In this SR, we investigated whether the presence of a cantilever affects the results of implant treatment for partial edentulism, including an analysis of the anterior and posterior regions of the dental arches. Further, we comprehensively searched for studies on the prognosis of implant treatment using cantilevered implant-supported fixed prostheses and selected eligible studies. Consequently, nine studies met the inclusion criteria. Of these, eight were cohort studies and only one was an RCT. Nine articles [15,16,17,18,19,20,21,22,23] included implant survival rate as an outcome. Most showed a tendency for the implant survival rate to be slightly lower in the cantilever group or comparable between the cantilever and control groups. Therefore, the consistency of the results between studies was assessed to be high. However, the number of placed implants differed among the studies: two studies placed one implant [18, 23], two studies placed two implants [17, 19], and the number of placed implants was unknown in the other five studies [15, 16, 20,21,22]. Furthermore, although implant length and diameter influenced the implant survival rate [24, 25], these factors were not standardized among the included studies.

Although patient satisfaction is clinically critical, only one study included it as an outcome [18]. The study concluded that satisfaction levels were the same regardless of the presence or absence of a cantilever. However, in this study, only the morphology and color of the crown and gingiva were evaluated as outcomes of patient satisfaction, and there was a lack of evaluation of items such as cleanability that may be degraded by the presence of cantilevers. In the long term, mechanical complications, which increase in the presence of a cantilever, could reduce patient satisfaction. Furthermore, the presence of a cantilever often reduces surgical invasion, treatment costs, and treatment duration, which may lead to increased patient satisfaction. Further studies that include more patient satisfaction outcomes are required.

With regard to the occurrence of mechanical complications as an outcome, all four studies demonstrated that the presence of a cantilever increased mechanical complications. Because the cantilever structure is not mechanically superior, it is easy to imagine that mechanical complications can increase. Furthermore, the prosthesis material has a significant effect on the occurrence of mechanical complications. Among the studies included in this SR, one study used porcelain-fused-to-metal crowns as a prosthesis, whereas the other three studies did not mention the material of the prostheses. Future studies using prostheses made of other materials, such as monolithic zirconia or all-ceramic crowns, may lead to new findings. Furthermore, the prosthesis fixation method can affect the occurrence of mechanical complications [26]. Of the included studies, one used screw fixation [15] and the type of fixation was unclear in the other three studies [16, 21, 22]. Moreover, although the use of screw-retaining abutments could affect the frequency of mechanical complications, such as implant fractures, only one study mentioned the use of screw-retaining abutments [15].

Regarding the amount of marginal bone loss around the implants, the type of implant-abutment connection is a factor that influences marginal bone loss, and systems with conical connections have been reported to have smaller marginal bone loss [27]. However, only one study [15] clearly indicated the implant-abutment connection; in all other studies, the implant-abutment connection was unclear, or multiple connections were mixed within the study. In addition, in the case of screw fixation, the presence of a screw-retaining abutment affects marginal bone loss [28]; however, among the studies included in this SR, only one study specified the use of a screw-retaining abutment [15]; in other studies, it was unclear whether a screw-retaining abutment was used. Among the factors that can cause marginal bone loss, mechanical factors and cleanability have been proposed as factors related to the cantilever. Regarding mechanical factors, the presence of cantilevers has a negative effect as per the finite element analysis [4]. However, poor cleanability may induce peri-implantitis, resulting in marginal bone loss. Because the presence of a cantilever may impair cleanability, future studies that include not only marginal bone loss, but also cleanability as an outcome are required.

It is clear from the finite element analysis that cantilever length affects stress distribution around the implants [29]. Therefore, it is necessary to consider the effect of cantilever length; however, none of the studies included in this SR investigated this effect of cantilever length. However, although not included in this SR, Palmer et al. reported it [30]. They reported the mechanical complications that occurred in 29 cases in which prostheses with a cantilever were attached to a single implant placed in the anterior region; the most common complication was screw loosening. Hence, the cantilever length did not significantly affect the mechanical complication and the cantilever length of the complication group and non-complication group were 7.6 (7.1–7.9) mm and 7.3 (7.0–8.0) mm, respectively. However, it is not possible to conclude from this study alone whether the cantilever length affects the prognosis of implant treatment, and further research is required.

Another factor that could be related to implant prognosis but could not be investigated in this SR was the direction of the cantilever. Of the nine studies included in this SR, one study included only mesial cantilevers [23], two included only distal cantilevers [15, 18], five included both mesial and distal cantilevers [16, 17, 19, 21, 22], and this was unknown in another study [20]. None of these studies compared the results of implant treatments for the mesial and distal cantilevers. In view of the above, it would be difficult to conclude the relationship between the direction of the cantilever and prognosis. The number of supported implants is another important factor that may influence prognosis. However, no study has investigated this relationship, and it remains unclear. Therefore, it is necessary to accumulate further research results.

Two previously published SRs of cantilevered implant treatment included four studies each, two of which were shared between the two for a total of six included studies [5, 6], three of which were ineligible for this SR due to issues with study design. On the other hand, this SR was able to include six new studies that were not included in previous SRs, four of which were published after 2020 [17, 19, 22, 23]. Comparing the results of this study with those of a previous SRs, there was agreement that the presence of a cantilever had no effect on implant survival rate or marginal bone loss, although cantilevers increased the incidence of mechanical complications. As the number of studies included in this study was significantly greater than those in previous SRs and the results were consistent, the results of this study were able to provide stronger evidence. However, only one study was included regarding the occurrence of mechanical complications that was not included in previous SRs [22], indicating that there is still a lack of research on this outcome. As implant and screw materials have improved [31], and the advent of CAD/CAM, which allows for the creation of precise prosthetic devices, is rapidly increasing [32], further research is needed, especially regarding mechanical complications as an outcome. Recently, a SR has been published that focused on implant-supported single-unit crowns with cantilever extension in posterior regions [33]. Compared to this SR, whose included studies and outcomes were somewhat different, the results were generally consistent, such as the implant survival rate not being affected by the cantilever and an increase in mechanical complications, which suggests the possibility that the prognoses of single implant-supported and multiple implant-supported cantilever prostheses may be comparable.

There are several limitations in this study. Although we attempted to perform a meta-analysis by site regarding the occurrence of mechanical complications, this was impossible because there was an insufficient number of studies. On the other hand, although the relationship between bruxism and implant failure was known [34], only one study stated that it excluded patients with bruxism [18], another study stated that it included patients with bruxism [22], and the other studies did not state whether patients with bruxism were included or not. Therefore, it is unclear whether many of the included studies included patients with bruxism, and it is important to note that it is unclear whether the results of this SR can be applied to patients with bruxism. Furthermore, the number of studies included in this SR was not large, therefore further research is required to confirm these results.