Stress Analysis of Denture Designs and Implant Number on Overdentures

Objectives: The purpose of the study was to evaluate the effect of number of supporting implants and different retentive mechanisms on load transfer characteristics of mandibular overdentures.

Methods: Two photoelastic models of edentulous mandible were fabricated having 2 and 4 cylindrical implants (Calcitek, 4 x 13mm) embedded in the parasymphyseal area. Four attachment systems were evaluated: single anchor attachment (ERA), bar-clip, bar with distally placed ball attachments, and bar with distally placed extracoronal rigid attachments (Easy slot). A vertical force of 30 lb was applied unilaterally to the central fossa of the right first molar. The resulting stresses of the models were observed and recorded photographically in the field of a circular polariscope.

Results: The highest stresses were observed with bar with distally placed extracoronal rigid attachment (Easy slot) design, followed by bar-ball, bar and the single anchor attachment (ERA) respectively for both models. The lowest stress was observed with the single anchor attachment (ERA) design for both models. There were slight differences in stress values around implants in both models.

Conclusions: The single anchor attachment (ERA) transferred the lowest stresses and bar with distally placed extracoronal rigid attachment (Easy slot) system transferred the highest stresses to the implants on both models. Varying number of implants had no significant effect on stress values around supporting implants.