The Effects of Calcium Phosphate Bone Cement Preparation Parameters on Injectability and Compressive Strength for Minimally Invasive Surgery

Abstract

Compared with biocompatibility, osteoconductivity, and mechanical properties, the poor injectability of calcium phosphate bone cements (CPCs) is always ignored, which actually hinders the development of CPC clinical transfer in minimally invasive orthopedic surgeries. Moreover, currently, CPC preparation in the clinic is labor-intensive and requires well-trained technicists, which might also result in the unstable quality of CPCs. In this work, we focused on three research objectives: (i) introducing a standardized preparation method for CPCs; (ii) studying the effects of preparation parameters on CPC injectability and compressive strength; and (iii) studying the injecting condition effects on CPC injectability, aiming to overcome CPCs' disadvantages in minimally invasive surgeries. Firstly, two strategies, named "variable mixing barrel control (VMBC)" and the "nested blade-baffle stirring rod (NBBSR)", were proposed in this study to solve the problems in the preparation of CPCs, which involved blending CPC powder and an agent to generate a paste, by enhancing the mixing performance and mimicking human manual stirring actions. Secondly, although the grinding parameter could significantly generate differences in the microstructure of CPCs, the compressive strength remained relatively stable. However, it was found to significantly affect the injectability of CPCs, leading to the inefficient injection of CPCs. Finally, the effects of syringe design, dimensions, and injecting conditions on CPC injectability were studied, and the results showed that the optimization of these factors enables the injection of CPCs, which has otherwise always been infeasible to implement in minimally invasive orthopedic surgeries.

Keywords: calcium phosphate bone cements (CPCs); injectability; microstructure; minimally invasive surgery; preparation procedure.

Figure 1

CPC forming process and rheological behavior: (A) CPC preparation process with four stages of molding. (B) CPC viscosity with shear rate. (C) Energy storage modulus (G′) and loss modulus (G″) of CPCs.

Figure 2

CPC preparation in a syringe barrel (similar to PMMA bone cements). Three typical phenomena of failed CPC preparation in testing: (i) unchurned CPC adhering to the barrel wall; (ii) pasty CPC adhering to the stirring rod; (iii) unfixed lumpy CPC clusters. The dotted-line boxes and arrows depict the uncontrollable material clusters in the barrel.

Figure 3

Simulation of the VMBC design for CPC mixing. (A) Validation of the VBMC Comsol simulation mechanism was conducted to analyze the effects using a built 2D syringe barrel and stirring rod model. The setting boundary condition and mesh are also shown here. (B) The secondary flow vortex and rotating vortex field in the cross-section of the barrel with a stirring rod at the high-volume VBMC condition. The streamline plot shows the combination of fluid flow velocity in the x and z directions. The background contour plot is associated with the pressure distribution, where the unit is Pascals (Pa). The right plot shows the rotating vortex in the three-dimensional simulation model. (C) The secondary flow vortex and rotating vortex field in the cross-section of the barrel with a stirring rod at the low-volume VBMC condition. The streamline is the combination of the fluid flow velocity in the x and z directions. The background contour is associated with the pressure distribution, and the unit is Pascals (Pa). The right plot shows the rotating vortex in the three-dimensional simulation model.

Figure 4

VMBC and NBBSR designs and the fabricated instrument. (A) Schematics and a real photograph of the proposed instrument based on the VMBC and NBBSR designs. (B) Manual preparation of CPCs by well-trained technicists included two main steps in the whole procedure: uniform mixing and cyclic grinding. The right images show the mimicking mixing and grinding actions using the proposed instrument. The direction of the arrow represents the direction in which the cement is mixed and ground by the tool.

Figure 5

Study of CPC mixing quality between skillful technicists. (A) CPCs prepared by skillful technicists. (i) Sampling lines 1 and 2 of CPCs. (ii) The analyzed RGB intensity of points on sampling line 1. (iii) The analyzed RGB intensity of points on sampling line 2. (B) CPCs prepared by our proposed prototype instrument. (i) Sampling lines 3 and 4 of CPCs. (ii) The analyzed RGB intensity of points on sampling line 3. (iii) The analyzed RGB intensity of points on sampling line 4.

Figure 6

Effects of grinding performance on CPC mechanical properties. (A) Anti-collapsibility of CPCs in deionized water. (B) Initial setting time tests. (C) Final setting time tests. (D) Compressive strength tests (ns, no statistical difference; *, p < 0.05). (E) SEM micrographs of CPC cross-sections after setting for 3 days. The picture frames highlight the large particles and voids that are created when the bone cement is not adequately mixed.

Figure 7

Effects of objective conditions on CPC injectability. (A) Customized syringe used in the tests. (B) Test scheme of CPC injectability. (C) The plot of the injecting force recorded in the process of the injecting test. (The box indicates the value of the injecting force taken by the cement for smooth injection.) (D) Effects of the syringe outlet length on CPC injectability. (E) Effects of the syringe outlet diameter on CPC injectability. (F) Effects of the injecting speed on CPC injectability. (G) Effects of ambient temperature on CPC injectability. (H) Effects of material on CPC injectability (ns, no statistical difference; *, p < 0.05; **, p < 0.01; ***, p < 0.001).

Figure 8

Effects of grinding performance on CPC injectability. (A) Wrapped solid microparticles in CPC pastes prepared by the PLA blade. (B) Blades with different widths. (C) Effects of blade width on CPC injectability (ns, no statistical difference; *, p < 0.05; **, p < 0.01; ***, p < 0.001). (D) SEM micrographs of CPC cross-sections prepared using different wide blades and open vessels. The picture frames highlight the large particles and voids that are created when the bone cement is not adequately mixed.