## 3 Discussion  
The present study demonstrated unusual and invasive involvement of the nasal cavity by late developed POMC after MSA. The patient presented with clinical symptoms 18 years after MSA. Due to the location and the extent of the lesion, the intraoral surgical approach was not considered to be appropriate, and thus the patient was referred to an otolaryngologist. The POMC was successfully treated with nasal endoscopic marsupialization without implant removal. No recurrence has been found to date. Evidence demonstrated that POMC development is related to invasive surgeries or trauma involving the maxillary sinus, such as Caldwell-Luc operation and orthognathic surgery [5,7]. During such events, the sinus mucosa might become entrapped and proliferate, eventually leading to POMC. However, a less invasive surgery was not exempted from causal factors for POMC. In the grafted bone, small fragment of the sinus mucosa might become accidentally entrapped [12]. To date, there have been nine cases of POMC related to MSA in five case studies (five case reports and one case series with a systematic review) [9, 10, 11, 12, 13]. Figure 4: Endoscopic marsupialization (**a-c**) and histologic finding of the removed specimen (**d**). (**a**) A dome-shaped cystic lesion occupied the inferior meatus, (**b**) The lesion was excised, and drainage was performed, (**c**) After partial removal of the cystic lesion, the medial portion of the previous augmentation was exposed, (**d**) Respiratory epithelium and infiltration of inflammatory cells were observed in the specimen (H-E stain, x100). Figure 5: Clinical, radiographic, and nasal endoscopic findings after endoscopic marsupialization. (**a**) Swelling on the left palate disappeared, (**b**) [PERSON]’s view, (**c**) Nasal endoscopic view. Regarding suspected causality, for example, sinus membrane perforation, interesting findings were noted; even though hypothesized by some authors [9,11], no previous studies provide definite evidence for the perforation because the patients underwent the initial MSA in other dental clinics and perforation (or intraoperative complication) was not detected (or not documented at least) [9, 10, 11, 12, 13]. In the present study, sinus membrane perforation was found near the access window area during MSA and repaired. In the CBCT taken after 13 years, a small radiolucent cystic lesion was found near the medial wall of the nasal cavity, and this lesion became significantly larger at 18 years. The location of the cystic lesion was somewhat remote concerning the site of the perforation. This may suggest that the folding or overlapping of the sinus membrane during the membrane elevation or repair procedure may affect the development of POMC. The development of POMC might not be necessarily preconditioned by the damage to the sinus membrane (perforation and tearing) [13]. Following MSA, the primary choice for POMC was enucleation by intraoral surgery, as demonstrated in all the above case studies [9, 10, 11, 12, 13]. However, in the present study, the location of the cystic lesion was hard to manage by intraoral approach due to the invasion of the nasal cavity. Moreover, enucleation of the entire cyst might have been caused by the large scar, collateral damage to the adjacent tissues, and implant removal owing to the extent of the lesion. Thus, the patient in the present study underwent endoscopic marsupialization. Other studies also demonstrated cases similar to the present one [15,16]. In one study, POMC (due to Caldwell-Luc operation 30 years ago) invaded the orbital floor and caused diplopia, vision impairment, and limitation of eye movement. In another case, a radicular cyst expanded to fill the entire maxillary sinus cavity, thinned palatal/sinus bone walls, and invaded the inferior means and nasal floor. There was elevation of the nostril, protrusion of the upper lip, and septal deviation in these cases. The above cases were successfully treated with endoscopic marsupialization. In the literature, the onset of POMC after MSA appears unclear.