## 4 Discussion
Even though no significant differences were found in any of the investigated variables between planned and unplanned pre-drilling, the need to insert, via a pre-drilling procedure, six mini-implants that were initially assigned to the no pre-drilling group may have had a major impact on the apical deviation in the full-arch/no pre-drilling group. Freshly frozen specimens/cadavers were unfrozen and utilized to mimic the clinical situation as much as possible. Nonetheless, and specifically owing to the advanced age of the donors, the bone might have been harder than in younger individuals, who are the most commonly treated patients in orthodontic practice. Additionally, it has to be noticed that pre-drilling is recommended in adult patients, and the present investigation further underlines the necessity of this recommendation. Full-arch 3D-printed tooth-supported guides have been described as successful and useful devices, and they are supposed to provide good anchorage and stability during mini-implant insertion [27] insertion. However, if braces are in place, the insertion of rigid full-arch guides can be challenging [25]. Therefore, skeletonized insertion guides can be a valid alternative in these cases. Moreover, they provide the advantage of a visual check of the palate during the insertion procedure. However, the insertion kit utilized in this study has been improved in the meantime with the introduction of vertical stops. A problem faced during the study was the reduced stability of skeletonized insertion guides where occlusal planes were flat or abraded. This might be due to the age of the donors, while it is unlikely to occur in young patients that represent the main target of this treatment. However, the exact age of the donors was not provided due to anonymization; therefore, this information could not be retrieved. Cuspidal coverage might be considered in some cases to improve the retention of the guide and the reproducibility of its insertion. Although ex vivo cadaver models have their own limitations, fresh frozen specimens highly mimic the human clinical situation. Another limitation of the present study consisted of the insertion of several mini-implants in each palate, as well as in uncommon clinical positions and in areas with reduced bone thickness. This strategy was chosen to reduce the number of donors needed, despite the fact that it may not accurately represent typical clinical scenarios and limit the generalizability of the results. For the same reason, the mini-implants were placed very close to each other compared to the common clinical conditions. These two factors might have been responsible for the introduction of errors at this stage since a marked deviation in the insertion of one implant could have compromised the correct placement in neighboring sites. For instance, implant tipping leading to the undesired contact of two mini-implants was observed in the paramedian region, which is frequently characterized by a reduced thickness of the bone. To reduce the error related to CBCT metal artefacts, STL datasets from the original mini-implant STLs were superimposed; thus, minor matching errors might have been generated at this stage. Indeed, the computer-based workflow was very extensive, since all datasets had to be registered, including the original implant without artefacts.