## 4 Discussion  
The VARO guide is a novel, fully-guided implant placement system characterized by the in-house, model-free, and zero-setup approaches. The present pilot study was performed to evaluate the accuracy and feasibility of the VARO guide system in the atrophic posterior maxilla for fully-guided flapless implant placement with simultaneous crestal sinus graft. From the four implants that had been placed using the VARO guide, the main findings of the study were: (i) the VARO guided surgery exhibited a range of deviation from the planned position of around 1 mm, (ii) simultaneous crestal sinus graft can be performed safely and without complication, and (iii) good stability was achieved for the placed implants at the initial and late stages. According to a recent meta-analysis of 669 implants placed in partially edentulous areas, the mean global deviation values were 2.68\({}^{\circ}\) in angle; 1.03 mm coronally; 1.33 mm apically; and 0.59 mm vertically [12]. In the current study, the deviation values were 4.76\({}^{\circ}\) in angle, 1.27 mm horizontally at the platform, 1.85 mm horizontally at apex, 0.84 mm vertically at the apex. The deviation values revealed here were slightly larger than that of the meta-analysis. This could be due to the fact that these implants were placed in a challenging situation of the atrophic posterior ridge, where there is a lack of bone support at the apical region of the implant fixture. Furthermore, all of the cases in this study were in the posterior region, the majority of them in the most posterior tooth of the arch, whereas the data from the meta-analysis was a combination of data from the anterior region. There is evidence in the literature that guided surgery in the anterior region produces more accurate results compared to the posterior since implant placement procedures in the molar region are more often faced with difficulties related to access and vision [13]. For evaluation of the guide suitability for clinical application, two criteria can be considered: (i) the possibility to deliver the immediate provisional prosthesis by passive fit (i.e., without chairside correction), and (ii) the safety margin deduced from the deviation data of the surgical guide. The recorded values in the present study were greater than the prosthetic requirement for the passive fit of immediate provisional restorations, which has been reported to be around 0.1-0.15 mm [14; 15; 16]. Achieving such a high degree of precision seems difficult  
Figure 5: Implant stability quotient (ISQ) values measured using the resonance frequency analysis. Case 1 having the least remaining vertical ridge height showed the least initial stability, and vice versa for Case 4.  
despite the current technological advances. Concerning safety, the vertical deviation at the apex is probably the most relevant outcome as it affects the proximity to vital anatomical structures [12]. With that being said, the vertical deviation of less than 1 mm demonstrated using the VARO guide could be considered \"safe\" in regards to preventing injuries to the inferior mandibular nerve and perforation of the Schneiderian membrane. Case 4, which was placed in the most-posterior single tooth region at the upper second molar, was the most accurate in all aspects among the four cases. This case also showed the greatest remaining ridge height of 8.39 mm. Case 2 had a comparable remaining ridge height of 8.19 mm; however, it was the least accurate in the vertical aspect. This was owed to the limited mouth opening in Case 2 and the subsequent difficulty in maneuvering the drill. Design solutions of the surgical guide to overcome the lack of vertical dimension include opening the buccal side of the drill sleeve to allow drill entry and reducing the sleeve length. Whether the buccal opening of the drill sleeve affects accuracy has been investigated previously [13]. In that study, the use of the open sleeve design resulted in less accurate implant placement in the posterior region.