress and section (r=0.966, P < 0.001; r=0.945, P < 0.001). The unlocked and locked steel plate stresses under upright + rotating state were significantly larger than those under the upright state (P < 0.05). The stress and strain values of the unlocked steel plate were significantly larger than those of the locked steel plate (P < 0.05) (Figures 4, 5, Tables 25). ISSN CN /R CODEN: ZLKHH 5693

4 Table 4 Comparison of the stress and strain values in the same part of non-locking plate in upright state and upright+rotating state (x±s, n=15) Section Upright Upright+rotating t P 3.23± ± ± ± C 29.56± ± ± ± ± ± Note: C were the sections in Figure 3, respectively. Table 5 Comparison of the stress and strain values in the same part of locking plate in upright state and upright+rotating state (x±s, n=15) Section Upright Upright+rotating t P 0.05± ± ± ± C 12.04± ± ± ± ± ± Note: C were the sections in Figure 3, respectively. DISCUSSION Locking anatomical steel plate is commonly used in high energy injuries in the distal tibia because it overcomes many disadvantages of traditional anatomical steel plates, including broad soft tissue dissection, postoperative adhesions, joint ankylosis or flexion limitation. Currently, the commonly used locking plates are screw locking plates [20-26]. The locked fixation system cause no additional pressure on the bone surface and can effectively protect the blood circulation in the fixed section of the bone. esides, the locking screw and the steel plate are integrated together and provide a better angulation stability for the broken ends of fractured bone. The distal inner implant and the distal tibia are closely fitted and more suitable for osteoporosis patients, and furthermore, the patients can exercise their knee joints in early period to avoid knee enstrophe and ecstrophy [27-29]. lthough the posterior anatomical locking plate fixation device for distal tibia has been extensively used, there are many clinical reports on screw loosening and plate and screw broken [30-33]. Thus we tried to analyze the commonly used fixation operation methods by finite element analysis to compare the stress on different sections of the plate, the screw hole edge and the screws in each fixation mode, so as to provide a mathematical physics model for the improvement of the fixation device. We successfully performed mesh division of the posterior anatomical locking plate for distal tibia using Mimics13.0, Geomagic Studiov9.0, Pro/Engineer3.0 and nsys11.0 based on the screw track data measured in section 2 and screw insertion principle. The established finite element model showed good morphological similarity with the real structure, the bone materials were assigned completely according to the gray values from the CT scan, and the fixed internal implants were assigned according to the labeling materials in manufacturer s manual. Thus the whole model resembled the real one and could simulate the actual stress in the patients at the upright position. In current study, since there was no contact between the outer circle cylinder of the bolt and the corresponding screw hole on the fixing plate, and the fixing plate was merely connected with the stepped surface of nut and screw thread portion in the 5694 non-locking steel plate, the deformation of the fixing plate was significant due to the great contact stress on the small contact area. Since the outer circle cylinder of the bolt was totally contacted with the corresponding screw hole on the fixing plate, and the fixing plate was also completely connected with the stepped surface of nut and screw thread portion in locking steel plate, the deformation of the fixing plate was not obvious due to the little contact stress on the large contact area. ased on the data from the simulation analysis on the locking and non-locking plates, the stress and deformation of the locking plate were only about half the non-locking plate, indicating that it is unlikely that the locking plate becomes loose in clinical application. Therefore, the posterior anatomical locking plate for distal tibia was more favorable for the fixation at the fracture site and could reduce the fixed plate and screws loosening, both of which could affect the fixed efficacy and result in more surgery. The stress of each part of the posterior anatomical locking plate fixation system for distal tibia was smaller than the yield strength of the titanium alloy screw, MPa [13, 34-38] in all the fixed and moving states; therefore, there was no plastic deformation or fracture in the fixation system. The imaging data of male youth with normal bone mineral density and bone tissue strength were used for modeling, and the finite element model stress analysis was only applied in one sample, thus lacking universality. The finite element analysis and in vitro validation test should be performed in more samples in the future to obtain data with universality to guide clinical application; bone loss and osteoporosis are induced by decline in estrogen level in the aged people, especially the elderly women; thus the effective pullout strength between the vertebra and vertebral nail is reduced [39-41] in such population, and the stress in the fixation system is different from that in the experimental model. Therefore, more studies should be carried out in patients with osteoporosis and other kinds of distal tibial fractures. In summary, the posterior anatomical locking plate for distal tibia is more favorable for the fixation at the fracture site and P.O. ox 10002, Shenyang

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