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. This evaluation was performed by superimposing images, a procedure that allows for a point-by-point evaluation of any discrepancy in the guide characteristics. It was found that milled guides had better reproducibility than prototyped guides, conferring greater precision to the surgical guides relative to the master model. These results corroborate those of a study by Park et al. [2], who observed that milled surgical guides had less deviation than prototyped guides (p < 0.05). Other studies also showed greater precision for milled guides in relation to the final implant position [8,9,12,13].
Results of the few 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 [7,9,14]. 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 prototyped guides. However, although this study’s results suggest that milled guides are superior to prototyped guides, various authors have already shown the advantages of using prototyped over conventional surgical guides produced on top of models and over implants that are positioned freehand [7,8,15].
The literature also shows that prototyped surgical guides may be associated with surgical complications caused by problems during their production. These problems include a lack of calibration of the prototyping 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 [6,7,16]. 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 prototyped 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 [9].
Despite the precision found in our 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 [2,14] and that cone-beam computed tomography images be used, in order to achieve a correct evaluation of the essential anatomical structures [7,13,17,18].
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 [3,12], 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.
According to the results obtained in this study, it is possible to suggest that prototyped surgical guides presented higher coefficient of variation than milled guides. Moreover, milled guides lead to fewer misalignments relative to the master model.