Effect of anchorage systems and extension base contact on load transfer with mandibular implant-retained overdentures

Controversy exists regarding the effect of anchorage systems and extension base contact on stress transfer to multiple implants by mandibular overdentures.

This simulation study measured photoelastically the biologic behavior of 4 implants retaining a mandibular overdenture. The purpose of the investigation was to compare the load transfer characteristics of different mandibular-retained overdenture designs, with and without edentulous ridge contact.

A photoelastic model of a human edentulous mandible was fabricated having 4 screw-type implants (3.75 x 10 mm) embedded in the parasymphyseal area. Substructure designs utilizing a cantilevered bar, spark erosion framework, noncantilevered bar, and solitary anchors were fabricated. A vertical load of 30 lb was applied to the first molar unilaterally on each of the 4 standardized overdenture prostheses, with and without a silicone tissue spacer, for a total of 8 tested conditions. Stresses that developed in the supporting structure were monitored photoelastically and recorded photographically.

Without the simulated tissue contact on the posterior edentulous ridge, the cantilevered bar framework caused the highest stresses to the terminal implant, followed by spark erosion framework, non-cantilevered bar, and solitary anchor design. With simulated tissue contact under the extension base, stress transfer to the distal implant was uniformly reduced to a low level.

Without intimate extension base contact with the posterior edentulous ridge, the cantilevered anchorage systems generated the highest stresses, under load, to the ipsilateral terminal implant and the solitary anchor design transferred the least. With simulated intimate extension base contact, all anchorage systems transferred low stress to the distal implant region. For all conditions and designs, low stress was transferred to the contralateral side of the arch.