For the majority of the evaluated variables, there was a tendency for the posterior implants to suffer from more deviation than anterior implants. This is in accordance with several published reports [5, 21, 22]. Interestingly, implants placed by the FG protocol seemed to be less vulnerable to inaccuracy by changing the implant sites, while the PG and FH protocols showed more horizontal and angle deviations for the posterior implants than anterior implants. The inferior outcome of the posterior implants can be due to the limited access, inferior visualization, additional drilling step for wider implants, and more difficult drill orientation for the PG and FH protocols. This also discloses an additional advantage of the FG protocol in being less susceptible to error by altering the implant surgical site, which increases the security during surgery [5, 21, 22].

In accordance with earlier studies, even for the FG protocol, a safety zone is needed and recommended to be in the range of 1–2 mm horizontally and vertically [5, 7, 8, 17], and up to an angle of 5° [7]. While it is tempting to propose a safety zone of 1 mm horizontal and 0.5 mm vertical deviations for the FG protocol as shown by the present study, more errors are expected clinically. Although this study aimed to simulate a clinical set-up, it has limitations that mandate caution while interpreting the results. For example, the models were produced from resin, which does not represent the structure and consistency of natural bone, and may contribute to the greater implant accuracy reported in this study. According to a recent systematic review, implant accuracy was lower in clinical and cadaver studies compared to laboratory studies [7]. The manikin heads with ideal mouth openings do not have natural limiting factors such as blood and saliva and patient movement, limited mouth opening, and restricted interarch clearance. These clinical limitations will interfere with the seating of the guides and orientation of the drills. The FG protocol may even be more influenced especially for posterior implants where the access is limited, that may mandate using the FG protocol guide according to PG protocol. As a result, several authors clearly stated that the use of digital technology does not eliminate the necessity of surgical experience and skills, and the clinicians should be comfortable shifting to conventional implant surgery in case of clinical complications [9, 11, 17]. Due to greater observed error for the PG protocol, it requires a greater safety zone during the planning and the clinician should be prepared to review the osteotomy during the different stages of implant surgery. While superiority of the FG protocol in the range of 0.5 mm–1.0 mm was observed, the deviations of the FG and PG protocols are clinically tolerable, and the differences between them may not be of clinical significance. Further, there is no clinical evidence of difference in implant survival and marginal bone loss of implants inserted conventionally and by the FG protocol [11]. Thus, clinical studies are needed to validate the actual benefit of the FG protocol to justify its routine use for the different clinical presentations [11]. Specifically, if the FG protocol will allow for clinically more esthetic implant restorations, a superior long-term outcome, better soft tissue management, cost-effectiveness, and patient-centered outcome [11]. In addition, the incidence of complications with the FG protocol such as guide misfit, fracture, limited drilling cooling, and lack of implant primary stability [17] should be determined. It is also necessary to emphasize that the results of this study are applicable for single implant placements, and different results may be observed for larger edentulous or longer span areas [4, 17]. This is important as the presence of well aligned teeth and a wide alveolar ridge can be used to guide implant placement to an acceptable orientation, which may explain the general similarity between the PG and FH protocols. Once the presentation becomes more complex, involving more than one implant, the FH implant placement will become more challenging [4, 20].