Estimation of the minimum number of test specimens for fatigue testing of acrylic bone cement

In the literature on fatigue testing of acrylic bone cements, data sets of various sizes have been used in different test series for the same cement formulation. There are two important consequences of this situation. First, it means that some test series last much longer than others, with all the implications for the cost of testing. Second, it makes drawing conclusions about the fatigue performance of a cement, based on the results of different literature series, a problematic issue. Clearly then, a recommendation as to what should be the minimum number of test specimens to use that would allow for confidence in the results of the statistical treatment of the test results (Gmin) would be desirable. In the present work, a method that could be used to culminate in such a recommendation is described. This method involves (i) obtaining experimental fatigue test results and (ii) analyzing those results using the Weibull probability distribution function and other statistical methods. This methodology is illustrated using fatigue life results obtained from uniaxial tension-compression fatigue tests on specimens fabricated from the polymerizing dough of one commercially available acrylic bone cement. For a tolerable error of 5%, we estimated Gmin to be either 7 (if the fatigue life results are treated using the two-parameter Weibull distribution function) or 11 (if the fatigue life results are treated using the three-parameter Weibull distribution function). To be on the conservative side, we therefore recommend that Gmin be 11. Three key limitations of the methodology presented here are discussed.     

Effect of two variables on the fatigue performance of acrylic bone cement: mixing method and viscosity

The goal of the present work was to establish the relative influence of one exogenous variable versus one endogenous variable on the fully-reversed tension-compression fatigue performance of bone cement. The method used to mix the cement constituents was the exogenous variable, while the viscosity of the mixed cement dough was the endogenous variable. Two commercial cement formulations (Palacos R and Osteopal) and two cement mixing methods (hand mixing and vacuum mixing) were used. It was found that for a given mixing method, cement viscosity exerts a marginal influence on fatigue performance. On the other hand, for a given cement formulation, vacuum mixing led to a statistically significant improvement in fatigue performance. The present results demonstrate the superior influence of mixing method over cement viscosity.     

Statistical distribution of the fatigue strength of porous bone cement

This paper reports on the damaging effects of different percentage porosities on the fatigue life of acrylic bone cement as used in the fixation of orthopaedic implants. Both hand-mixed (HM) and vacuum-mixed (VM) specimens containing different levels of porosity were fatigue tested to failure. A negative correlation between porosity level and fatigue life was demonstrated for both techniques. Considerable scatter was present in the data. Using the pore size distributions for HM and VM cement virtual HM and VM specimens were created containing various levels of porosity. Incorporating the effect of pore size and pore clustering quantified previously using the theory of critical distances a fatigue life prediction could be obtained for the virtual specimens. The virtual data agreed strongly with the experimental findings, predicting the correlation and more significantly the scatter in the experimental results. Using the virtual porosity failure model, it was demonstrated that given a constant porosity the fatigue life can vary by over an order of magnitude in both HM and VM cement. This suggests that not only porosity level but pore size distribution is extremely important in controlling the fatigue life of bone cement. It was verified that pore clustering and pore size are the major contributors to failure in HM and VM cement respectively. Furthermore, given the beneficial effects of porosity it has been proposed that an even distribution of small pores would provide an optimal bone cement mantle. Using the virtual model, it was determined that neither technique was capable of achieving such a distribution indicating a need for a new more reliable technique. The TCD based virtual porosity failure model should prove to be a powerful tool in the design of such a technique.     

聚甲基丙烯酸甲酯骨水泥与Cortoss骨水泥生物力学性能及聚合温度比较

背景：聚甲基丙烯酸甲酯骨水泥是其最常用的一种填充材料,但由于其存在聚合时放热、单体毒性等缺点,所以目前出现了以Cortoss为代表的生物新型骨水泥。 目的：对比两种骨水泥在体外的力学性能。 方法：将聚甲基丙烯酸甲酯与新型骨水泥Cortoss按照ISO5833:2002标准分别制作成抗压及抗弯模型,将制作的模型使用生物力学机测试两种骨水泥的抗弯及抗压模量,同时在制模过程中测量骨水泥聚合温度。 结果与结论：与聚甲基丙烯酸甲酯骨水泥比较,Cortoss骨水泥在聚合过程中最高温度较低、抗压强度较强、抗弯模量降低,差异均有显著性意义(P < 0.05),而抗弯强度间差异无显著性意义(P > 0.05)。说明Cortoss生物力学强度优于聚甲基丙烯酸甲酯骨水泥,由于Cortoss在聚合时放热较少,所以其用于手术时有较好的安全性。      

高黏度骨水泥与传统骨水泥治疗骨质疏松性椎体压缩性骨折的疗效比较

目的比较高黏度骨水泥与传统聚甲基丙烯酸甲酯(PMMA)骨水泥治疗骨质疏松性椎体压缩性骨折的疗效,探讨高黏度骨水泥在临床应用中的优势。方法选择2009年7月~2013年7月治疗并获得随访160例骨质疏松性椎体压缩性骨折患者,其中男性64例,女性96例;年龄61~88岁,平均年龄69.1岁。分高黏度骨水泥组[91例(112个椎体)]和传统骨水泥组[69例(86个椎体)]。高黏度骨水泥组,采用以色列Disc-O-Tech公司Confidence骨水泥,施行经皮椎体成形术(PVP);传统骨水泥组,采用PMMA骨水泥,施行PVP。术后对比两组患者视觉疼痛模拟评分(VAS)、责任椎Cobb角的恢复情况及术后骨水泥渗漏情况,并随访观察。结果高黏度骨水泥组与传统骨水泥组VAS评分(1.5±0.8vs1.4±0.9)比较差异无统计学意义(P0.05);高黏度骨水泥组Cobb角恢复优于传统骨水泥组(13.6°±3.1°vs 19.8°±3.0°),差异有统计学意义(P0.05);高黏度骨水泥组渗漏率远低于传统骨水泥组(19.6%vs 41.9%),差异有统计学意义(P0.05)。所有患者术后获得3~48个月(平均16个月)随访,其中3例出现神经根症状,所有患者未出现感染、肺栓塞等并发症。结论高黏度骨水泥与传统PMMA骨水泥相比,在纠正椎体Cobb角及降低骨水泥渗漏发生率方面效果更佳,明显提高了PVP的安全性及有效性。     

The contradictory effects of pores on fatigue cracking of bone cement

The beneficial effect of porosity reduction on the fatigue life of bone cement has been demonstrated in numerous experimental studies. Clinically, however, it seems that the beneficial effect of porosity reduction of cement around total hip replacement components can only be found in large follow-up studies. Little is known about the actual mechanical effect of a pore on fatigue crack formation in cement mantles. We studied the effect of pores on the crack formation process in a finite element model of a transverse slice of a total hip reconstruction. We created models with a single large pore and models with multiple pores at levels of 2, 4, and 9%. The models were cyclically torque-loaded, causing macrocracks to appear in the cement mantle. In all models, we found that pores acted as microcrack initiators. However, pores could have both a detrimental and a beneficial effect on the macrocrack propagation in the cement mantle. Both effects were seen in the models with a single large pore and in the models with multiple pores. Pores would either accelerate, deviate, or decelerate the macrocrack propagation in the cement mantle. The effect of the pores depended on the location of the pores with respect to the stress intensities in the model, but was independent of the pore size or the level of porosity. The results may explain why the beneficial effect of vacuum mixing is difficult to demonstrate clinically. Stress intensities that are present in a cement mantle in an in vivo situation may overshadow the detrimental effect of a pore, while the beneficial effect may become more pronounced.     

Comparison of two microdilution methods for testing susceptibility of Candida spp. to voriconazole

The growing number of fungal infections, coupled with emerging resistance to classical antifungal agents, has led to the development of new agents, among them voriconazole. Susceptibility to voriconazole was tested by using two microdilution techniques: the reference method described in National Committee for Clinical Laboratory Standards document M27-A2 and a colorimetric method, Sensititre YeastOne. A total of 272 Candida isolates (132 of Candida albicans, 62 of C. parapsilosis, 33 of Candida glabrata, 21 of C. krusei, 15 of C. tropicalis, and 9 of C. lusitaniae) and two control strains (C. parapsilosis ATCC 22019 and C. krusei ATCC 6258) were tested. There was a high rate of agreement between the two methods used (97 to 100%).     

A controlled-notch specimen to study fatigue crack initiation in bone cement

Despite the extensive literature on the mechanical characteristics and failure properties of poly(methyl methacrylate) bone cement, little is known of its fatigue crack initiation process. The most likely in vivo bone cement fatigue crack initiation sites are internal flaws and irregularities on the bone cement surface. The stress concentration created by a flaw, and subsequently the stress state at that flaw, depends on the flaw geometry. To model the fatigue crack initiation process of a flaw, it is necessary to reproduce the stress state at that flaw. In this study, a special mold was designed to introduce notches with specific tip radii into fatigue specimens. The notch was molded into the specimen to simulate the in vivo flaw formation process. The molding method allows control of the stress concentration by specifying the notch tip radius. We created notched specimens where the tip radii of the notches ranged from "sharp" (< 3 microm) to 400 microm. The results demonstrated that notched specimens created by the special mold satisfied two necessary requirements for fatigue crack initiation studies: (1) the material microstructure at the notch tip must not be disrupted by the notching process, and (2) the notch tip stress field, determined by the notch tip geometry, must be reproducible.     

Comprehensive biocompatibility testing of a new PMMA-hA bone cement versus conventional PMMA cement in vitro

For more than 50 years PMMA bone cements have been used in orthopaedic surgery. In this study attempts were made to show whether cultured human bone marrow cells (HBMC) show an osteogenetic response resulting in new bone formation, production of extracellular matrix (ECM) and cell differentiation when they were cultured onto polymerized polymethylmethacrylate (PMMA)-hydroxyapatite (HA), conventional PMMA bone cement being taken as reference. Biocompatibility parameters were collagen-I and -II synthesis, the detection of the osteoblast markers alkaline phosphatase (ALP) and osteocalcin, the number of adherent cells and the cytodifferentiation of immunocompetent cells. Cement surface structure, HA stability in culture medium and chemical element analysis of specimens were considered. Fresh marrow cells were obtained from the human femora during hip replacement. Incubation time was up to ten weeks. We used atomic forced microscopy (AFM) and scanning electron microscopy (SEM) for cement specimen analysis. Fluorescent activated cell sorter (FACS), immunohistochemical staining. SEM and light microscopy (LM) served us to judge the cellular morphology. Products of the extracellular matrix were analyzed by protein dot blot analysis, SEM energy dispersive X-ray analysis (SEM-EDX) and Ca2+/PO(4)3- detection. HA particles increased the osteogenetic potential of PMMA bone cement regarding the cellular production of collagen, alkaline phosphatase (AP), the number of osteoblasts and the cellular differentiation pattern in vitro. Both tested cements showed good biocompatibility in a human long-term bone marrow cell-culture system.     

Comprehensive biocompatibility testing of a new PMMA-HA bone cement versus conventional PMMA cement in vitro

For more than 50 years PMMA bone cements have been used in orthopaedic surgery. In this study attempts were made to show whether cultured human bone marrow cells (HBMC) show an osteogenetic response resulting in new bone formation, production of extracellular matrix (ECM) and cell differentiation when they were cultured onto polymerized polymethylmethacrylate (PMMA)-hydroxyapatite (HA), conventional PMMA bone cement being taken as reference. Biocompatibility parameters were collagen-I and -III synthesis, the detection of the osteoblast markers alkaline phosphatase (ALP) and osteocalcin, the number of adherent cells and the cytodifferentiation of immunocompetent cells. Cement surface structure, HA stability in culture medium and chemical element analysis of specimens were considered. Fresh marrow cells were obtained from the human femora during hip replacement. Incubation time was up to ten weeks. We used atomic forced microscopy (AFM) and scanning electron microscopy (SEM) for cement specimen analysis. Fluorescent activated cell sorter (FACS), immunohistochemical staining, SEM and light microscopy (LM) served us to judge the cellular morphology. Products of the extracellular matrix were analyzed by protein dot blot analysis, SEM energy dispersive X-ray analysis (SEM-EDX) and Ca²⁺/PO₄ ^3- detection. HA particles increased the osteogenetic potential of PMMA bone cement regarding the cellular production of collagen, alkaline phosphatase (AP), the number of osteoblasts and the cellular differentiation pattern in vitro. Both tested cements showed good biocompatibility in a human long-term bone marrow cell-culture system.     

Damage accumulation, fatigue and creep behaviour of vacuum mixed bone cement

The behaviour of bone cement under fatigue loading is of interest to assess the long-term in vivo performance. In this study, uniaxial tensile fatigue tests were performed on CMW-1 bone cement. Acoustic emission sensors and an extensometer were attached to monitor damage accumulation and creep deformation respectively. The S-N data exhibited the scatter synonymous with bone cement fatigue, with large pores generally responsible for premature failure; at 20 MPa specimens failed between 2 x 10(3) and 2 x 10(4) load cycles, while at 7 MPa specimens failed from 3 x 10(5) load cycles but others were still intact after 3 x 10(6) load cycles. Acoustic emission data revealed a non-linear accumulation of damage with respect to time, with increasing non-linearity at higher stress levels. The damage accumulation process was not continuous, but occurred in bursts separated by periods of inactivity. Damage in the specimen was located by acoustic emissions, and allowed the failure site to be predicted. Acoustic emission data were also used to predict when failure was not imminent. When this was the case at 3 million load cycles, the tests were terminated. Creep strain was plotted against the number of load cycles and a linear relationship was found when a double logarithmic scale was employed. This is the first time a brand of cement has been characterised in such detail, i.e. fatigue life, creep and damage accumulation. Results are presented in a manner that allows direct comparison with published data for other cements. The data can also be used to characterise CMW-1 in computational simulations of the damage accumulation process. Further evidence is provided for the condition-monitoring capabilities of the acoustic emission technique in orthopaedic applications.     

Quantitative analysis of the effect of porosity on the fatigue strength of bone cement

This paper reports on the effects of porosity and its distribution on the fatigue strength of bone cement. Hand-mixed (HM) and vacuum-mixed (VM) bone cement samples were fatigue tested to failure. The point of failure commonly coincided with large single pores (in the VM materials) and multiple pores in clusters (in the HM material). The effect of pores was analysed using the Theory of Critical Distances (TCD), a theory previously developed to explain the effect of notches and other stress concentrations on fatigue and fracture. Clusters of pores were analysed by developing a criterion to decide whether local cracking would act to link pores together, forming a single stress concentration of more complex shape. This approach enabled us to predict the high-cycle fatigue strength of samples containing clusters of pores, with good accuracy (errors less than 13%). We then used the analysis to develop general rules for the effect of pore size and proximity on fatigue strength. For example, we showed that a single pore of 2mm diameter or more would cause a significant decrease in the fatigue strength (compared to that of pore-free material); however, two pores of only 1mm diameter in close proximity would be equally damaging. This demonstrates the importance not only of pore size but also of pore density and distribution. However, pores do have beneficial effects such as improved drug dispersion, bone ingrowth and crack tip blunting. Therefore, given the findings from this study, a possible step forward in the development of surgical bone cements may involve a compromise in which relatively small pores are evenly distributed throughout the material.     

Effect of test frequency on the in vitro fatigue life of acrylic bone cement

The goal of the present work was to test the hypothesis that test frequency, f, does not have a statistically significant effect on the in vitro fatigue life of an acrylic bone cement. Uniaxial constant-amplitude tension-compression fatigue tests were conducted on 12 sets of cements, covering three formulations with three very different viscosities, two different methods of mixing the cement constituents, and two values of f (1 and 10 Hz). The test results (number of fatigue stress cycles, N(f)) were analyzed using the linearized form of the three-parameter Weibull equation, allowing the values of the Weibull mean (N(WM)) to be determined for each set. Statistical analysis of the lnN(f) data, together with an examination of the N(WM) estimates, showed support for the hypothesis over the range of f used. The principal use and explanation of the present finding are presented.     

Effect of fabrication pressure on the fatigue performance of Cemex XL acrylic bone cement

During a cemented arthroplasty, the prepared polymerizing dough of acrylic bone cement is subjected to pressurization in a number of ways; first, during delivery into the freshly prepared bone bed, second, during packing in that bed (either digitally or with the aid of a mechanical device), and, third, during the insertion of the prosthesis. Only a few studies have reported on the influence of the level of pressurization experienced during these events (which, depending on the cementing technique used, has been put at between 8 and 273 kPa) on various properties of the cement. That was the focus of the present study, in which the fully reversed tension-compression (+/-15 MPa; 5 Hz) fatigue lives (expressed as number of cycles to fracture, N(f)) of rectangular cross-sectioned "dog-bone" specimens (Type V, per ASTM D 638) fabricated from Cemex XL cement, at pressure applied continuously to the cement dough during curing in the specimen mold, p=75,150, and 300 kPa, were determined. The N(f) results were analyzed using the linearized transformation of the three-parameter Weibull relationship to obtain estimates of the Weibull mean, N(WM), which was taken to be the index of fatigue performance of the specimen set. Over the range of p studied, N(WM) increased as p increased (for example, from 329,118 cycles when p was 75 kPa to 388,496 cycles when p was 300 kPa); however, the increase was not significant over any pair of p increment steps (Mann-Whitney U-test; alpha<0.05).     

实验用犬两种麻醉方法的比较

犬常被用于手术学教学及科研实验,其良好的麻醉效果,是保证手术操作完成及科研过程实施的必备先决条件。现将犬的二种麻醉方法及麻醉比较报告如下。1材料与方法1.1犬的选择和术前处理随机选择手术学教学犬30条,雄性19条,雌性11条,年龄3~8岁,体重5~21 kg,平均体重16.6 kg;科研实     

Standard setting: Comparison of two methods

Background  

The outcome of assessments is determined by the standard-setting method used. There is a wide range of standard – setting methods and the two used most extensively in undergraduate medical education in the UK are the norm-reference and the criterion-reference methods. The aims of the study were to compare these two standard-setting methods for a multiple-choice question examination and to estimate the test-retest and inter-rater reliability of the modified Angoff method.     

Investigation of fatigue crack growth in acrylic bone cement using the acoustic emission technique

Failure of the bone cement mantle has been implicated in the loosening process of cemented hip stems. Current methods of investigating degradation of the cement mantle in vitro often require sectioning of the sample to confirm failure paths. The present research investigates acoustic emission as a passive experimental method for the assessment of bone cement failure. Damage in bone cement was monitored during four point bending fatigue tests through an analysis of the peak amplitude, duration, rise time (RT) and energy of the events emitted from the damage sections. A difference in AE trends was observed during failure for specimens aged and tested in (i) air and (ii) Ringer's solution at 37 degrees C. It was noted that the acoustic behaviour varied according to applied load level; events of higher duration and RT were emitted during fatigue at lower stresses. A good correlation was observed between crack location and source of acoustic emission, and the nature of the acoustic parameters that were most suited to bone cement failure characterisation was identified. The methodology employed in this study could potentially be used as a pre-clinical assessment tool for the integrity of cemented load bearing implants.     

Effect of mixing method and storage temperature of cement constituents on the fatigue and porosity of acrylic bone cement.

The influence of the storage temperature of the cement constituents prior to mixing (21 vs. 4 degrees C) and the mixing method (hand mixing vs. vacuum mixing) on the uniaxial tension-compression fatigue performance and porosity of Palacos R acrylic bone cement was studied. The fatigue results were analyzed using the three-parameter Weibull equation. The fatigue performance was expressed as an index I, which was defined as the product of the Weibull characteristic fatigue life and the square root of the Weibull slope. Statistical analyses of these results show that although the mixing method (for a given storage temperature) exerts a significant influence on the fatigue performance and areal porosity, the effect of storage temperature (for a given mixing method) on either of these parameters is not significant.     

两种横纹肌染色方法的比较

<正>横纹肌包括骨骼肌和心肌,常规的H-E染色方法对正常横纹肌中横纹显示不清楚,所以为了显示与观察横纹,常常需要用到特殊的横纹肌染色方法。传统的横纹肌染色包括Mallory磷钨酸苏木精染色法(PTAH)和Heidenhain氏铁苏木素染色法等。作为医学检验方法,以简单快速为首选,而要制作教学切片,则对染色的效果,切片存放时间等要求高。为了寻求一种稳定的,效果好的,适合制作大量教学切片的染色方法,我们将上述2种横纹肌的染色方法做了比较。