Prospect of short plateau implants in atrophic posterior maxilla- biomechanical study

Background

Poor bone quality and anatomic restrictions significantly influence implant success in posterior maxilla. Short implants were proposed as a reasonable choice. Implant prognosis is predetermined by stress magnitudes in bone-implant interface, which are sensitive to bone and implant parameters. Plateau implants are often preferred since they reduce bone stresses and improve implant prognosis. Precise analysis of complex biomechanical systems can only be performed by finite element (FE) method.

Aim/Hypothesis

The aim of the study was to evaluate and compare the prospect of different short plateau implants placed in atrophic posterior maxilla under 120.92 N mean maximal functional load (Mericske-Stern & Zarb, 1996).

Material and Methods

5.0 mm length and 4.0 (N), 5.0 (M), 6.0 (W) mm diameter Bicon SHORT ® implants were studied. Their 3D models were placed in eighteen posterior maxilla segment models with types III and IV bone. They were designed in Solidworks 2016 software and had three geometries- (A) 1.0 4.0 mm, (B) 0.75 4.25 mm and (C) 0.5 4.5 mm cortical cancellous bone layer, their size was 10 × 30 × 19 mm. Implant and bone were assumed as linearly elastic and isotropic. Elasticity modulus of cortical bone was 13.7 GPa, cancellous bone – 1.37 0.69 (type III IV). Bone-implant assemblies were simulated in FE software Solidworks Simulation. 4-node 3D FEs were generated with a total number of up to 5,064,000. 120.92 N mean maximal oblique load (molar area) was applied to the center of 7.0 mm abutment. Von Mises equivalent stress (MES) distributions were studied to determine areas of bone overload with magnitude greater than 100 MPa in cortical and 5 MPa in cancellous bone adopted as bone tissues ultimate strength.

Results

MES maximal values were found in crestal bone. The spectrum of maximal MESs in cortical bone was between 17 MPa (III, A, W) and 55 MPa (IV, C, N). They were influenced by cortical bone thickness, bone quality and implant dimensions. MES reduction due to cortical bone thickness increase from 0.5 to 1.0 mm was 25, 35, 17% for N, M and W implants and type IV bone, while for type III it was 25, 34, 19%. Cancellous bone quality was found to have a substantial impact on biomechanical state of cortical bone- two-fold reduction of elasticity modulus (1.37 versus 0.69 GPa) corresponded to 24.2, 30.2 and 26.5% MES rise for N, M and W implants and 1.0 mm cortical bone, 26.6, 23.6 and 20.5% MES rise for N, M and W implants and 0.75 mm cortical bone, and 25.0, 23.1 and 23.8% MES rise for N, M and W implants and 0.5 mm cortical bone. MESs magnitudes in cancellous bone were found below its ultimate strength (5 MPa) only for M and W implants placed into 1.0 mm cortical bone.

Conclusions and Clinical Implications

Stresses in posterior maxilla were influenced by cortical bone thickness, bone quality and especially implant diameter. Under 120.92 N load and 0.5–1.0 mm cortical bone, failure of 4.0 × 5.0 mm, 5.0 × 5.0 mm, 6.0 × 5.0 mm Bicon SHORT® implants was highly unlikely from the viewpoint of cortical bone overload. To avoid cancellous bone overstress, both 5.0 × 5.0 and 6.0 × 5.0 mm implants were found applicable, but only in case of 1.0 mm cortical bone.