Effect of two different soft liners and thicknesses mediating stress transfer for immediately loaded 2-implant supported mandibular overdentures: A finite element analysis study

Statement of problem. The use of soft liners as female receptacles for ball attachments retaining immediately loaded implant overdenture has been recommended to enhance osseointegration and provide a shock absorbing effect. However, which liner and which thickness is still unclear. Purpose. The purpose of this finite element analysis (FEA) study was to evaluate the effect of a 2 and 4-mm thickness of thermoplastic resin (TRL) and silicone-based liners (SBL) on the displacement and stresses transmitted to immediately loaded implants retaining a mandibular overdenture. Material and methods. Four 3-dimensional (3D) FEA models of a mandibular implant overdenture retained by 2 ball attachments (2 models for each lining material with 2- or 4-mm liner thicknesses) were developed. Implants were placed in the canine regions and surrounded by a 1-mm cylinder of immature bone simulating immediate loading. A vertical and an oblique load of 150 N were applied in the right premolar molar regions. Stresses and displacement were set as output variables. Results. Replacing the TRL by the SBL was associated with a decrease in stresses by 73% and in displacement by at least 46%. Increasing the thickness of any liner decreases stresses by 45% during vertical loading and by 25% during oblique loading and decreases displacement by 61.5% during vertical loading and by an average of 32.5% during oblique loading. None of the liners exceeded the experimental risk limits for micromotion at the bone-implant interface (50 mu m). Conclusion. In immediately loaded mandibular implant overdentures, both SBL and TRL decrease the micromotion of implants and the stresses at the bond-implant interface. However, SBL is more effective. The thickness of both liners seems to play a major role in decreasing the stresses and displacement of periimplant tissues.