Outlook of short finned implants in the posterior maxilla- the role of cortical bone thickness

Background Short finned implants are often applied in critical cases of edentulous posterior maxilla with no available bone for subcrestal implant placement. Resulting higher stresses in crestal cortical bone lead to its overload and subsequent implant failure. Finite element (FE) method provides biomechanical evaluation of bone-implant structures and influence of bone quality and implant parameters on bone stresses. Aim/Hypothesis The aim of the study was to evaluate the impact of crestal placement of short finned implants on stress magnitudes in atrophic posterior maxilla under oblique functional loading to predict bone overload and implant failure. Material and Methods Four Bicon Integra-CPô implants with 4.5, 6.0 mm diameter and 5.0, 8.0 mm length were selected for this evaluation. Their 3D models were placed in 12 posterior maxilla segment models with type IV bone, 1.5, 1.0 and 0.5 mm crestal cortical bone thickness. These models were designed using CT images in Solidworks 2016 software. Implant and bone were assumed as linearly elastic and isotropic. Elasticity modulus of cortical bone was 13.7 GPa, cancellous bone - 0.69 GPa. Bone-implant assemblies were analyzed in FE software Solidworks Simulation. 4-node 3D FEs were generated with a total number of up to 2,820,000. 120.92 N mean maximal oblique load (molar area) was applied to the center of 7 Series Low 0° abutment. Von Mises equivalent stress (MES) distributions in surrounding bone were studied to determine the areas of bone overload with magnitude > 100 MPa in cortical bone. Results Maximal magnitudes of MESs were found in crestal cortical bone. The spectrum of maximal MESs was between 14.0 and 47.0 MPa. The highest MESs were found for 4.5◊5.0 mm implant, while the smallest magnitudes were determined for 6.0 × 8.0 mm implant. For tested implants, maximal MES magnitudes were significantly influenced by cortical bone thickness and implant dimensions- for 0.5, 1.0 and 1.5 mm cortical bone thickness and diameter increase from 4.5 to 6.0 mm, MES drop was 48, 47, 43% for 5.0 mm length implants, while for 8.0 mm length implants it was 48, 48, 46%. In 0.5 mm cortical bone thickness, maximal MESs varied within 15-17% range. For 4.5 6.0 mm diameter implants, bone thickness increase from 0.5 to 1.5 mm corresponded to 43 35% MES drop for 5.0 mm length implants and 33 30% - for 8.0 mm length implants, while two-fold bone thickness increase from 0.5 to 1.0 mm corresponded to 21 19% MES drop for 5.0 mm length implants and 15 15% - for 8.0 mm length implants. Conclusion and Clinical Implications The study enhances perception of bone stress issues in the posterior maxilla relative to cortical bone thickness. Evaluated finned implants have not exceeded 100 MPa ultimate cortical bone strength. For 1.5 mm cortical bone thickness, no sensitivity to implant length was found. This FE study approves the finned implants clinical success in posterior maxilla due to their low susceptibility to poor bone quality. It provides a rationale for appropriate implant selection.