V. Krüger, S. Jönsson, E. Lidén (Lund, Sweden)
INTRODUCTION: It has long been known that the rheological behaviour of bone cement for vertebroplasty is both important and complex. Considering that the bone cement will be injected into a porous structure it is necessary to complete the conventional shear rheometer data with extensional flow data to fully characterize the flow properties. The objective of the study was to thoroughly examine the rheological behaviour and flow properties of two hydraulic bone cements which had the same composition but different viscosity.
METHODS: The hydraulic bone cements examined consisted of 60wt% calcium sulfate, 40wt% hydroxyapatite and an iodine containing liquid. However the properties of the two materials – Cement 1 and Cement 2 – had been modified so that Cement 1 was less viscous than Cement 2.
Initially the two cements were characterized using strain sweep measurements (VOR Bohlin rheometer). The measurements were performed between 2 and 30 minutes increasing the strain from 0.002% to 2%. Then the extensional flow properties were determined with the contraction flow method . The paste was extruded through a contraction nozzle at different injection speeds (1, 2, 4 and 8ml/min). Finally, the distribution of the cements within a synthetic vertebra was tested by injecting the paste, with constant rate, from filled 10ml syringes through 11G needle into porous blocks (open cell rigid foam 7.5pcf, Sawbones Europe AB). The sawbone blocks were immersed in water during the tests.
RESULTS: The strain sweep measurements show that Cement 1 needed about 25 minutes to build the inner structure. Cement 2 built up its structure faster. It took only 3.5 minutes. The extensional flow properties of Cement 1 and 2 were the same. Both were extensional thinning. Bone cement 1 and 2 distribute without problems in the porous blocks. There was no difference in the results (see fig. 1).
Fig. 1: (A) Distribution of bone cement 1 (B) Distribution of bone cement 2
CONCLUSIONS: This study has shown that extensional flow properties in addition to conventional rheometer data are necessary to completely describe the rheological properties of hydraulic bone cements. Considering only conventional data Cement 1 and 2 would be treated as materials with different properties but they work both equally for injection into porous structures because the extensional flow properties are of greater importance in this application.
 Stading M., Bohlin L. (2000) Measurements of extensional flow properties of semi-solid foods in contraction flow. Proceedings of the 2nd International Symposium on Food Rheology and Structure 2, 117-120
ACKNOWLEDGEMENTS: The authors would like to thank Leif Bohlin for the inspiring and fruitful discussions.