An in vitro study using confocal laser scanning microscopy to evaluate the marginal misfits of different implant-supported frameworks.

PURPOSE: This study aimed to develop and evaluate a simple, non-destructive method for assessing the misfit and passivity of implant-retained prostheses frameworks. MATERIALS AND METHODS: To simulate the rehabilitation of a mandible posterior partially edentulous area using 3-unit screw-retained frameworks supported by two implants were fabricated and divided into the following five groups (n = 10 in each group): OP = one-piece framework cast in Co-Cr with the conventional method (control-group); Co-Cr frameworks sectioned and welded by laser (=LAS) or tungsten inert gas (=TIG); Co-Cr CAD-CAM = milled Co-Cr framework; Zir CAD-CAM = milled zirconia framework. The horizontal |X| and vertical |Y| misfits were measured using confocal laser scanning microscopy with one or both screws tightened. Data were analyzed by a two-way ANOVA with repeated measures and Bonferroni correction (α = 0.05). RESULTS: The greatest |X| misfit was observed in the OP group with both screws tightened (290 µm) and one screw tightened (388 and 340 µm). The conventional casting groups sectioned and welded by laser or TIG had lower mean values (235.35 µm, both screws tightened; and 275 µm, one screw tightened) than the OP framework. However, these values still exceeded those of the milled Co-Cr and zirconia frameworks (190 and 216 µm with both screws tightened). Across all reading conditions, every framework subjected to testing consistently maintained vertical |Y| misfit levels below the threshold of 53 µm; however, the milled frameworks exhibited higher vertical misfits than the frameworks obtained by the conventional cast method. CONCLUSIONS: The frameworks, whether cast and sectioned with laser welding or milled from Co-Cr, exhibit improved marginal misfit and enhanced passive fit when compared to other fabrication methods. Additionally, the use of confocal laser scanning microscopy is highly effective for passivity and misfit analysis.

Comparison of the correlation of photoelasticity and digital imaging to characterize the load transfer of implant-supported restorations.

STATEMENT OF PROBLEM: Whether splinting or not splinting adjacent implants together can optimize the stress/strain transfer to the supporting structures remains controversial. PURPOSE: The purpose of this study was to compare the photoelasticity and digital image correlation (DIC) in analyzing the stresses/strains transferred by an implant-supported prosthesis. MATERIAL AND METHODS: A polymethylmethacrylate model was made with a combination of acrylic resin replicas of a mandibular first premolar and second molar and threaded implants replacing the second premolar and first molar. Splinted (G1/G3) and nonsplinted (G2/G4) metal-ceramic screw-retained crowns were loaded with (G1/G2) and without (G3/G4) the presence of the second molar. Vertical static loads were applied to the first molar implant-supported crown (50 N-photoelasticity; 250 N-DIC). The resulting isochromatic fringes in the photoelastic models were photographed, and a single-camera 2-dimensional DIC system recorded the deformation at the surface of the resin models. RESULTS: Residual stresses were present in the photoelastic model after screw fixation of the crowns. The following average photoelastic stress results (MPa) were found around the loaded implant: G1 (20.06), G2 (23.49), G3 (30.86), G4 (37.64). Horizontal strains (εxx, %) between the molars averaged over the length of the loaded implant were found by DIC: G1 (0.08 ± 0.09), G2 (0.13 ± 0.10), G3 (0.13 ± 0.11), G4 (0.16 ± 0.11). Splinted crowns transferred lower stresses to the supporting bone when the second molar was absent. The second molar optimized the stress distribution between the supporting structures even for nonsplinted restorations. CONCLUSIONS: Both methods presented similar results and seemed capable of indicating where issues associated with stress/strain concentrations might arise. However, DIC, while apparently less sensitive than photoelasticity, is not restricted to the use of light-polarizing materials.

Digital image correlation analysis of the load transfer by implant-supported restorations.

This study compared splinted and non-splinted implant-supported prosthesis with and without a distal proximal contact using a digital image correlation method. An epoxy resin model was made with acrylic resin replicas of a mandibular first premolar and second molar and with threaded implants replacing the second premolar and first molar. Splinted and non-splinted metal-ceramic screw-retained crowns were fabricated and loaded with and without the presence of the second molar. A single-camera measuring system was used to record the in-plane deformation on the model surface at a frequency of 1.0Hz under a load from 0 to 250N. The images were then analyzed with specialist software to determine the direct (horizontal) and shear strains along the model. Not splinting the crowns resulted in higher stress transfer to the supporting implants when the second molar replica was absent. The presence of a second molar and an effective interproximal contact contributed to lower stress transfer to the supporting structures even for non-splinted restorations. Shear strains were higher in the region between the molars when the second molar was absent, regardless of splinting. The opposite was found for the region between the implants, which had higher shear strain values when the second molar was present. When an effective distal contact is absent, non-splinted implant-supported restorations introduce higher direct strains to the supporting structures under loading. Shear strains appear to be dependent also on the region within the model, with different regions showing different trends in strain changes in the absence of an effective distal contact.