Biomechanical behaviour and retention of 2-implant mandibular overdentures with different positions, angulations, and diameter of implants: A narrative review

Objectives: Two-implant retained mandibular overdenture (2IMO) is a popular treatment option in completely edentulous patients. The purpose of this review is to compare varying implant position, angulation, and diameter on biomechanical behavior, crestal strain, and retentive strength of 2IMO. Materials and Methods: The literature search was conducted using a single search term “Mandibular implant overdenture” in the PubMed-MEDLINE and Cochrane Central Register of Controlled Trials (CENTRAL) databases. All types of clinical or laboratory studies related to different positions, angulations, and diameter of the implants used in 2IMO were included. The insights of the studies about stress and strain evaluation and the retention of the 2IMO patients using different implant positions, angulations, and diameters were presented. Different methodologies and radiographic techniques that evaluated crestal bone level changes in clinical studies using 2IMO were reviewed. Results: Total 2238 studies in PubMed and 535 studies in Cochrane CENTRAL databases appeared during the initial search and 21 studies (14 on biomechanical behavior and bone loss and 7 on retention) were shortlisted to evaluate different outcomes in 2IMO with different positions, angulations, and diameter of implants. The studies regarding biomechanical behavior indicated increased stresses with increased positions of the implants from midline and increased angulations. The studies regarding the diameter of the implants have shown better stress distribution pattern with standard sized (or greater diameter implants) than reduced diameter of implants. Higher survival rate was observed with standard sized implants than mini-implants and greater bone loss was observed with premolar position than canine position. The retention is increased with increase in the position of the implants from midline. The retention of the 2IMO found to have contradicting results with different angulations of implants. Conclusions: In 2IMO, increased stresses were observed with increase in position of the implants from midline and increased angulations of the implants. The retention values were also increased with increase in position of the implants from midline. Inconclusive results were found with different angulations of the implants. Further clinical and in vitro studies are advocated to evaluate retention of the 2IMO with different angulations, positions, and diameter of the implants.