Calcium phosphate fillers have been shown to increase cement osteoconductivity, but have caused drawbacks in cement properties. Hydroxyapatite and Brushite were introduced in an acrylic two-solution cement at varying concentrations. Novel composite bone cements were developed and characterized using rheology, injectability, and mechanical tests. It was hypothesized that the ample swelling time allowed by the premixed two-solution cement would enable thorough dispersion of the additives in the solutions, resulting in no detrimental effects after polymerization. The addition of Hydroxyapatite and Brushite both caused an increase in cement viscosity; however, these cements exhibited high shear-thinning, which facilitated injection. In gel point studies, the composite cements showed no detectable change in gel point time compared to an all-acrylic control cement. Hydroxyapatite and Brushite composite cements were observed to have high mechanical strengths even at high loads of calcium phosphate fillers. These cements showed an average compressive strength of 85 MPa and flexural strength of 65 MPa. A calcium phosphate-containing cement exhibiting a combination of high viscosity, pseudoplasticity and high mechanical strength can provide the essential bioactivity factor for osseointegration without sacrificing load-bearing capability.

Powder-liquid poly (methyl methacrylate) (PMMA) bone cements are widely utilized for augmentation of bone fractures and fixation of orthopedic implants. These cements typically have an abundance of beneficial qualities, however their lack of bioactivity allows for continued development. To enhance osseointegration and bioactivity, calcium phosphate cements prepared with hydroxyapatite, brushite or tricalcium phosphates have been introduced with rather unsuccessful results due to increased cement viscosity, poor handling and reduced mechanical performance. This has limited the use of such cements in applications requiring delivery through small cannulas and in load bearing. The goal of this study is to design an alternative cement system that can better accommodate calcium-phosphate additives while preserving cement rheological properties and performance. In the present work, a number of brushite-filled two-solution bone cements were prepared and characterized by studying their complex viscosity-versus-test frequency, extrusion stress, clumping tendency during injection through a syringe, extent of fill of a machined void in cortical bone analog specimens, and compressive strength. The addition of brushite into the two-solution cement formulations investigated did not affect the pseudoplastic behavior and handling properties of the materials as demonstrated by rheological experiments. Extrusion stress was observed to vary with brushite concentration with values lower or in the range of control PMMA-based cements. The materials were observed to completely fill pre-formed voids in bone analog specimens. Cement compressive strength was observed to decrease with increasing concentration of fillers; however, the materials exhibited high enough strength for consideration in load bearing applications. The results indicated that partially substituting the PMMA phase of the two-solution cement with brushite at a 40% by mass concentration provided the best combination of the properties investigated. This alternative material may find applications in systems requiring highly injectable and viscous cements such as in the treatment of spinal fractures and bone defects.

Introduction: Acrylic bone cements are used in a variety of orthopedic applications including vertebral decompression and implant augmentation. However, they are bioinert with no ability to integrate with bone. We hypothesize that correction of this drawback can be achieved through the addition of the calcium phosphate (CaP), Brushite, providing a porous scaffold for bone interdigitation while retaining desirable mechanical, rheological, and handling properties. Methods: Composite two solution cement samples were prepared by Brushite addition yielding a pre‐mixed cement with up to 75% of the polymer replaced. Viscosity was assessed using frequency sweeps on non‐setting cements with a DHR‐3 Rheometer. Compression tests followed the ASTM F451 standard using a Bionix MTS 370. To study injectability, a pneumatic gun was used to extrude 1mL of cement through the system and the time measured. Results: CaP addition was shown to be feasible with no deleterious effect on cement mechanical properties. Rheology showed that the variation in viscosities between samples is not significant (p<0.05) and pseudoplasticity was sustained. Injectability of the cement formulations varied considerably, while some showed “plugging” after the cartridge was emptied. Significance: The versatility of the pre‐mixed two solution bone cement allows for the incorporation of CaP fillers thereby enhancing interdigitation with bone. Grant Funding Source: Supported by The University of Texas System, UT Transform Award (Dr. Danieli Rodrigues)