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.
Acrylic bone cements, although successful in the field of orthopedics, suffer from a lack of bioactivity, not truly integrating with surrounding bone. Bioactive fixation is expected to enhance cement performance because of the natural interlocking and bonding with bone, which can improve the augmentative potential of the material in applications such as vertebroplasty (VP). In a recent study, two composite cements (PMMA-hydroxyapatite and PMMA-brushite) showed promising results demonstrating no deterioration in rheological and mechanical properties after CaP filler addition. In this study, the dynamic properties of the cements were investigated in vitro and in vivo. The hypothesis was that these composite cements will provide osseointegration around the implanted cement and increase new bone formation, thus decreasing the risk of bone structural failure. The effects of CaP elution were thus analyzed in vitro using these cements. Mass-loss, pore formation, and mechanical changes were tracked after cement immersion in Hank’s salt solution. PMMA-brushite was the only cement with a significant mass loss; however it showed low bulk porosity. Surface porosity increases were observed in both composite cements. Mechanical properties were maintained after cement immersion. In vitro culture studies tested preosteoblast cell viability and differentiation on the cement surface. Cell viability was demonstrated with MTT assay and confirmed on the cement surface. ALP assays showed no inhibition of osteoblast differentiation on the cement surface. In vivo experiments were performed using a rat tibiae model to demonstrate bone ingrowth around the implanted cements. Critical size defects were created and then filled with the cements. The animal studies showed no loss in mechanical strength after implantation and increased bone ingrowth around the composite cements. In summary, the composite cements provided bioactivity without sacrificing mechanical strength.
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