The secondary outcome of this review was the analysis of the clinical and radiological complications related to the penetration of implants in the maxillary sinus. Clinical complication among the different authors ranges from 0 to 14.3%, with a weighted mean complication rate of 3.4%, without finding statistical difference according to the level of implant penetration. The most common clinical complication was epistaxis, which can be considered a minor complication and that did not lead to major complications. Radiographic complication has also been shown low, weighted complication rate of 14.8%, without finding statistical difference according to the level of implant penetration. The most common complication was thickening of sinus membrane without having relevance at the clinical level. This is in concordance with the mongrel-dog study of Jung et al., who observed after 6 months follow-up that the mucosa in the maxillary sinus cavity showed no inflammatory signs when dental implants perforating inside the maxillary sinus, suggesting that the extending implants do not make the maxillary sinus vulnerable to complications and do not cause any effect to the sinus physiology and resulting with no local or systemic pathology at all.

Consequently, it seems that maxillary sinus changes in relation to protruded implants inside the sinus cavity and does not statically affect to implant survival rate neither to clinical nor radiographic complications.

Several limitations could be described for the present review. Firstly, there is a lack of a control group in the included studies, to compare outcomes and complications, with implant placed in native bone, or with regenerative techniques. Second limitation was related to the types of included studies being seven retrospectives and one prospective study. Third, the lack of a reliable method to evaluate the millimeters of implants intruded inside the maxillary sinus and to assess sinus membrane perforation or not. Fourth, the analysis of the radiographic complications has been analyzed only in three studies with computerized tomography and in two studies by orthopantomographies and periapical radiographs, taking into account the difficulties of analyzing radiological complications in periapicals and orthopantomographies.

Future research should focus on performing randomized studies comparing implant intrusion in the maxillary sinus versus regenerative techniques, focusing on the rate of complications and patient outcomes. Further, it would be interesting to study the possibility of placing short implants and thus avoid access to the maxillary sinus.

The current review showed that the exposure of dental implants in the sinus cavity without the augmentation procedure or graft materials shows a high survival rate of 95.6%, without statistically significant differences according to the level of penetration (lower or higher to 4 mm). Changes in maxillary sinuses in relation to protruding implants within the sinus cavity do not statically affect clinical or radiographic complications of 3.4% and 14.8% respectively. Although caution is necessary, it is not advised to carry out this technique intentionally, since the supporting literature is based only on retrospective studies. Further research is needed, with prospective and randomized studies that directly compare different techniques in equal local and systemic conditions to explore the complications and outcomes of the patient center.