## 4 Discussion
Strictly correlated with the experience is the learning curve concept, which expresses that there are a progressive number of attempts needed to reach a repeatable level of precision. The awareness of this data helps standardize and optimize the method of training. With reference to the d-CAI, e.g., [PERSON] et al., in a clinical trial, reported the presence of a learning curve, and set the twentieth attempt as the initial plateau case [30]. [PERSON] et al. did not find a learning curve for s-CAI after analyzing the results from two different implantologists, both experts in the freehand procedure but inexpert with the s-CAI, who were working on totally edentulous patients. At the same time, a significant correlation between coronal and angular deviation and the progressive number of treated cases over time, emerged for partially edentulous subjects [16]. Nevertheless, the authors accept that a reduced sample size and the absence of the operative timing evaluation are limits of their trial. No in vitro studies have correlated the accuracy of implant placement with the progressive number of operative performances. Even if a real learning curve reaching the same accuracy level plateau was not identified, a continuous enhancement of accuracy for both operators over time resulted in the present survey. The habit of managing the physical opposition of the bone during the implant placement phases by keeping a steady position of the handle-piece, has maintained its importance [7; 15]. Nevertheless, the reported deviations in the present study were so low that the differences between the two operators did not compromise the final results of the procedure. The major reliability of s-CAI with respect to the freehand approach was confirmed. On the basis that the fitting between the sleeves and the drills is considered crucial for implant placement accuracy, the adopted device's characteristics may have positively affected the outcomes. The present study adopted specific disposable poly-ether-ether-ketone (PEEK) sleeves, as used in another similar trial [31]. This innovation, proposed for the same purposes in orthopedics [32], should give less rigidity to the system while keeping a good fitting with the drills, thanks to the thermoplastic properties of the PEEK. The templates with tunnels directly printed according to the digital project appear more customized than those with standard metallic sleeves inserted subsequently in the device at the designed positions. Nevertheless, particularly in partial edentlunism with five residual teeth at least, these devices reached a similar level of accuracy [33; 34]. Before the advent of optical scanners, the way for evaluating the superimposition of virtually placed with actually placed implants was through the comparison of the pre-and postoperative CT scans [35; 36]. The \"indirect\" methods, with the EOS or IOS, avoid a second x-ray exam for the patient, but are prone to the above-cited shortcomings in the phase of imaging data acquisition. Impression distortions and related cast imprecisions for EOS, intraoral conditions, and the movements of the patients or erroneous device movements during IOS, can affect the accuracy of STL data [20; 34] along with causing an imprecise connection with the scanning pins for both techniques. Furthermore, the quality of the 3D planning software algorithms for matching DICOM and STL files affects the absolute reliability of the s-CAI using optical scanners [35]. Regarding in vitro surveys, [PERSON] et al. reported a minimal discrepancy between a new type of IOS with improved scan bodies and the EOS [37]; and [PERSON] et al. did not find a significant difference between the outcomes from radiographic, EOS, and IOS methods of accuracy evaluation as compared to free-hand implantology, s-CAI, and d-CAI methods,, as accomplished by a relevant expert in each [31]. A notable discrepancy among the outcomes of scanning and radiologic evaluation methods emerged in the literature [19; 20; 34; 37].