Flow characteristics of curing polymethyl methacrylate bone cement

Abstract

During polymerization, polymethyl methacrylate bone cements have complex viscoelastic characteristics. Within a short working time they transform from dough-like consistencies to solid cements. Therefore, the time at which a cement is introduced to cancellous bone surfaces and subjected to pressure is important, to achieve optimum flow and mechanical interdigitation. Achieving adequate mechanical interlock increases the area for load transfer and reduces localized bone-cement interface stresses. The aim of this study was to measure the flow characteristics for commercial bone cements as a function of time and calculate the apparent viscosities for the curing bone cements The capillary extrusion method was used to measure the rate of flow of the curing cement, by means of a melt flow index apparatus, which was manufactured in-house. The tests were conducted using nozzles of different lengths and under two loads. This enabled the power index value, n, and the pressure at the die entry, Po, to be calculated for each material with respect to time. Once the flow characteristics were determined, a series of formulae were used to calculate the shear rates, y, the shear stresses, r, and the apparent viscosities, na, of the curing bone cements. The results indicated that acrylic bone cements are non-Newtonian, pseudoplastic materials, since the power index values are less than 1.0 during the curing stage. The consistency indices, K, were calculated from the shear stress versus shear rate data. The apparent viscosities of the cements were found to increase with respect to increases in time. Clinically, it was considered desirable to inject and pressurize the cement into the medullary canal while its viscosity is relatively low in order to obtain maximum interdigitation into cancellous bone, provided adequate containment and a means of pressurization can be achieved. The pseudoplastic character of bone cements is responsible for their reduction in viscosity with increased shear rate, a property that may be exploited to enhance penetration with appropriate delivery.