Porcelain veneering of titanium : clinical and technical aspects
Abstract: Gold and other alloys have since long been used for production of crowns and bridges as replacement for damaged or lost teeth. However, doubts have been arisen on the suitability of using these materials for dental restorations, since also gold has shown capacity to cause side-effects as allergic reactions. This is especially valid for alloys which during the last decades have been used as porcelain- fused-to metal restorations. This fact created an interest in using titanium instead of these alloys. Early in the eighties trials to use titanium for this purpose were started in Japan. Titanium as an unalloyed metal differs in two aspects from mentioned alloys: it has a phase transformation at 882° C which changes its outer and inner properties, and furthermore, has an expansion located in between what the on the market available types of porcelain had. In Japan a technique for casting titanium was developed, where the after- treatment of the casting was elaborate, in order to re-establish the original properties of titanium. The porcelain developed for veneering had shortcomings as rendering a rough surface and non satisfactory esthetics. In Sweden a new concept was introduced in 1989, where the processing of titanium was performed by industrial methods as milling, spark erosion and laser welding. The idea with this was to avoid the phase transformation. During the nineties also a number of porcelain products were launched and a vast number of studies, laboratory and clinical were performed and published, with varying results. In the first study of this thesis a prospective clinical trial was performed at a Public Dental Health Clinic, Sweden. Twenty-five patients were provided with 40 copings of pure titanium, which were veneered with porcelain. After 2 years 36 of these crowns could be evaluated and the patients were also interviewed regarding problems like shooting pain or difficulties to cleanse around the teeth which were crowned. This evaluation showed generally unchanged values for color, form, surface and fit. Regarding surfaces, 1 porcelain fracture was registered (3%). The patients´ response was positive and no case of sensitivity was at hand after 2 years, but in 3 cases food impaction was at hand. The second study is a systematic review of published articles on bond strength between titanium and porcelain. The review has made comparisons of bond strength at three-point bending test between different porcelain bond to different alloys and to titanium, between different brands of porcelain and titanium, with porcelain after various types of processing the titanium surface, with different compositions of the porcelain and at different firing conditions. Generally could be stated that with this type of test, three-point bending, the bond strength between titanium and porcelain is lower than with alloys. It was also stated that there are differences on bond strength between different brands of porcelain, that processing the titanium surface and composition of the porcelain affects bond strength, as also firing conditions do. The third study has the intention to enlighten the firing accuracy with different types of dental furnaces, and how maintenance and quality control is performed at Swedish dental laboratories. Since titanium porcelain is fired at a temperature which is 200° C below what is used with most conventional alloys, specific demands are put upon the furnaces used. The optimum firing temperature is judged to be 750° C for porcelain veneering of titanium, according published studies. In this study the real firing temperature at the holding period of 1 minute was recorded by a thermo element connected to a digital temperature measurement apparatus. The accuracy of tested furnaces demonstrated a wide variation, and in almost all cases the real temperature was above what the furnace display indicated, even high above in some cases. This means a risk for an unwanted augmentation of the oxide layer on the titanium which could fracture at loading. Regarding maintenance and quality control on interview with 62 laboratories, most of these did not attain the standard which could be expected and claimed. The fourth study has the intention to study how the bond strength between titanium and porcelain is affected by a temperature raise of 30° C, performed with two firing concepts for titanium porcelain, in a three- point bending test. The fractured surfaces were also analyzed with SEM and EDX. These two concepts for titanium porcelain implies that one has an oxide firing of the titanium metal as the first firing step, while the other is fired with a bonding agent as the first step in the firing procedure. Furthermore, half of the test bodies are aged by thermo-cycling. The study has shown that a moderate raise of the firing temperature does not affect the bond strength in this case. Comparing bond strength between the two different firing concepts, the three- point bending test showed that the test bodies which had undergone an oxidation firing had significantly higher bond strength, in all but one situation. Though, these results are contradicted by the SEM and EDX analysis. These showed that at oxidation firing, the fractures occurred in the brittle and probably thickened oxide layer of the titanium, while when firing without oxidation, the fractures occurred in a well integrated interface with titanium oxide and porcelain components. The reasons for these contradictory results might be that the oxidation firing changed the ductility of the titanium, creating a higher stiffness which better could withstand the deflection of the specimens created at the three-point bending test. For the same reason it might also be irrelevant to test bond strength between porcelain and metals with differing stiffness. Considering this, and results from other studies, the validity of the test standard for metal- ceramic bond strength today might be questioned.
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