The stress relaxation of cement clinkers under high temperature

The energy consumption of crushing is directly affected by the mechanical properties of cement materials. This research provides a theoretical proof for the mechanism of the stress relaxation of cement clinkers under high temperature. Compression stress relaxation under various high temperatures is discussed using a specially developed load cell, which can measure stress and displacement under high temperatures inside an autoclave. The cell shows that stress relaxation dramatically increases and that the remaining stress rapidly decreases with an increase in temperature. Mechanical experiments are conducted under various temperatures during the cooling process to study the changes in the grinding resistance of the cement clinker with temperature. The effects of high temperature on the load-displacement curve, compressive strength, and elastic modulus of cement clinkers are systematically studied. Results show that the hardening phenomenon of the clinker becomes apparent with a decrease in temperature and that post-peak behaviors manifest characteristics of the transformation from plasticity to brittleness. The elastic modulus and compressive strength of cement clinkers increase with a decrease in temperature. The elastic modulus increases greatly when the temperature is lower than 1000 °C. The compressive strength of clinkers increases by 73.4% when the temperature drops from 1100 to 800 °C.     

Radiopacity of tantalum-loaded acrylic bone cement

Radiopacifying agents are commonly added to bone cements to enhance the visibility of the cement in radiography. The radiopacifiers usually employed may, however, have undesired effects on the mechanical properties of the cement. A potentially new radiopacifier is tantalum, which in the present work was evaluated in terms of radiopacity. Bone cements containing different percentages of tantalum were compared with plain bone cement as well as with formulations containing different percentages of the commonly used radiopacifier barium sulphate. The radiopacity was assessed quantitatively and qualitatively, by measuring with a digital densitometer the optical density of the cement on X-ray films, and consulting the expertise of ten orthopaedic surgeons. It was found that tantalum does present radiopacity, but not as high as barium sulphate under the specific conditions applied to this study.     

A finite element rheological model for polymethylmethacrylate flow: analysis of the cement delivery in vertebroplasty

Polymethylmethacrylate (PMMA) is increasingly used in orthopaedics. Finite element (FE) modelling can play an important role in understanding the PMMA flow behaviour. However, FE models have not been used so far because conventional FE packages do not allow for the rheopectic and pseudoplastic behaviour of PMMA to be taken into consideration and because it requires multiple expertise to incorporate these behaviours into an FE package. The objectives of the present paper are to: (a) propose a rheological model that describes PMMA flow behaviour; (b) implement this model into ANSYS using FORTRAN; and (c) validate the implementation by comparing it with analytical solutions. After the validation showed good agreement, an FE model of PMMA delivery through an eight-gauge cannula was developed to examine the extra-vertebral flow conditions of vertebroplasty. The FE analysis showed a logarithmic increase of the injection pressure, where it almost doubled from 1.2 to 2.3 MPa over two minutes. This unanticipated non-linear increase is due to the highly non-uniform viscosity profile in the cannula. It can be concluded that: (a) the rheological model implemented in ANSYS can be used to analyse practical flow problems related to PMMA and (b) time and shear-rate effects of PMMA are crucial to estimate its flow behaviour accurately.     

Reinforcement of bone cement using metal meshes

Following on from previous work reported in this journal, a practical system has been developed for in situ fibre reinforcement of bone cement in the hip joint prosthesis. A fibre preform is inserted during the operation, surrounding the metal stem of the prosthesis, forming a composite with the cement. Tests performed on full-scale models of the joint show improvements in fatigue life by more than an order of magnitude when a preform of metal fibres is used; this significantly delays the onset of cement cracking and stem loosening. Initial results reported previously suggest that other fibre materials such as carbon may give even greater improvements. Clinical trials have begun, using a form of the metal mesh.     

Short-fibre reinforcement of calcium phosphate bone cement

Calcium phosphate cement (CPC) sets to form hydroxyapatite, a major component of mineral bone, and is gaining increasing interest in bone repair applications. However, concerns regarding its brittleness and tendency to fragment have limited its widespread use. In the present study, short-fibre reinforcement of an apatitic calcium phosphate has been investigated to improve the fracture behaviour. The fibres used were polypropylene (PP) fibres, 50 microm in diameter and reduced in length by cryogenic grinding. The compressive strength and fracture behaviour were examined. Fibre addition of up to 10 wt % had a significant effect on composite properties, with the energy absorbed during failure being significantly increased, although this tended to be accompanied with a slight drop in compressive strength. The fibre reinforcement mechanisms appeared to be crack bridging and fibre pull-out. The setting time of the CPC with fibre reinforcement was also investigated and was found to increase with fibre volume fraction.     

断路器弹簧应力松弛研究

利用商用有限元软件ANSYS,对真空断路器弹簧操动机构超程螺旋弹簧的非线性蠕变应力松弛现象进行了研究。根据所建立的断路器超程弹簧三维有限元分析模型,求得了弹簧在位移栽荷作用下弹簧体内蠕变松弛应力的衰减情况。分析表明在外部载荷作用下,由于弹簧中的弹性变形会向微塑性变形转变,使得弹簧内剩余应力逐渐减小,并表现出了较明显的非线性。并且初始应力越大,蠕变应力松弛速率也会越大,但在稳态阶段,初始应力对残余应力的衰减速度几乎没有影响,研究结果对于今后断路器弹簧操动机构超程弹簧及其它弹簧的强度设计和维护有一定的参考价值。     

MicroCT examination of human bone specimens: effects of polymethylmethacrylate embedding on structural parameters

X‐ray microtomography permits the nondestructive investigation of trabecular and cortical bone specimens without special preparation of the sample. To do a quantitative characterization, the cross‐section images have to be binarized, separating bone from nonbone. For this purpose, a widely used method is uniform thresholding. However, for commonly available microtomography scanners which use a polychromatic X‐ray source, it is unclear what effect the surrounding medium (e.g. air, saline solution, polymethylmethacrylate) has on the threshold value used for the binarization. In the literature an easy procedure to find the optimal uniform threshold value for a given acquisition condition is reported. By applying this procedure, the present work investigated whether a microtomography scan of trabecular bone samples in air or embedded in polymethylmethacrylate gave the same results in terms of structural parameters. The gold standard, that is, histological sections, was used as a reference. Two fixed threshold values were found, one for the microtomography scans performed in air and one for the scans with the same samples embedded in polymethylmethacrylate. These were applied on the correspondent microtomography images for the estimation of structural parameters, such as bone volume fraction, direct trabecular thickness, direct trabecular separation and structure model index. Paired comparisons were made in bone volume fraction between histological sections and microtomography cross‐sections for the same bone samples scanned first in air and then embedded in polymethylmethacrylate, by which no significant differences were found. Paired comparisons were also made in bone volume fraction, direct trabecular thickness, direct trabecular separation and structure model index for the same samples over volumes of interest of 4 × 4 × 4 mm³ between microtomography scans in air and scans with the samples embedded in polymethylmethacrylate. Neither these comparisons showed significant differences. This leads to the conclusion that structural parameters estimated by microtomography for human trabecular bone samples scanned either in air or embedded in polymethylmethacrylate are not affected by the surrounding medium (i.e. presence or absence of polymethylmethacrylate), provided that the corresponding optimal threshold value is applied for each acquisition condition.     

Fretting corrosion of orthopaedic implant materials by bone cement

Debonding of bone cement from the stem of the femoral component of a hip prosthesis can result in local tangential oscillatory movement, i.e. fretting, between the two contacting materials as the limb is moved. Patches where such rubbing has occurred are frequently seen on removed implants. Fretting fatigue experiments have been carried out in Hanks solution on austenitic stainless steel and Ti-6Al-4V (IMI 318) with bridges of bone cement clamped to the specimens. Fretting appears to have little effect on the fatigue life of either material but the scanning electron microscope reveals the formation of thick oxide layers which subsequently give rise to loose debris particles by a process of delamination. Further experiments carried out in Hanks solution in an electrolytic cell have shown that there are potential changes when a bone cement rider is fretted against a stainless steel or titanium alloy plate although the change in potential is only one tenth that obtained with a metal-on-metal contact. Fretting by bone cement appears to be producing damage to the metal surfaces which manifests itself as mild wear rather than a diminution in fatigue strength.     

Properties of composite specimens of old and new bone cement

The most common cause of failure of a total hip replacement is aseptic loosening of an implant. In a number of cases, the cement-bone interface of at least one component is not compromised. In cases of aseptic cup loosening, removal of a well-fixed femoral stem may be undertaken to facilitate exposure of the acetabulum for cup revision, and the surgeon may choose to leave the functional cement-bone interfaces in the femur undisturbed. After cup revision, new cement is pressurized within the old cement mantle and a stem is cemented into this 'old-new cement' composite. Retaining the old cement mantle is an attractive option as it reduces the duration of surgery, minimizes bleeding, and preserves the bone stock. Excellent results have been shown with this technique of 'in-cement femoral revision' using a double-tapered polished stem. While considerable literature is available on the short- and long-term properties of PMMA bone cement, very little is known about the mechanical properties of old-new composite cement specimens where the old cement is more than a few days old. This paper tests the properties of such old-new composite specimens where the 'old' cement is aged between 3.3 and 17.7 years, better reflecting clinical situations.     

车灯塑件退火状态下应力松弛规律研究

针对注塑件中普遍存在残余应力这一问题,以材料为聚碳酸酯(PC)的车灯塑件为例,对注塑件在退火工艺下的应力松弛规律进行了研究,对工业玻璃在退火状态下的应力松弛计算公式进行了分析,提出了一种针对聚碳酸酯(PC)材料在退火状态下的应力松弛的数学模型。采取实验的方法,利用钻孔法对3种不同退火状态下得到的车灯塑件的残余应力进行了检测,根据其中测得的两组残余应力数据计算出了应力松弛数学模型中的待定参数,建立起了退火时间、退火温度、退火前残余应力以及退火后残余应力四者之间的关系,并将由该数学模型计算得到的残余应力与第3组测得的残余应力数据进行了对比。研究结果表明,通过由模型和实验得到的结果两者的误差非常小,因此说明该数学模型具有较高的可靠性与准确性。     

Stress relaxation behaviour in polymers for combined bending and torsion

This paper describes a machine for stress relaxation test in polymers using a method of combined bending and torsion. As an example, the experimental data for polyamide has been given. The X-ray diffractogram analysis of structure changes of outer surface of examined polymer has also been made.     

A procedure and criterion for bone cement fracture toughness tests

Nowadays, two procedures, based on the recommendation of two American standards (ASTM E399 and ASTM D5045), are used to determine the fracture toughness, KIc, of bone cement. However, there is a lack of knowledge about the equivalence of the two testing methods applied to bone cement. Additionally, in spite of the recommendation of several authors to introduce a rejection criterion for specimens based on the size of defects found in the fracture surface, no data are available about the effect of porosity within the material on the KIc of bone cement. The aims of this study were to verify whether the KIc values calculated for bone cement using the two procedures are comparable and whether macroporosity within the tested samples affects the KIc value of bone cement, and, if so, to establish a rejection criterion for specimen selection. Samples of pure polymethyl methacrylate (PMMA) were tested by both procedures. Additionally, samples showing defects (macroporosity) of different sizes and located in different positions within the specimen were tested. The KIc value determined following the ASTM E399 procedure was 13 per cent lower than that calculated following the ASTM D5045 procedure. In the first series a lower data scatter was observed. Also, the presence of macroporosity on the fracture surface of the specimen affected the KIc value of bone cement. Therefore, the mechanical behaviour of samples was affected by defects within the material. Since it is possible to mould specimens without macroporosity, it seems recommendable to reject specimens with macroporosity on the fracture surface before calculating the KIc value of bone cement.     

Atomic force microscope nanolithography of polymethylmethacrylate polymer

We describe a nanolithography process for a polymethylmethacrylate (PMMA) surface using scanning contact atomic force microscopy. Parallel furrows were scribed with a pyramidal silicon tip using the same scan mechanism as used to image samples. The PMMA was first electron beam irradiated using a scanning electron microscope and developed. The topography formed is reproducible and predictable. Material from the region where the tip scribes is moved to nearby regions, and aligned, elongated PMMA fragments are seen to decorate the valleys between furrows.     

Temperature and ageing condition effects on the characterization of acrylic bone cement

This study investigates the effect of the environmental temperature and ageing condition on the characterization of acrylic bone cement. The tests were performed according to ISO 5833. The testing parameters were allowed to vary within the limits defined by the standard, in order to assess their effect on the results of the test. In certain cases the tests were also performed under conditions which the standard does not provide for but which are likely to occur clinically. This investigation showed that the cement behaviour may also change in the temperature range specified in the standard. Therefore, it is deemed appropriate to correlate the curing parameters of the bone cement to the environmental temperature, performing the test at different temperatures. In this way the effect of temperature on the duration of the phases in the cement curing could be assessed. The resultant graphical representation of the effect of temperature on the duration of the phases in cement curing has direct clinical relevance. Furthermore, this study showed that the ageing conditions of the mechanically tested specimens affected the results. Hence, it is deemed advisable to modify the ageing conditions of the specimens, fixing them closer to the in vivo conditions.     

Direct milling of polymethylmethacrylate for cranioplasty applications

The manufacturing of customized implants for cranioplasty applications has been recognized as a valuable alternative in order to reduce time and surgical costs and to produce better cosmetic results. Generally, the machining processes, in special milling, have been adopted as an intermediary step to manufacture a dedicated mold for casting the designed implant. In this work, we proposed the use of direct milling of simpler implants to shorten the process chain without compromising its dimensional quality. A set of computed tomography (CT) images were acquired from a dried human skull, vetorized, and further used to reconstruct a solid model. Such solid model was processed with computer-aided design (CAD) and computer-aided manufacturing systems to the machining planning. An acrylic implant was directly milled in a three-axis machining center and its external surface was measured with a coordinated measuring machine and compared with its corresponding in the original model (skull) and the digital ones (CT images and CAD). Based on the assessment of dimensional deviations, it is possible to affirm that direct milling can be considered the ultimate phase in the production of customized implants.     

A comparison between micro-CT and histology for the evaluation of cortical bone: effect of polymethylmethacrylate embedding on structural parameters

Cortical bone microstructure is an important parameter in the evaluation of bone strength. The aim of this study was to validate the characterization of human cortical bone microarchitecture using microcomputed tomography. In order to do this, microcomputed tomography structural measurements were compared with those obtained through histological examination (the gold standard). Moreover, to calculate structural parameters, microcomputed tomography images have to be binarized with the separation between bone and nonbone structures throughout a global thresholding. As the effect of the surrounding medium on the threshold value is not clear, an easy procedure to find the global uniform threshold for a given acquisition condition is applied. This work also compared the structural parameters of microcomputed tomography cortical sample scan in air or embedded in polymethylmethacrylate; histology was used as a reference. For each acquisition condition, a fixed threshold value was found and was applied on the corresponding microcomputed tomography image for the parameters assessment. Twenty cortical bone samples were collected from human femur and tibia diaphyses. All samples were microcomputed tomography scanned in air, embedded in polymethylmethacrylate, rescanned by microcomputed tomography, examined by histology and finally compared. A good correspondence between the microcomputed tomography images and the histological sections was found. Paired comparisons in cortical porosity, Haversian canal diameter and Haversian canal separation between histological sections and microcomputed tomography cross sections, first in air and then embedded in PolyMethylMethAcrylate, were made: no significant differences were found. None of the comparisons showed significant differences for cortical porosity, Haversian canal diameter and Haversian separation over a three-dimensional volume of interest, between microcomputed tomography scans in air and with samples embedded in PolyMethylMethAcrylate. The very good correlation between bone structural measures obtained from microcomputed tomography datasets and from two-dimensional histological sections confirms that microcomputed tomography may be an efficient tool for the characterization of cortical bone microstructure. Moreover, when the corresponding threshold value for each condition is used, structural parameters determined by microcomputed tomography are not affected by the surrounding medium (PolyMethylMethAcrylate).     

Smooth surface micro finite element modelling of a cancellous bone analogue material

Tetrahedral finite element meshes with smooth surfaces can be created from computed tomography scans of cancellous bone in order to evaluate its mechanical properties. Image processing before creation of the mesh can affect the accuracy of determined mechanical properties. For a cancellous bone analogue, threshold, mesh density and surface smoothing parameters used in mesh generation were varied and the mechanical properties predicted by the resulting meshes were compared to experimental results. This study has shown that threshold selection is vital for accurate determination of volume fraction and resulting mechanical properties.     

The effect on the fatigue strength of bone cement of adding sodium fluoride

New bone cements that include several additives are currently being investigated and tested. One such additive is sodium fluoride (NaF), which promotes bone formation, facilitating implant integration and success. The influence of NaF on the fatigue performance of the cement as used in biomedical applications was tested in this paper. In fact fatigue failure of the cement mantle is a major factor limiting the longevity of a cemented implant. An experimental bone cement with added NaF (12 wt%) was investigated. The fatigue strength of the novel bone cement was evaluated in comparison with the cement without additives; fatigue tests were conducted according to current standards. The load levels were arranged based on a validated, statistically based optimization algorithm. The curve of stress against number of load cycles and the endurance limit were obtained and compared for both formulations. The results showed that the addition of NaF (12 wt%) to polymethylmethacrylate (PMMA) bone cement does not affect the fatigue resistance of the material. Sodium fluoride can safely be added to the bone cement without altering the fatigue performance of the PMMA bone cement.