The use of stackable guides is a developing practice. Given the lack of studies in this specific field of guided surgery, the authors concluded that a systematic review was irrelevant. The results should be noted with caution, without the possibility of recommendations or conclusions for clinical practice.

This scoping review has enabled to identify several important points, both positive and negative.

- Placement of the surgical guide:

When there are still teeth on the jaw, the authors seemed to favour anchoring the base using the remaining teeth [11, 18, 26], which is also reported in the literature to improve the accuracy of guide positioning [26,27,28]. When the teeth are too mobile (e.g. due to periodontal disease), a splint could be made before the CBCT to prevent any movement that would destabilize the positioning of the base [25]. An optical impression is recommended to avoid tooth movement, which would result in a distorted planning [29].

In addition, in cases of simultaneous extraction/implant placement, the dislocation movement of teeth during extraction may deform the bony tables which may complicate the placement of the guide on these modified supporting tissues. Even without bone deformation, stabilizing the guide on extraction sockets could be challenging [20]. The stackable guide provides a secure base with bone-pins and accurate placement with the dental support used prior to extractions.

In edentulous patients, attention must be paid to the compression zone between the supporting tissues and the surgical guide so as not to destabilize the latter. When the base of the stackable guide is supported by soft tissues, oedema caused by local anesthesia may lead to positioning errors. In these clinical situations, it is important to infiltrate the anesthetic solution away from the support area [14].

In cases of limited mouth opening, stacking of the guide stages may be very complicated or even impossible and the use of these guides is therefore contraindicated [24].

Although selected studies were of low level of evidence, few of them have reported data between planed and final bone reduction or implant placement [11, 16, 19, 22]. Obviously, these data can only give a trend that needs to be supported by further studies, but they seem to show small discrepancies between planning and realization.

- Stacking of the guide stages:

According to Costa et al., magnetic attachments allow for stable and reproducible stacking of guide bases [16]. The characteristics of attachment devices were not well developed in the selected studies.

- Possible planning in case of bone reduction of the ridge:

In completely edentulous patients, the drill guide will be bone-supported; if bone resection is required, a full flap is lifted so mucosal support is not possible. The surgeon will not be able to achieve an accurate free-hand bone reduction to fit the drill guide. After ridge resection, anatomical landmarks are modified, making it difficult to position the guide later [14].

The interest in guiding bone reduction is major as it would allow the use of drill guides even in patients requiring bone resection. Once the intraosseous pins of the base are placed, the reduction can be performed up to the limit of the guide, which represents bone margins planned on software before the surgery. Then a drill guide is “nested” on the base to allow a fully guided surgery from ridge reduction to implant placement. In this way, the resection is guided, and the new bone level is true to plan [11, 14, 16, 18, 21,22,23,24].

- Placement of the provisional prosthesis:

The immediate loading of the screw-retained temporary prosthesis avoids repetitive screwing/unscrewing of the implant superstructures. The provisional prosthesis is directly placed until complete osseointegration of the implants. D’Haese et al. have shown that mobilizing the implants too early during screwing/unscrewing can lead to deviation of the implant axes from the planning [30].

In addition, the prosthesis is manufactured before surgery, thus avoiding postoperative impressions which are more complex to manage at the end of surgery. When the provisional prosthesis is obtained by transforming of the temporary removable complete denture, the risk of fracture of the prosthesis is high [31]. Moreover, of the various milled materials available, polymethyl methacrylate can be used to manufacture prostheses with improved mechanical properties [29, 32].

During the digital design of the temporary prosthesis, the implant positions lead to the choice of abutment height and angulation. This step ensures the necessary passivity of the prosthesis to avoid any iatrogenic stresses on the implants during the osseointegration phase [20]. Planning is a time-consuming step, but it saves time during surgery and limits prosthetic sessions. Indeed, the temporary restoration can be placed directly after implant placement. This avoids leaving the patient without a prosthesis for 24–48 h as well as an often poorly tolerated impression and bite registration session at the end of the surgery [4, 16, 23, 28].

- Decrease in patient comorbidities:

These static guides eliminate the need for full flap surgery [33]. Indeed, it is no longer necessary to see the underlying bone structure because the guide has been designed based on a careful and accurate CBCT analysis. A flapless surgery can be performed, operating time is significantly shortened, and postoperative comorbidities decrease (bleeding, pain, oedema, hematoma) [4, 23].

Patient and surgeon comfort is improved, and the immediate loading of the implants also avoids a difficult impression session [20].

- Digital workflow:

Digital workflow is an interesting procedure that could help in treatment planning and virtual prosthetic project. Indeed, planning is done virtually on dedicated software with superimposition of DICOM files and STL files (initial situation from intraoral scanning and virtual prosthetic project). Then printing surgical templates made it possible to transfer this prosthetically guided project from the computer to the patient [34]. Several authors have reported no differences in the clinical accuracy of implant placement between additive and subtractive manufactured guides [35, 36]. In addition, acrylic resin is a suitable material for the manufacture of surgical guides, with the benefits of ease fabrication, reduced cost and less time wasted by technicians [36, 37].

There also are many advantages to this digital workflow: fewer clinical sessions are required, the provisional prosthesis fits better, and the placement of implants is more accurate [16, 38].

Limits.

- Persistent risk of error:

Despite the promising concept of stackable guides, errors can accumulate throughout treatment steps (CBCT acquisition, wax up, distortion of the planned manufacture of the guide, calibration of the 3D printer, surgical phase, anchoring the guide during surgery, stacking the different parts). The practitioner’s experience is essential in order to remain focus on the progress of each step during the surgery and be able to fix any problem or mistakes [14, 15, 19, 21].

Main limits are in the need to acquire knowledge and experience in this field; as most of new techniques, it requires a learning curve for both surgeons and dental technicians, and the need for a specific software (initial investment required).

Obviously, limits in the use of conventional stereolithographic guide are available for stackable guide (mouth opening, remaining errors in positioning the base…).

The absence of postoperative control radiography in most studies (6/12) made it impossible to objectively assess the real effectiveness of the technique presented, or the correct temporary prosthesis positioning with regard to biological or prosthetic requirements. The results should therefore be interpreted with caution.

- Financial cost:

Manufacturing the guide adds a cost to the treatment, particularly when using magnetic attachments [14, 16].

Clinical relevance:

There is no study with good levels of evidence to evaluate and measure the benefit of stackable surgery guide. However, this surgical technique appears promising in order to improve surgical precision. It differs from the use of conventional stereolithographic guides which are already used daily by several practitioners. Deep analysis and complete planning of each clinical case are time-consuming but essential steps to achieve an optimal temporary the comfort for both surgeon and patient.

In return, the comfort for both surgeon and patient is clearly increased and the surgery becomes more reproducible. (Table 4)