Mechanical testing of a flexible fixation device for the lumbar spine

The aim of this study was to test mechanically a new flexible fixation system for the lumbar spine. This device incorporates loops of polyester braid which are secured by a crimped titanium sleeve. In tensile tests, all loops failed, by slippage through the crimped sleeve, at 434 +/- 25 N (mean +/- standard deviation from five loops) for a single crimp and 415 +/- 15 N (from five loops) for two crimps. The intact system was then tested according to the ASTM standard. In a static test, all five specimens failed by slippage of the braid through the sleeve. Initial slippage occurred between 600 and 700 N, but the mean maximum load sustained was 1090 +/- 140 N. Dynamic tests were performed on ten constructs at a frequency of 5 Hz, under a range of loading conditions. The maximum load applied in any of the tests was 825 N. Two constructs did not complete the required 5 x 10(6) test cycles because of fracture of their spinous process hooks. However, other tests, under the same conditions, showed no signs of failure. Fracture occurred as a result of fretting damage from the recommended stainless steel roll pins.     

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.     

The effects of different wire and screw combinations on the stiffness of a hybrid external fixator

Hybrid external fixators that use tensioned wires in the metaphysis and screws in the diaphysis combine the advantages of both unilateral and circular fixators and provide considerable flexibility in frame construction. This study aims to determine the effects of wire and screw combinations on the mechanical characteristics of such systems. Acrylic rods were used to simulate a long bone with a transverse fracture gap. A standard hybrid fixator design was used to stabilize the fracture in accordance with modern concepts with wires being used on one side of the fracture and screws on the other side. Twelve different wire and screw combinations were tested in axial compression and four-point bending in two planes. All the fixators tested demonstrated stiffening under increasing axial load, the extent of which appeared to correlate inversely with the number of wires used. Increasing the number of wires by one increased the axial stiffness by 7-16 N/mm (average 11 N/mm). The bending stiffness was largely controlled by the number of wires and increased by 0.42-1.03 N m/mm (average 0.74 N m/mm) for each additional wire. The axial stiffness of fixators with three screws was 4-7 N/mm (average 6 N/mm) higher than those with two screws if the offset screw was not in use. When the offset screw was used no stiffness advantages were found for three screws over two screws. The use of an offset screw substantially increased axial stiffness (by 12-26 N/mm, average 20 N/mm) and is recommended for unstable fractures. The two-ring hybrid fixators with four wires in one ring and three screws in the other had stiffness characteristics similar to the conventional four-ring Ilizarov fixator. Knowledge gained regarding the effects of wire and screw combinations on overall stiffness will be helpful both in the construction of frames and in destabilization during fracture healing.     

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.     

基于有限元的多股螺旋弹簧疲劳寿命预测

针对多股螺旋弹簧（多股簧）的疲劳失效问题,以阿联酋某企业使用的特种多股簧为研究对象,通过有限元仿真分析多股簧的应力应变历程并利用子模型技术进行了应力应变的精确计算,通过静态响应试验验证了有限元模型的正确性;进行了钢丝材料试验,结合Manson模型推导出多股簧材料的疲劳性能参数;结合多股簧的应变和疲劳寿命预测模型预测了多股簧的疲劳寿命,并进行试验验证。研究结果表明：多股簧承载时,弹簧端部承受的应力较大,最大应力点位于外层钢丝且外层钢丝主要承受弯曲应力,与弹簧端部位置处外层钢丝疲劳断裂现象吻合; 静态响应仿真与实测值误差不超过5%;由SWT准则预测得到的疲劳寿命与试验疲劳寿命较为吻合,且预测疲劳断裂区与实际断裂区一致。     

滚珠丝杠副和行星式滚柱丝杠副静刚度的比较研究

首先论述了滚珠丝杠副的静刚度,而后结合三角螺纹的变形推导出行星式滚柱丝杠副刚度计算方法。由于载荷分布对静刚度影响较大,所以先推导出载荷分布规律,再求出静刚度,从而使结果更为精确,通过两种丝杠副的比较,得出行星式滚柱丝杠副的静刚度比滚珠丝杠副的静刚度约大５０％的结论。     

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.     

Axial force on the screw of a single-flow high-pressure submersible pump

The calculated and experimental axial forces on the screws of the EVNOP 5-16-1200 and EVNOP 5-30-1200 high-pressure single-screw single-flow pumps are compared. The pumps are used in oil-extraction systems. On the basis of test results for single-flow pumps with a bearing, the actual axial force may be measured. This permits correction of the formula for the axial force on the screw in production conditions.     

某型直升机涵道装配螺钉的改进设计

某型直升机涵道装配过程中,存在装配螺钉安装不到位的故障现象。针对该故障现象,笔者分析了故障原因,并提出了装配螺钉采用弧面结构的改进方案。通过对改进后螺钉的工作原理和受力状态的分析,建立了力学平衡模型,运用Pro/E和MSC.Patran软件进行各零部件三维模型建立、虚拟装配和有限元分析,验证了改进后的螺钉结构可以解决安装不到位的故障现象。     

Robotic-assisted internal fixation of hip fractures: a fluoroscopy-based intraoperative registration technique

The internal fixation of proximal femoral (hip) fractures is the most frequently performed orthopaedic surgery procedure. When using a sliding compression hip screw, a commonly used fixation device, accurate positioning of the device within the femoral neck-head is achieved by initially drilling a pilot hole. A cannulated component of the hip screw is then inserted over the guide wire (surgical drill bit), which is used to drill the pilot hole. However, in practice, this fluoroscopically controlled drilling process is severely complicated by a depth perception problem and, as such, a surgeon can require several attempts to achieve a satisfactory guide wire placement. A prototype robotic-assisted orthopaedic surgery system has therefore been developed, with a view to achieving accurate right-first-time guide wire insertions. This paper describes the non-invasive digital X-ray photogrammetry-based registration technique which supports the proposed robotic-assisted drilling scenario. Results from preliminary laboratory (in vitro) trials employing this registration technique indicate that the cumulative error associated with the entire X-ray guided robotic system is within acceptable limits for the guide wire insertion process.     

牵引销紧固螺钉断裂分析

分析了牵引销紧固螺钉失效断裂的原因，对失效螺钉的化学成分、显微组织和裂纹及断口特征等进行了综合分析。结果表明：失效螺钉表层脱碳，在交变应力作用下，致螺纹齿根处疲劳损伤直至断裂。建议应严格控制热处理质量，避免脱碳；或以螺栓取代螺钉或将螺钉尺寸加大。     

结构静力/疲劳试验控制模式转换技术研究

根据飞机结构静力/疲劳试验中起落架试验、试验支持、操纵系统试验、破坏试验的需求,对模式转换技术进行了研究,并利用FlexTest 200协调加载控制系统进行了模式转换验证试验。试验结果表明,模式转换技术可以应用于飞机结构静力/疲劳试验。     

一体化框架在某型民机疲劳试验中的设计与应用

在以往的全机静力/疲劳试验中,常采用立柱-龙门架加载框架。该加载方式不便于设计检查平台,不适用于疲劳试验。针对疲劳试验周期长、试验加载设备反复使用、经常检查试验件等特点,设计了一体化加载框架,主要包括整体框架式加载系统、检查平台、油路线槽系统。一体化加载框架可以一次安装到位,减轻了试验人员的重复劳动,有利于试验件的损伤检查,减少疲劳试验裂纹的漏检率。试验表明,该加载方式可在后续全机和部件的静力/疲劳试验中应用。     

Design and biomechanical testing of pedicle screw for osteoporotic incidents

In this study, geometrical features of pedicle screws have been modified and their performances are compared. Performance analysis has been made in terms of pull-out strength and torsional strength. The parameters investigated are core diameter, holes drilled normal to screw axis, angle between sequential holes and distance between holes. Three different core diameter have been studied, which are 4 mm (normal core diameter), 5 mm (medium core diameter) and 5.5 mm (high core diameter). Distance between sequential holes has been arranged such that there is either one hole per pitch or one hole per two pitches. Angle between sequential holes is either 90 degrees or 120 degrees. According to the test results, the screw, with medium core diameter (5 mm) containing one hole per two pitches with 90 degrees angle between sequential holes, has exhibited the optimum performance considering torsional strength and pull-out strength requirements. Its torsional strength is slightly higher than and, when Grade 40 polyurethane foam was used as bone simulating material, its pull-out strength is as good as, an undrilled normal core diameter screw, which is already being used in surgical operations. The fatigue performance of this best performed screw has also been found satisfactory according to the related standard. Its pull-out strength is also tested on a calf vertebra and a promising result has been obtained.     

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.     

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。     

具有大承载能力的环路解耦两转动并联机构综合方法研究

为解决解耦机构承载能力弱的问题,提出一种具有大承载能力的环路解耦两转动并联机构型综合方法。基于螺旋理论,提出分支传递力螺旋矩阵与输出运动螺旋矩阵的互易积为对角矩阵（主对角线元素全不为零）的机构解耦判别准则,分析机构输出运动螺旋与传递力螺旋之间的关系,确定出驱动螺旋和非驱动螺旋的形式,将上述运动螺旋进行合理的排序和组合得到基础解耦分支。为使解耦机构更好的应用于工业中,提出冗余驱动分支构造方法,在基础解耦分支中添加冗余驱动分支,构造环路,实现环路内反解同步,构造环路之间彼此解耦的环路驱动分支,既不破坏机构的解耦特性又增加了机构的承载能力,综合出环路解耦两转动并联机构。运动学分析和静力学分析表明,所综合出的机构雅可比矩阵为广义对角阵、同一环路内驱动位移同步,并且承载能力强于传统解耦机构,验证了所提出的综合方法的正确性。通过该方法综合出来的机构,在汽车耐久性测试台架方面具有一定的应用前景。     

金属橡胶材料疲劳损伤性能研究

对设计制作的固支圆盘形金属橡胶构件进行疲劳损伤特性试验研究,对其疲劳损伤性能的分析表明金属橡胶材料的疲劳破坏不同于实体材料,它不会发生突然断裂失效,其疲劳破坏形式主要是其内部的金属丝产生局部断裂、破碎和磨损,从而在构件内部产生累积损伤,宏观上表现为承载能力和耗能性能不断衰减;通过对其内部金属丝两种典型勾连结构耗能方式随振动周期变化情况的分析,揭示金属橡胶材料疲劳损伤机理主要是其内部勾连结构滑移挤压方式的不同及金属丝间的微动摩擦、磨损及断裂;用刚度损伤和阻尼损伤描述金属橡胶的疲劳损伤过程,并提出综合损伤因子D作为金属橡胶疲劳失效判据。     

某电子设备连接螺钉冲击松动原因分析

螺纹连接被广泛应用于机载电子设备中。某机载电子设备在冲击试验中,连接隔振器的螺钉出现松动。文中首先提出了引起该设备螺钉松动的宏观原因,主要是冲击响应过大和连接螺钉太小,仿真分析发现,螺钉至少增大为M10才能满足强度要求。考虑到仿真误差和强度余量,同时提出更换大阻尼隔振器的改进措施,改进后的设备顺利通过冲击试验。文中针对螺钉松动进行的原因分析和改进措施对同类型电子设备螺钉和隔振器选型具有一定的借鉴意义。     

某轻型卡车驱动桥桥壳疲劳分析及优化

后桥作为汽车主要的承载件和传力件,对其进行疲劳分析,对提高整车安全性有重要意义。笔者对新开发的后桥进行CAE分析,发现桥壳钢托附近存在断裂风险,因此对其进行疲劳台架试验验证,试验结果确定易在此位置发生断裂。针对断裂位置,提出两种优化方案,利用疲劳分析软件对两种优化方案进行对比,通过台架验证,使得桥壳疲劳寿命达到企业标准,并为以后的后桥壳设计提供依据。