Free Engineering Materials Thesis Example
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In chemistry, at a room temperature a different structure varieties of methylmethacrylate monomer permits to produce solid PMMA with help of polymerization. Reaction is usually fastened by the cupidity of the polymer, which is dissolved in polymer (Webb and Spencer, 2007). The dynamic viscosity (η) of substances is usually calculated by diving stress (F) by rate (S) (η = F/S). When we have viscosity decrease and shear rate increase – cement in liquid state is formed. This process has shear thinning performance and has a definition of “pseudo plastic” (Webb and Spencer, 2007). As we know, while polymer chains grow in length, the viscosity of all cements increases during process of polymerization and many manufacturers can to idea of changing the viscosity of all cements by increasing or decreasing molecular weight of the cement (Webb and Spencer, 2007). Two most common methods for that is copolymers and applying different kind of sterillisation. To obtain micro-interlock and penetrate cancellous bone interstices, cement have to be liquid at its best to be able to flow under pressure and remain its form. During curing of the bone cement, there is always a bulk contraction followed by polymerization process (Webb and Spencer, 2007). The main reason for this contraction is distance between molecules which keep decreasing. Air inclusions in the cement bring to a result, which shrinkage gets to a point lower than 6%. Porosity which were caused by inclusion of air, brings us to a result of bone cement weakening.
Centrifugation may be one of those methods, which can limit porosity. Increased amount of radiopacifiers in the bone cement enhanced the visualization of cement flow during injection process and prevented leakage (Provenzano, Murphy and Riley, 2004). Combination of such elements as barium sulfate and zirconium dioxide are the most popular radiopacifiers, which are used in the bone element. Bone cement mechanical behavior shows that mechanical strength does not decrease with 10 percent of powder formation and is not related with weight base. This particles may lead to exhaustion cracks and badly affect the fatigue of the cement. Local stress factors can be responsible for decrease in tensile and fatigue. There three common factors which can affect cement flow: Bone related parameters, cement properties and type of injections. PMMA is a mixture consisted of two elements, which go under the exponential curing prior coming to solid state (Provenzano, Murphy and Riley, 2004). Cement viscosity in not constant and transfers from liquid to solid. In addition, the time interval between the injections can influence viscosity.
Webb, J. and Spencer, R. (2007). The role of polymethylmethacrylate bone cement in modern orthopaedic surgery. Journal of Bone and Joint Surgery - British Volume, 89-B(7), pp.851-857. Available at: http://www.bjj.boneandjoint.org.uk/content/89-B/7/851.full [Accessed 1 Feb. 2015]
Boger A, e. (2015). Properties of an injectable low modulus PMMA bone cement for osteop - PubMed - NCBI. [online] Ncbi.nlm.nih.gov. Available at: http://www.ncbi.nlm.nih.gov/pubmed/18288697 [Accessed 1 Feb. 2015].
Provenzano, M., Murphy, K. and Riley, L. (2004). Bone Cements: Review of Their Physiochemical and Biochemical Properties in Percutaneous Vertebroplasty. American Journal of Neuroradiology, [online] 25(7), pp.1286-1290. Available at: http://www.ajnr.org/content/25/7/1286.full [Accessed 1 Feb. 2015]
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