Using finite element analysis, we evaluated the effects of implant number, attachment type, and framework material on stress distribution in bone and in implant and prosthetic components of mandibular overdentures in completely edentulous mandibles. Straumann implants (4.1 mm diameter and 12 mm length) were placed in a completely edentulous mandible in two configurations. For the two-implant models, implants were positioned axially in the lateral–canine region while for the three-implant models, one implant was placed at the midline and two were positioned bilaterally in the canine regions. Overdentures were retained with ball or locator attachments and fabricated with either a cobalt–chromium (Co-Cr) framework, a polyether-ether-ketone (PEEK) framework, or no framework (acrylic base only). Three-dimensional finite element analysis was performed for 12 models in total. Mastication was simulated by applying bilateral vertical loads to a rigid, circular food bolus (radius 1 cm): 100 N at the first-molar central fossae and 65 N at the canine incisal edges. The reported outcomes were maximum (tensile) and minimum (compressive) principal stresses in cortical and trabecular bone, and von Mises stresses in the implant and prosthetic components. Across all models, stresses were higher in cortical than in trabecular bone. Three-implant configurations yielded a more balanced stress distribution than two-implant configurations, with notably reduced midline stresses. When attachment types were compared, locator systems generated higher von Mises stresses in the implant and prosthetic components, whereas stress distribution within the surrounding bone was similar between ball and locator attachments. With respect to framework material, PEEK frameworks exhibited higher stress values than Co–Cr frameworks. This study shows that increasing the number of implants in overdentures improves load sharing, although it can shift the location of local stress concentrations. Greater framework rigidity is associated with lower and more homogeneous stress distributions in both bone and implant components.
Sai̇p et al. (Mon,) studied this question.