## 4 Discussion
As the window where a graft material can be displaced becomes smaller, it is beneficial to maintain the graft material in the sinus cavity. In the present study, a barrier membrane for covering the lateral window was not used in most surgeries. All surgeries except two (47 surgeries) had a minimum-sized lateral window (20-30 mm\({}^{2}\)). Figure 5a,b shows the healing state of a minimum-sized lateral window without barrier membrane at 5 and 48 months after a sinus floor elevation, respectively. The regenerated new bone and xenograft granules completely filled the lateral window, with small amounts of xenograft granules attaching to the adjacent periosteum (Figure 5a). Figure 6 shows the axial and coronal views of computed tomography at six months after minimally invasive MSFE without using a barrier membrane. Displacement of graft material was not observed, and the osteotomy site was filled with graft material having an irregular surface. Although a barrier membrane is not utilized, reducing the lateral window size might decrease soft tissue ingrowth from the periosteum into the graft material, leading to less displacement of the graft material during the healing process [19]. \begin{table}
\begin{tabular}{p{113.8 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt}} \hline \hline  & **Number of Surgery** & **Average Window Size and Shape** & **Average Number of Implant Placement** & **Timing of Implant Placement** & **Incidence of Sinus Perforation** \\ \hline Present study. & & 20–30 mm\({}^{2}\) & & & \\
2021 & 49 & except 2 surgeries (circular shape) & 1.69 & Simultaneous & 2.0\% \\ \hline Shunsuke & & & & & \\ Kawakami & 20 & SW, 48.7 mm\({}^{2}\) & - & Delayed (6 months) & SW 16.7\% LW 16.7\% LW 16.7\% LW 16.7\% \\ \hline Yu and Qiu 2017 & 20 & \begin{tabular}{c} SW, 81.66 mm\({}^{2}\) \\ (two ovoid window) \\ LW, 118.04 mm\({}^{2}\) \\ one ovoid window) \\ \end{tabular}  & 20 & \begin{tabular}{c} SW, 81.66 mm\({}^{2}\) \\ (two ovoid window) \\ LW 118.04 mm\({}^{2}\) \\ LW 3.22 \\ \end{tabular}  & 20 & \begin{tabular}{c} SW, 81.66 mm\({}^{2}\) \\ (6 months) \\ \end{tabular}  & 20 & \begin{tabular}{c} SW 3.00 \\ SW 9.1\% LW 0.0\% LW 0.0\% LW 0.0\% LW 0.0\% \\ \end{tabular}  \\ \hline [PERSON], & & & & & \\ [PERSON] et al., 2012 & 23 & \begin{tabular}{c} 69.71 mm\({}^{2}\) \\ (square shape) \\ \end{tabular}  & - & \begin{tabular}{c} Delayed \\ (6 months) \\ \end{tabular}  & 21.70\% LW \\ \hline [PERSON], [PERSON] et al., 2017 & 32 & \begin{tabular}{c} SW, 30.9 mm\({}^{2}\) \\ LW, 73.7 mm\({}^{2}\) \\ \end{tabular}  & \begin{tabular}{c} SW 2.12 \\ LW 2.06 \\ LW 2.06 \\ \end{tabular}  & \begin{tabular}{c} Delayed \\ 6 month) \\ \end{tabular}  & \begin{tabular}{c} SW 18.8\% LW 25.0\% LW 25.0\% LW 25.0\% LW 4.2\% \\ \end{tabular}  \\ \hline [PERSON], [PERSON] et al., 2018 & 49 & \begin{tabular}{c} SW, 35.25 mm\({}^{2}\) \\ LW, 47.49 mm\({}^{2}\) \\ \end{tabular}  & \ \begin{tabular}{c} SW 1.32 \\ LW 1.46 \\ \end{tabular}  & Simultaneous &
\begin{tabular}{c} SW 8.0\% LW 4.2\% LW 4. When the window size was reduced, the possibility of encountering the intraosseu anastomosis located on the lateral wall of the maxillary sinus was also decreased. If arteries or arterioles are ruptured during surgery, maintaining a visual field becomes difficult. Thus, post-operative swelling, bleeding, and discomfort can show increases. In an imaging study  
Figure 5: (**a**) Healing state of a minimum-sized lateral window at 5 months after the surgery. Without utilizing barrier membrane, particles of anorganic bovine bone could be observed on bone surface and periosteum; (**b**) Healing state of a minimum-sized lateral window without utilizing barrier membrane at 48 months after the surgery (anorganic bovine bone graft).