Original study - ZZI 03/2017

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

Optical Coherence Tomography (OCT) is a non-invasive procedure and allows microstructures to be displayed on and in organic and inorganic material. (Fig. 1). In addition to current investigations on diagnostics and on the evaluation of carious lesions [14], above all examinations of interfaces at composite fillings confirm the great potential of this approach in evaluating restorations [10, 11]. The OCT uses light in the near-infrared wavelength range. Due to the attenuation of light in the sample by dispersion, reflection and absorption depending on the refractive index of the materials, structures can be displayed up to a maximum depth of approx. 2–2.5 mm. These signal-emitting structures (signal lines) become visible at interfaces of various different phases, e.g. in the case of inhomogeneous materials, such as air inclusions or gaps which might indicate an impaired bond.

In order to produce a permanent and stable bond with an adhesive bonding material, a tribochemical pre-treatment of the zirconium oxide surfaces is advantageous [3, 16]. At the same time such a pre-treatment in the case of some implant manufacturers under certain circumstances does lead to forfeiture of the guarantee. To date non-invasive information as to what the interface looks like between the implant surface, bonding material and restoration is lacking and whether changes under chewing simulation are identified. If applicable, additional OCT information will permit a premature indication of possible weak points in the bond.

This study aimed at identifying changes to interfaces prior to and after a dynamic impact in the case of adhesively bonded Resin Nano Ceramic (RNC) crowns on one-piece ZrO2 implants by means of optical coherence tomography (OCT).

Material and methods

A total of 20 zirconium dioxide (ZrO2, Y-TZP) test implants were produced (Fig. 2). The basic form of the test implants was lathed (workshop for fine mechanics, Faculty for Physics and Geosciences, University of Leipzig, Germany) from the presintered blocks (In-Ceram YZ-55, VITA, Bad Säckingen, Germany) and then the sintering process was performed at the dental laboratory (VITA Zyrcomat 6000 MS, Bad Säckingen, Germany).

The convergence angle of the implant structure was 3° and the height 6 mm. The mould was designed in such a way to enable the incorporation of a premolar crown with sufficient material thickness in all areas. As this was an in-vitro study a schematic thread was constructed to guarantee the necessary retention in the embedding material and thus to simulate a complete osseointegration. Additional modifications to the thread surface were deemed to be unnecessary.

A total of 20 premolar crowns were produced from Lava Ultimate (3M ESPE, Seefeld, Germany) in the CAD/CAM procedure. Scanning was performed with the BlueCam of the Cerec 3D-System (Sirona, Bensheim, Germany). In the next step the standard design of the crowns (tooth 14, Biogeneric Programme No. 48) at the virtual model was performed according to the customary construction steps of the Cerec System. With the Cerec-MC-XL Grinding Unit (Sirona, Bensheim, Germany) the crowns were produced and then polished in the next step using the Lava Ultimate Polishing Set (Meisinger, Neuss, Germany) (Fig. 3).

After that all crowns were pre-treated from the inside and half of the implant abutment structures were subjected to a tribochemical pre-treatment. For this purpose the surfaces to be treated were marked with a black felt-tip pen and jet blasted with CoJet (distance approx. 5–10 mm, 30 µm, 2 bar, 3M ESPE, Seefeld, Germany) in order to obtain an evenly and completely silicated surface (Fig. 4).

After cleaning all treated surfaces with alcohol and drying these with oil-free air, the crowns were adhesively bonded to the implants. Firstly Scotchbond Universal (SU, 3M ESPE, Seefeld, Germany) was massaged into the inner crown sides and appropriate implant surfaces for 20 sec. In the next step these surfaces were dried for 5 sec using a gentle flow of air and larger build-ups of Scotchbond were removed using a dry microbrush.

The crowns were then filled with RelyX Ultimate (3M ESPE, Seefeld, Germany) using the Intraoral Tip and then positioned on the implants. The surplus was removed using foam pellets. After applying glycerine gel the samples were light-cured under constant manual pressure for 20 sec per surface (Elipar, 3M ESPE, Seefeld, Germany) (Fig. 5).

The implants were embedded in a cold polymerising synthetic material (Technovit 4000, Heraeus Kulzer, Hanau, Germany). This was performed directly in the sample holder of the chewing simulator with an individualised fixture in the parallelometer.

Sectional views of the fastened crowns before and after chewing simulation (CS) and thermocycling by means of optical coherence tomography were produced (OCT, Telesto II, Thorlabs GmbH, Dachau, Lübeck, Germany) (Fig. 6).

The samples were scanned from mesial, distal and occlusal (parameter: 1310 nm, 48 kHz, 1024 × 1024 pixels in B-Scan). Per OCT-Scan 300 frames (optical cutting planes) were generated whereby the region of interest (ROI; well visible, central area of the ZrO2 -abutment) was found in the frame area No. 100–200. In each case 5 photos from this region were used for the purpose of evaluation (frame No. 100, 125, 150, 175, 200) resulting in a cumulative evaluation for the respective surface.

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