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.

Formation of radicals by mechanical processes

The application of e.s.r. spectroscopy to the study of radicals formed by mechanical means (grinding, shearing, bending, cutting, etc.) is discussed. Two mechanisms are considered, one being the homolytic breaking of main-chain bonds in polymers, and the other being charge-transfer (tribo-electric effect). The former is thought to occur for high polymers whilst the latter may be important for certain inorganic materials especially ionic solids. Specific examples are given for the generation of radicals by cutting finger nails and by grinding bone. In both cases, well-defined e.s.r. spectra were obtained for which identifications are offered. Possible implications of the mechanical formation of radicals in bone are considered.

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.

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.

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

In order to determine the feasibility of using electron paramagnetic resonance (EPR) to monitor the molecular processes occurring in polymethylmethacrylate (PMMA) bone cement, a preliminary study has been conducted on the bone cement in vitro. It has been found that a sufficient concentration of free radicals is generated in the bone cement during the polymerization process so that both the polymerization and the curing can be monitored by EPR. The results of these measurements on both radiolucent and radiopaque Surgical SimplexR P bone cement are presented. It has also been found that free radicals are present in the radiopaque bone cement powder as received from the manufacturer. These radicals which apparently are produced in the radiation sterilization process have been found to be unaffected by the polymerization/curing process and to be stable for temperatures below about 100 degrees C. It has clearly been shown that EPR can used to monitor the molecular processes of bone cement in vitro. It is hoped that this technique can be utilized to monitor the in vivo ageing process of bone cement used to fix orthopedic implants.