The influences of electrical potential and surface finish on the fatigue life of surgical implant materials.

The influence of both imposed anodic potential in Ringer's solution and surface finish on the fatigue lives of annealed 316 type stainless steel and annealed pure titanium were measured and statistically compared to fatigue data run in air. The applied potentials in simulated extracellular fluid approximated conditions existing within the body while also producing the types of surface defects actually found on removed long time implants within the time interval of the accelerated R. R. Moor fatigue tests. Differentiating tests were run at single levels of applied cyclic stress well above the endurance limits. In Ringer's solution, the fatigue life of the 316 stainless steel decreased with increasing applied potential, and at +500 mV was significantly shorter than when run in air. At each condition, the 316 stainless steel was independent of initial surface finish. In contrast, the fatigue life of titanium improved rapidly with increasingly fine surface finishes. Furthermore, compared to air, the application of +500 mV in Ringer's solution improved the life of the rough surface finished material and markedly increased to number of cycles to failure for the electopolished specimens.     

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.     

Parametric analysis of rank transformed data for statistical assessment of genotoxicity data with examples from cultured mammalian cells

The utility of rank transformation followed by parametric analysisof the ranks has been assessed for determination of the statisticalsignificance of genotoxicity data. Both non-parametric and parametricanalytical methods have defects when used to assess the significanceof results from routine regulatory tests. Superficially, therank transformation method followed by parametric analysis ofranks appears to be an ideal solution. However, we consideredthat such a test might suffer a substantial loss of power whenused to analyse normally distributed data with very low samplereplication. To test this hypothesis we took 22 data sets fromfive ‘borderline’ positive compounds in mouse lymphomatreat-and-plate assays where treatment-related increases werebetween 1.5- and 3-fold the control and analysed these resultsby Dunnett's t-test using rank transformed data and weighted,untransformed data. In theory these mouse lymphoma data shouldshow the rank transformation system at its worst in comparisonwith parametric methodology using weighted data. Surprisingly,the rank transformation methodology showed no loss of powerand, overall, performed more consistently than the weighteddata methodology. Based on this limited number of data sets,rank transformation followed by parametric analysis of ranksseems to be an approach very suitable for genotoxicity assaysin general, particularly where distributions are non-normalor of uncertain form. It combines the general applicabilityof non-parametric methods with the power of parametric analyses.However, the methodology still requires to be further validatedin use and by computer simulation.     

The detection and importance of outliers in the in vivo micronucleus assay

Micronucleus tests are generally analysed statistically fordifferences between the means of treated and control groups.‘Outliers’ may either be rejected or grouped togetherwith data from less responsive animals. In either case, a valuableindicator of a small, more sensitive (responder) populationsub-group may then be missed. To alleviate this problem, wehave developed an additional strategy, based on historic data,for the detection of any single animal with a significant increasein micronucleated polychromatic erythrocytes in an otherwiseinsignificant treatment group. Forty-one sets of negative controldata (of five male and five female CD-1 mice each) have beenanalysed. Within each set there were no significant male tofemale differences and data were consistent with a Poisson distribution.Pooled data from all 41 sets showed slightly hyper-Poisson variationand were adequately described by the negative binomial distribution.The negative binomial probability generating function was usedto show that six or more micronuclei per 1000 polychromaticcells from one treated animal would be significant for our laboratory,methodology and strain of mouse, provided that concurrent negativecontrol data conformed with historic values. Changes in methodologydesirable for this type of analysis include increasing the numberof mice in each test group and possible compensation by a reductionin the number of test groups.     

Effect of Fatiguing Exercise on Longitudinal Bone Strain as Related to Stress Fracture in Humans

Muscular fatigue in the training athlete or military recruit has been hypothesized to cause increased bone strain that may contribute to the development of a stress fracture. Under normal circumstances, muscles exert a protective effect by contracting to reduce bending strains on cortical bone surfaces. In vivo strain studies in dogs show that muscle fatigue following strenuous exercise elevates bone strain and changes strain distribution. However, a similar experiment has yet to be performed in humans. The purpose of this work was to test the hypothesis in humans that strenuous fatiguing exercise causes an elevation in bone strain. It was also hypothesized that this elevation is greater in younger people than in older people due to the decline in muscle strength and endurance that normally occurs with age. To test these hypotheses, strain in the tibiae of seven human volunteers was measured during walking before and after a period of fatiguing exercise. Neither hypothesis was sustained. Post-hoc analysis of the strain data suggests that strain rate increases after fatigue with a greater increase in younger as opposed to older persons. Although not conclusive, this suggests that it is strain rate, rather than strain magnitude, that may be causal for stress fracture. © 1998 Biomedical Engineering Society. PAC98: 8745Dr, 8745Bp, 0180+b     

Practical implementation of a multiplex PCR for acute respiratory tract infections in children

Molecular testing for acute respiratory infections (ARIs) has documented value but limited implementation due to questions that typically slow the acceptance of new tests. This study sought to address these questions and achieve implementation. Rhinovirus was added to a nested multiplex PCR (M-PCR), increasing its diagnostic yield. Over one winter, three hospital pediatric departments used the M-PCR to complement their direct fluorescent-antibody assay (DFA) for respiratory syncytial virus (RSV). Clinicians recorded "pretest probability estimates" (using continuous scales for various pathogen groups) for comparison with test results; treatments and test turnaround times were also recorded. Transnasal and throat swabs, with or without nasopharyngeal aspirate (NPA), were M-PCR tested. NPA-containing sample sets found to be RSV positive by DFA were not further tested. Single PCR for human metapneumovirus (hMPV) was performed retrospectively. Of 178 ARI episodes representing 172 patients, NPA was included in 97 sample sets; 54 (56%) were determined to be RSV positive. The other NPA-containing sample sets (n = 43) yielded 27 findings (63%), and the swab-only sets (n = 81) yielded 47 findings (58%); rhinovirus was found most often. Testing for hMPV yielded seven positive results. M-PCR median turnaround times were 4 days in swab-only samples and 5 days with NPA. Antibiotics were prescribed in 50 episodes, at rates similar for RSV and rhinovirus. Pretest probability estimates of a viral cause were lower in episodes caused by rhinovirus than in episodes caused by RSV. The hospitals continued to use M-PCR for NPA-containing samples found to be RSV negative by DFA. Test implementation is more likely with higher diagnostic yield and a protocol that reflects day-to-day clinical and laboratory operations.     

Reliability of nucleic acid amplification methods for detection of Chlamydia trachomatis in urine: results of the first international collaborative quality control study among 96 laboratories

The first European Quality Control Concerted Action study was organized to assess the ability of laboratories to detect Chlamydia trachomatis in a panel of urine samples by nucleic acid amplification tests (NATs). The panel consisted of lyophilized urine samples, including three negative, two strongly positive, and five weakly positive samples. Ninety-six laboratories in 22 countries participated with a total of 102 data sets. Of 204 strongly positive samples 199 (97.5%) were correctly reported, and of 506 weakly positive samples 466 (92.1%) were correctly reported. In 74 (72.5%) data sets correct results were reported on all samples, and 17 data sets (16.7%) showed either one false-negative or one false-positive result. In another 11 data sets, two or more incorrect results were reported, and two data sets reported a false-positive result on one negative sample. The Roche COBAS Amplicor test was performed in 44 (43%) data sets, the Abbott LCx assay was performed in 31 (30%) data sets, the Roche Amplicor manual assay was performed in 9 (9%) data sets, an in-house PCR was performed in 9 (9%) data sets, the Becton Dickinson ProbeTec ET assay was performed in 5 (4.9%) data sets, and the GenProbe TMA assay was performed in 4 (3.9%) data sets. The results of the Roche Amplicor manual (95.6% correct), COBAS Amplicor (97.0%), and Abbott LCx (94.8%) tests were comparable (P = 0.48). The results with the in-house PCR, BD ProbeTec ET, and GenProbe TMA tests were reported correctly in 88.6, 98, and 92.5% of the tests, respectively. Freeze-drying of clinical urine specimens proved to be a successful method for generating standardized, stable, and easy-to-transport samples for the detection of C. trachomatis by using NATs. Although the results, especially the specificity, for this proficiency panel were better than most quality control studies, sensitivity problems occurred frequently, underlining the need for good laboratory practice and reference reagents to monitor the performance of these assays.     

The influence of the viscosity classification of an acrylic bone cement on its in vitro fatigue performance

The goal of the present work was to investigate the influence of the viscosity classification of an acrylic bone cement on its in vitro fatigue performance, as determined in fully-reversed tension-compression (+/-15 MPa) fatigue tests. The test matrix comprised six commercially available bone cements [Orthoset1, (OS1), Orthoset(R)3 (OS3), CemexRX (CRX), Cemex XL (CXL), Palacos R (PR) and Osteopal (OP)], two methods of mixing the cement constituents (hand-mixing and vacuum-mixing), two methods of fabricating the test specimens (direct molding and molding followed by machining), two specimen cross-sectional shapes (rectangular or "flat" and circular or "round"), and four test frequencies (1, 2, 5, and 10 Hz). In total, 185 specimens, distributed among 20 sets, were tested. The test results (number of fatigue stress cycles, N_f) were processed using the linearized transformation of the three-parameter Weibull distribution, whence estimates of the Weibull mean, N_[WM], were obtained. Statistical analysis of the ln N_f results (Mann-Whitney test; alpha<0.05) and a comparison of the N_[WM] estimates for specimen sets in which the formulations have essentially the same composition but different viscosity classification (namely, OS1 versus OS3, CRX versus CXL, and PR versus OP) showed that, in the majority of the comparisons carried out, the viscosity classification of a bone cement does not exert a significant influence on its in vitro fatigue performance.     

How does flow cytometry express Gaussian distributed biological information?

The analogue-to-digital conversion step in flow cytometric analysis introduces a positive skew into the distribution of any measured parameter. In this communication we describe a semi-analytical method to determine if a distribution skewed by the analogue-to-digital conversion step is compatible with a Gaussian origin. The procedure involves determining the errors associated with calculating the coefficient of variation from the skewed distribution and applying systematic corrections. The method was tested with a number of experimental data sets and is illustrated with 3 distributions. One of these was not compatible with a Gaussian origin, but the other 2 were, in spite of considerable positive skewness in the data.     

The relationship between stress, porosity, and nonlinear damage accumulation in acrylic bone cement.

The long-term survival of cemented hip replacements depends on the ability of the cemented fixation to resist fatigue damage. Damage has been assumed to accumulate linearly (Miner's law) even though it is unlikely to be the case in such a porous brittle material. This study addresses the nonlinear stress-dependent nature of fatigue damage accumulation in acrylic bone cement. Specimens were subjected to a zero-to-tension fatigue load in water at 37 degrees C. A total of 15 specimens were tested, i.e., five specimens at each of three stress levels. The specimens were cyclically loaded to a certain fraction of their fatigue lives and the amount of microcracking present at that time was quantified by counting each crack and measuring its length. This procedure was repeated until the specimen failed. A total of 801 cracks formed in the 15 specimens. All cracks were found to initiate at pores. Crack propagation directions were distributed normally about the direction perpendicular to the applied load at the lower stress levels, but at higher stress, the distribution tended to be broader. At higher stresses, more cracks were produced per pore. The damage accumulation process in acrylic bone cement was found to be nonlinear with the degree of nonlinearity increasing with stress. Furthermore, great variability was found which was attributed to the differences in porosity between specimens. A power law equation is given which describes the predicted relationship between damage accumulation and number of loading cycles as a function of the stress level.     

The improved Jellyfish Valve: durability enhancement with sufficient blood compatibility

The Jellyfish Valve is one of the most promising polymer valves for artificial hearts. The present problems to be solved are 1) how to prevent a membrane fracture and 2) how to eliminate a calcification, because both of these problems were observed in experiments with goats after 312 days and 414 days of pumping. Finite element analysis demonstrated that mechanical tensile strain induced in the membrane at valve closure was clearly consistent with the fracture location as well as calcification area in in vivo experiments. Based on this finding, a new valve seat with an additional concentric ring 14 mm in diameter and 0.5 mm in width was finally developed. The maximum strain was dramatically reduced to 52% by the design improvement. Moreover, accelerated fatigue tests demonstrated that the improved valve was 10 times more durable as compared with the original valve, which was equivalent to an in vivo duration of 8.3 years. In animal experiments, including 31 days and 46 days use in a total artificial heart (TAH), no thrombus was found despite the lack of anticoagulant or antiplatelet therapies. These results indicate that the improved Jellyfish Valve might be one of the most durable polymer valves, able to perform in artificial hearts for a long period of time.     

A simple multi-specimen apparatus for fixed stress fatigue testing.

To cope with the time-consuming characteristics of fatigue tests, a multi-specimen fatigue testing apparatus, which could test 10 specimens at a time, was designed, constructed, and tested. The specimens are fixed around a rotating axis, and the required stresses are applied by weights attached on the other end of each specimen. The test mode can be categorized as a stress-controlled flexural fatigue test. Its performance was tested by comparing it with a commercial three-point bending fatigue testing apparatus. The stress versus number of cycles to failure curves of poly(methylmethacrylate) (PMMA), which were obtained from both fatigue testing equipment, showed results that were similar to each other. The fatigue test results of acrylic bone cement in a fixed-stress mode also showed good agreement between the data obtained from the new apparatus and the commercial apparatus. The test results seem quite reliable and show feasibility of significantly reducing the overall test periods. It may be valuable, especially for the fatigue tests, which must be done with a low frequency and a low applied stress level such as a fatigue test of bone cements.     

Search for lesions in mammograms: statistical characterization of observer responses

We investigate human performance for visually detecting simulated microcalcifications and tumors embedded in x-ray mammograms as a function of signal contrast and the number of possible signal locations. Our results show that performance degradation with an increasing number of locations is well approximated by signal detection theory (SDT) with the usual Gaussian assumption. However, more stringent statistical analysis finds a departure from Gaussian assumptions for the detection of microcalcifications. We investigated whether these departures from the SDT Gaussian model could be accounted for by an increase in human internal response correlations arising from the image-pixel correlations present in 1/f spectrum backgrounds and/or observer internal response distributions that departed from the Gaussian assumption. Results were consistent with a departure from the Gaussian response distributions and suggested that the human observer internal responses were more compact than the Gaussian distribution. Finally, we conducted a free search experiment where the signal could appear anywhere within the image. Results show that human performance in a multiple-alternative forced-choice experiment can be used to predict performance in the clinically realistic free search experiment when the investigator takes into account the search area and the observers' inherent spatial imprecision to localize the targets.     

Standardization of direct susceptibility test for blood cultures.

Insufficient data are available to establish the reliability of direct disk diffusion susceptibility tests performed utilizing positive blood culture broth as inoculum. When Staphylococcus aureus ATCC 25923, Escherichia coli ATCC 25922, and Pseudomonas aeruginosa ATCC 27853 were used, 0.03 ml of turbid overnight blood culture broth was found to produce zone diameters closely approximating the size of diameters obtained by a standardized method. Results of direct (0.03 ml of inoculum) and standardized susceptibility tests were then compared for 116 positive blood cultures (1,069 individual disk comparisons). There were 1,011 test agreements (94.6%). There were also 48 (4.5%) minor discrepancies (change between sensitive and intermediate or between intermediate and resistant) and 10 (0.9%) major discrepancies (change between sensitive and resistant). The major discrepancies were randomly distributed among several organisms and antibiotics. Discrepancies occurred most frequently in the more clinically acceptable direction; i.e., in 79.3% the direct test indicted greater resistance than the standardized test. These data establish that 0.03 ml of turbid overnight blood culture broth produces results which compare closely to those obtained with standard methods, and in practice yield direct susceptibility results with a clinically acceptable level of reliability.     

3D real time methodology monitoring cement failures in THA

The present work proposed a methodology to monitor cement microcrack formation in the cemented femoral stem construct using the acoustic emission technique. This technique provides a unique means to automatically tally the number of microcracks, to visualize microcrack distribution, and to animate the progress of crack formation in a given time window of a fatigue test. In this work, the formulation of microcrack source location was derived and a computer program was developed specifically for the proposed application. The program was validated using computer simulation and standard pencil lead break tests. It was found that the mathematical errors complied with the acceptable minimal error. Based on the pencil lead break tests, the average technical error used to estimate the resolution of this technique was 4.7 mm at the present stage. The program was then used to monitor the fatigue damage in precoated cemented femoral hip constructs loaded for a total of more than five million cycles. Two types of microcrack activities were observed in the experiments: Type I and Type II microcracks. A Type I microcrack was a crack that was captured by four or more sensors, and therefore its location was defined uniquely by a set of coordinates. A Type II microcrack was a crack that was captured by three or less sensors, therefore it was unlocatable. Both counts of Type I and Type II microcrack were tallied with respect to the day of fatigue tests. Acoustic emission microcrack graphs were used to visualize the distribution of Type I microcracks in the construct. It was found that the Type I microcracks distributed mainly over the proximal third of the stem. The amount of microcrack events decreased significantly as the number of loading cycles increased.     

The statistics of quantifiable homeostasis. I. Simple linear homeostasis

The statistical properties are explored of the least-squares estimators of the parameters of a deterministic linear model for homeostasis with random, normally distributed technical errors. The parameters are: the amplitude of perturbation (A), the homing value (H), the restoration constant (B), and the lag time (L). The former two, although important in their own right, tell us nothing about the processes of biologic restitution and are thus considered "nuisance" parameters; the latter two are the "business" parameters, which tell us about the processes of biologic adjustment. The properties studied include bias and precision and their relationships to the number of data points and to the size of the technical error. Efforts have been concentrated on data points equally spaced over six lag times (which corresponds roughly to the period of observation used in loading studies, such as glucose tolerance tests). The distributions of these four estimators are studied in sets of 100 Monte Carlo simulations, each with 15, 30, and 100 data points, respectively. The estimators are virtually unbiased and all satisfactorily close to Gaussian. The residual mean-square errors (the divisor being four less than the number of data points, since four parameters are estimated) seem to be unbiased estimators of the variances of the errors. Moreover, when suitably scaled, the residual sums of squares have distributions close to the chi-square distribution for the appropriate degrees of freedom and with the corresponding means and variances. The correlations among the estimators are modest and, except where A is involved, small.     

Evaluation of automated large-scale screening tests for syphilis.

Two methods of performing serological screening tests for syphilis are compared. One consisted of the Venereal Diseases Reference Laboratory (VDRL) slide test, the cardiolipin Wassermann reaction (CWR), and the Reiter protein complement fixation test (RPCFT) performed manually; the other was a fully automated system using two Technicon AutoAnalyzers (AAII), one for the automated reagin test (ART) and the other for automated complement fixation tests. The absorbed fluorescent treponemal antibody test (FTA-ABS) was used as a final arbiter in all cases found to be seropositive by either method. A pooled antigen consisting of a mixture of cardiolipin and Reiter protein was used for the automated complement fixation test, thus increasing the scope and capacity of the system. The AutoAnalyzer was shown to be capable of performing 400 cardiolipin and Reiter complement fixation tests and 700 automated reagin tests in an 8-hour day. Modification of the complement fixation test method to take advantage of the highly sensitive colorimeter resulted in a significant increase in sensitivity and a corresponding saving in reagents. Of the 7843 sera tested, 258 gave a positive result in one or more of the screening tests. The automated test detected many more Reiter positive sera (127) than the manual test (83). Conversely, fewer CWR positive sera were detected by the automated test (60) than by the manual test (82). There was little difference between the number of positive sera detected by the ART (73) and the VDRL slide test (71). In 19 instances the automated tests detected positive sera which registered as completely negative in the manual tests, and four seropositive cases which the automated tests had failed to detect were detected by the manual tests, and four seropositive cases which the automated tests had failed to detect were detected by the manual tests. It was concluded that a combination of the ART and automated Reiter protein complement fixation test (ARPCFT) would be ideal for use in a large-scale screening programme for the detection of syphilis.     

Biodegradation of polyurethane under fatigue loading

A method utilizing expansion of a diaphragm-type film specimen was developed to study in vitro biodegradation of poly(etherurethane urea) (PEUU) under conditions of dynamic loading (fatigue). A finite element model was used to describe the strain state, which ranged from uniaxial at the edges of the film to balanced biaxial tensile strain at the center. During testing, the film was exposed to a H(2)O(2)/CoCl(2) solution, which simulated in vivo oxidative biodegradation of PEUU. The extent of chemical degradation was determined by infrared analysis. Physical damage of the film surface was characterized by optical microscopy and scanning electron microscopy. Dynamic loading did not affect the rate of degradation relative to unstressed and constant stress (creep) controls in regions of the film that experienced primarily uniaxial fatigue; however, degradation was accelerated in regions that experienced balanced biaxial or almost balanced biaxial fatigue. It was concluded that the combination of dynamic loading and biaxial tensile strain accelerated oxidative degradation in this system. Chemical degradation produced a brittle surface layer that was marked by numerous pits and dimples. Physical damage of the surface in the form of cracking occurred only in fatigue experiments. Cracking was not observed in unstressed or creep tests. Cracks initiated at the dimples produced by chemical degradation, and propagated in a direction that was determined by the strain state.     

An international external quality assessment of nucleic acid amplification of herpes simplex virus.

BACKGROUND: There is an increasing awareness of the need for external quality control of diagnostic virology. OBJECTIVES: To assess the quality of nucleic acid amplification tests (NAT) of herpes simplex within Europe. STUDY DESIGN: Herpes simplex virus (HSV) proficiency panels were produced at the Swedish Institute for Infectious Disease Control on behalf of the European Union Concerted Action for Quality Control of Nucleic Acid Amplification in 1999 and 2000. Nine reference laboratories evaluated the production process. Each panel consisted of 12 coded samples with various concentrations of inactivated, freeze-dried HSV type 1 (HSV-1), and HSV type 2 (HSV-2), or negative controls. Positive samples included HSV-1 and HSV-2 in a range of concentrations (2 x 10(2) to 2 x 10(7) genome copies per ml) similar to those found in cerebrospinal fluids from patients with HSV encephalitis. RESULTS: Sixty-six participants reported a total of 76 data sets for panel 1, and 71 reported 78 data sets for panel 2. The majority of the participants employed qualitative 'in-house' polymerase chain reaction (PCR) methods, either in a single, nested or semi-nested format. For panel 2, 9 laboratories reported use of 'real-time' PCR in contrast to 3 for panel 1. Three laboratories submitted quantitative results on both panels. Thirty percent of the data sets had correct results for the entire panel 1. In 6 data sets (8%) a total of 11 false positive results were reported. For panel 2, 28% of the data sets had correct result. Nineteen false positive results were reported in 14 data sets (18%), but most of the incorrect results reflected a lack of test sensitivity. CONCLUSIONS: The relatively high frequency of false positive results and the large number of false-negative results, albeit at low copy number, stress the need for improvement in the quality of HSV NAT and for external quality control programmes.