## 4 Discussion
Figure 6: Maximly sinus floor elevation through minimally invasive lateral approach (anorganic bovine bone graft without using barrier membrane): (**a**) panoramic radiograph immediately after maxillary sinus floor elevation; (**b**) after 6 months follow-up; (**c**) axial and (**d**) coronal view of computed tomography at 6 months after surgery. The osteotomy site was filled with graft material having an irregular surface (red arrows). Displacement of graft material was not observed. involving 58 Koreans, an artery passing through the lateral wall of the maxillary sinus was observed in about 50% of patients. The average distance from the alveolar ridge to the posterior superior alveolar artery (PSAA) was about 18-20 mm in the premolar region and about 15-17 mm in the molar region [20]. In the present study, the upper margin of the lateral window was positioned at a distance of 10 to 12 mm from the alveolar bone crest, and the anteroposterior distance of the lateral window was 5 to 6 mm. Thus, the possibility of damaging the PSAA or its dental branch is low during an osteotomy. Actually, in the present study of 49 surgeries, severe intraoperative bleeding did not occur. The position of the minimum-sized lateral window is important for successful lateral sinus floor elevation. In the present study, the position of the window was set in relation to the outline of the maxillary sinus and the alveolar crest. The mesial boundary of the window was decided to be about 3 mm distal to the sloping anterior maxillary wall which was the most mesial border where direct instrumentation could be made [21]. The superior boundary was planned to be approximately 10-12 mm above the alveolar crest considering the inserted implant length. Because the position of the window was constant in mesial and superior boundaries, the size of the window could be minimized with a restricted extension of the distal, inferior boundary within 5 mm. If the circular-shape window with a diameter of 5 mm (area of 20 mm\({}^{2}\)) had restricted instrumental access, it was gradually enlarged to have a diameter of 6 mm (area of 30 mm\({}^{2}\)). To create the lateral bony window, the present study used a diamond round bur with a diameter of 3 mm. A piezoelectric device was not chosen because of its prolonged surgery duration period [22]. There was no occurrence of membrane perforation upon the surgical phase of osteotomy utilizing a rotary instrument with careful manipulation. However, a higher risk of membrane perforation rate has been reported when utilizing conventional rotary instruments compared with piezoelectric devices [23; 24]. Piezoelectric bone surgery is considered to minimize adjacent soft tissue injury that can be recommended for a minimally invasive surgical approach for MSFE [25]. As the window became smaller, the increased incidence of membrane perforation could be predicted because instrument access and visual field were restricted. However, the incidence of membrane perforation did not have a significant relationship with the window size. A systematic review reported that the overall incidence of membrane perforation during lateral approach sinus elevation was 15.7% [23]. In spite of a small window (20-30 mm\({}^{2}\)) in the present study, the incidence of membrane perforation was only 2.0%. This incidence was lower than those (8-18%) reported in other studies using a small-sized window of approximately 30-50 mm\({}^{2}\)[8; 11; 12; 18]. These other studies also showed that there was no significant difference in the incidence of membrane perforation between small and large window groups. Within careful surgical manipulation, the incidence of membrane perforation was not affected by the minimally invasive lateral approach. Many studies have reported that membrane perforation is closely related to irregular sinus floors such as sinus septum, root protruding into the sinus, and bone exostosis [26; 27; 28]. In 49 surgeries of the present study, there was one case of membrane perforation.