Role of autogenous bone grafting on success of Bicon short implants- FE study.

Background Placement of harvested autogenous bone graft is a standard step in Bicon short implant insertion. The quality of bone graft remains unclear, though it was proven that its low grade may result in overstain (strain level above minimum effective strain pathological (MESp) by Frost). Moreover, implant prognosis depends on cortical and especially cancellous bone quality. Finite element (FE) method allows precise analysis of this complex biomechanical system from the viewpoint of bone turnover. Aim/Hypothesis The aim of the study was to evaluate the role of autogenous bone grafting around Bicon short implants on strain level in adjacent bone to predict implant success. Material and Methods 5.0 mm length and 4.0 (N), 5.0 (M), 6.0 (W) mm diameter Bicon SHORT ® implants were chosen for this analysis. Their 3D models were placed crestally in posterior maxilla models with type III bone. They were designed in Solidworks 2016 for 2 scenarios- 1.0 mm crestal sinus cortical and 4.0 mm cancellous bone (A), and 0.75 mm crestal sinus cortical and 4.25 mm cancellous bone (B). Materials were linearly elastic and isotropic. Cortical bone Young modulus was 13.7 GPa, cancellous bone – 1.37 GPa. Four degrees of autogenous bone graft quality were studied- 100% (E1 = 13.7 GPa), 75% (E2 = 10.3 GPa), 50% (E3 = 6.85 GPa) and 25% (E4 = 3.43 GPa). Bone-implant assemblies were analyzed in Solidworks Simulation. 3D FEs were generated with a total number of up to 4,062,000. 120.9 N mean maximal oblique load was applied to the center of seven Series Low 0° abutment. First principal strain (FPS) distributions were analyzed and correlated with 3000 microstrain MESp to evaluate implant success. Results Analysis of FPS distributions in cortical bone has shown that there were no overstrains at the implant neck area. Maximal FPSs were found at the first fin located in cancellous bone. They were influenced by implant diameter, bone graft quality and cortical bone thickness. For E1, E2, E3 and E4 bone quality FPS reduction due to diameter increase from 4.0 to 6.0 mm was 54.5, 57.1, 62.5, 71.4% for A, 56.0, 58.8, 65.0, 76.2% for B. Reduction of bone graft quality caused significant overstrain of cancellous bone- for N, M and W implants, FPS rise due to bone graft quality decrease (E1 versus E2) was 56.3 55.0, 52.4 58.3 and 67.7 69.4% (A B). Decrease of cortical bone thickness from 1.0 to 0.75 mm resulted in FPS rise- for N, M and W implants, it was 25.0, 42.9, 16.7% (E2), 21.4, 37.5, 16.7 (E3) and 13.6, 36.0, 14.6% (E4). Conclusion and Clinical Implications Under mean maximal functional loading, sufficient influence of bone graft quality on bone-implant interface was determined. Crestal bone overstrain was highly unlikely because FPSs were found to be less than 3000 microstrain threshold. Contrarily, E2-E4 bone graft quality is critical from the viewpoint of cancellous bone overstrain and implant failure. N implants were found to be the most susceptible to the graft quality. Placement of 6.0 mm diameter implant allows to decrease this risk.