implantaten und Aufbau - BEGO

Weitere Magazine

Empfehlungen

Info

8 Originalia – Materials Science Table 3 Short-circuit current densities in nA/cm 2 Metal pair Implant with Table 2 Potential differences of the various implant/superstructure combinations in mV Metal-pair NaCl solution in both half-cells Implant in NaCl solution and superstructure in lactic acid solution + Superstructure forms the cathode/– Superstructure forms the anode [mV] Implant with lowest value NaCl solution in both half-cells [nA/cm 2 ] lowest value highest value highest value mean mean lowest value NaCl solution and lactic acid solution [nA/cm 2 ] lowest value highest value highest value mean Gold alloy –230 +95 –81 –290 +139 –30 Titanium superstructure –290 +180 –18 –460 –38 –219 CoCrMo alloy +106 +129 +120 –75 +113 +37 mean Gold alloy 7 128 57,8 24 472 177,2 Titanium superstructure 44 160 61,1 10 216 102,4 CoCrMo alloy 1,9 6,1 3,1 3,6 5,4 2,9 wool thread in the bridge tube ensures that the minimum cross-section of approx. 0.5 mm 2 is maintained. The implant was placed in normal NaCl solution with a pH of 6.0. The NaCl solution is intended to imitate the environment in the bone/ skin tissue. Two different solutions were used for the superstructures: firstly, normal NaCl solution and, secondly, the same solution with the addition of lactic acid, which reduced the pH from 6.0 to 2.3. This simulates the oral situation in which the pH is lowered in the plaque by anaerobic degradation of sugars. In a galvanic cell, metal ions dissolve out of the anode, and if there is an electrically conductive connection with the cathode, an electric current is created. The current flowing in the galvanic cell represents a measure of the corrosion. As a large surface permits a stronger electron flow than a small surface, a correlation can be made between current and surface area. The maximum rate of corrosion occurs at maximum current flow, i.e., a short circuit. For this reason, the flow of current, caused by the voltage potential of the galvanic cell, is measured with various load resistances. Fig. 4 shows the equivalent circuit diagram of the test setup with the internal resistance Ri, the load resistance RL and the voltmeter. First, the voltage was measured after 24 hours, after removing RL so that only the internal resistance of the voltmeter (vacuum tube voltmeter of accuracy class 0.5% with 1 volt recorder output, from Colora Messtechnik GmbH, Berlin, Ri > 100 GΩ) applied. Then, after a further 24 hours, further measurements were taken under a load of 3 MΩ and finally, again after 24 hours, under a load of 1 MΩ (metallicfilm resistors of accuracy class 1%, load rating 0.125 watts). Within these periods of time, the voltage values stabilised and were documented with an xt recorder (Siemens potentiometric recorder X-T C1011). Corrosion current measurements can be carried out as voltage measurements at ohmic resistances, as described by Heitz et al. (1990) [2]. However, with this method, short-circuit currents in galvanic cells cannot be determined, because that would require the voltage to be measured at a resistance of 0 Ω, which is of course impossible. Therefore the “short-circuit current” was calculated by linear extrapolation of the voltage-current characteristic for a voltage of 0 volts (short circuit). Although this method assumes an ohmic internal resistance of the galvanic cell (Fig. 4), it still delivers a good approximation for relatively small deviations of the current source from Ohm’s law. An alternative would be to use a potentiostat, with which the internal resistance of the ammeter can be electronically compensated. For measurements with the superstructure in the NaCl/lactic acid solution, the pH in the implant test-tube was influenced by diffusion via the salt bridge. According to the measurement, it was between 4 and 3.5. No change was detected in the pH of the lactic acid solution, As an example, Fig. 5 shows the evaluation of measurements with the implant/CoCr alloy pair. Four pairs were measured for each of the three superstructure materials. Results and discussion The potential differences for the unloaded galvanic cell are shown in Table 2. Positive figures mean that the implant formed the anode, while negative figures indicate that the superstructure formed the anode. The corrosion and also a possible biological effect of a galvanic cell depend on the quantity of metal ions which go into solution per unit of time. This is determined not by the potential difference, but by the short-circuit current, so the calculated short-circuit currents are discussed below on the basis of the figures in Table 3. The lowest current density was measured between the titanium implant and the CoCr superstructure, in both the saline solution and the corrosion solution. Evidently, the pH has no appreciable influence on the corrosion of the CoCr ZWR ̶ Das Deutsche Zahnärzteblatt 2008; 117 (10)
Originalia – Materials Science 9 Fig. 1 Implant with sealed connection point Load resistance Wool thread Salt bridge Superstructure Implant Electrolyte Fig. 3 The half-cells were joined by a salt bridge to form a galvanic cell. A plastic tube with a wool thread passing through it served as the salt bridge. (The tube was constricted in the middle by a thread loop in order to slow down the ion exchange. The connecting tube was filled with the NaCl solution, and when examining the superstructures in corrosion solution, the tube was filled half-and-half with NaCl solution and corrosion solution.) alloy, in contrast to the other superstructure materials. Kulmburg (2006) reports that corrosion currents with the commercially pure titanium/CoCrMoNb pairing are smaller than with the commercially pure titanium/high-gold alloy pairing by a factor of 10 to 100. This is consistent with the results of our measurements [4]. The unexpected potential difference and the flow of current between titanium implant and titanium superstructure are presumably attributable to differences in the microstructure of the metal. The surface of both the superstructure and the implant were sandblasted and therefore were no longer homogeneous because the blasting generates a certain compressive stress in the metal surface [3]. Mechanical stresses can, in principle, change the corrosion behaviour of metals. As a rule, the areas that have been deformed display increased susceptibility to corrosion [1]. A further indication of inhomogeneities is the fact that the titanium superstructures reacted both more nobly and less nobly than the implant surfaces in the NaCl solution at a pH of 6.0. Only at a lower pH, following the addition of lactic acid, did the titanium superstructures react consistently less nobly. The material of the CoCrMo superstructure is far harder than that of the titanium superstructure. Measurements with a Vickers hardness testing device (CV-400DAT, CV-Instruments, Maastricht, Netherlands) produced values of 420 +/– 10 HV and 320 +/– 10 HV under a load of 500 g. It could be that the surface of the CoCrMo superstructure was changed less by the sandblasting due to its greater hardness compared to the softer titanium surface, and therefore the spread of measurements is smaller for this alloy. In terms of the corrosion currents, our investigation showed the titanium implant/CoCrMo superstructure pair to be the most favourable combination. The sandblasted surface of the CoCrMo alloy reacted more nobly than the titanium surface, as was the case in Kulmburg’s investigation using a CoCrMoNb alloy. Only in the case of the lower pH in the NaCl/lactic acid solution did the superstructure react less nobly in one out of four tests. Overall, all the current densities that we measured were considerably lower than those reported by Kulmburg (1986) for the gold alloy/CoCr alloy and gold alloy/NiCr alloy pairings, for which he obtained values of 100 to 1000 nA/cm 2 [5]. They are also lower than the values recorded by Taher and Al Jabab (2003), which were 10 to 30 nA/cm 2 for the titanium/CoCr cast alloy pairing. This may be attributable to the fact that we did not use a cast CoCr alloy, but, according to the manufacturer, a homogeneous, industrially prefabricated alloy [6]. There is a good overall match with the findings of Venugopalan et al. (1998), who reported currents of 1–3 nA/cm 2 for the titanium/ CoCrMo pairing and 66–154 nA/cm 2 for titanium/high-gold alloy [7]. It may seem surprising that the cast gold alloy reacted less nobly than the surface of the titanium implant. According to Kulmburg (2006), a more noble reaction would be expected [4]. He states that commercially pure titanium has a very base potential, which can be raised by around 2400 mV by means of passivation, so falling just short of the values of precious-metal alloys. The decisive factor influencing the potential of the implant surface is thus the development of the passivation layer. According to the implant manufacturer, the implant surface was produced by a special sandblasting process. Evidently, in the case in hand, the passivated titanium surface reacts more nobly than the sandblasted precious-metal surface. As the sandblasting had the effect of increasing the surface area of our samples, it may be assu- Fig. 2 CoCr implant superstructure with sealed connection point Fig. 4 Equivalent circuit diagram for the examined galvanic cell with implant/ superstructure in the test set up used. Uo and Ri simulate the cell; RL is the load resistance. The voltage at RL is measured with the external high-impedance (R>100 GΩ) voltmeter V. ZWR ̶ Das Deutsche Zahnärzteblatt 2008; 117 (10)
Seite 1 und 2: ZWRDas deutsche Zahnärzteblatt ww
Seite 3 und 4: Originalia -- Materials Werkstoffku
Seite 5 und 6: Originalia - Werkstoffkunde 5 Die K
Seite 7: Originalia - Materials Science 7 In
Seite 11 und 12: GO FOR GOLD. GO FOR GOLD. Bionic En

implantaten und Aufbau - BEGO

Sie wollen auch ein ePaper? Erhöhen Sie die Reichweite Ihrer Titel.

Template löschen?

Als Template speichern?