Abstract: Most studies investigating human lumbar vertebral trabecular bone (HVTB) mechanical property-density relationships have presented results for the superior-inferior (SI), or “ on-axis” direction. Equivalent, directly measured data from mechanical testing in the transverse (TR) direction are sparse and quantitative computed tomography (QCT) density-dependent variations in the anisotropy ratio of HVTB have not been adequately studied. The current study aimed to investigate the dependence of HVTB mechanical anisotropy ratio on QCT density by quantifying the empirical relationships between QCT-based apparent density of HVTB and its apparent compressive mechanical propertieselastic modulus (E-app), yield strength (sigma(y)), and yield strain (epsilon(y))-in the SI and TR directions for future clinical QCT-based continuum finite element modeling of HVTB. A total of 51 cylindrical cores (33 axial and 18 transverse) were extracted from four L1 human lumbar cadaveric vertebrae. Intact vertebrae were scanned in a clinical resolution computed tomography (CT) scanner prior to specimen extraction to obtain QCT density, rho(CT). Additionally, physically measured apparent density, computed as ash weight over wet, bulk volume, rho(app), showed significant correlation with rho(CT) [rho(CT) = 1.0568 x rho(app), r = 0.86]. Specimens were compression tested at room temperature using the Zetos bone loading and bioreactor system. Apparent elastic modulus (E-app) and yield strength (sigma(y)) were linearly related to the rho(CT) in the axial direction [E-SI = 1493.8 x (rho(CT)), r = 0.77, p < 0.01; sigma(Y,SI) = 6.9 x (rho(CT)) = 0.13, r = 0.76, p < 0.01] while a power-law relation provided the best fit in the transverse direction [E-TR 3349.1 x (rho(CT))(1.94), r = 0.89, p < 0.01; sigma(Y,TR) 18.81 x (rho(CT)) 1.83, r = 0.83, p < 0.01]. No significant correlation was found between epsilon(y) and rho(CT) in either direction. E-app and sigma(y) in the axial direction were larger compared to the transverse direction by a factor of 3.2 and 2.3, respectively, on average. Furthermore, the degree of anisotropy decreased with increasing density. Comparatively, epsilon(y) exhibited only a mild, but statistically significant anisotropy: transverse strains were larger than those in the axial direction by 30%, on average. Ability to map apparent mechanical properties in the transverse direction, in addition to the axial direction, from CT-based densitometric measures allows incorporation of transverse properties in finite element models based on clinical CT data, partially offsetting the inability of continuum models to accurately represent trabecular architectural variations.

Abstract: Although the architectural design parameters of 3D-printed polymer-based scaffolds-porosity, height-to-diameter (H/D) ratio and pore size-are significant determinants of their mechanical integrity, their impact has not been explicitly discussed when reporting bulk mechanical properties. Controlled architectures were designed by systematically varying porosity (30-75%, H/D ratio (0.5-2.0) and pore size (0.25-1.0 mm) and fabricated using fused filament fabrication technique. The influence of the three parameters on compressive mechanical properties-apparent elastic modulus E-app, bulk yield stress sigma(y) and yield strain epsilon(y)-were investigated through a multiple linear regression analysis. H/D ratio and porosity exhibited strong influence on the mechanical behavior, resulting in variations in mean E-app of 60% and 95%, respectively. sigma(y) was comparatively less sensitive to H/D ratio over the range investigated in this study, with 15% variation in mean values. In contrast, porosity resulted in almost 100% variation in mean sigma(y) values. Pore size was not a significant factor for mechanical behavior, although it is a critical factor in the biological behavior of the scaffolds. Quantifying the influence of porosity, H/D ratio and pore size on bench-top tested bulk mechanical properties can help optimize the development of bone scaffolds from a biomechanical perspective.

Abstract: Acrylic bone cement is widely used to anchor orthopedic implants to bone and mechanical failure of the cement mantle surrounding an implant can contribute to aseptic loosening. In an effort to enhance the mechanical properties of bone cement, a variety of nanoparticles and fibers can be incorporated into the cement matrix. Mesoporous silica nanoparticles (MSNs) are a class of particles that display high potential for use as reinforcement within bone cement. Therefore, the purpose of this study was to quantify the impact of modifying an acrylic cement with various low-loadings of mesoporous silica. Three types of MSNs (one plain variety and two modified with functional groups) at two loading ratios (0.1 and 0.2 wt/wt) were incorporated into a commercially available bone cement. The mechanical properties were characterized using four-point bending, microindentation and nanoindentation (static, stress relaxation, and creep) while material properties were assessed through dynamic mechanical analysis, differential scanning calorimetry, thermogravimetric analysis, FTIR spectroscopy, and scanning electron microscopy. Four-point flexural testing and nanoindentation revealed minimal impact on the properties of the cements, except for several changes in the nano-level static mechanical properties. Conversely, microindentation testing demonstrated that the addition of MSNs significantly increased the microhardness. The stress relaxation and creep properties of the cements measured with nanoindentation displayed no effect resulting from the addition of MSNs. The measured material properties were consistent among all cements. Analysis of scanning electron micrographs images revealed that surface functionalization enhanced particle dispersion within the cement matrix and resulted in fewer particle agglomerates. These results suggest that the loading ratios of mesoporous silica used in this study were not an effective reinforcement material. Future work should be conducted to determine the impact of higher MSN loading ratios and alternative functional groups. (C) 2014 Elsevier Ltd. All rights reserved.

Abstract: Prosthetic joint infection is one of the most serious complications that can lead to failure of a total joint replacement. Recently, the rise of multidrug resistant bacteria has substantially reduced the efficacy of antibiotics that are typically incorporated into acrylic bone cement. Silver nanoparticles (AgNPs) are an attractive alternative to traditional antibiotics resulting from their broad-spectrum antimicrobial activity and low bacterial resistance. The purpose of this study, therefore, was to incorporate metallic silver nanoparticles into acrylic bone cement and quantify the effects on the cement's mechanical, material and antimicrobial properties. AgNPs at three loading ratios (025, 0.5, and 1.0% wt/wt) were incorporated into a commercial bone cement using a probe sonication technique. The resulting cements demonstrated mechanical and material properties that were not substantially different from the standard cement. Testing against Staphylococcus aureus and Staphylococcus epidermidis using Kirby-Bauer and time-kill assays demonstrated no antimicrobial activity against planktonic bacteria. In contrast, cements modified with AgNPs significantly reduced biofilm formation on the surface of the cement. These results indicate that AgNP-loaded cement is of high potential for use in primary arthroplasty where prevention of bacterial surface colonization is vital. (C) 2014 Elsevier B.V. All rights reserved.

Abstract: Soluble particulate fillers can be incorporated into antibiotic-loaded acrylic bone cement in an effort to enhance antibiotic elution. Xylitol is a material that shows potential for use as a filler due to its high solubility and potential to inhibit biofilm formation. The objective of this work, therefore, was to investigate the usage of low concentrations of xylitol in a gentamicin-loaded cement. Five different cements were prepared with various xylitol loadings (0, 1, 2.5, 5 or 10 g) per cement unit, and the resulting impact on the mechanical properties, cumulative antibiotic release, biofilm inhibition, and thermal characteristics were quantified. Xylitol significantly increased cement porosity and a sustained increase in gentamicin elution was observed in all samples containing xylitol with a maximum cumulative release of 41.3%. Xylitol had no significant inhibitory effect on biofilm formation. All measured mechanical properties tended to decrease with increasing xylitol concentration; however, these effects were not always significant. Polymerization characteristics were consistent among all groups with no significant differences found. The results from this study indicate that xylitol-modified bone cement may not be appropriate for implant fixation but could be used in instances where sustained, increased antibiotic elution is warranted, such as in cement spacers or beads. (C) 2014 Elsevier B.V. All rights reserved.

Abstract: The purpose of this study was to compare computed tomography density (rho(CT)) obtained using typical clinical computed tomography scan parameters to ash density (rho(ash)), for the prediction of densities of femoral head trabecular bone from hip fracture patients. An experimental study was conducted to investigate the relationships between rho(ash) and rho(CT) and between each of these densities and rho(bulk) and rho(dry). Seven human femoral heads from hip fracture patients were computed tomography-scanned ex vivo, and 76 cylindrical trabecular bone specimens were collected. Computed tomography density was computed from computed tomography images by using a calibration Hounsfield units-based equation, whereas rho(bulk), rho(dry) and rho(ash) were determined experimentally. A large variation was found in the mean Hounsfield units of the bone cores (HUcore) with a constant bias from rho(CT) to rho(ash) of 42.5 mg/cm(3). Computed tomography and ash densities were linearly correlated (R-2 = 0.55, p < 0.001). It was demonstrated that rho(ash) provided a good estimate of rho(bulk) (R-2 = 0.78, p < 0.001) and is a strong predictor of rho(dry) (R-2 = 0.99, p < 0.001). In addition, the rho(CT) was linearly related to rho(bulk) (R-2 = 0.43, p < 0.001) and rho(dry) (R-2 = 0.56, p < 0.001). In conclusion, mineral density was an appropriate predictor of rho(bulk) and rho(dry), and rho(CT) was not a surrogate for rho(ash). There were linear relationships between rho(CT) and physical densities; however, following the experimental protocols of this study to determine rho(CT), considerable scatter was present in the rho(CT) relationships.