Porcelain veneering of titanium--clinical and technical aspects

Gold and other alloys have long been used for the production of crowns and bridges as replacements for damaged or lost teeth. However, doubts have arisen on the suitability of using these materials for dental restorations, as gold has also shown a capacity to cause side-effects such 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 has led to an interest in using titanium instead of these alloys. Trials to use titanium for this purpose were initiated in Japan in the early 1980s. Titanium as an unalloyed metal differs in two aspects from the above named alloys: it has a phase transformation at 882 degrees C, which changes its outer and inner properties, and it has an expansion that lies between that of the porcelain types available on the market at the time. In Japan a technique for casting titanium was developed, where the after-treatment of the casting was elaborate, to re-establish the original properties of titanium. The porcelain developed for veneering had shortcomings as the rendering produced a rough surface and non satisfactory esthetics. In Sweden a new concept was introduced in 1989. Here the processing of titanium was performed by industrial methods such as milling, spark erosion and laser welding. The idea behind this was to avoid phase transformation. During the 1990s a number of porcelain products were launched and a vast number of both laboratory and clinical studies 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 in Sweden. Twenty-five patients were provided with 40 copings of pure titanium, which were veneered with porcelain. After 2 years 36 of these crowns were evaluated and the patients were also interviewed regarding problems such as shooting pains or difficulties in cleaning around the teeth that were crowned. This evaluation showed generally unchanged values for color, form, surface and fit. Regarding surfaces, one porcelain fracture was registered (3%). The patient responses were positive and no case of sensitivity was reported after 2 years, but in 3 cases food impaction was reported. The second study is a systematic review of published articles on bond strength between titanium and porcelain. The review made comparisons of bond strength using three-point bending tests between different porcelain bonds to different alloys and to titanium, between different brands of porcelain and titanium, with porcelain following various types of processing of the titanium surface, with different compositions of the porcelain and with different firing conditions. Generally it could be seen that with this type of test (three-point bending) the bond strength between porcelain and titanium was lower than with alloys. It was also seen that there are differences in bond strengths between different brands of porcelain, that processing the titanium surface and composition of the porcelain affected bond strength, and that firing conditions were also important. The third study was performed with the intention of examining the firing accuracy of different types of dental furnaces and to investigate how maintenance and quality control is performed at Swedish dental laboratories. Since titanium porcelain is fired at a temperature which is 200 degrees C below that used for most conventional alloys, there are specific demands on the furnaces used. The optimum firing temperature is judged to be 750 degrees C for porcelain veneering of titanium, according to 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 higher than the temperature indicated by the furnace display; in some cases this was very much higher than the temperature displayed. This means a risk for an unwanted augmentation of the oxide layer on the titanium, which could fracture on loading. Regarding maintenance and quality control, interviews performed at 62 laboratories revealed that most of these did not attain the standards expected and claimed. The fourth study was performed with the intention of studying how the bond strength between titanium and porcelain is affected by a temperature increase of 30 degrees C, performed with two firing concepts for titanium porcelain and examined by three-point bending tests. The fractured surfaces were also analyzed with SEM and EDX. These two concepts for titanium porcelain differ in 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 were aged by thermo-cycling. This study has shown that a moderate elevation in the firing temperature does not affect the bond strength in this case. Comparing bond strengths between the two different firing concepts, three-point bending tests showed that the test bodies that had undergone an oxidation firing had significantly higher bond strengths in all but one situation. These results were contradicted by the SEM and EDX analysis. These showed that with oxidation firing the fractures occurred in the brittle (and probably thickened) oxide layer of the titanium, while the fractures occurred in the well integrated interface with titanium oxide and porcelain components when firing without oxidation. The reasons for these contradictory results might be that oxidation firing changed the ductility of the titanium, creating a higher stiffness which could better withstand the deflection of the specimens created during the three-point bending tests. For the same reason it might also be irrelevant to test bond strength between porcelain and metals with differing properties. Considering these results and the results from other studies, the validity of the current test standard for metal-ceramic bond strengths may be questioned.