129. Mécanismes sous-tendant l’injectabilitée limitée des ciments phospho-calciques: étude de séparation des particules - Mechanisms underlying the limited injectability of hydraulic calcium phosphate paste: part II particle separation study

M. Habib, F. Gitzhofer, M. Bohner, G. Baroud (Sherbrooke, Canada)

INTRODUCTION: Calcium-phosphate cements are of great interest for the bone augmentation procedure. In it, the hydraulic calcium-phosphate paste is injected through a small bore needle into the bone. The injectablity of these pastes is relatively poor resulting into a non-uniformly extruded paste [1]. The earlier studies [1,2], show that it is the phase separation in that the liquid of the suspension separates under pressure leading to a compaction of the powder inside the delivery syringe causing a halt of the injection process. However, it is unclear whether or not particle separation contributes to the poor injectabilty of such pastes. It is hypothesized that fine particle may behave like a liquid and thus separating under the injection, leaving larger particles behind.

METHODS: β-TCP as a model powder was used to investigate the behaviour of calcium phosphate paste on injection. The particle size and distribution (PSD) of the extrudate has been measured over the course of the injection (four samples, with 30 sec as an interval between each sample) by using a laser diffraction granulometer and further inspected using scanning electron microscopy (SEM). Four protocols were followed to measure the PSD of the samples. Water was used as a dispersant in protocols no.1 and 2, and the ultrasonic vibration was applied for 1 and 0.5 min respectively. In protocols 3 and 4 the isoprobanol was used as a dispersant and the ultrasonic vibration was applied for 1 and 0.5 min respectively. After choosing the most reliable conditions for PSD measurement (protocol no.1), a factorial design with two factors, two levels each [(A) LPR (40% or 50%), and (B) location (inside Syringe or outside it)] was implemented to observe the responses d0.1, d0.5 and d0.9.
RESULTS: The statistical analyses show that, the effect of the first factor (LPR) and the sample location (Factor B) are not significant on the responses D0.1, D0.5 and D0.9 in the volume and number distributions measurements as all the « Prob > F » values are greater than 0.01 [Table 1]. More specifically, there is no significant difference in the particles sizes D0.1, D0.5 and D0.9 of what is remained inside syringe and the extrudate samples. Again, there is no interaction between the two factors (AB). There is no evidence of particle separation based on the SEM images.
Table 1 ANOVA for the selected Factorial design according the size distribution in volume percent for the three responses

: D0.1 D0.5 D0.9
Source F P F P F P
A 0.48 0.5064 0.65 0.4429 0.026 0.8751
B 4.35 0.0704 0.17 0.6877 2.39 0.1604
AB 1.18 0.3086 0.66 0.4396 0.24 0.6344
Fig. 1 SEM images of the samples collected from inside syringe and from out side syringe (position no.1).

CONCLUSIONS: There is no doubt that there is a liquid phase separation associated with the injectability of CaP pastes. This study shows that this liquid phase separation do not cause a fine particle separation while injection. The particle size and distribution measurements of the irregular shape powder are to some extent tricky. No fine particle separation was detected in both laser diffraction and SEM analysis.

REFERENCES: (1) M. Habib et al Acta Biomaterialia 4 (2008), pp. 1465-1471.
(2) M. Bohner, G. Baroud, Biomaterials 26 (2005) 1553-1563.

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