The overall outcome of this study indicates the superiority of the FG protocol in comparison to PG and FH protocols for placing single implants. With the exception of vertical deviation, this was obvious for horizontal neck, horizontal apex, and angle deviations that were closer to the planned implant for the FG protocol than the other protocols. In addition, this superiority was shown for anterior and posterior implants. Such observations confirm the advantage of the FG protocol in controlling all the steps of osteotomy and implant placement [7, 9, 15, 16]. On the contrary, the inaccuracies of the PG and FH protocols were generally similar and tended to be approximately double the inaccuracies of the FG protocol. Thus, the hypothesis that the accuracies of all the protocols were similar was rejected. In the present study, different software programs were needed to design guides for the FG and PG protocols. While this may have influenced the outcome, the differences between the FG and PG protocols seem to be related to the variations in drilling and implant placement. In addition, since the anterior and posterior implants of the FG protocol had similar accuracy, the hypothesis that there is no influence of the location of the implant on the accuracy of implant placement was accepted. However, this hypothesis cannot be accepted for the PG and FH protocols as the anterior implants were generally more accurate than the posterior implants. Therefore, as per earlier studies, inexperienced clinicians may benefit from FG implant placement [12,13,14]. For example, Rungcharassaeng et al. found that the FG protocol reduced differences between experienced and inexperienced operators for placing single posterior implants [12]. Likewise, Park et al. and Marheineke et al. found no difference between experienced and inexperienced operators when FG protocol guides were used, while the differences became obvious when implants were placed without surgical guides [13, 14]. According to Schulz et al., in the hand of final-year dental students, FG implant placement was more accurate than PG implant placement [15].
The superior accuracy and the less variation of the FG protocol is most likely related to the control of all the drilling steps and the implant placement via sequential use of precision sleeves. This eliminated the manual orientation and handling of the drills at any stage of drilling or implant placement. In accordance with these observations, Noharet et al. reported a better accuracy of the FG protocol compared with the conventional surgical guide [5]. Likewise, Vermeulen found the FG protocol to be more accurate than the FH protocol [19]. Further, several clinical studies reported that the PG protocol is associated with approximately double the errors of the FG protocol [16, 20, 21]. On the contrary, the PG protocol inaccuracy seemed comparable to the FH protocol, which could be due to the execution of most of the drilling steps and implant placement without the use of guides, leading to inevitable deviation of the drills and implant placement. This is further accentuated in the hands of inexperienced operators who may not precisely control the subsequent drilling steps [14, 15]. While the actual difference between the FG and PG protocols in all the variables is minimal, and still within the recommended safety zone of 2 mm [17], it can still be of clinical significance in cases where the available bone is limited, surgical site is compromised, and the implant is in close proximity to natural teeth and vital anatomical structures [17]. Further, clinically, this will be accentuated with the self-taping abilities of implant threads and inhomogeneity of natural bone that can lead to more implant deviations [20, 21]. Thus, where great accuracy is desirable, the clinician should aim to complete all the drilling procedures and implant placement through the guide [7].
The observed accuracy of the FG protocol (approximate vertical deviation = 0.4 mm, neck deviation = 0.5 mm, apex deviation = 0.7 mm, and angle deviation = 2.5o) confirms earlier studies [7, 17, 18] that reported neck deviation in the range of 0.4–0.9 mm [5, 19, 22], apex deviation of 0.5–1.2 mm [5, 19] and angle deviation of 0.3–4.0° [5, 21, 22]. Thus, despite the technological advancement, the FG protocol is still prone to error [4, 7, 9, 12, 18] that is an accumulation of deviations introduced from every step of the planning, guide fabrication, and implant placement procedures [17]. For example, the planning process involves scanning and segmentation of the oral and vital tissues, and any deficiency of the resolution will influence the accuracy of the virtually designed guides [23, 24]. Guides are produced from 3D printing or milling, and both fabrication techniques are susceptible to surface and dimensional errors [6, 25] that may affect the intraoral fit and sleeve orientation. A study that specifically evaluated the errors in the production of guides found that the sleeve centers deviated in the range of 0.07 mm to 0.38 mm, and the angle deviated in the range of 0.4°–3.3° [26]. Nevertheless, the greatest errors seem to occur during the surgical procedure. For example, improper seating of the guide and the deformation of the guide inside the mouth [9, 27]. The deformation of the guide from the present study and from previous studies seem to be more prominent on the buccolingual direction [4, 12, 21, 22], which does not have a rigid structure such as the teeth at the mesiodistal direction. The mechanical tolerance between the drills and the interchangeable sleeves can further contribute to implant deviation [23, 28, 29]. A recent study reported that the length of the sleeve and the drilling distance influenced the accuracy of guided surgery [18]. Further, the presence of debris within the osteotomy can prevent complete seating of the implant [14], which was observed in our study and another study, where the FG protocol implants were more coronal than the planned implants [12]. In clinical situations, more errors are anticipated from CBCT and 3D segmentation of the hard tissues prior to virtual implant planning [23, 24] and patient-related factors such as movement, limited visibility due to the presence of blood, and limited visual access [7, 8].
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].