Abstract: BACKGROUND: There are many studies on the stress distribution of the Schneiderian membrane. However, domestic and international research on the stress analysis of the Schneiderian membrane mainly focuses on the stress-strain analysis during the maxillary sinus elevation surgery or of the isolated Schneiderian membrane, while the research on the stress-strain of the Schneiderian membrane during occlusion after maxillary sinus elevation and implant restoration is relatively rare.
OBJECTIVE: To investigate the stress distribution, stress peak, and displacement of the Schneiderian membrane during occlusion after maxillary sinus elevation and implant restoration with different remaining bone volumes in the maxillary molar area and under different bone quality conditions using finite element analysis. 
METHODS: Based on cone beam CT scan files of healthy adult volunteers, models of maxillary sinus lift and implants and related accessories were established. Four bone properties (class I-IV cancellous bone), two residual bone volumes in the maxillary first molar area (residual bone height 3 mm and 5 mm), three implant morphologies (large thread implant, cylindrical implant, and tapered implant) and three models after maxillary sinus lift [maxillary sinus lift without bone grafting or bone grafting but no bone formation after lift (S group), maxillary sinus lift with bone grafting around implant but no bone formation at the apex (T group), and maxillary sinus lift with bone grafting around implant and at the apex (U group)] were set. Finite element analysis was performed on the stress distribution, stress peak and displacement of Schneiderian membrane in each group of models under static load. 
RESULTS AND CONCLUSION: (1) When the bone quality gradually changed from Class I to Class IV, the Von Mises stress peak and displacement peak of Schneiderian membrane in each group of models showed a gradual upward trend, reaching the maximum value in Class IV bone. When the remaining bone volume in the maxillary first molar area was 3 mm, the bone quality was Class IV, and large threaded implants were implanted, the Von Mises stress peak of Schneiderian membrane in the S group model was the largest (0.558 MPa) and the displacement peak was the largest (278 μm). When the bone quality gradually changed from Class I to Class IV, the displacement peak of Schneiderian membrane in each group of models showed a gradual upward trend. Under the same bone quality conditions, the displacement peak of Schneiderian membrane in the models implanted with cylindrical, conical, and large threaded implants showed the order of S group > T group > U group. (2) The results show that when the remaining bone volume was small and the bone density was poor, large threaded implants should be used with caution for maxillary sinus lift surgery. When selecting the implant system before maxillary sinus lift surgery, the state of the maxillary sinus mucosa should be taken into consideration before surgery, and the implant plan should be planned accordingly. The height, density and bone formation of the remaining bone volume and the bone formation in the maxillary sinus determined the stress peak and displacement peak of the occlusal force transmitted to the Schneiderian membrane after implant restoration. The patient's remaining bone volume and bone density should be fully considered during maxillary sinus lift surgery. The Schneiderian membrane should be peeled off as much as possible during surgery to increase the bone formation area. The cylindrical implant and the conical implant had no significant effect on the stress distribution and displacement deformation of the Schneiderian membrane after maxillary sinus lift implant restoration. When the remaining bone volume was less than 3 mm for maxillary sinus lift implant restoration, the use of bone transplant materials could significantly reduce the implant mobility and the stress on the Schneiderian membrane during occlusion after restoration. When the remaining bone volume in the maxillary molar area was greater than 5 mm, the peak stress of Schneiderian membrane was much lower than the average perforation tension of healthy Schneiderian membrane, regardless of whether there was bone formation around the implant and at the top.

Key words: Schneiderian membrane, finite element analysis, bone density, biomechanics, maxillary sinus elevation surgery, bone condition