Influence of Implant Number on Peri-Implant and Posterior Edentulous Area Strains in Mandibular Overdentures Retained by the New Ti–Zr (Roxolid®) Mini-Implants as Single-Units: In Vitro Study

The new Ti–Zr (Roxolid®) mini-implants have not yet been fully researched. We analyzed peri-implant and posterior edentulous area microstrains during mandibular overdenture (OD) loading at different sites with different extents of forces when one-, two-, three-, or four- mini dental implants (MDIs) as single-units supported the respective ODs. The models were designed from cone beam computed tomography (CBCT) scans of an appropriate patient with narrow ridges. The mucosal thickness was 2 mm. Strain gauges were bonded on the vestibular and oral peri-implant sites, and in the distal edentulous area under the saddles. The loads were applied posteriorly bilaterally and unilaterally with 50, 100 and 150 N forces, and anteriorly with 50 and 100 N forces. Each loading was repeated 15 times. Statistical analysis included descriptive statistics, boxplots and the MANOVA. Higher forces induced higher peri-implant microstrains, as well as unilateral loadings, especially on the loaded side, in all models except the one-MDI model where anterior loads (100 N) elicited the highest peri-implant microstrain (1719.35 ± 76.0). The highest microstrains during unilateral posterior loading (right side) with 150 N force were registered from the right MDI in the two-MDI model (1836.64 ± 63.0). High microstrains were also recorded on the left side (1444.48 ± 54.6). By increasing the number of implants, peri-implant microstrains and those in the edentulous area decreased. In the three- and four-MDI models, higher microstrains were found in the posterior than in the anterior MDIs under posterior loadings. None of the recorded microstrains exceeded bone reparatory mechanisms, although precaution and additional research should be provided when only one or two MDIs support ODs.