## 4 Discussion  
This study aimed to measure the trueness and precision of different implant level impression techniques in an all-on-four fully dedentulous cast with anterior parallel implants and posteriorly tilted implants. The first null hypothesis was partially rejected. In terms of trueness, statistically significant differences were found between the DIOS and DPG groups in comparison with the true value. However, in terms of precision, scans were consistently reproducible within each group when analyzing angular deviations, yet DPG had fewer discrepancies when comparing 3D distortion datasets. The second null hypothesis was also partially rejected. Trueness within ISBs revealed significant differences but did not have a clear direction since the results varied between parallel and angulated ISBs. However, parallel and distal implants were equally precise in terms of angular deviation and 3D deviation for both groups. This is important since the correlation between trueness and precision is a substantial aspect in choosing a proper impression technique for the intended application. Several factors can influence impression accuracy which may project in the passivity of the prosthesis including implant angulation, implant depth, implant connection type, and inter-implant distance [11; 12; 13]. These factors were highlighted with conventional impressions in previous studies. However, different paths of results can be found when studying scans of digital impressions. With regards to implant angulation, the fidelity of digital impressions must not be affected by the angulation of implants as the worry of impression material distortion during removal, or movement of impression transfer is not a problem in this technique [11; 12]. However, the results of this study showed no clear results for trueness when comparing parallel and distal implants where alternating values were recorded, having parallel implants more accurate in some cases and distal ones more legitimate in others. In the DIOS group, higher angular deviations were observed at scan body of the LLI and RFP followed by RLI and LFP. A possible analysis of this can be directional inaccuracy when bending the IOS as it approaches a different plane disfavoring the capture of scan bodies located at the curve. In other words, errors may depend not on implant angulation but rather on the arch shape and how the scanner is oriented to capture the needed image unlike photogrammetry system being fixed in a certain position and at a predetermined standardized distance is not influenced by motion or camera's inclination. However, this was not the case with 3D deviations of DIOS where RFP and LFP were truer than RLI and LLI. This means that implant angulation favors accuracy of IOSs where results were consistent with [PERSON] and [PERSON] [35]. In contrast, a systematic review by [PERSON] et al. stated that angulations larger than 15 degrees can influence intraoral scanning accuracy [13]. It is important to note that differences in the results between 3D distortions and angular deviations are due to the lack of measurement uniformity between both variables despite the similarity of the purpose traced. 3D distortions are calculated from the surface of the ISB while the latter are calculated from the center axis projected by the software. In addition, RMS used to calculate the surface fit is more sensitive to outliers [48] than the mean absolute deviation used to calculate angular deviations. However, RMS was used instead of mean values since the best fit algorithm matching produces positive and negative deviations between reference and test objects which could lead to results canceling each other and not representing the real divergence [47]. The findings in this study did not relate directly to the previous work. Congruence is made difficult by the lack of standardization of measurement methodology, comparison programs, number of implants, origin of the reference dataset, and IOS and DPG technologies used.