Photoelastic Stress Distribution Produced by Different Retention Systems for a Single-Implant Mandibular Overdenture

Purpose: For patients poorly adapted to conventional dentures, the single-implant mandibular overdenture has been proposed as a simplified alternative for the two-implant treatment, together with the use of immediate loading of the implant. The aim of this in vitro study was to compare the photoelastic characteristics of stress transfer around the implant in a single-implant mandibular overdenture using different types of attachments. Materials and Methods: A photoelastic model of an edentulous mandible with a resilient edentulous ridge and a unique implant located at the symphyseal region was obtained to reproduce a single implant-retained mandibular overdenture. Six 2.0 mm height attachments were selected and inserted in the same photoelastic model in a random order. A universal testing machine was used to induce axial vertical loads of 70 Ncm applied at the central incisor and at the central region of the first right molar without contact on the contralateral side. The photoelastic analysis was performed using a polariscope integrated into the testing machine. Standard separate views were photographed, using only one model per system. A visual qualitative analysis of stress-induced fringes was performed to comparatively rank the different attachment systems. Results: All attachments showed a similar tension distribution concentrated in the apical third, and the highest stress concentration was at the apical level. There was a low stress concentration at the coronal third of the implant, with no discernible stress in the first threads of the implants, except for Dalla Bona, which showed low stress at the coronal part of the implant (1 fringe). No identifiable fringes were observed when the load was applied in the molar region, which resulted from the low amount of stress transmitted by the implant for all attachments. Conclusion: The load transferred to a single mandibular implant was evenly distributed around the implant with low stress concentration, irrespective of the type of retention system.