The process of acid cleaning of ceramic-veneered restorations has been a standard process in dental technology for decades. Aqueous hydrogen fluoride (hydrofluoric acid, HF) in a concentration of up to 40% has frequently been used. In addition to the known problems connected with the use of dental alloys, questions also arise as to how the dental technician should handle the modern all-ceramic materials in such cases.
Although the view that zirconium dioxide cannot be etched with hydrofluoric acid is very widespread in dental technology, this is a misconception. By using a high concentration of hydrofluoric acid (>40%) together with heat, it is possible to effect selective changes in the material, but this may also result in damage to the surface, adversely affecting strength.
Objects to be acid-cleaned are not infrequently placed in an ultrasonic device on a Friday night, and the device is then allowed to run throughout the entire weekend. This results in intense generation of heat and produces an uncontrolled surface, which often makes it impossible to re-use the framework because of reduced mechanical fatigue strength, particularly in the case of bridge restorations.
Products for use in the dental laboratory developed specifically for the removal of such ceramic veneering residues have been available on the market for years. A well-known and reliable product of this type is Ceramex, manufactured by Renfert, which markedly differs in composition from other products on the market. In addition to about 10% sulfuric acid (H2SO4), Ceramex also contains hydrofluoric acid, but in a concentration of only < 7%.
The object of this study is to determine whether this combination, which has been proven to be effective with metal alloys, specifically causes surface changes with the modern framework material zirconium oxide and would thus significantly affect the mechanical properties and fatigue strength.
10 small plates (3 x 3 cm²) with a thickness of 0.7 mm composed of VITA In-Ceram YZ and 6 small plates composed of NPM (Wirobond 280, BEGO) were produced and then veneered using the accompanying veneering ceramic as indicated in the processing instructions, i.e. VITA VMK Master, VITA VM13 for the NPM framework and VITA VM9 for the Y-TZP framework. The uniform thickness of the veneering ceramic was exactly 1 mm.
In order to test application according to the indications of the manufacturer (Renfert), the ceramics were subjected to mild surface abrasion treatment with 130 µm and at a pressure of 3 bar. Moreover, only a small amount of veneering ceramic was abraded in order to allow better observation of the actual application (in this case etching) and the etching process as well. The abrasion process is necessary in order to provide sufficient surface area for the acid to act upon. Glazed or highly-polished surfaces should be avoided.
After this, the test pieces were stored once in an ultrasonic device (VITASONIC) at a temperature of approx. 30 °C for various periods (15 min. to 24 h). The same process was also conducted – this time without the ultrasonic device – with samples in a Ceramex bath at room temperature. The etching process was subjected to qualitative testing under a scanning electron microscope. Loss was recorded by means of measurement with callipers.
The images (Fig. 1) of VITA VM9 definitely show that the veneering ceramic is clearly etched away by Ceramex. It can also be seen that a longer residence time results in a clear increase in etching depth. Thus an average of about 0.1 mm more veneering ceramic can be etched away in 24 h. Further prolongation of this time would result in even deeper etching.
The comparative images (Fig. 2) of the various veneering ceramics VITA VM9, VITA VM13 and VITA VMK Master clearly show that the veneering ceramics were all etched differently. For example, VITA VM9 is acted on differently by Ceramex than the two metal ceramics VITA VM13 and VITA VMK Master, which are dissolved to a comparable extent. This is caused by the somewhat differing structure of the individual veneering ceramics. Although glass is acted on strongly by hydrofluoric acid, dissolving of the individual crystal structures from the glass matrix of the veneering ceramics works somewhat better with Ceramex.
VITA VM13 and VITA VMK Master show only slightly differing behavior in the etching process. In this case as well, the structure of the ceramic is of decisive importance. For example, VITA VM13 shows highly homogenous fine distribution of the leucite crystals, and VITA VMK Master is characterized by a few agglomerations of the crystals, which then gives the impression that the ceramic is more strongly dissolved. However, no difference could be detected in the measured etching depth.
The respective opaquers were also tested. These are also partially dissolved by Ceramex, although not to the same extent as the veneering ceramics. Sand-blasting is therefore to be preferred in the case of the opaquer. The pressure and grain size to be used should be recommended by the respective alloy manufacturers.
Within the 24 h test phase, no effect and thus no etching process could be detected on the Y-TZP framework or on the alloy (Fig. 3). This was also confirmed by scanning electron microscopy and x-ray diffractometry with a Bruker D8. However, it is not advisable to prolong this duration of use as desired without checking the intermediate Status.
Ceramex, manufactured by Renfert, therefore has been proven to show quite favourable etching of veneering ceramics. The average etching depth within 24 h was 0.1 mm in this test, with the tested metal ceramics showing minimally better Dissolution.
Moreover, it was clearly found that the use of an ultrasonic equipment with a temperature slightly higher than room temperature (30 °C) allows a 1.5-fold etching process. A possible effect on Y-TZP and the NPM used was not observed during this period (24 h).
Although etching of opaquers would be possible in principle, sand-blasting or even removal by blasting of the opaquer layer is more advisable.
Ceramex provides a favourable option for carefully freeing frameworks from their veneering ceramic, and this also applies to the framework material zirconium dioxide (3Y-TZP-(A)). In any case, one should ensure that the veneered frameworks, especially if they have been glazed, have a roughened veneering ceramic surface (e.g. using diamond abrasives or the sand-blasting process). In the case of zirconium dioxide, however, the materials must by no means be abraded or sand-blasted down to the framework; the last residue of the veneering ceramic should be removed in the ultrasonic bath using Ceramex. This prevents transformation of the crystalline framework from a tetragonal to a monocline form, which would cause a decrease in the fatigue strength of the restoration.
Because of the countless variants on the market, when using dental alloys, one should determine compatibility with Ceramex by prior testing in the dental laboratory in order to rule out the possibility of irreparable damage to the restoration. Intermediate checks are recommended after using Ceramex for a maximum of 24 h, regardless of the framework material used.
A study by VITA Zahnfabrik H. Rauter GmbH & Co. KG and Renfert GmbH / Authors: Dr. Michael Tholey, VITA Zahnfabrik and MDT Oliver Bothe, Renfert GmbH