# Influence of housing retaining materials on fracture resistance of zirconia-reinforced implant overdentures: an in vitro study

## TL;DR

Adding zirconium oxide nanoparticles improves the strength of implant overdenture materials, especially when using autopolymerising PMMA.

## Contribution

Demonstrates that ZrO₂ nanoparticle reinforcement significantly enhances flexural strength of overdenture bases.

## Key findings

- 7% ZrO₂ reinforcement increased flexural strength by 75-90% compared to non-reinforced controls.
- Autopolymerising PMMA showed higher strength than UfiGel Hard as a retaining material.
- Nanoparticle dispersion and surface roughness increased with higher ZrO₂ concentrations.

## Abstract

To evaluate the influence of zirconium oxide (ZrO₂) nanoparticle reinforcement and housing retaining material on the flexural performance of implant-supported overdenture acrylic resin bases.

Thirty-six standardized bar-shaped heat-cured polymethylmethacrylate (PMMA) specimens were fabricated in accordance with ISO 20795-1 and divided into six groups (n = 6) based on ZrO₂ nanoparticle concentration (0%, 3%, and 7% by weight) and housing retaining material (autopolymerising PMMA or UfiGel Hard). Specimens were subjected to thermocycling to simulate oral aging and tested under three-point bending to determine flexural strength. Surface characteristics were qualitatively assessed using scanning electron microscopy (SEM) and atomic force microscopy (AFM). Data were analyzed using one-way ANOVA with Tukey's post hoc test (α = 0.05).

ZrO₂ nanoparticle reinforcement significantly enhanced flexural strength across both retaining materials. Specimens containing 7 wt% ZrO₂ demonstrated approximately 75%–90% higher flexural strength compared with non-reinforced controls, depending on the retaining material employed. All reinforced groups exceeded the minimum flexural strength requirement of 65 MPa specified by ISO 20795-1 for denture base polymers. Autopolymerising PMMA consistently exhibited higher flexural strength than UfiGel Hard. SEM revealed differences in nanoparticle dispersion with increasing concentration, while AFM demonstrated increased surface roughness at higher ZrO₂ levels. These improvements suggest enhanced resistance to housing-related fracture, a common clinical complication in implant-supported overdentures.

Zirconium oxide nanoparticle reinforcement significantly enhanced the flexural strength of implant-supported overdenture acrylic resin bases, with superior performance observed when autopolymerising PMMA was used as the housing retaining material. These findings suggest a material-based approach to improving overdenture base fracture resistance; however, clinical extrapolation should be approached with caution, and further studies incorporating fatigue loading and long-term aging are warranted.