Pull-out strength of screws from cortical bone in the maxillo-facial region

The fixation of maxillofacial fractures is an important clinical procedure, which may be achieved by the attachment of plates across the fracture. The stability of the fracture will depend on the stiffness of the fracture fixation plates and the security of the fixation screws to the thin maxillofacial cortical bone. The design of screws, manufactured by Champy and AO were tested from both mini-and micro-fixation systems. Pull-out tests were conducted on cortical bone plates, ranging in thickness from 0.6 to 3.5 mm. No significant differences were observed in the ultimate pull-out forces achieved for both mini-systems of 2 mm outer diameter. However, these pull-out forces were generally greater than those obtained for the micro-screws, even at the lower bone thicknesses. Two models were developed which attempted to predict the behaviour of screw pull-out failure. The failure mechanism was primarily dependent on the thickness of the bone, with secondary influences related to the shear strength of the bone and a geometrical factor of the screw.     

动物骨折治疗用金属骨板局部增量成形新工艺

目的目前动物骨折常用的锁定骨板内固定技术(Point-contact Reconstruction Compress Locking, PRCL)需要采用多个工具配合手动完成骨板成形,针对该过程中精度不可控、效率低等问题,提出一种弯扭复合成形模具,发展一种局部增量成形金属骨板的方法。方法 PRCL骨板固定治疗中,为了贴合受伤骨骼,治疗前骨板需经过面内弯曲、弯曲以及扭转3类变形。通过调整弯扭复合成形模具的空间位置及模具不同组成部分的相对位置,实现不同区域内不同变形量的面内弯曲、弯曲或扭转。应用数值方法分析验证弯扭复合成形模具及成形方法的适用性,基于DEFORM软件建立工业纯钛TA2骨板局部增量成形过程有限元模型,分析具有两个成形区的TA2骨板局部增量成形特征。结果塑性变形仅发生在复合模具附近,对已变形区无影响,会引起未成形区的刚性位移;骨板长度方向受力小于其宽度和厚度方向受力,面内弯曲需要较大的成形载荷。结论所发展的模具和方法可实现预期的骨板成形,也适用于其他PRCL金属骨板的成形。     

Theoretical relationship between maximum pore size and toughness in experimental inflammatory arthritis

Rheumatoid arthritis increases the risk of fracture. In an animal model of inflammatory arthritis, femoral diaphysis had a decreased toughness as well as increased cortical porosity, when compared to normal bone. Based on the hypothesis that stress concentration from the large porous defects reduces the ability of the cortical bone to resist failure, this work determined if the changes observed in porosity could explain the changes observed in toughness. Using theoretical relationships of the stress concentration and stress states, a model of the observed conditions was considered. A relationship was developed that indicated the relative difference in toughness between normal and arthritic specimens as a function of pore size. Results indicated that the increase in cortical pore size could theoretically reduce toughness by 55%. This decrease compares with the experimentally observed drop in toughness of 61%. Furthermore, the critical parameter for fracture in this situation is the ratio of pore diameter to cortical thickness. Efforts to reduce cortical porosity seen in inflammatory arthritis would be effective in enhancing the toughness of bone and may reduce morbidity in a human population. ©2000 Kluwer Academic Publishers     

Mesoscale simulations of a dart penetrating sand

Historically, hydrodynamic calculations have utilized continuum constitutive models to simulate the coupled dynamic response of a solid projectile penetrating a heterogeneous target system such as concrete, foam or a granular porous medium. Continuum models fail to capture the complicated grain level response within the heterogeneous target which can result in asymmetric loading of the projectile leading to variations in projectile performance. These grain level effects can be crucial to predicting the penetration depth or overall effectiveness of the projectile. In order to assess the possibility of using mesoscale simulations to resolve the grain level dynamics, hydrodynamic simulations were performed for an 11.4 cm long, 0.9 cm diameter dart penetrating a bed of porous granular dry sand with an initial velocity of 366 m/s. Simulations were performed using the Eulerian hydrocode CTH in a two-dimensional planar configuration. The goal of the mesoscale simulations is to determine the viability of using these techniques as an alternative to continuum models and to assess the effects of grain level variability such as anisotropic material distributions and variations in the dynamic yield and fracture strength. The results indicate that variations in the size distribution of aggregate added and the fracture strength of the sand system can have a significant effect on penetration performance of the dart; whereas variations in the dynamic strength of the sand had little effect on the dart penetration.     

Effects of Ti2448 half-pin with low elastic modulus on pin loosening in unilateral external fixation

The objective of this study was to compare the benefits of titanium 2448 (Ti2448) half-pin and titanium-6 aluminium-4 vanadium (TAV) half-pin on reducing pin loosening during external fracture fixation. Although having similar strength, Ti2448 half-pin had even lower elastic modules(33 GPa)when compared with TAV half-pin (110 GPa), which was similar to that of cortical bone (20 GPa). In the external fixation of tibial model fractures and canine cadaveric tibia fractures, Ti2448 half-pin had greater recoverable deformation and less stress concentration at the pin–bone interface in compression, torsion, and four-points bending test. Then, tibial fractures were created in 24 dogs and stabilized with four half-pins of either Ti2448 or TAV in each animal. At 4 and 8 weeks postoperatively, fracture healing and pin loosening was assessed by radiographic grading scale. The scores of Ti2448 group were significantly higher than those of TAV group. Micro-CT analysis also indicated larger quantity and higher quality of newly formed bone at pin–bone interface in Ti2448 group. Histology observation showed the newly formed bone integrated well into the threads of Ti2448 half-pins. In contrast, there was a layer of necrotic tissue between the bone tissue and TAV half-pin at pin–bone interface in TAV group. The extraction torque values of Ti2448 half-pins near the fracture line were significantly higher than those TAV pins. In conclusion, the Ti2448 half-pin with low elastic modulus could enhance osseointegration and reduce pin loosening when compared with TAV half-pin. It is a promising biomaterial for constructing external fixation system in clinical application.     

Experiment study on fracture fixation with low rigidity titanium alloy

In order to investigate bone tissue reaction to the low rigidity titanium alloy of TNTZ in bone plate fixation, animal experiment with rabbit was performed with X-ray follow-up and histological observation. Experimental fractures were made in rabbit tibiae, and fixed by different bone plates of SUS316L, Ti–6Al–4V and TNTZ. Although there was no significant difference in fracture healing, bone atrophy was observed in cortical bone especially under the bone plate, which was different in time course among three materials. The bone atrophy under the bone plate was confirmed as porous or poor bone tissue in histological observation. In addition, the diameter of the tibia bone was increased in TNTZ as the result of bone remodeling with a new cortical bone. It is confirmed that the elastic modulus of the bone plate will naturally influence bone tissue reaction to the bone plate fixation according to the Wolff’s law of functional restoration.     

碳纤维增强聚乳酸复合材料体外降解特性

制备了碳纤维增强聚乳酸(C/ PLA) 骨折内固定复合材料, 研究了体外降解过程中其力学性能的变化。结果表明: 在体外降解过程中, C/ PLA 复合材料的各项力学性能均有不同程度的下降, 但经过硝酸处理后的C/PLA 复合材料降解速度缓慢, 表明界面结合强度的提高对降解过程起抑制作用。      

Evaluation of fracture toughness of cartilage by micropenetration

Failure properties of cartilage are important in injury repair and disease, but few methods exist for measuring these properties, especially in small animals. To meet this need, a new indentation/penetration method for measuring fracture toughness of cartilage is proposed. During indentation, a conical tip is displaced into the surface of the cartilage, causing first a non-penetrating indentation, and then a penetration into the tissue. The method assumes that tissue penetration occurs during periods of rapid work, which are identified from a curve of work rate vs. time. Total penetration depth is determined by summing the displacement during these periods. Fracture work is the work that occurs during rapid work, or penetration, and fracture toughness defined as the fracture work divided by one-half the penetrated surface area of the indenting tip. The method was validated by indentation testing of bovine cartilage. Penetrating indentations with a conical tip were performed in bovine patellar cartilage and depth of penetration and fracture toughness predicted. For comparison with the indentation data, depth of penetration was measured in histological sections. These measurements agreed well with the predicted depth. Predicted fracture toughness also agreed with values measured via a macroscopic test. This newly described method has promise as a general method for measuring fracture toughness in cartilage, particularly in small animals, since penetrating tips with small tip radius can be manufactured and penetration may be accomplished in cartilage of minimal thickness.     

表面处理对碳纤维增强聚乳酸材料界面性能的影响

制备了碳纤维增强聚乳酸（C／PLA）骨折内固定材料，对碳纤维进行了浓硝酸表面处理。测量了C／PLA复合材料的力学性能，并通过XPS分析了表面处理前后碳纤维表面官能团的变化。结果表明，在复合材料界面区发生了某种酯化反应，这是其界面粘结性能得到改善的根本原因。     

H80薄板拉伸性能的尺寸效应

为了研究薄板拉伸性能的尺寸效应及其产生的微观机制,对晶粒尺寸相似但厚度不同的H80薄板试样进行了拉伸试验和显微硬度试验.通过建立晶界区域强化模型和晶粒间的弱弱组合变形机制,研究了薄板的厚度尺寸、晶粒尺寸及显微硬度分布对拉伸力学性能的影响.结果显示:强度指标值和塑性指标值均随厚度尺寸的减小而降低,呈现越小越弱的尺寸效应现象,但强度指标和塑性指标的变异因子值却呈现越小越强的相反规律.薄板的横向硬度均值随厚度尺寸的增大而增大,而横向硬度值的变异因子却随厚度尺寸的增大而减小.随厚度尺寸的减少,晶界区域所占百分比的减少和单个晶粒所占百分比的增加是产生拉伸性能尺寸效应现象的根本原因.此外,显微硬度分析表明:易变形区域形成的概率随厚度尺寸的减小而增大,易变形区域的集中变形是引起薄板试样过早断裂的原因.     

A Model of the Process of Penetration of a Compound Elongated Projectile into a Shielded Target

We present the results of theoretical investigation of the mechanism of deformation and fracture of compound elongated projectiles penetrating into a shielded target. The process of penetration is analyzed by taking into account the thermal and mechanical characteristics of the materials, the thickness of the shield, the geometric parameters of the (leading and head) components, and the conditions of collision with the target. The numerical simulation is carried out by using the LasTan 2D-Impact software package based on the finite-element method. The proposed model of penetration enables us to study the distinctive features of deformation and fracture of compound elongated projectiles in the process of their interaction with shielded targets. The analysis of the accumulated results shows that the maximum degree of damage to compound elongated projectiles is attained in their leading part and caused by shear and tensile strains.     

Higher order Cauchy–Born rule based multiscale cohesive zone model and prediction of fracture toughness of silicon thin films

In this work, we extend the multiscale cohesive zone model (MCZM) (Zeng and Li in Comput Methods Appl Mech Eng 199:547–556, 2010), in which interatomic potential is embedded into constitutive relation to express cohesive law in fracture process zone, to include the hierarchical Cauchy–Born rule in the process zone and to simulate three dimensional fracture in silicon thin films. The model has been applied to simulate fracture stress and fracture toughness of single-crystal silicon thin film by using the Tersoff potential. In this study, a new approach has been developed to capture inhomogeneous deformation inside the cohesive zone. For this purpose, we introduce higher order Cauchy–Born rules to construct constitutive relations for corresponding higher order process zone elements, and we introduce a sigmoidal function supported bubble mode in finite element shape function of those higher order cohesive zone elements to capture the nonlinear inhomogeneous deformation inside the cohesive zone elements. Benchmark tests with simple 3D models have confirmed that the present method can predict the fracture toughness of silicon thin films. Interestingly, this is accomplished without increasing of computational cost, because the present model does not require quadratic elements to represent heterogeneous deformation, which is the inherent weakness of the previous MCZM model. Quantitative comparisons with experimental results are performed by computing crack propagation in non-notched and initially notched silicon thin films, and it is found that our model can reproduce essential material properties, such as Young’s modulus, fracture stress, and fracture toughness of single-crystal silicon thin films.     

An analytical model of hole size in finite plates for both normal and oblique hypervelocity impact for all target thicknesses up to the ballistic limit

This paper presents, for the first time, a single comprehensive analytical model for the hole size produced by hypervelocity impact into finite plates. This model is based on experimental data for 2017 aluminum spheres impacting 2014, 2024 and 6061 aluminum plates.

The significance of this model is that it spans the entire range of target thickness from very thin to very thick, which makes it possible to determine when the impact conditions are those of thin target behavior (where the hole size increases with increasing target thickness and debris formation and damage is important) and when the impact conditions are those of thick target behavior (where the hole size decreased with increasing target thickness and the debris formation is significantly decreased). The model makes it clear that the target thickness that divides the thin target regime from the thick target regime is a function of velocity. This means that an impact configuration which exhibits thick target behavior at common experimental velocities could actually exhibit thin target behavior at velocities in the tens of kilometers per second such as that of meteroid impacts. This hole size model also includes the effects of oblique impact and computes both the major and the minor diameters of the hole.

This paper also raises, for the first time, the possibility that the commonly accepted models for crater diameter (and by implication those for penetration depth as well), which are taken to be a power function of velocity, might be wrong. Only a linear dependence on velocity for the crater diameter is consistent with the linear velocity dependence of this and all other accepted models of hole diameter in finite plates. If this is correct, it would raise questions about the validity of using any target damage computer models, that are based on the old crater modeling equations, to extrapolate to higher velocities.     

Effect of cryo-induced microcracks on microindentation of hydrated cortical bone tissue

Microcracks accumulate in cortical bone tissue as a consequence of everyday cyclic loading. However, it remains unclear to what extent microdamage accumulation contributes to an increase in fracture risk. A cryo-preparation technique was applied to induce microcracks in cortical bone tissue. Microcracks with lengths up to approximately 20 μm, which were initiated mainly on the boundaries of haversian canals, were observed with cryo-scanning electron microscopy. A microindentation technique was applied to study the mechanical loading effect on the microcracked hydrated bone tissue. The microindentation patterns were section-scanned using confocal laser scanning microscopy to understand the deformation and bone damage mechanisms made by mechanical loading. The results show that there was no significant difference with respect to microhardness between the original and microcracked hydrated cortical bone tissues (ANOVA, p > 0.05). The cryo-induced microcracks in the bone tissue were not propagated further under the mechanical loads applied. The deformation mechanism of the microcracked cortical bone tissue was plastic deformation, not brittle fracture.     

静电纺丝法制备小口径胶原-聚乳酸人工血管

以聚乳酸(PLA)和胶原(Col)为原料,通过静电纺丝法制备了小口径(d=3.0mm)Col-PLA人工血管。采用扫描电镜、孔径分析仪和万能拉力机对Col-PLA人工血管的外层纤维形貌、孔径和拉伸性能进行了表征,测定了人工血管的管壁厚度及爆破强度。研究了纺丝电压、纺丝液质量分数、PLA与Col质量比对人工血管结构及性能的影响。结果表明:随纺丝电压的增加,纤维排列由杂乱变为规整,最佳的纺丝电压为15~20kV之间;当纺丝液质量分数增大时,Col-PLA人工血管的纤维直径增大,孔径及孔隙率均变小,拉伸强度和爆破强度提高;随PLA与Col质量比提高,人工血管的内层管壁厚度减少,外层管壁厚度增加,使拉伸强度和爆破强度提高,PLA与Col质量比分别为70∶30和90∶10时,制得人工血管力学强度能够满足使用要求。     

Polylactic acid coating on a biodegradable magnesium alloy: An in vitro degradation study by electrochemical impedance spectroscopy

Polylactic acid (PLA) was coated on a biodegradable magnesium alloy, AZ91, using spin coating technique for temporary implant applications. The degradation behaviour of the coated alloy samples was evaluated using electrochemical impedance spectroscopy (EIS) method in simulated body fluid (SBF). EIS results suggested that the PLA coating enhanced the degradation resistance of the alloy significantly. Increase in the PLA coating thickness was found to increase the degradation resistance, but resulted in poor adhesion. Long-term EIS experiments of the PLA coated samples suggested that their degradation resistance gradually decreased with increase in SBF exposure time. However, the degradation resistance of the PLA coated samples was significantly higher than that of the bare metal even after a 48 h exposure to SBF.     

The modified temperature term on Johnson Cook model for AZ31 magnesium alloy

The quasi‐state and dynamic mechanism of AZ31 magnesium alloy at a strain rates range of 0.001 s^‐1–2500 s^‐1 under a temperature range of 20 °C–250 °C were researched by compression tests using the electronic universal testing machine and split Hopkinson pressure bar system. The true stress‐strain curves at different strain rates and evaluated temperatures were obtained. The result shows that the thermal soften effect of AZ31 magnesium alloy is significant. By modifying the temperature term of the original Johnson Cook model of AZ31 magnesium alloy, a modified Johnson Cook model of AZ31 magnesium alloy has been proposed to reveal thermal soften effect on the deformation behavior of AZ31 magnesium alloy more precisely. With the modified Johnson Cook model and fracture model, the finite element method simulation of AZ31 magnesium alloy hat shaped specimen under impacting was conducted. The numerical simulation result is consistent with the experimental result, which indicates that the modified Johnson Cook model and fracture model are greatly valid to predict the deformation and fracture behavior of the AZ31 magnesium alloy hat shaped specimen under impacting.     

Length scales for the fracture of nanostructures

Length scales are essential to the understanding of small volume deformation and fracture in emerging technologies. Recent analysis by two groups at the atomistic (Horstmeyer and Baskes, 1999) and mesoscopic (Gerberich et al., 2002) levels have shown the importance of the volume to surface ratio to the indentation size effect (ISE) at small depths of penetration. We have interpreted this in terms of the plastic work under the contact and the surface work associated with the creation of new surface or the excess surface stress. Treating this as a modified Griffith criterion the case is made that this same length scale should apply to the delamination of thin films. By making this simple equivalency in length scales, an R-curve analysis for crack growth resistance, G _R, in thin film delamination emerges. This recovers the classic ² _ys h/E term as well as the fact that interfacial toughness should scale with the square root of incremental crack growth. Here _ys is yield strength, h is thickness and E is modulus of the film. As applied to thin Cu and Au films bonded to silicon substrates, the model is in good agreement.     

Fracture process in cortical bone: X-FEM analysis of microstructured models

Bones tissues are heterogeneous materials that consist of various microstructural features at different length scales. The fracture process in cortical bone is affected significantly by the microstructural constituents and their heterogeneous distribution. Understanding mechanics of bone fracture is necessary for reduction and prevention of risks related to bone fracture. The aim of this study is to develop a finite-element approach to evaluate the fracture process in cortical bone at micro-scale. In this study, three microstructural models with various random distributions based on statistical realizations were constructed using the global model’s framework together with a submodelling technique to investigate the effect of microstructural features on macroscopic fracture toughness and microscopic crack-propagation behaviour. Analysis of processes of crack initiation and propagation utilized the extended finite-element method using energy-based cohesive-segment scheme. The obtained results were compared with our experimental data and observations and demonstrated good agreement. Additionally, the microstructured cortical bone models adequately captured various damage and toughening mechanisms observed in experiments. The studies of crack length and fracture propagation elucidated the effect of microstructural constituents and their mechanical properties on the microscopic fracture propagation process.     

Comparative Study about Degradation of High-purity Magnesium Screw in Intact Femoral Intracondyle and in Fixation of Femoral Intracondylar Fracture

Bone screws encounter complex mechanical environment in fracture fixation of weight-bearing bone.In the present study, high-purity magnesium(HP Mg) screws were applied in fixation of rabbit femoral intracondylar fracture with 3 mm gap. In the control group, HP Mg screws of the same design were implanted at corresponding position of contralateral leg. At 4, 8 and 16 weeks after surgery, retrieved femurs went through micro-computed tomography(micro-CT) scanning and hard tissue processing. Under mechanical stress involved in fracture fixation, bending of screw bolt was observed at the portion exposed to facture gap at 4 weeks. Then local corrosion at the same portion was detected 16 weeks after surgery,which indicated the accumulation effect of mechanical stress on Mg corrosion. HP Mg screws in the fracture group had no significant difference with the control group in screw volume, surface area, surfaceto-volume ratio(S/V). And peri-implant bone volume/tissues volume(BV/TV) and bone volume density(BMD) in the fracture group was comparable to that in the control group. Furthermore, histological analysis showed new formed bone tissues in fracture gap and fracture healing 16 weeks after surgery. Under mechanical stress, HP Mg screw suffered bolt bending and local corrosion at the portion exposed to fracture gap. But it had no influence on the integral corrosion behaviors, osseointegration of HP Mg screw and the fracture healing. Therefore, HP Mg screws possessed good potential in fracture fixation of weightbearing bones.