## 2 Materials and Methods
### OCT Imaging, Image Analysis  
In parallel with the clinical evaluation of the restorations, according to the FDI criteria [1], they were imaged with spectral domain OCT (SD-OCT, Telesto II SP5, Thorlabs GmbH, Dachau, Germany). A detailed description of OCT and the equipment used has already been provided [12; 17]. The restored tooth surfaces were scanned axially with a beam of broadband light in a field of view of a maximum of 10 mm \(\times\) 10 mm \(\times\) 3.5 mm (air, pixels maximum 1000 \(\times\) 1000 \(\times\) 512) point-by-point and line-by-line. The axial/lateral resolution was <5.5 (air) \(\upmu\)m/20 \(\upmu\)m. Further technical specifications of the OCT system were: center wavelength 1310 nm, imaging speed 76 kHz, sensitivity \(\leq\) 106 dB, and an A-scan average of one.  
\begin{table}
\begin{tabular}{c c c} \hline
**Adhesive\_Tooth** & **Enamel, \%** & **Dentin, \%** \\ \hline FbM\_13 & 19.8 & 4.9 \\ \hline SyC\_24 & 7.4 & 18.8 \\ \hline SbM\_45 & 12.9 & 2.4 \\ \hline \end{tabular}
\end{table}
Table 1: Mean fractions (%) of interfacial adhesive defect length at enamel and dentin of the samples, determined as described in the Materials and Methods Section 2.2.  
OCT imaging was performed by an experienced, calibrated dentist (cleaning, drying of the restored surface, placement of the probe, separation of the lips and cheeks) who was blinded to the restorative materials. The operator-stabilized probe was perpendicularly positioned to the surface of the restoration at a distance of 30 to 35 mm. Depending on the mesial-distal extent of the restoration, 3D OCT images with 150 to 500 B-scans per image stack were generated and exported (Thorlabs SDOCT v. 3.2, Thorlabs GmbH).  
An increasing number of evenly distributed B-scans (N: 1, 3, 5, 7, 9, 15, 21, 45, 61) were selected per restoration after alignment of the image stacks (registry plugin StackReg [18], FijI (ImageJ 1.48, [PERSON], National Institutes of Health, Bethesda, MD, USA)). Image analysis was performed, as previously described [10], at the same screen by one trained, blinded, calibrated examiner, who was introduced into the methodology of image evaluation. For this purpose, in each B-scan, the lengths of the interfaces on enamel and dentin, as well as the signals for the interfacial gap (Figure 1), were determined using ImageJ and CTAn 1.1.4.4.1+ (64 bit, Bruker microCT N.V., Kontich, B). A gap signal at the cavity floor is a cluster of a minimum of three image pixels with enhanced brightness compared to the image background, defined by rising and falling slopes in the A-scans located at the interface with a height above the maximum signal value of the range of the background.  
The percentage for the length of adhesive defects in a B-scan was determined as follows:  
total length defect signal/length specific interface \(\times\) 100, %.  
For each restoration, a weighted mean fraction of the interfacial gap length was calculated for enamel and dentin. Explanation: due to the cavity geometries, the lengths of the interfaces at enamel and dentin are unevenly distributed in the B-scans. Therefore, each B-scan contributes differently to the weighted mean fraction of the interfacial gap length with the proportion of the inherent length of the respective interface to the total length in the volume scan. In addition, interfacial gaps are unevenly distributed in the cavity. Furthermore, therefore, each B-scan does not contribute equally to the calculated mean value. Subsequently, the weighted means were graphically plotted as a function of the number of B-scans.  
Semi-automatic segmentation of the enamel/dentin-composite interface for the restoration of tooth 24 was performed using Seg3D* 2.1.5, discriminating between areas with and without interfacial gap formation (NIH/NIGMS, University of Utah, Salt Lake City, UT, USA).  
Figure 1: _Cont_.