EPR study of free radicals in PMMA bone cement: a feasibility study

Real-time synchronous measurement of curing characteristics and polymerization stress in bone cements with a cantilever-beam based instrument

An instrumentation capable of simultaneously determining degree of conversion (DC), polymerization stress (PS), and polymerization exotherm (PE) in real time was introduced to self-curing bone cements. This comprises the combination of an in situ high-speed near-infrared spectrometer, a cantilever-beam instrument with compliance-variable feature, and a microprobe thermocouple. Two polymethylmethacrylate-based commercial bone cements, containing essentially the same raw materials but differ in their viscosity for orthopedic applications, were used to demonstrate the applicability of the instrumentation. The results show that for both the cements studied the final DC was marginally different, the final PS was different at the low compliance, the peak of the PE was similar, and their polymerization rates were significantly different. Systematic variation of instrumental compliance for testing reveals differences in the characteristics of PS profiles of both the cements. This emphasizes the importance of instrumental compliance in obtaining an accurate understanding of PS evaluation. Finally, the key advantage for the simultaneous measurements is that these polymerization properties can be correlated directly, thus providing higher measurement confidence and enables a more in-depth understanding of the network formation process.

In vitro evaluation of orthopedic composite cytotoxicity: assessing the potential for postsurgical production of hydroxyl radicals

Hydroxyl radical (*OH)-induced inflammation is a primary mode for in vivo cytotoxicity. A legitimate concern is whether particulate wear debris from orthopedic composites can stimulate inflammation via ferrous ion (Fe2+)-mediated production of *OH. The purpose of this research was to utilize electron paramagnetic resonance (EPR) spin trapping in investigating and comparing the potential for postsurgical cytotoxicity induced specifically by *OH in the presence of two composites: Simplex P and the novel, hybrid, CORTOSS. Cytotoxicity is evaluated based on the composites competitively chelating catalytic Fe2+ or readily reducing ferric ions (Fe3+), in facilitating the Fenton reaction (FR). *OH that are produced were then validated by a radical scavenger to confirm a genuine radical signal and mechanism. Spin adduct peak areas decreased in the presence of CORTOSS as opposed to increasing in the presence of Simplex P, evaluated against their respective controls. A plausible theory elucidating this finding is that CORTOSS may sequester the Fe form, by virtue of its monomers. Principally, direct comparison of composites indicated that Simplex P had greater tendency to produce *OH, yielding 25.6 and 48.7% greater spin adduct peak areas when chelated and non-chelated Fe2+ are used, respectively. Moreover, the rate of FR accelerated when chelated Fe2+ was used, leading to the formation of a ternary complex with the composites. This was more prominent in Simplex P, as coordination of chelated Fe2+ occurs on its surface via an electrostatic attraction to allow a seventh coordination site for ligand exchange in the ternary complex, stabilized by Ba2+. Conversely, the silica found in CORTOSS possesses radical quenching abilities that deactivate generated *OH in impeding the efficiency of FR. Neither composite demonstrated a capacity to readily reduce Fe3+ to the relevant Fe2+, as validated by a non-radical pathway. Instead, the artificial spin adduct signal attained when employing chelated Fe3+ was due to the nucleophilic addition of water onto DMPO. Simplex P may also serve as a template for surface catalysis of the nucleophilic addition of water onto DMPO involving chelated Fe3+. CORTOSS is thought not to induce cytotoxicity, whereas the propensity of Simplex P in promoting Fenton chemistry is a serious issue that must be addressed.

Histologic changes after vertebroplasty

BACKGROUND

Vertebroplasty with use of polymethylmethacrylate cement is gaining popularity in the treatment of some specific painful lesions of the spine. It remains unclear, however, what possible side effects this type of cement might have upon the vertebral body. We performed a histologic and radiographic analysis of the end plate and disc to determine whether there was a difference between vertebroplasty with polymethylmethacrylate cement and vertebroplasty with calcium phosphate cement in the surrounding tissue of the goat spine. Furthermore, we assessed whether a defect in the end plate, simulating end-plate fracture and allowing for direct contact of cement with disc tissue, had any effect on end-plate or disc degeneration.

METHODS

Twenty-four mature goats were divided between two follow-up periods (six weeks and six months). All animals underwent a bilateral transpedicular vertebroplasty at two lumbar levels, where one of the following treatments was applied: vertebroplasty with calcium phosphate cement with or without an end-plate defect, and vertebroplasty with polymethylmethacrylate cement with or without an end-plate defect. The effect of the various treatments on the integrity of the intervertebral disc, end plate, and surrounding tissue was examined with semiquantitative histologic analysis and radiography.

RESULTS

No sign of disc or end-plate degeneration was seen in any of the analyzed sections. The mean disc height did not decrease from the postoperative period to the time that the animals were killed in any group, thereby supporting the histologic findings. A mild inflammatory reaction was found in four vertebral bodies in the polymethylmethacrylate groups only.

CONCLUSIONS

Calcium phosphate cement and polymethylmethacrylate cement both seem to be adequate bone-void fillers in terms of biological behavior in the vertebral body.

Fracture and fatigue properties of acrylic bone cement: the effects of mixing method, sterilization treatment, and molecular weight

The purpose of this study was to characterize the relative and combined effects of sterilization, molecular weight, and mixing method on the fracture and fatigue performance of acrylic bone cement. Palacos R brand bone cement powder was sterilized using ethylene oxide gas (EtO) or gamma irradiation. Nonsterile material was used as a control. Molecular weights of the bone-cement powders and cured cements were measured using gel permeation chromatography. Hand and vacuum mixing were employed to mold single edge-notched bend specimens for fracture toughness testing. Molded dog-bone specimens were used for fatigue tests. Electron microscopy was used to study fracture mechanisms. Analysis of variance and Student t-tests were used to compare fracture and fatigue performance between sterilization and mixing groups. Our results indicate that vacuum mixing improved significantly the fracture and fatigue resistance (P<.05, P<.07) over hand mixing in radiation-sterilized and EtO-sterilized groups. In vacuum-mixed cement, the degradation in molecular weight resulting from gamma irradiation decreased fracture resistance significantly when compared with EtO sterilization and control (P<.05). A corresponding decrease in fatigue resistance was observed in the cement that was degraded severely by a radiation dose of 10 MRad (P<.05). In contrast, EtO sterilization did not result in a significantly different fracture resistance when compared with unsterilized controls for vacuum-mixed cement (P>.1). For hand-mixed cement, fracture and fatigue resistance appeared to be independent of sterilization method. This independence is believed to be the result of higher porosity that compromised the mechanical properties and obscures any effect of sterilization. Our results indicate that a combination of nonionizing sterilization and vacuum mixing resulted in the best mechanical performance and is most likely to contribute to enhanced longevity in vivo.

ESR study of MMA polymerization by a peroxide/amine system: bone cement formation

Electron spin resonance (ESR) spectroscopy was used to gain insight at the molecular level into the curing of bone cement. Methyl methacrylate was polymerized using a N,N-dimethyl-p-toluidine (TD)/benzoyl peroxide (BPO) redox system in the presence of polymethyl methacrylate (PMMA) powder. The conventional nine-line ESR spectrum for the growing polymer radical was detected at the gel stage of polymerization. While the optimum free radical concentration was observed near the equimolar amine/BPO concentration, excess amine led to a change in the chemical structure of the trapped radical and inhibited the polymerization process. At a high amine/BPO ratio the nine-line signal disappeared and a three-line nitroxide-based radical appeared. The appearance of this nitroxide signal seems to depend on the amine/BPO molar ratio and on the presence of PMMA. An excess amount of amine with respect to BPO was found to inhibit the polymerization process. When BPO was removed, the system still polymerized but with a longer gelation time and a lower radical concentration. These results demonstrate that trapped free radicals in the bulk polymerization of MMA convert to polymeric peroxides that act as initiators in bone cement. When the accelerator 4-dimethylamino phenethyl alcohol (TDOH) was used, a higher radical concentration was observed in the polymerizing system. TDOH shows potential for being a more effective accelerator than TD for bone cement curing.

Molecular and mechanical property changes during aging of bone cement in vitro and in vivo

The changes in mechanical properties and free radical concentration of curing Simplex P Radiopaque Bone Cement in vivo and in vitro conditions were studied. Samples were prepared so that each in vivo sample that cured and aged in the canine femoral intramedullary cavities had an in vitro counterpart that was cured and aged in a constant-temperature saline bath at 37 degrees C. An electron paramagnetic resonance (EPR) spectrometer was used to measure the growth and decay (curing) of polymerization radicals. The results of EPR measurements showed that the curing (disappearance of free radicals) of in vivo samples takes a much longer time (more than 4 weeks) than in vitro curing (less than 2 weeks). The mechanical tests indicate that, whether aged in vivo or in vitro, the strength increased rapidly for the first 1-2 weeks and then slight increases were seen for up to 6 months.

Free radical decay kinetics in PMMA bone cement

Electron-spin resonance has been used to measure the decay of the concentration of polymerization radicals in polymethyl methacrylate (PMMA) bone cement and the kinetics of the decay determined. Thermal annealing at various temperatures has shown that the logarithm of the concentration of radicals varies linearly with time, but with a nonzero intercept. These results have been analyzed by including both first- and second-order decay processes. The first-order process has an activation energy of 39 +/- 2 kcal/mol and is probably due to a diffusion-limited termination. The second-order process has an activation energy of 36 +/- 5 kcal/mol and is probably due to bimolecular termination.

Acrylic bone cement: in vitro and in vivo property-structure relationship--a selective review

The nature and curing characteristics of acrylic bone cement are presented to give some basic understanding of the key to improving its performance in in vivo clinical use. The use of electron paramagnetic resonance (EPR) spectroscopy in the (polymerization) setting and curing/aging of bone cement under in vitro and in vivo conditions is presented. The current research effort to improve the implant fixation by precoating the prosthesis with acrylic polymers is also discussed.

The effect of initial temperature on free radical decay in PMMA bone cement

The effect of varying the initial temperature of the components of poly(methyl methacrylate) (PMMA) bone cement has been investigated. Electron paramagnetic resonance (EPR) spectroscopy was used to monitor free radical decay during the curing of the cement. Samples cured in saline at 37 degrees C were found to exhibit first-order decay kinetics for the polymerization radicals for approximately one week after mixing. This indicates that the decay did not take place by combination or disproportionation and was probably due to a transfer process. Decreasing the temperature of the bone cement components prior to mixing resulted in smaller decay rates, but still with first-order kinetics. This decrease in decay rate with lower initial temperature may be due to decreased porosity of the cement, possibly due to decreased monomer evaporation. SEM micrographs of the samples were consistent with this change in porosity.

Molecular and macroscopic properties of PMMA bone cement: free-radical generation and temperature change versus mixing ratio

The molecular and macroscopic changes occurring during the polymerization of poly(methyl methacrylate) (PMMA) bone cement have been investigated. Electron paramagnetic resonance (EPR) spectroscopy was used to monitor free-radical generation and this was compared to temperature changes occurring in the cement for various ratios of polymer powder to liquid monomer (P/L ratio) used in the sample preparation. Both the concentration and the characteristic growth time of the free radicals associated with the polymerization of the bone cement depended on the P/L ratio used. Larger P/L ratio resulted in shorter characteristic growth time for the free radicals as well as a shorter time for the occurrence of the peak sample temperature. Smaller P/L ratios gave smaller maximum concentrations of free radicals and larger peak temperatures. These results are explained on the basis of (1) more initiators present at higher P/L ratios resulting in faster polymerization and (2) less initiators and more monomers present at smaller P/L ratios resulting in fewer radicals but more exothermic reactions. The free radicals present in the bone cement due to the manufacturer's sterilization process were found to be proportional to the fraction of powder used in the preparation, indicating negligible monomer loss during sample mixing.