INTRODUCTION: Introduction of centrifugation and vacuum mixing has helped to decrease cement porosity to approximately 5-8% and <2% respectively, but has led to a marked increase in volume shrinkage, which has been implicated as a factor in aseptic loosening.The purpose of this study was to compare the reaction of six of the most commonly used bone cements to centrifugation, hand- and vacuum-mixing, and to address the need to establish a mixer/cement combination offering good porosity reduction with minimal increase in shrinkage.
METHODS: The following cements were used: CMW1, CMW3, Endurance, Osteobond, Pallacos R, Simplex P. For hand mixing, we used a Stryker Prism Mixing Bowl; the cement was then transferred to the syringe using a spatulum. For vacuum mixing, we used a Stryker Advanced Cement Mixing Bowl and transferred cement to an attached syringe, all under 21in Hg of vacuum. For centrifugation we used hand-mixed cement with subsequent centrifugation at 2000rpm for 1min in a Zimmer Centrifuge System. Density was determined at 5 and 20min after mixing, utilizing the flotation technique with a resolution of 10?L. Volume shrinkage was calculated using the ratio of these two density values.
RESULTS: Across the six cements, average shrinkage and density were calculated as 4.98% and 1.21g/cm3 for hand mixing, 5.76% and 1.24g/cm3 for centrifuge mixing, and 6.60% and 1.26g/cm3 for vacuum-mixed cements. These variations were highly significant (unpaired t-test, p<0.0001). To name just two examples of different reactions to advanced mixing techniques, shrinkage in vacuum-mixed Palacos increased to 6.7% from 3.7% in hand-mixing, while centrifuging of Osteobond increased shrinkage from 5.2% to 6.5%.
DISCUSSION AND CONCLUSION: The concern of reduced fatigue life due to high porosity has been addressed by the introduction of modern techniques for cement preparation. Both advanced mixing techniques in this study reduced porosity and increased density, although vacuum mixing proved to be most efficient. Noteworthy was the varying extent to which different bone cements benefitted from these techniques. For comparable levels of porosity reduction, significantly different increases in volume shrinkage were seen. Volume shrinkage is of concern, as it is perceived to influence debonding at the implant-cement interface, which can lead to aseptic loosening. Further research on the composition and mixing conditions of standard PMMA bone cements is required to investigate a compromise between reduction of porosity and increased shrinkage.
For more information contact Wolfgang Fitz, MD
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