Development of a functional fixator system for bone deformity near joints

A functional external fixator system for bone deformity near the joints using worm gear was developed for curing the angle difference in fracture bones while the lengthening bar was developed for curing the differences in length, also in fracture bones. Both experiments and FE analysis were performed to compare the elastic stiffness in several loading modes and to improve the functional external fixator system for bone deformity near joints. The FE model using compressive and bending FE analysis was applied due to the angle differentiations. The results indicate that compressive stiffness value in the experiment was 175.43 N/mm, bending stiffness value in the experiment was 259.74 N/mm, compressive stiffness value in the FEA was 188.67 N/mm, and bending stiffness value in the FEA was 285.71 N/mm. Errors between experiments and FEA were less than 10% in both the compressive stiffness and the bending stiffness. The maximum stress (l57 MPa) applied to the angle of the clamp was lower than the yield stress (176.4 MPa) of SUS316L. The degree of stiffness in both axial compression and bending of the new fixator are about 2 times greater than other products, with the exception of EBI (2003).     

A simple way to model wires used in ring fixators: analysis of the wire stiffness effect on overall fixator stiffness

The Ilizarov fixator has become widely used in the treatment of complex trauma and deformity correction in the United Kingdom over the last few decades. As such, it has been widely researched. Within this fixator, the characteristic fine wires are the weakest and most flexible component. The wires allow interfragmentary micromotion, which is believed to be beneficial to bone healing. In this paper, a novel and simple numerical model of the stiffness and deflection of the wires when the fixator is subject to axial and torsional loading is presented. A range of wire lengths and pretensions are analysed, and the results compared to the findings of other researchers, with generally good agreement. This indicates that fixator stiffness can be determined simply and without recourse to sophisticated finite element analyses or extensive physical testing. In addition, the model is used to predict the contribution of the wires to the overall stiffness of the fixator.     

Analytical modelling of kirschner wires in Ilizarov circular external fixators using a tensile model

Mechanical characteristics of orthopaedic fixators, such as the stiffness and stability, directly influence the mechanobiological environment in which the bone is healed. In circular external fixators, the transfixing Kirschner wires are the major contributors to the biomechanics involved. A comprehensive understanding of the mechanical behaviour of the wires is therefore the key to biomechanical analysis of the Ilizarov fixator. In this study, to model the behaviour of the wires, a purely theoretical approach has been adopted to obtain explicit equations for a solely tension-based model formulation. Mathematical modelling leads to new algebraic polynomials whose solutions are the exact maximum deflection, angle of deflection, and total (final) tension in the wire. The predictions are compared with published experimental and computational findings on the deflection and stiffness of the wires, and analytical explanations are provided for the previously observed behaviours. Parametric (practical) implications of this type of abstraction include the fact that the angle of deflection as well as the tension in the wire are independent of its length. The inverse proportionality of wire stiffness to its length is also deduced. The findings are applicable to tensile elements (ropes, chains, etc.), provided that the tensile deformation (elongation) can be deemed to be the dominant mode of deformation.     

Salvage procedures for trochanteric femoral fractures after internal fixation failure: biomechanical comparison of a plate fixator and the dynamic condylar screw

The aim of this study was the biomechanical evaluation of the reversed less invasive stabilization system (LISS) internal fixation as a joint-preserving salvage procedure for trochanteric fractures. Five LISS plates and five dynamic condylar screws (DCS) were tested using synthetic femora (Sawbones) with an osteotomy model similar to a type-A2.3 pertrochanteric fracture. The constructs were subjected to axial loading up to 1000 N for five cycles. Then, the force was continuously increased until fixation failure. For the evaluation of the biomechanical behaviour, the stiffness levels were recorded and the osteotomy gap displacement was mapped three-dimensionally. The average stiffness for the constructs with LISS plates was 412 N/mm (with a standard deviation (SD) of 103N/mm) and 572N/mm (SD of 116 N/mm) for the DCS constructs (p=0.051). Local displacement at the osteotomy gap did not yield any significant differences. The LISS constructs failed at a mean axial compression of 2103N (SD of 519N) and the DCS constructs at a mean of 2572N (SD of 372N) (p=0.14). It is concluded that the LISS plate offers a reliable fixation alternative for salvage procedures.     

Fatigue strength of a wire passing through a cannulated screw: implications for closure of the sternum following cardiac surgery

It has been proposed that the incidence of sternal dehiscence can be decreased by passing the wires used for sternotomy closure through cannulated screws. However, there is a potential risk of fatigue failure as a result of the wire moving against the screw, e.g. during coughing and sneezing. The system of cannulated screws and wire was subjected to static tensile testing to failure. Five tests were performed and failure occurred at 388 +/- 34 N (mean +/- SD). Ten cyclic tests were then performed. Sinusoidal loading was applied at 10 Hz with peak forces in the range 10-90 per cent of the static failure force, at a constant load ratio R = 10. The test with the lowest peak force reached run-out at 6 x 10(6) cycles. The others failed by the ends of the wire closures becoming untwisted (one test), the wire fracturing at the twist (three tests) or the wire fracturing at the screw (five tests). However, calculations based on these results suggest that fatigue failure is unlikely to occur as a result of regular breathing or continuous coughing or sneezing.     

Yielding of the clamped-wire system in the Ilizarov external fixator

This study demonstrates that the clamped-wire system used to suspend bones within an Ilizarov external fixator yields when the perpendicular load exceeds 50 N per wire. Cyclic loading was applied to tensioned wires clamped within an Ilizarov ring component, with steadily increasing load amplitude. Wires were tested at four initial tension settings. The amount of energy lost within the clamped-wire system per load cycle was calculated for every test. The results showed that there was a consistent trend to increasing non-recoverable energy loss per load cycle when peak loads exceed 50 N for all initial tension settings. A finite element (FE) model replicating the experimental conditions was performed to investigate the levels of stress within the loaded wires. The FE model analyses showed that high stresses were generated in the wires close to the clamping sites, and that the stress levels could reasonably be expected to exceed the material yield stress when loaded to about 55 N, for all initial tension settings. The results show that material yield, accompanied by some wire slippage through the clamps, is responsible for system yield, in agreement with previous studies. Although the initial wire tension has an appreciable effect on the wire stiffness, it did not affect the elastic load range of the clamped-wire system. To prevent yield of the clamped-wire system in practice, the fixator should be assembled with sufficient wires to ensure that the load transmitted to each wire by the patient does not exceed 50 N.     

Apparatus to test insertion and removal torque of bone screws

This paper describes affordable equipment for testing bone screw torque, corresponding to ASTM standard F543-00 for testing metallic medical bone screws. Correct testing of thin and long bone screws is essential due to screw failures during insertion and removal of the screws. Furthermore, insertion torque is an important factor in predicting fixation strength, screw pull-out force and effects of surface treatment of screws. The capability of the custom-built tester was determined using polytetrafluoroethylene and wood disc samples and bone screws. Bovine cortical bones allowed testing to the failure limit, i.e. the torque increased in long screws to the fracture limit. For 2.7 and 3.5 mm thick self-tapping cortical bone screws, the failure torques were 30-50 per cent higher than the minimum values required by the standard (1.0 and 2.3 N m respectively). The equipment provided reproducible results and fulfilled the ASTM standard very well. Preliminary testing with amorphous diamond coated bone screws showed good durability of the coating and on average 10-15 per cent lower torque values compared with uncoated screws. The equipment can be used to measure insertion and removal torques as described in the standard. Furthermore, it also allows testing of normal screws and bolts.     

Bending fatigue characteristics of corroded wire ropes

The corrosion of a wire rope reduces its life expectancy. In this study, repeated bending tests were conducted using a bending fatigue tester by changing the tensile load and corrosion time of wire ropes, which were the same type as those used in elevators. The number of broken wires was studied, and a tensile test was conducted for cases in which the fracture was severe. The effect of corrosion fatigue on life expectancy was considered by comparing fracture strength values and observing fracture surfaces. The results indicate that an increase in accumulated corrosion fatigue, a greater tensile load, and repeated bending cycles may yield a rapid decrease in fracture strength and an increase in the number of broken wires. Therefore, it is concluded that corrosion fatigue is an important factor that decreases the life expectancy of wire rope.     

Mechanical testing of the wire clamping characteristics of the Fixclip internal fixation system

Stainless steel wires (Kirschner's wires) have been used for over 60 years to fix internally a wide range of osteotomies and fractures in bones. Fixclips (Corifix, Cirencester, UK) shaped like winged washers have been used to clamp pairs of K wires of diameters ranging from 0.8 to 3.0 mm to standard orthopaedic screws and to fix bones internally in more than 400 patients. The system has proved to be biologically and mechanically very effective. This paper describes the design and some biomechanical test results of the Fixclip. Preliminary strength tests conducted using a Hounsfield Tensometer show that the pull-out force, reflecting the grip of the clip on the wire, is dependent on the wire size and, throughout the clinically relevant range, increases linearly with increasing screw torque. Changing the surface finish of the clip did not confer any advantage over the existing polished clip in terms of grip strength.     

Finite element analysis of stress around a sternum screw used to prevent sternal dehiscence after heart surgery

The sternum screw has been proposed as a means of preventing sternal dehiscence, following heart surgery, by increasing the contact area between the wire used to close the median sternotomy and the surrounding bone; as a result, the contact stress is reduced. A finite element model was constructed of a cylindrical wire or screw passing through a block of sternum which consisted of cancellous bone sandwiched within a cortical shell. The thickness of the cortical shell and the material properties of bone were varied between reasonable values. The stress distribution in the sternum was calculated for each model when the wire was subjected to a tension (250 N) which would be required for six wires to withstand a strong cough (40 kPa). Results were validated by comparison with a simple analytical model in which the bone and wire were considered incompressible. They show that the screw reduces the contact stress to almost one-seventh of its value when wire is used alone. Contact stresses are especially high if the cortical shell is thin. The high stress in the bone around a screw falls off within a few millimetres. As a result, no problems are anticipated in placing six screws in each half-sternum so that the sternotomy may be closed with the usual six wires.     

Biomechanical comparison of three second-generation reconstruction nails in an unstable subtrochanteric femur fracture model

The purpose of this study is to determine if a biomechanical difference exists in dynamic stiffness, fatigue life, and fracture site displacement by comparing three cephalomedullary reconstruction nails. An examination was made of the Biomet Uniflex reconstruction nail, the Biomet Vector nail, and the Stryker Howmedica Long Gamma nail in the fixation of an unstable subtrochanteric femur fracture model, using a synthetic bone model. Mean stiffness for each nail was initially determined in control specimens (i.e. no fracture and no instrumentation). The nail stiffness values were 1764.0 N/mm (controls), 373.61 N/mm (Uniflex), 294.27 N/mm (vector), and 656.36 N/mm (Gamma). The Gamma was statistically stiffer than the Uniflex and Vector (p < 0.002). Mean fatigue life measurements were: Uniflex at 52,891 cycles, failing at the most distal of the proximal two screw holes; Vector at 45,344 cycles, failing in the nail at the level of the fracture site; Gamma at 88,748 cycles failing at the lag screw hole. The p value between the Gamma and Vector was less than 0.01 and between Gamma and Uniflex was less than 0.05. The mean maximal axial displacement at the fracture site was 2.448 mm, 2.305 mm, and 0.790 mm for the Uniflex, Vector and Gamma, respectively. The p value between the Gamma and the other nails was < 0.01. The mean maximal transverse displacement at the fracture site was 1.223 mm, 1.197 mm, and 0.280 mm respectively. The p value between the Long Gamma and the other two nails was < 0.01. In conclusion, the Long Gamma nail demonstrated statistically significant fixation superiority in stiffness, resistance to fatigue, and fracture site displacement compared to the Uniflex and Vector nails. This biomechanical information may aid in choosing implants for fixation of unstable, subtrochanteric femur fractures.     

滚珠丝杠-螺母副结合部轴向动态特性参数测试与分析

为了获得滚珠丝杠-螺母副结合面轴向动态特性参数,建立了滚珠丝杠-螺母副滚动结合面轴向动力学参数识别模型并研发了测试平台,分析了不同丝杠结构尺寸参数对丝杠轴向动态刚度的影响规律。实验结果表明,丝杠直径、导程、螺旋升角、工作圈数的增大均可提高其轴向动态刚度,而丝杠装配过程中的预拉伸所产生的轴向应变量会减小丝杠结合面轴向动态刚度。最后建立了滚珠丝杠-螺母副结合面轴向动态刚度神经网络预测模型,预测结果表明计算刚度与实测值相差不超过8%。     

Reduction in wire tension caused by wire clamping and wire tensioner removal: an experimental Ilizarov frame study

The stability of an external ring fixator mainly depends on wire tension. Wire fixators should maintain the tension during both wire clamping to the ring and removal of the tensioner device. In the present study the loss in wire tension related to fixator clamping and wire tensioner removal using three different wire fixator designs was studied. The fixators were based on two different cannulated bolts and a washer. Effects from two different pretension levels in combination with three different bolt torque levels upon loss in wire tension were described. Emitted wire vibration frequency was used to assess the corresponding wire tension. Wire tension was determined after each wire fixator tightening and after the removal of the wire tensioner. Increased bolt torque led to a small decrease in tension for both pretension levels. A considerable higher tension loss was measured when removing the wire tensioner. In all cases, the combination of a new cannulated bolt and a washer maintained the highest tension.     

The biomechanics of plate fixation of periprosthetic femoral fractures near the tip of a total hip implant: cables, screws, or both?

Femoral shaft fractures after total hip arthroplasty (THA) remain a serious problem, since there is no optimal surgical repair method. Virtually all studies that examined surgical repair methods have done so clinically or experimentally. The present study assessed injury patterns computationally by developing three-dimensional (3D) finite element (FE) models that were validated experimentally. The investigation evaluated three different constructs for the fixation of Vancouver B1 periprosthetic femoral shaft fractures following THA. Experimentally, three bone plate repair methods were applied to a synthetic femur with a 5 mm fracture gap near the tip of a total hip implant. Repair methods were identical distal to the fracture gap, but used cables only (construct A), screws only (construct B), or cables plus screws (construct C) proximal to the fracture gap. Specimens were oriented in 15 degrees adduction to simulate the single-legged stance phase of walking, subjected to 1000 N of axial force, and instrumented with strain gauges. Computationally, a linearly elastic and isotropic 3D FE model was developed to mimic experiments. Results showed excellent agreement between experimental and FE strains, yielding a Pearson linearity coefficient, R2, of 0.92 and a slope for the line of best data fit of 1.06. FE-computed axial stiffnesses were 768 N/mm (construct A), 1023 N/mm (construct B), and 1102 N/mm (construct C). FE surfaces stress maps for cortical bone showed Von Mises stresses, excluding peaks, of 0-8 MPa (construct A), 0-15 MPa (construct B), and 0-20 MPa (construct C). Cables absorbed the majority of load, followed by the plates and then the screws. Construct A yielded peak stress at one of the empty holes in the plate. Constructs B and C had similar bone stress patterns, and can achieve optimal fixation.     

微米尺度铜键合丝疲劳性能研究

目前,铜键合丝广泛用于集成电路、电子封装等领域,但是铜丝在制造和键合的过程中受到的局部应力和摆动,会不可避免地产生疲劳问题。通过一套以自激振动为原理组装的微结构疲劳试验装置,对不同直径的微米级铜键合丝进行对称弯曲疲劳性能测试。试验结果表明:该试验装置能够成功地对微米级铜丝进行对称弯曲疲劳性能试验;无论是屈服强度、抗拉强度还是弹性模量,直径20μm的铜丝均高于直径30μm、40μm的铜丝,表现出明显的尺寸效应;所有铜丝的疲劳寿命集中在4.5×104~1×107;在相同应力条件下,铜丝的疲劳寿命随着铜丝直径的增加而减小;直径20μm、30μm、40μm的铜丝对应的疲劳强度(N=106)分别为147 MPa、97 MPa、70 MPa。从扫描电子显微镜的断口分析结果可以看出拉伸断口为凿峰状,断口周围表面出现许多相间的条状拉拔痕迹;疲劳断口为平齐正断,两条裂纹起源于试样表面,瞬断区为窄条状。     

基于等效刚度仿真应力的螺钉强度校核

为解决系统级仿真中螺钉连接难以覆盖的难题,提出利用理论计算获得连接螺钉的轴向刚度与剪切刚度。通过等效刚度模型表征螺钉的承载特性,建立了带简化螺钉的系统级有限元模型,准确地获得了系统连接中螺钉部位的支反力。通过搭建冲击试验环境,测试相同部位的应力水平并加以比对,反向佐证了连接部位的支反力与实际情况相符。采用第四强度理论对螺钉的强度与寿命进行校核,结果表明设计合理可行。该方法具有一定的工程应用价值,可供同类项目借鉴。     

The fatigue life of contoured cobalt chrome posterior spinal fusion rods

Intraoperative contouring of posterior rods in lumbar arthrodesis constructs introduces stress concentrations that can substantially reduce fatigue life. The sensitivity of titanium (Ti) and stainless steel (SS) to intraoperative contouring has been established in the literature; however, notch sensitivity has yet to be quantified for cobalt chrome (CoCr), which is now being advocated for use in posterior arthrodesis constructs. The goal of this study is to evaluate the sensitivity of CoCr rods to intraoperative contouring for posterior lumbar screwrod arthrodesis constructs. In this paper lumbar bilateral vertebrectomy models are constructed based on ASTM F1717-01 with curved rods (26-30 degrees total curvature) and poly-axial pedicle screws. Three types of constructs are assembled: first, 5.5 mm SS rods with SS screws (6.5 x 35 mm), second, 6.0 mm Ti rods with Ti screws (7.5 x 35 mm), and third, 6.0 mm CoCr rods with Ti screws (7.5 x 35 mm). All specimens are tested at 4 Hz in dynamic axial compression-bending with a load ratio of ten and maximum load levels of 250, 400, and 700 N until run-out at 2 000 000 cycles. Results are presented that show that the fatigue life of CoCr constructs tend to be greater than Ti constructs at all levels. At the 400 N maximum loading, CoCr lasts an average of 350 000 cycles longer than the Ti constructs. The CoCr constructs are able to sustain the 250 N load until run-out at 2 000 000 cycles but they fail at high load levels (maximum 700 N). The CoCr constructs fail at the neck of the Ti screw at high loads whereas Ti screws fail at the notch induced by contouring. Since CoCr is compatible with magnetic resonance imaging and has high static strength characteristics, the results of this study suggest that it may be an appropriate substitute for Ti.     

肠道微机器人柔性运动系统

提出一种适用于肠道微机器人的柔性运动系统来提高肠道机器人微创诊断的主动运动能力.柔性运动系统采用尺蠖型运动方式,由柔性运动机构和柔性驱动机构组成.柔性运动机构包括径向气囊软足和轴向伸缩推杆,并用万向节连接微机器人前后腔体从而提高运动柔性;柔性驱动机构利用尼龙线绳牵引波纹管泵驱动气囊软足和伸缩推杆激励微机器人伸缩.微机器人样机直径为12.2 mm,长度为78 mm,质量为14.8g,最大径向钳位外径为20.2 mm,最大轴向行程为16.4 mm.实验结果表明,柔性驱动机构可以为波纹管泵和伸缩推杆分别提供最大为0.67N和0.65 N的驱动力;微机器人样机能够在不同倾斜角度的刚性有机玻璃管中运动,在水平和竖直管道中的平均运行速度为0.38 mm/s和0.25 mm/s;能通过最小曲率半径为49.3 mm的塑料软管,在离体肠道中也能实现有效运动.本柔性运动系统为肠道微机器人提供了一种安全有效的自主运动方案.     

高速双螺母滚珠丝杠副轴向接触刚度研究

对双螺母预紧的滚珠丝杠副进行了接触受力分析,推导出接触变形的公式,建立了滚珠丝杠副的轴向刚度计算公式。研究分析了影响滚珠丝杠副轴向刚度的几个主要因素,包括预紧力、螺旋升角、接触角等,给出了这几个因素与滚珠丝杠副轴向刚度对应的关系曲线,为新型高速滚珠丝杠的合理设计提供了理论基础。     

A device for improved reduction of tibial fractures treated with external fixation

A widely used method of treatment for unstable tibial shaft fractures is unilateral external fixation. The majority of fixators act as three distinct devices: an intra-operative reduction device, a device to maintain fracture alignment during healing and an aid to healing by allowing movement at the fracture site. Conventional operative techniques require the surgeon to manipulate a number of degrees of freedom at once, making reduction of the fracture difficult, and results in the fixator being out of alignment with the long axis of the bone. An operative method has been developed that separates reduction and fixation. A dedicated device has been designed to improve the per-operative control of fracture fragments during fracture reduction. The device has been used in clinical trials for the reduction of 22 diaphyseal tibial fractures. Compared with previous operative techniques there has been a saving of 53 per cent in fracture reduction time and an overall saving of 10 per cent in operating time. Fracture alignment has been improved compared with reductions achieved with a fixator which potentially improves healing and lowers the rate of malunion. In each case the fixator has been applied in alignment with the bone, improving dynamization and reducing the likelihood of malunion due to fixator cam slippage.