## 1 Introduction  
Dental implant treatment is needed to restore the function and aesthetics of an edentulous patient. Adequate quality and quantity of bone are necessary for the proper placement of the dental implant. After extraction of maxillary posterior teeth, bone atrophy and maxillary sinus pneumaticization gradually progress. Thus, insufficient bone for implant placement is frequently observed for a maxillary posterior alveolar bone [1]. For such cases, a maxillary sinus floor elevation (MSFE) can be considered to place the dental implant. The Schneiderian membrane which is located on the maxillary sinus cavity can be elevated and bone-graffing materials can be used to fill in the space between the elevated membrane and the maxillary sinus floor. This concept of MSFE was introduced by [PERSON] in 1977 [2]. Since then, various advanced techniques have been developed and progressed. A MSFE can be performed with a lateral or a crestal approach according to the method of accessing the Schneiderian membrane. With the crestal approach, the sinus floor is elevated through the site of implant installation. The Schneiderian membrane is elevated by graft material filling or hydraulic pressure. This technique has shown successful results in many cases when the height of the alveolar bone is more than 5 mm [3, 4]. A major disadvantage of the crestal approach is the lack of direct visibility of the maxillary sinus cavity. Unlike the crestal approach, the lateral approach needs the lateral window for better access and visibility into the sinus cavity. The Schneiderian membrane can be seen and evaluated through the lateral window. It can then be carefully detached and elevated from the sinus cavity. The graft material is filled into the space between the sinus floor and the elevated membrane. The lateral approach may be preferred when the alveolar bone height is less than 6 mm and/or when the crestal approach is difficult to perform. The lateral approach is a more invasive technique than the crestal approach because osteotomy should be performed for the formation of a lateral window. However, this technique has benefits as it is possible to directly observe and manipulate the Schneiderian membrane. A minimally invasive lateral approach is a technique that allows sinus bone augmentation with reduced window size. Various surgical techniques have been proposed to reduce the size of the lateral window. A slim-linear shape of the lateral window for a lateral approach with a simultaneous bone-added osteetome technique has been introduced to decrease the size of the lateral window while maintaining proper access to the instrument [5,6]. [PERSON], [PERSON] et al. (2019) have proposed a vertically oval shape window (4 \(\times\) 8 mm\({}^{2}\)) for a single implant and a horizontally oval shape window (10 \(\times\) 5 mm\({}^{2}\)) for multiple implants [7]. There was no difference in the obtained augmented bone height when the height of the window was 4 mm or 8 mm [8]. Two mini windows with a height of 5-6 mm in a horizontally oval shape (total size of 52 mm\({}^{2}\)) have demonstrated similar results to a conventional window technique [9]. It has been suggested that a minimally invasive lateral sinus elevation with a window size of about 3-5 mm in height can result in a favorable patient morbidity [10]. [PERSON], [PERSON] et al. (2017) have compared a small window (6 \(\times\) 6 mm\({}^{2}\)) with a large one (8 \(\times\) 10 mm\({}^{2}\)) and found that the small window can result in less postoperative discomfort [11]. In a retrospective study, [PERSON], [PERSON] et al. (2018) have used a small window (35 mm\({}^{2}\)) and a large one (47 mm\({}^{2}\)) and demonstrated no significant difference in terms of implant survival, augmented bone stability, or postoperative complication [12]. Although a reduced window dimension has demonstrated clinical success, reported minimally invasive lateral windows to differ in size, shape, and position.