The effect of the quality of the bone and of the cement pressurization magnitude and duration on the fixation achieved with polymethylmethacrylate (PMMA) bone cement is studied in vitro. Seventy-one cement-bone interface specimens, prepared under various conditions of pressurization of low-viscosity bone cement, are tested in tension. The load at failure and the maximum cement penetration are measured to assess the fixation achieved, and the quality of the bone is assessed by determining the compressive strength of each of the bone specimens. Statistical analysis of the data indicates that the pressure magnitude is the most influential of the factors considered in the cement penetration behavior and in the development of failure load capacity. The duration of the pressure does not appear to be a significant factor. The cement penetration is a decreasing function of the bone strength, reflecting a decrease in the porosity and an increase in the area fraction. Although not directly measured in these tests, these latter bone properties are indirectly measured by the bone compressive strength. The effect of increasing bone strength on the failure load is nonlinear. The development of adequate failure load capacity is the result of a balance between the cement penetration allowed by the porosity of the bone and the inherent strength of the cancellous bone itself. Weak bone, although adequately penetrated by cement, cannot provide strong fixation. Stronger, denser bone limits cement penetration, but pressurization enhances development of failure load capacity through more complete infusion and interlocking of the cement in the available pore space. The strength of the fixation achievable for any bone is limited by the intrinsic strength of the bone.(ABSTRACT TRUNCATED AT 250 WORDS)
A theoretical basis for understanding polymerization shrinkage of bone cement is presented based on density changes in converting monomer to polymer. Also, an experimental method, based on dilatometry and the Archimedes' principle is presented for highly precise and accurate measurement of unconstrained volumetric shrinkage of bone cement. Furthermore, a theoretical and experimental analysis of polymerization shrinkage in a constrained deformational state is presented to demonstrate that porosity can develop due to shrinkage. Six bone-cement conditions (Simplex-Ptrade mark vacuum and hand mixed, Endurancetrade mark vacuum mixed, and three two-solution experimental bone cements with higher initial monomer levels) were tested for volumetric shrinkage. It was found that shrinkage varied statistically (p< or = 0.05) from 5.1% (hand-mixed Simplex-Ptrade mark) to 6.7% (vacuum-mixed Simplex-Ptrade mark) to 10.5% for a 0.6:1 (polymer g/monomer mL) two-solution bone cement. Shrinkage was highly correlated with initial monomer content (R(2) = 0.912) but with a lower than theoretically expected rate. This discrepancy was due to the presence of residual monomer after polymerization. Using previously determined residual monomer levels, the theoretic shrinkage analysis was shown to be predictive of the shrinkage results with some residual monomer left after polymerization. Polymerization of a two-solution bone cement in a constrained state resulted in pores developing with volumes predicted by the theory that they are the result of shrinkage. The results of this study show that shrinkage of bone cement under certain constrained conditions may result in the development of porosity at the implant-bone cement interface and elsewhere in the polymerizing cement mantle.
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