1,720,975 research outputs found
Characterization and computational modelling of acrylic bone cement polymerisation
Full text linkTotal joint replacement is one of the most successful surgical procedures and is a
proven treatment for arthritis. Despite low failure rates, the wide application of the
treatment means that large numbers of prostheses fail and must be revised. Improved
pre-clinical testing methods for these orthopaedic devices may assist in developing
new prostheses with improved clinical results. Computational modelling of biological
systems is becoming increasingly accurate and is a much quicker and cheaper
alternative to physical testing, but continued development is necessary to ensure
computational models produce accurate and reliable predictions of implant behaviour.
Acrylic bone cements have been used as a method of fixation for over 50 years but
despite improvements in cement handling techniques and numerous attempts to
improve the mechanical properties of the cement in other ways, the cement is often
highlighted as the weak link in the joint replacement system. Aseptic loosening is
cited as the cause for the majority of revision operations and cement degradation has
been shown to be a contributor to the loosening process. In-vivo, cement is subject to
cyclic loads and these are the primary cause of cement damage. Residual stresses
generated during the polymerisation of the cement are now thought to play a
significant role in cement failure.
This thesis examines the development of residual stresses as a result of thermal and
chemical changes during polymerisation of the cement. Experimental techniques for
characterising the evolution of materials properties during the polymerisation reaction
are discussed. Differential scanning calorimetry was used to measure the reaction
variables such as the activation energy of polymerisation. The development of an
ultrasonic rheometry technique for monitoring the mechanical property evolution
within a bone cement specimen is discussed. Computational models were generated to
predict the reaction behaviour of the cement in terms of the heat produced and the
evolution of the physical properties of the curing mass. Some advantages and
disadvantages of candidate mathematical models have been evaluated and are
discussed, along with applications in several implant fixation scenarios.. The model
compared well with experimental data and was used to predict thermal necrosis in the
bone surrounding both a hip resurfacing implant and a knee replacement. Using the
output reaction path produced by the thermal model a mechanical model was also
produced simulating the shrinkage and mechanical property evolution exhibited by
the polymerising cement. Two material models were compared with and without the
effects of plasticity. Residual stress magnitudes were assessed in comparison with
published values and showed better agreement when plasticity was included. Peak
stresses were observed to occur during polymerisation. The location of the peak
stresses were compared with experimental data on pre-load crack locations in the
literature and showed good agreement
Medial-lateral loading and wear in TKA
No full textPre-clinical wear testing of Total Knee Arthroplasty has traditionally been carried out in a physical experiment [Fisher, 2002]. Recent computational models have been shown to have sufficient accuracy to be considered alongside these experiments [Knight, 2007]. These computational models use a relationship of wear volume proportional to the product of contact pressure, sliding distance and cross shear [Maxian, 1996]. Instrumented knees have recently shown that medial-lateral (ML) loads may be of similar magnitudes to that of the anterior-posterior (AP) load. The AP load is known to have a significant effect on the kinematics of the total knee replacement and so it is reasonable to assume that application of an ML load may have a similar degree of influence on kinematics. The effect of the ML load is hypothesised to increase the cross shear and hence the wear rate. At present, the ISO standard for testing TKA contains no provision for a ML load
Does a PEEK femoral TKA implant preserve intact femoral surface strains compared with CoCr? A preliminary laboratory study
Full text linkBACKGROUND: Both the material and geometry of a total knee arthroplasty (TKA) component influence the induced periprosthetic bone strain field. Strain, a measure of the local relative deformation in a structure, corresponds to the mechanical stimulus that governs bone remodeling and is therefore a useful in vitro biomechanical measure for assessing the response of bone to new implant designs and materials. A polyetheretherketone (PEEK) femoral implant has the potential to promote bone strains closer to that of natural bone as a result of its low elastic modulus compared with cobalt-chromium (CoCr).QUESTIONS/PURPOSES: In the present study, we used a Digital Image Correlation (DIC) technique to answer the following question: Does a PEEK TKA femoral component induce a more physiologically normal bone strain distribution than a CoCr component? To achieve this, a DIC test protocol was developed for periprosthetic bone strain assessment using an analog model; the protocol aimed to minimize errors in strain assessment through the selection of appropriate analysis parameters.METHODS: Three synthetic bone femurs were used in this experiment. One was implanted with a CoCr femoral component and one with a PEEK femoral component. The third (unimplanted) femur was intact and used as the physiological reference (control) model. All models were subjected to standing loads on the corresponding polyethylene (ultrahigh-molecular-weight polyethylene) tibial component, and speckle image data were acquired for surface strain analysis using DIC in six repeat tests. The strain in 16 regions of interest on the lateral surface of each of the implanted bone models was plotted for comparison with the corresponding strains in the intact case. A Wilcoxon signed-rank test was used to test for difference at the 5% significance level.RESULTS: Surface analog bone strain after CoCr implantation indicated strain shielding (R2 = 0.6178 with slope, ? = 0.4314) and was lower than the intact case (p = 0.014). The strain after implantation with the PEEK implant deviated less from the intact case (R2 = 0.7972 with slope ? = 0.939) with no difference (p = 0.231).CONCLUSIONS: The strain shielding observed with the contemporary CoCr implant, consistent with clinical bone mineral density change data reported by others, may be reduced by using a PEEK implant.CLINICAL RELEVANCE: This bone analog in vitro study suggests that a PEEK femoral component could transfer more physiologically normal bone strains with a potentially reduced stress shielding effect, which may improve long-term bone preservation. Additional studies including paired cadaver tests are necessary to test the hypothesis further
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
- …
