Maxillary sinus augmentation is a viable technique that can be used to insert implants in a maxilla with reduced height. However, several complications can occur with this technique which can be classified into intraoperative, acute postoperative and chronic postoperative complications. In particular, the most frequent complications may be perforation of the Schneiderian membrane, intraoperative hemorrhage, infraorbital nerve injury, orbital wall perforation, implant displacement within the sinus, edema, infection of the inserted graft, flap dehiscence and formation of a fistula [14-25].
Thus, a valid alternative could be to use the ZI procedure, it has been reported to have a very high survival, but even with this procedure there are numerous studies which mention that the use of the Zi is not without complications.. The insertion of ZI represents a major surgical procedure that should be performed under general anesthesia by properly trained surgeons. The learning curve can be very long, and the brain and orbita may be interested in the procedure [
26]. In recent years, static and dynamic navigation techniques have been used [
27]. Sometimes, in the case of implants inserted in a more palatal position, a more complicated prosthetic restoration may be necessary [
26]. Also, the eventual removal of failed implant could be a more complex procedure. Sinusitis has been reported in 3.9% of the cases, and a failure to obtain implant osseointegration in 2.44% [
27]. In another review, the rate of complications was 7.2% when using an intrasinusal technique and 1.8% with an extrasinusal technique [
16]. A 4.9% prosthetic complication rate was reported, with a 0.69 implant failure rate. In a clinical study on 141 implants inserted in 45 patients, an overall complication rate of 5,67% was reported [
28]. In another clinical study on 182 ZI in 82 patients, a low complication rate was reported, sinusitis 10% and peri-implant hyperplasia 0.8%. Other reported complications have been oro-antral fistula, foreign body reaction, difficulties in maintaining proper oral hygiene in the posterior palatal region, transversal mobility, paresthesia, bruising, laceration of the lips, injury to the orbit and periorbital hematoma, cranial penetration, temporary deficts of some sensory nerves, soft tissue hypertrophy, abutment and prosthetic screw loosening, mucositis, prothesis fracture [28-30]. In a recent systematic review, the Authors conclude that ZI are not recommended as a first therapeutical option [
25]. In pterygoid implants (PI) a success rate, after one year of 97.05% was reported [
4]. In a systematic review, a cumulative survival over a 10 years period was riported for PI [
23]. In another systematic review on 1983 PI in 634 patients a mean survival rate of 94.87 was reported [
31]. Bidra et al reported [
3] in another systematic review of PI, a 95.5% cumulative survival rate after 6 years. Recently, Wilkirson et al. reported on a FEA study on PI [
32] . Subperiosteal implants were proposed and introduced in Sweden during the ‘40s, and have been extensively used in the ’50 and ’60 [
33]. However, they had a too high percentage of complications and failures. In those years, the construction technique of this type of implant was extremely complex, a perfect adaptation of the implant to the underlying bone was very difficult to obtain, the surgical technique was very complicated and very time consuming. Moreover, these implants tended to be very large and they required very large flaps to position them on the underlying bone bed. Two surgeries were, furthermore, needed, with a high biological cost: one for the impression of the bone, the second for implant positioning. The used materials were chrome-cobalt and vitallium [
34]. Stvrecky e coll.,[
35] in a 15-year retrospective study, reported a 5–10-year survival rate of 58.3%. In the past few years, thanks to the introduction and widespread use of new digital technologies and metals, subperiosteal implants have been proposed again [36-38]. 3D metal printing has allowed a much better accuracy during the implant manufacturing [
37,
38]. The use of different metals, i.e., titanium, has allowed to obtain a smaller implant structure [
36]. Nowadays, subperiosteal implants could offer advantages, e.g., reduction of treatment period, reduction of costs, avoidance of complex and risky surgical procedures [
39]. In recent years, several studies point to the fact that with these subperioseal implant of the next generation no resorption of the underlying bone, mobility or fracture of the implant have been reported [
39]. Moreover, 95% survival rates have been reported. However, only small numbers of patients with very short periods of follow-up can be found in the literature. Some reported complications have been swelling, edema, pain, implant exposure [
37]. Overall, the response of the patients has been extremely positive regarding their comfort, chewing capabilities and the stability of the prosthetic restauration [
38]. These results are probably related to the use of all the new digital technologies, which allow an extremely precise and close apposition of the implant structure to the underlying bone. Ten patients have been followed for one year; the Authors reported a 100% implant survival percentage, and 10% of early complications and 20% of late complications [
39]. No complications or implant loss were reported in 16 patients, with a one year follow-up [
40]. In a clinical study on 16 patients, with a 6 months follow-up, only one implant loss, without other relevant complications, was present.
The stresses caused by the prosthesis during chewing cannot be measured in vivo. The methods used can be experimental such as strain gauges using electrical strain gauges and photoelasticity. However, each method has its limitations, for example the disadvantage of strain gauges is that the measurement is limited by the area where the strain gauge is applied, which may not include the area of interest. The method instead of photoelasticity allows instead to identify gradients of stress over the entire structure. Its disadvantage consists in recreating a reflective model that in the case of complex structures is difficult [41-43]. FEA has proven to be a useful tool for estimating stress and strain in this innovative implant system. One of the peculiarities of FEA lies in its physical similarity between real results in vivo and numerical ones. However, further simplification of the geometry can inevitably lead to inconsistent results [41-44]. Consistent results in FEA must provide the complete geometry of the implant and surrounding bone to be modeled, material properties, loading constraints and conditions, and mesh convergence tests. The main advantages of the FEA methodology are essentially to be found in the non-invasive technique, static and dynamic tests can be performed, the study can be carried out several times and there is no need to sacrifice animals, therefore also from an ethical point of view it is extremely useful.
However, this methodology also has drawbacks which substantially concern the knowledge of the software, the fact that the results are dependent on the configuration parameters and the need for in-depth information on the behavior of the analyzed components.