Abstract: BACKGROUND: Bone tissue remodeling is closely related to stress loading. Currently, there are few studies or guidelines on the relationship between bone and occlusal adjustment of implant prostheses and there is also a lack of scientific evidence.
OBJECTIVE: To investigate the effects of different implant occlusal gaps on stress distribution, stress peak and displacement at the implant-bone interface under I-IV bone conditions by a finite element method.
METHODS: After scanning the equal-scale tooth model with an optical scanner, equal-scale models of the upper right first molar Straumann 4.8×8 mm BL RC implant and its related components was constructed using Solidworks 2022. Then, using Mimics, Geomagic, and Solidworks software, the maxillary and mandibular bone models of class I-IV bones were established based on the bone classification proposed by ZARB and LEKHOLM in the literature, and the NORTON and TRISI bone density classification method. The models were assembled with the occlusal gaps of 0, 20, 40, 60, 80, and 100 μm for the restorations, and an additional set of homogeneous models without density ratio settings was constructed for comparison. After the above models were imported into Hypermesh for meshing, the material assignment, boundary constraints and parameter setting were performed for the finite element analysis. Finally, 250 N was used as the loading force to simulate the maxillary and mandibular stress conditions. Stress distribution, peak stress and displacement of the implant-bone interface in each group of models were analyzed and compared.
RESULTS AND CONCLUSION: Under the same loading conditions, the stresses in the implant restorations were evenly distributed with the occlusal contact points. When the occlusal gap reached 80 and 100 μm, stress interruptions occurred in the implant crowns under class I bone and class II, III and IV bones, respectively. The displacement of the implant-bone interface was mainly concentrated in the cortical bone region around the implant and transmitted down the long axis of the implant to the cancellous bone region at the bottom. With the changes of class I-IV jaw bones, the displacement and Von Mises stress in the cortical bone region increased in all groups, and were greater than those in the cancellous bone region. The Von Mises stress in the cancellous bone region was similar to that in the cortical bone region except that it showed a downward trend from class II bone. However, when the occlusal gap increased, the stress and displacement peak values in the cortical bone and the cancellous bone showed a decreasing trend. The stress of the implant-bone interface was between 20 MPa and 60 MPa when the occlusal gap was 0-40 μm for class II-IV bones and 60 μm for class IV bone, and the stress of the other groups was less than     20 MPa. The Von Mises stress was mainly concentrated in the neck of the implant, and the peak value of von Mises stress in class II-IV bones with the occlusal gap of 20 μm was higher than that (144.10 MPa) in class I bone with the occlusal gap of 0 μm. In the homogeneous model with different elastic moduli, the distribution of stress and displacement was more uniform than that in the heterogeneous model and the occlusal space should increase with the decrease of jaw bone density in clinical practice. To conclude, from the perspective of biomechanics, the alveolar bone should be taken into account in the occlusal adjustment of implant denture. An occlusal gap of 20-40 μm between a single dental implant and a natural tooth in the opposite jaw is a relatively suitable solution for occlusal adjustment under different bone conditions. However, due to the particularity of finite element analysis method, it needs to be further studied in combination with clinical practice. 

Key words: implant, bone, stress, occlusal gap, occlusal contact, three-dimensional finite element