A theoretical and experimental analysis of polymerization shrinkage of bone cement: A potential major source of porosity

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-P(TM) vacuum and hand mixed, Endurance(TM) 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 ≤ 0.05) from 5.1% (hand-mixed Simplex-P(TM)) to 6.7% (vacuum-mixed Simplex-P(TM)) to 10.5% for a 0.6:1 (polymer g/monomer mL) two- solution bone cement. Shrinkage was highly correlated with initial monomer content (R² = 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. (C) 2000 John Wiley and Sons, Inc.