## 4 Discussion  
In our in vitro study, we proposed a method which evaluated the position of the implant, comparing laboratory scanning (gold standard) versus intra-oral scanning with three different scan abutments by superimposed digital STL files. The deriving of the spatial position of the implant was measured in the longitudinal axis, rotational angle, and absolute distance. When using the digital impression for implants, we must use a scan abutment that will capture the spatial position of the implant related to the intra-oral tissues. The data from the scanning are converted to STL file and this should be as accurate as possible to achieve the best clinical result. Accuracy may be affected by the scanner technology (confocal versus triangulation), the experience of the dentist and the scanning trajectory [20,21]. A study by [PERSON] et al. determined that the maximum tolerance of the bone around one implant is 50 microns [22,23]. Hence, in this study, we assumed that movement in any of the axes (B-L, M-L, or I-G) of more than 50 microns may cause distortion in the rehabilitative work (single crown):  
1. M-D displacement (x-axis) causes inaccurate seating due to inadequate contact with adjacent teeth. 2. B-L displacement (y-axis) causes inaccurate buccal or lingual insertion which may lead to cheek or tongue biting, respectively. 3. G-I displacement (z-axis) causes improper occlusal contacts. The parameter D is affecting the pressure on the bone, can affect the passive fit, increase pre-load tension, and cause more mechanical problems. In this study, only the ZZ scan abutment showed D < 50 microns (46 microns). Our hypothesis for this result can be explained by the scan abutment production, the ZZ is the only one-piece scan abutment which may lead to less distortion when connecting this abutment to the analog. The MIS scanning abutment is the only one which demonstrated no statistically significant displacement in the spatial position in relation to the X and Z axes (Table 1 shows the statistical significance), we assume that perhaps the square geometry versus the cylinder geometry of the other two scan abutments may affect this result. Another assumption is that the MIS abutment is composed of two pieces made from the same material (titanium) while the ZZ abutment is one-piece (PEEK) and the AB abutment is composed of two pieces from different materials (titanium base and body from PEEK). The rotational angle is one of the most clinically important because a large deviation in the angle may prevent passive fit of the restoration. All three scan abutments showed a rotational deviation of the implant axis between the laboratory scanner and the intra-oral scanner. The AB abutment showed a deviation of 1.04 degrees while the other two abutments showed deviations of about half a degree. It is important to remember that the AB scan abutment is made from two different parts, a cylinder PEEK material and titanium base which is cemented to the cylinder. This may be the reason for the high deviation in the rotational angle. It is reasonable to assume that a scanning error of one-degree may not clinically affect a single implant-based crown, but may affect the clinical result of two or more implants; however, there is not enough evidence in the literature. We also found that with the one-piece scan abutment (ZZ), the rotational angle was the lowest compared to the two-piece scan abutments. This is confirmed with the study of [PERSON] et al [24], which examined the influence of scan body design on the accuracy of the implant position; they used only one-piece scan abutments which showed no difference in the linear and angular position between the two abutments; however, they used a CMN machine for measuring the spatial position, which is different from our measuring method. We may suggest that one reason for the differences between the two scanners is related to the technology.