The objective of this study was to compare two different surgical guides confection in terms of their reproducibility and precision relative to the initial virtual projection. Despite being an in vitro study, this study evaluated the accuracy on the reproductivity of the main methods used to fabricate surgical guides in the era of digital dentistry, since no other study performed this comparison. This evaluation was performed by superimposing images, a procedure that allows for a point-by-point evaluation of any discrepancy in the guide characteristics. Although no difference was found regarding the best fit alignment between the groups, 3D printing group presented higher variation on the reproducibility of the surgical guides. These results corroborate those of a study by Park et al. [2], who observed that milled surgical guides had less deviation than 3D printing guides (p < 0.05). Other studies also showed greater precision for milled guides in relation to the final implant position [10, 12, 15, 20]. Clinically, this error seems not to influence in the final result of the rehabilitation. Bell et al. [16] evaluated 2 different surgical guide material, in relation to the angular deviation of the implant inserted with using a thermoplastic surgical guide and 3D-printed one. The authors demonstrated that no clinical difference was shown between both groups, although implants placed using the thermoplastic surgical guide are less accurate on the apex positioning.
Results of the other studies that compared the final implant position obtained using both guides are questionable, as various factors can influence the precision of guided surgery, such as scanning errors, errors in producing the guides, mechanical errors, data transmission errors, and human error [8,-10,12,17,21]. These factors are cumulative and interactive and can occur at any time during the process. This study supports the findings of these authors because it used the reproduction of guides based on the same scan of the same model, confirming that using CAD/CAM-assisted surgical guides is a more precise technique than using 3D printing guides. However, although this results suggest that milled guides are superior to 3D printing guides, the literature still controversial, since some authors have shown the advantages of using 3D printing over conventional surgical guides produced on top of models and over implants that are positioned freehand [9,10,19,22], while other studies showed no significant difference implant survival rate and effectiveness using conventional or digital implant placement procedures [11].
The literature also shows that 3D printing surgical guides may be associated with surgical complications caused by problems during their production. These problems include a lack of calibration of the printing equipment, changes to the physical properties of the resin, difficulty in positioning or fixing the guide in the oral cavity, or limitations in mouth opening [7,9,23]. It is important to know the limitations of the prototyping guide technique to minimize the potential for complications during the surgical procedure. Van Assche et al. [1] observed that, to avoid deforming 3D printing guides, it is essential that the guide have a total thickness of 2.5 to 3.0 mm. This deformity is not observed in milled guides because the resin blocks are ready to be machined, without suffering any change to their structure [12].
Despite the precision found in this study’s results, the literature suggests that errors may occur during the manufacture of either type of surgical guide. Thus, it is recommended that a 2-mm safety margin be maintained around important and vital structures [2,21] and that cone-beam computed tomography images be used, in order to achieve a correct evaluation of the essential anatomical structures [9,20,24, 25].
Clinically, the goal of precise surgical guides is to avoid damaging the noble structures and to offer an ideal treatment plan that meets the patient’s aesthetic and functional objectives [3,15], with a shorter duration of surgery and fewer complications during surgery. Although the results of this study showed a difference in reproducibility and precision for the different methods of making surgical guides, future studies are needed to gauge the implications that such differences might have on surgical positioning. It is also necessary to evaluate the cost/benefit ratio of both types of guides for the patient and dental surgeon.