Original study - ZZI 03/2017

A pilot study for evaluating interfaces by OCT: loading of a Resin Nano Ceramic on one-piece ZrO2 implants

Despite utmost care the manual pre-treatment and bonding of the RNC crowns on the implants could be seen as possible error source.

A suspicion that the ZrO2 surface with a mat appearance as a result of CoJet pre-treatment in the SB samples has a reduced reflection and therefore generates a weaker signal even before CS could not be confirmed in previous tests. Both surfaces (SB and non-SB) were reflected in the same way in the OCT.

A further limitation of the study existed in the fact that the OCT depending on the refractive index of the various materials only has a penetration depth of max. 2–2.5 mm. The crowns produced were therefore constructed as thinly as possible in considering the minimum wall thicknesses of ? 1 mm and ? 1.5 mm in the occlusal area. After dynamic chewing simulation a substance abrasion occurred so that the minimum thickness of occlusal areas was even underrun.

Material ZrO

2

Furthermore, the material zirconium oxide possibly limits the significance of OCT due to its refractive index in comparison with other materials. Due to its high refractive index ZrO2 reflects a lot of light (air ? 1, water ? 1.3, composite enamel ? 1.5, natural enamel 1.62, ZrO2 ? 1.9). The signal in the OCT-B-Scan at the appropriate interface must not be per se connected to a gap due to this property of ZrO2 or even be put on the same level as “no composite bonding”. As it is evaluated within the sense of “more pronounced“ after a simulated chewing load, the reflected light must, however, have increased. This can be explained by the surfaces RU+SB/ZrO2 moving apart as a result of the load.

A void is created by the distance increasing between the two surfaces. This void is filled with air, thus representing a gap. As a result of the additional air phase between the interfaces RU+SB and ZrO2 , the reflection is raised at the transition of light between the phases and the signal in the B-Scan is „more pronounced“ (Fig. 10). Why both interfaces were not perceived as two signal lines which are separated from each other may have possibly been due to the fact that the interfaces had not moved far enough apart in order to represent this in the area under review despite the high resolutions with the OCT. It is possible that the distance which the light had to cover in mesial and distal photos in comparison with occlusal photos was too large for a detailed presentation. The occurrence of a second signal line in occlusal B-Scans speaks in favour of that. In CS a direct application of force occlusal can be associated with a higher load. A deviation of the interfaces should therefore be initially viewed occlusal as probable (Fig. 11).

To verify the gap formation scanning electron microscope recordings are made in an ongoing test section (Fig. 12).

Our CS simulated a 5-year clinical application and was able to document that within the limitations of this study no debonding of the RNC crowns was observed during dynamic loading. In order to be able to evaluate the quality of the compound with regard to a completed CoJet pre-treatment, further conventional examinations (pull-out tests) are required.

In consequence thereof a conflictive interpretation of the current limited indication of Lava Ultimate for the treatment of crowns can be derived from the results presented. In this study under review, however, only a load force of 50 N during chewing simulation was laid down, whereas physiological forces of 60–75 N, when chewing in the area of the anterior teeth can arise [4]. A current randomised clinical study in which 40 RNC crowns are adhesively bonded extraoral to ZrO2 abutments and then intraorally screwed on, supports the change of indication despite interrupting the integrity of the crown through the screw canal on the one hand with an identified bonding failure of 80 % within the first year at the SU+RU/ZrO2-interface [13]. On the other hand these clinical data support the first results of our pilot study as the remarkable gap signal not only for SB, but also for non-SB in our examination at the SU+RU/ZrO2-interface and its change following the chewing simulation refers to a possible “weak point” in the bond.

Conclusions

The pilot study presented shows that a non-invasive presentation of changes to RNC interfaces is possible using OCT and that additional visual information can be derived. In addition to the conventional methods of materials testing the OCT is able to supply information on possible weak points in the bond („more pronounced“ signal line, double line). Despite promising values of our examination with a survival rate of 100 % the initial OCT results refer to the need for optimising the adhesive bond from RNC to ZrO2, irrespective of the pre-treatment performed. In particular, as a result of meagre clinical data, there is thus the need for further experimental and clinical examinations with focus on interfaces and suitable material combinations to also clinically benefit from the advantages of RNC restorations on one-piece ZrO2-implants. With OCT a non-invasive procedure is additionally available.

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