Stress transfer of four mandibular implant overdenture cantilever designs

The influence of implant number and cantilever design on stress distribution on bone has not been sufficiently assessed for the mandibular overdenture.

The purpose of this simulation study was to measure, photoelastically, the biologic behavior of 2 or 3 implants retaining different designs of cantilevered bar mandibular overdentures and to compare load characteristics.

Two photoelastic models of a human edentulous mandible were fabricated having 2 or 3 screw-type implants (Nobel Biocare, 3.75 x 10mm) embedded in the parasymphyseal area. Bar frameworks using a 7-mm cantilever were fabricated for both models. A clip-retained and a plunger-retained (SwissLoc) prosthesis were fabricated as superstructures for each framework. Vertical loads of 15 and 30 pounds were applied unilaterally to the first molar and 15 pounds to the first premolar on each of the 4 standardized overdenture prostheses. The cantilever was removed from the 2-implant framework and the clip-retained prosthesis was loaded similarly on the first molar with 25 pounds. Stresses that developed in the supporting structure were monitored photoelastically and recorded photographically.

While all 4 prostheses demonstrated low stress transfer to the implants, the plunger-retained prosthesis caused more uniform stress distribution to the ipsilateral terminal abutment compared to the clip-retained prosthesis and provided retention security under tested loads. The plunger-retained prosthesis retained by 2 implants provided better load sharing from the ipsilateral edentulous ridge than the clip-retained prosthesis retained by 3 implants, and lower resultant stresses were seen on the implants.

Under load, all prosthetic designs demonstrated a low stress transfer to the ipsilateral abutment and to the contralateral side of the arch. The plunger-retained prosthesis retained by 2 implants demonstrated a more uniform stress transfer to the ipsilateral terminal abutment than the clip-retained prosthesis retained by 3 implants and provided more retention, given the implant configuration, prosthetic design and arch form.