Μελέτη του πολυμερισμού του οστικού τσιμέντου με την μέθοδο του πυρηνικού μαγνητικού συντονισμού

Abstract

There are still many questions about the polymerization of the bone cement (PMMA), which constitutes the second crucial factor for the strength of the PMMA and particularly about the time of its completion, when acquires its full strength. In the existing studies about the strength of PMMA there is unanimity concerning the general principle that the PMMA acquires its final strength gradually. The differences that exist refer only to the period of time that this happens. In the oldest study it is referred that the PMMA acquires the 80% of its strength 15min after it’s mixing and the rest 20% in 18h.85 However, in the other studies that followed, the time above increased: there is increase of the strength in compression and tension after 40 days, 145 greatest strength in tension after 4 months, 142 greatest strength in bending after 12 months, 3 greatest fracture toughness from tension after 2 months, 46 increase of strength in bending with consequent increase of molecular weight after 2 weeks, 6 and finally greatest strength in compression in 3 months.110 From the above it is concluded that a further examination of the polymerization of the material is necessary.The PMMA follows a chain polymerization with the formation and maintenance of free radicals. It is particularly active in the first stage and decreases gradually. More specifically the decrease of free radicals, which is indicative of the polymerization progress, is linear logarithmically with time as it was shown by the only study with EPR (Electron Paramagnetic Resonance) about the Simplex PMMA.142 This happens because both the solid environment and the continuously greater length of chains make them hard to move in order to be connected with other free-radicals, whereas additives entrap some of them.The nuclear magnetic resonance that was used for the study of CMW-1 Radiopaque bone cement polymerization has been used for the study of polymerization of other polymers for many years.14Our material was mixed with the common manual way and with the use of the Mitab vacuum device, which has been proved to decrease the porous and to increase the strength of the material.54,88 With the magnetic resonance it is recorded the resultant of the particularly complicated movements of protons in the mass of the polymerized bone cement. These movements decrease with the progress of the PMMA polymerization because of the continuous increase of the length of the molecular chain, having as a result the registered sign (time T2 short) to become continually weaker. The smallest rate of this time at which it was fixed, was registered in 400h-1000h after the PMMA mixing in both cases, with or without vacuum. At this period of time the polymerization of the material is considered that it is completed. These results are in accordance with the relevant study about EPR for the Simplex Radiopaque bone cement, where the complete elimination of free radicals and the completion of polymerization happened after 400h from mixing.142 The second component which was observed in our experiment could be justified from the presence of free radicals in the propolymerized powder of PMMA, which are formed during the sterilization of the material and are considered fixed, not participating in the polymerization.168In conclusion, we could argue that the bone cement polymerization of our study is completed at least 1000h after its mixing, when it is expected to acquire the greatest degree of its final strength. Therefore we should treat with enough skepticism the initial - before full weight bearing- mobilization of patients who underwent joint arthroplasty with bone cement, because of the decreased strength of PMMA at this period of time and its increased fracture danger.In addition, the method of vacuum mixing which is beneficial for the mechanical properties of the material seems to affect the rate of the PMMA polymerization at least for the first 72 hours, as the statistical analysis showed a statistical significant difference when the vacuum mixing compared with the common manual mixing. This statistical significant difference is vanished when the PMMA polymerization is completed.