The incidence of implant displacement into the maxillary sinus remains unknown because of the lack of cohort studies and the relatively few published case reports [9]. However, the fact that the number of publications on this subject over the past 8 years has doubled compared to previous years, might imply a tendency toward an increasing incidence of implant displacement inside the maxillary sinus. An obvious explanation for this finding could be the rise in the number of patients treated with dental implants over the last years in combination with the fact that implant placement is performed by dentists with short of experience in many cases [11].
Implant displacement into the maxillary sinus can occur intraoperatively or postoperatively either prior to implant loading of after functional loading [9, 10].
Several factors can lead to this complication. At the intraoperative stage implant migration to the maxillary sinus can be a result of incorrect surgical planning (placement of implants in sites with inadequate bone height and volume), surgical inexperience of the anatomic landmarks of the maxillary sinus and improper surgical procedures (overpreparation of the recipient site, application of heavy force during implant placement, perforation of the sinus membrane during drilling sequence) [9]. Moreover, an unsuccessful sinus floor elevation procedure can result in uneven bone regeneration leading to less residual bone for implantation [8, 16]. These factors can affect primary stability of the implant, which is the main cause of implant migration at this stage by allowing implant micromovement that prevents clot formation and revascularization and thus inhibits new bone formation [9, 17]. Primary stability is defined as the absence of mobility in the bone bed after the implant has been placed [18]. It depends on the mechanical engagement of an implant with the fresh bone socket [19]. Besides the adequate level of surgical experience and the way the surgical procedure is performed, primary stability is affected also by the quantity and quality of the bone [18, 20], implant morphology [18], implant surface roughness, and topography [21, 22]. As a result, lack of primary stability can lead to early failure of the implant which in combination with the close proximity of the implant with the maxillary sinus may eventually lead to implant migration into the sinus. Lack of primary implant stability may be the cause of osseointegration failure in most cases, as well as in both cases that are demonstrated in the present paper. This notion can be derived by examining the bone height in CBCTs of both patients. It seems also that sinus floor elevation in case 1 was unsuccessful since there are no signs of bone regeneration into the right sinus.
At the postoperative stage, prior to implant loading, migration can be caused by impaired osseointegration as a result of infection, clinical, or subclinical oroantral fistulae and sinusitis [9]. Moreover, failure in osseointegration can be the result of pre-existent infection of the bone at the site of implant placement leading to bone destruction, or a particular deterioration of the bone structure such as osteoporosis or osteopenia [2]. Last but not least, there is one report of implant migration at the time of abutment connection due to the lack of implant osseointegration [23].
Migration of an implant after functional loading is extremely rare, and it is usually related to implant fracture [10], incorrect masticatory forces, exert destructive forces on the bone around the implant and implant loading in a time interval of less than 3 weeks after placement [2, 23].
Concerning implantation planning, the most frequently involved site associated with implant migration into the maxillary sinus is the upper first molar area (58.3%), followed by second premolar, second molar (16.6%), and first premolar (8.3%) sites [9]. The most frequently displaced implants are shown to be cylindrical implants (62.5%) without the association of the implant diameter with the fact of displacement [9]. Regarding implant length, most displaced implants are shown to be more than 10 mm in length size, while shorter implants appear to have a smaller incidence of implant migration [9]. Therefore, caution is needed especially in the posterior maxilla where many authors suggest the use of wider and longer implants due to the poor bone quality in this area (IV bone type) [23, 24]. Taking bone quantity into account, the height of residual bone is a key factor when deciding about implant length [9]. In any case, the inserted implant must not penetrate more than 4 mm into the maxillary sinus in order to prevent sinusitis or implant migration [15].
In the aim to explain the migration of an implant into the maxillary sinus, three main mechanisms have been proposed [2]. One is the changes in the intrasinus and nasal pressure: such changes can produce a suction effect because of the negative pressure exerted by the intrasinus and nasal cavity [2, 25]. Another mechanism is based on the autoimmune reaction to the implant, taking into account bone destruction secondary to infections at the implant site either before or after the implantation [2]. Such an example is the pre-existent apical foci involving the teeth, producing osteitis and bone weakening with resorption of certain parts of the maxilla, or peri-implantitis that leads to progressive resorption of the bone around the implant, eventually compromising osseointegration [2, 23]. One last mechanism may be the incorrect distribution of occlusal forces produced by the prosthetic restoration [26].
Since implants may move inside the sinus due to postural reasons, a CBCT should be performed immediately before surgical management, thus facilitating the location of the implants intraoperatively. Concerning the two cases that are discussed in the present paper, this was not possible since CBCT was not available in the hospital. Therefore, it was performed within 48 h prior to surgery.
Concerning the time of surgical management, one should take into account that foreign bodies in the paranasal sinuses, such as implants, should be removed immediately because, although they may remain asymptomatic for a long period, they may also lead to several complications, the most common of which is sinusitis [1,2,3, 7,8,9,10,11,12,13, 23]. Sinusitis is caused due to interruption of the mucociliary clearance or due to tissue reaction [7, 9]. It can also facilitate bacterial colonization, or even fungal infections, such as aspergillosis [7, 9, 27, 28]. Maxillary sinus infection can further lead to orbital cellulitis and damage of the optic nerve [29]. There has also been reported one case of cluster-like-headache associated with implant migration in the maxillary sinus [30]. A study on the tissues from the removed implant threads has reported various degenerative changes in the maxillary sinus mucosa associated with chronic inflammation, while others state that a foreign body inside the maxillary sinus can lead to cancer because of chronic irritation [7, 9, 31, 32].
Management of implant displacement into the maxillary sinus can be achieved with the implementation of four different surgical techniques, as follows. The first one involves functional endoscopic sinus surgery [9, 11, 12]. This procedure begins with a partial uncinectomy and middle meatal antrostomy with enlargement of the maxillary sinus ostium. By these means, better access to the maxillary sinus is achieved and the displaced implant can be retrieved [11]. This procedure is suitable for cases with implant displacement into the maxillary sinus without the existence of oroantral communication [9, 11]. The choice for performing endoscopic implant retrieval is not affected by symptoms of paranasal sinusitis and/or obstruction of the natural maxillary ostium [11]. Advantages of this procedure are the following: (a) the fact that it is a less invasive procedure, (b) the possibility of endoscopic control and treatment of maxillary antrum, nasal mucosa, ethmoid cells, frontal sinus, and sphenoidal sinus pathology, (c) the surgical ‘toilette’ and enlargement of the obstructed maxillary ostium, and (d) the quick recovery of maxillary sinus functions [11]. However, this procedure might lead to some complications, including the formation of ‘synechias’ due to scar formation between the inferior turbinate and the nasal septum [9, 11].
The second procedure is based on the intraoral removal of the displaced implant by performing the Caldwell-Luc technique [11]. This involves the creation of a bony window in the anterior-lateral aspect of the maxillary sinus and the retrieval of the displaced implant through that window [11]. The main limitation of this surgical procedure is the obligatory absence of any signs and symptoms of paranasal sinusitis and the patency of the maxillary ostium [11].
In case of a pre-existent oroantral fistula, and only when a sinus pathology is absent, implant retrieval can also be managed by taking advantage of the existing communication between the maxillary sinus and the oral cavity through the alveolar bone [9].
Finally, the fourth choice of surgical management of this complication involves the combination of the endoscopic and the intraoral approach (Caldwell-Luc technique). This combined procedure is being performed whenever implant displacement into the maxillary sinus is associated with signs and symptoms of sinusitis, obstruction of the maxillary ostium and oroantral communication [11].