## 4 Discussion  
Zygomatic implants (ZIs) represent one of the most used therapeutical choices in response to severe atrophic maxillae (Cawood-Howell V and VI) rehabilitation. There are different techniques to place ZIs. [PERSON] et al. (2018) used a crestal incision to displace a large flap facilitating the exposure of the zygomatic bone, followed by the creation of a window for the displacement of the sinus membrane. The technique of [PERSON] (2018) differed from the original technique, as there was no need for a classical window opening on the wall of the maxillary sinus. Instead, they proposed a slot acting as a smaller antrostomy to orient the twist drill for implant placement. The third technique is completely performed outside the maxillary sinus, without windows opening or channels in the wall of the maxillary sinus. This has been used for patients with a pronounced maxillary sinus lateral wall concavity ([PERSON] and [PERSON], 2018). The most used ZI positioning protocol for edentulous maxilla usually involves two zygomatic implants in the premolar area and two to four standard dental implants placed anteriorly to provide additional support for a full-arch maxillary prosthesis ([PERSON], 2018). Furthermore, the Quad technique provides the use of four ZIs, two for each side, where the premaxilla does not allow positioning of traditional implants. This protocol can support the whole prosthesis. [PERSON] and [PERSON] (2018) evaluated patients treated with the Quad approach and reported that the cumulative survival rate after 40 months was 96%; generally; the weighted average success in zygomatic implant positioning is 97.05% ([PERSON], 2018). In comparison to conventional treatment, ZIs preserve the patient from multiple surgical procedures and reduce the entire rehabilitation process (immediate loading is also described), time for healing and the risk of infections. Moreover, use of ZIs avoid extraoral harvesting for osseointegration, providing predictable success and absolute retention. Even if it prosthetic failure was rare, some authors ([PERSON] and [PERSON], 2018; [PERSON], 2018; [PERSON], 2018) demonstrated some drawbacks related to ZIs. They are positioned closed to delicate structures in the midface (e.g., maxillary and palatine artery, facial and V2 nerve branches), and because of this the surgical procedures should be performed by a trained surgeon. In addition, any problems related to ZIs are more difficult to approach than in conventional dental implant rehabilitation. During the early stages after surgery, sinusitis appears to be one of the most common complications. The 13 publications included in this review evaluated 478 patients and 1156 zygomatic implants overall, with only 69 patients (14.5%) with sinusitis, of which 42 (8.8%) showed clinical relevance in the follow up. From a diagnostic point of view, maxillary sinusitis may be observed radiologically (usually by orthopantomography or, more often, maxillofacial computed tomography with clinical assessment. These diagnostic tools are useful not only for proper evaluation and surgical planning before ZI placement, but for monitoring the clinical and prosthetic outcomes in more detail afterwards. Otherwise, in the literature examined, there was no evidence of precise diagnostic analysis of sinusitis itself. From this review, clinical outcomes of sinusitis seemed difficult to find when compared with radiological outcomes ([PERSON] and [PERSON], 2018), which is a critical point considering the greater evidence of radiological signs ([PERSON], 2018) that can cause silent sinusitis with otorhinolaryngological consequences, both chronically and over time ([PERSON] and [PERSON], 2018; [PERSON], 2018). Indeed, both symptomatic and silent sinusitis were taken into account in this review to extend the analysis to the evaluation of sinusitis diagnosis (nasal endoscopy and radiological assessment) and treatment (FESS vs. simple antibiotic vs. follow-up).