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

Abstract

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.

Loss of Cement-bone Interlock in Retrieved Tibial Components from Total Knee Arthroplasties

“…In cases where there is sclerotic bone, drilling the cut surface has been advocated to allow penetration of the cement [2]. Even with careful preparation, the immediate postoperative contact fraction will likely be less than 100% because the intertrabecular spaces may contain some residual lavage fluid and marrow, and because the cement shrinks on curing [7]. Our study supports the concept of obtaining sufficient initial interlock, with the acknowledgment that there will likely be some loss of interlock with long time in service.…”

CT volumetry of intravertebral cement after kyphoplasty. Comparison of polymethylmethacrylate and calcium phosphate in a 12-month follow-up

“…Thus we were able to measure absolute values of the cement volume. Baseline CT was not performed immediately after kyphoplasty because there are known dynamic volume changes of PMMA during the process of polymerization and temperature variations [30,31]. We considered a 48-h interval appropriate for PMMA and expected no dimensional changes from that time.…”

A comparison of the shrinkage of commercial bone cements when mixed under vacuum

“…The figures for shrinkage in our study are in the range reported by previous studies. 6,9 Theoretical calculations predicted that the molecular rearrangement occurring during polymerisation would produce a volumetric shrinkage of 7.6%. 11 Espehaug et al 12 reported the differences in the long-term outcome of a shape-closed stem, the Charnley stem, with different types of bone cement.…”

“…However, it is difficult to correlate this material property with the long-term outcome because of the changes in formation of the cement over time and the relatively recent introduction of vacuum mixing 9 which is known to increase shrinkage. 6,10,13 Vacuum mixing also affects other biomechanical properties of bone cement in a dose-dependent fashion. The porosity of bone cement when mixed is inversely proportional to the vacuum pressure.…”

“…[4][5][6] Cement shrinkage can also initiate damage within the cement mantle as a result of the residual stress occurring during curing. 7,8 Although shrinkage of cement has been quantified in a few isolated commercial brands and its dynamic volume changes investigated, 6,9 there is to date, to our knowledge, no published study of a direct comparison of cements.…”