The surface micromechanical properties of ultrahigh-molecular-weight polyethylene (UHMWPE) are critical in determining the wear, deformation, and fracture in the surface region. These properties have not been accessible to simple mechanical testing on a spatial scale relevant to these mechanical processes until recently. The structural factors associated with surface mechanical properties (crosslinking, oxidation state, local orientation of polymer, crystallinity, etc.) can be highly variable and localized and may vary on micron spatial scales or smaller. Furthermore, time/frequency-dependent behavior of the surface may have an important role in the overall surface mechanical behavior. Recent work has shown the utility of depth sensing microindentation/nanoindentation testing to interrogate local surface mechanical properties. The goal of the present study was to measure the effect of loading rate on the depth-sensing microindentation testing of UHMWPE. Three different UHMWPE materials (Hylamer, a large crystal material; GUR 1020, a standard medical-grade material; and Marathon, a crosslinked material) were tested using a microindentation method at loading rates ranging from 0.01 to 1 Hz. Similarly, a gamma-irradiated in air and 15-year shelf-aged tibial component was tested through its cross-section to assess the variations in mechanical properties with location and to compare the micromechanical profile with the oxidation profile. It was found that rate of testing affected the microhardness of each material, however, only GUR 1020 and Hylamer showed rate-dependent behavior for modulus and energy dissipation factor. Micromechanical profiles through oxidized regions of the tibial component showed a high correlation with the oxidation profile. Increases in modulus, hardness, and energy dissipation factor were seen with increasing oxidation and each property was loading-rate dependent. These results show that depth-sensing microindentation/nanoindentation testing on the micron scale provides highly consistent and reproducible measurements of surface mechanical properties. This scale of testing minimizes the potential variations caused by local heterogeneity in crystallinity, surface orientation, and other submicron structural features.

The release of gentamicin as a function of time was measured for Palacos and two-solution bone cements by using a novel pH technique. The pH of an aqueous solution of gentamicin is a function of the gentamicin concentration and it decreases linearly over concentrations of 0.0-0.1 wt %. Therefore, a new, direct, and inexpensive in vitro technique was developed based on continuous readings of the pH in phosphate-buffered saline (PBS) at 37 degrees C to determine the release kinetics of gentamicin from poly(methyl methacrylate) (PMMA)-based bone cement. In addition, this method was used to compare the release profiles of Palacos R-40 bone cement with a two-solution bone cement developed in our laboratory and loaded with two different concentrations of gentamicin sulfate. Finally, the pH-based method was used to track the elution of gentamicin in both mixed and static conditions to determine the effect of mixing on the diffusion of gentamicin out of the cement. It was found that Palacos R-40 released 4.95 +/- 0.22 wt % of its gentamicin after 24 h in PBS solution. This data compares favorably with previously reported values of gentamicin elution from Palacos R-40, which ranged from 3 to 8 wt % of the total amount of incorporated gentamicin, depending on the size and the surface area of the samples. The results show that Palacos samples released 4.84 +/- 0.27 mg after 24 h, a two-solution cement loaded with an equivalent concentration of gentamicin sulfate released 3.81 +/- 0.52 mg, and two-solution cement loaded with twice the concentration of Palacos released 5.53 +/- 0.26 mg of gentamicin. A higher percentage of release was recorded from Palacos than from the two-solution bone cement, and the effect of PBS mixing conditions on the release kinetics was only significant in the early stages of release and not at 24 h. It was concluded that monitoring the pH is an effective technique to measure gentamicin release from PMMA-based bone cements in PBS solution.

Methyl methacrylate monomer can evaporate from bone cement to reach cytotoxic levels of concentrations in the implant bed of total joint prosthesis. Therefore, this study was performed by using a novel Fourier transform infrared spectroscopy method to quantify the release of monomer vapor from experimental two-solution bone cement in vitro during polymerization, to examine the effect of surface area versus cement mass, and to explore the effect of initiation chemistry. The results revealed that monomer vapor release is a surface phenomenon. In addition, initiation chemistry plays a major role in controlling the reaction time, and therefore heat concentration and dissipation, which resulted in a higher absorbance peak as initiation chemicals concentration increased. It was concluded that using the FTIR to monitor MMA vapors is an effective technique to obtain quantitative information about monomer vapor release from bone cements during polymerization and provides insight on the polymerization kinetics of two-solution acrylic bone cement.