Fatigue resistance of yttria-stabilized tetragonal zirconia polycrystal clasps for removable partial dentures

Different Undercut Depths Influence on Fatigue Behavior and Retentive Force of Removable Partial Denture Clasp Materials: A Systematic Review

PURPOSE

To perform a systematic review that provides an overview of the current literature on fatigue behavior of removable partial denture (RPD) clasp materials based on different retentive areas.

MATERIALS AND METHODS

The review followed the Preferred Reporting Items for Systematic Review and Meta-Analyses (PRISMA) guidelines. Electronic searches were done via PubMed, Scopus, and OVID for studies reporting on RPD clasps and the fatigue failure of clasp materials. Inclusion criteria were English language with full text and in vitro studies only. Exclusion criteria were studies that did not assess the fatigue of RPD clasp materials. A quality assessment and selection of full-text articles were performed according to Consolidated Standards of Reporting Trials criteria.

RESULTS

A total of 182 articles were initially identified and screened. After applying inclusion and exclusion criteria, 15 articles were selected for the final analysis. Seven of the included studies utilized a vertical insertion/removal testing approach. Ten studies used the constant deflection test. Three studies used untapered specimens, and 12 studies used tapered specimens. Ten studies performed post-test analysis.

CONCLUSIONS

Cobalt-chromium (CoCr) is the strongest material in terms of fatigue in relation to the undercut depth and the modern, digitally manufactured RPD clasp materials also exhibit comparable fatigue behavior. Recent RPD clasp materials such as polyetheretherketone (PEEK) or laser sintered CoCr, however, require further study in terms of their fatigue behavior. In order to improve the quality of future studies, a standardized and calibrated fatigue testing method is needed with standardized specimen size and shape, which will reduce the risk of bias and enable meta-analysis for bulk comparison between studies.

Fatigue behavior of removable partial denture cast and laser-sintered cobalt-chromium (CoCr) and polyetheretherketone (PEEK) clasp materials

OBJECTIVE

To investigate the fatigue behavior of cast and laser-sintered cobalt-chromium (CoCr) and polyetheretherketone (PEEK) material for a removable partial denture (RPD) clasps.

METHODS

Dumbbell-shaped specimens were digitally designed with the center part of the dumbbell being a half-round shape at the cross-sectional dimension of 1.25 mm to simulate a typical clasp design and dimensions. A fatigue machine with an offset axis rotation system simulated a typical undercut depth of 0.25, 0.50, and 0.75 mm. Each group was subjected to 30,000 fatigue cycles (simulating 21 years) or till specimen failure. Before testing, the stress value at each undercut depth for each specimen was established in a universal testing machine and SN curves were plotted for each group. Data were statistically analyzed using Kruskal-Wallis and post hoc tests. The fractured surfaces were analyzed using SEM.

RESULTS

The average fatigue cycles with 0.25, 0.50, and 0.75 mm undercuts were 27,155 ± 6277, 9298 ± 4033, 5642 ± 8785 for cast CoCr and 26,765 ± 6150, 11,318 ± 7931, 2861 ± 4803, for laser-sintered CoCr, respectively. Apart from three specimens, the PEEK groups did not fail during the simulation period. Clasps with 0.25 mm deflection showed significantly higher fatigue resistance than other groups (p < .001). There was no significant difference in fatigue behavior between the cast and laser-sintered CoCr. Microporosities at the fractured site along with irregular crack propagation were observed for cast and laser-sintered CoCr specimens. Fatigue-induced broken polymer crosslinking chains were observed in PEEK specimens.

CONCLUSION

PEEK material exhibited the highest fatigue resistance and significantly lower deflection resistance. Cast and laser-sintered CoCr showed similar fatigue resistance and behavior.

Comparison between Integrated and Parallel Interlock Designs of an Extracoronal Attachment-retained Distal Extension Removable Partial Dentures: A Clinical Trial

Objective

Precision attachments may exert unfavorable stresses on abutments in distal extension bases. This study compared between two reciprocation designs in attachment removable partial dentures (RPDs).

Materials and Methods

Fourteen patients were allocated into two groups. Each patient received an attachment-retained RPD with one of the two types of attachments being studied. Group I received the integrated interlock type of reciprocation and group II received the parallel interlock type. Abutments were examined for modified plaque index, modified bleeding index, periodontal probing pocket depth, clinical attachment level, and modified papillary bleeding Index.

Results

Comparisons of periodontal parameters between mesial and distal abutments within each group revealed no statistically significant difference. Means of these parameters were used for the comparisons. There was a significant difference at P < 0.05 in all parameters between the two groups at time of insertion and at 3, 6, and 9 months of follow-up with values of group (II) higher than group (I).

Conclusion

RPDs of both designs showed an increase in periodontal parameters. Integrated interlock design showed better scores. It is preferable to use the attachment-retained RPD with integrated interlock instead of parallel interlock design.

Bonding and Thermal Cycling Performances of Two (Poly)Aryl-Ether-Ketone (PAEKs) Materials to an Acrylic Denture Base Resin

Poly(aryl–ether–ketone) materials (PAEKs) are gaining interest in everyday dental practices because of their natural properties. This study aims to analyze the bonding performance of PAEKs to a denture acrylic. Testing materials were pretreated by grinding, sandblasting, and priming prior to polymerization with the denture acrylic. The surface morphologies were observed using a scanning electron microscope and the surface roughness was measured using atomic force microscopy. The shear bond strength (SBS) values were determined after 0 and 2500 thermal cycles. The obtained data were analyzed using a paired samples t-test and Tukey’s honestly significant difference (HSD) test (α = 0.05). The surface characteristics of testing materials after different surface pretreatments showed obvious differences. PAEKs showed lower surface roughness values (0.02–0.03 MPa) than Co-Cr (0.16 MPa) and zirconia (0.22 MPa) after priming and sandblasting treatments (p < 0.05). The SBS values of PAEKs (7.60–8.38 MPa) met the clinical requirements suggested by ISO 10477 (5 MPa). Moreover, PAEKs showed significantly lower SBS reductions (p < 0.05) after thermal cycling fatigue testing compared to Co-Cr and zirconia. Bonding performance is essential for denture materials, and our results demonstrated that PAEKs possess good resistance to thermal cycling fatigue, which is an advantage in clinical applications. The results imply that PAEKs are potential alternative materials for the removable of prosthetic frameworks.

Color accuracy of different types of monolithic multilayer precolored zirconia ceramics

STATEMENT OF PROBLEM

The use of monolithic multilayer precolored zirconia facilitates the production of esthetic restorations in a convenient and straightforward manner. However, the accuracy of the final color is not clear.

PURPOSE

The purpose of this in vitro study was to determine the color accuracy of different types of monolithic multilayer precolored zirconia ceramics of varying thicknesses.

MATERIAL AND METHODS

Eighty cubical A2-shade monolithic multilayer precolored zirconia ceramic specimens (15×15 mm) of 2 different thicknesses (1.0 mm and 1.5 mm) and 4 zirconia brands (UPCERA EXPLORE [UPEX], KATANA Zirconia STML [STML], Enamel ZR Multi-5 [EZM5], and Aidite 3D Pro Zir [A3DM]) were fabricated and divided into 8 groups (n=10). The Commission Internationale de l'Eclairage (CIELab) values were measured against 3 different backing substrates (gray, transparent, and A2) by using a spectrophotometer. The color difference (ΔE) between the backing substrates of each group and the Vita A2 Shade Guide, translucency parameter (TP), and chroma (C) values were calculated to evaluate the accuracy of the final color. Statistical analysis was performed by using ANOVA and the post hoc Tukey HSD tests (α=.05).

RESULTS

The ΔE values for UPEX and STML exceeded the clinically acceptable thresholds for gray and transparent backing substrates (ΔE>3.7) and were higher than those for the A2 backing substrate. A3DM showed less ΔE from the shade guide for all the backing substrates (P<.05), and the corresponding ΔE values were within the clinically acceptable threshold (ΔE<3.7). The TP value was inversely proportional, and the C value was proportional to thickness. For the zirconia ceramics with identical thicknesses, UPEX and STML exhibited the highest TP values, and A3DM showed the lowest C value (P<.05).

CONCLUSIONS

At a specific thickness, color accuracy was mainly affected by the type of monolithic multilayer precolored zirconia ceramic, and the high transparency of ceramics will cause color differences.

Laser-Milled Microslits Improve the Bonding Strength of Acrylic Resin to Zirconia Ceramics

Heightened aesthetic considerations in modern dentistry have generated increased interest in metal-free “zirconia-supported dentures.” The lifespan of the denture is largely determined by the strength of adhesion between zirconia and the acrylic resin. Thus, the effect on shear bond strength (SBS) was investigated by using an acrylic resin on two types of zirconia ceramics with differently sized microslits. Micromechanical reticular retention was created on the zirconia surface as the novel treatment (microslits (MS)), and air-abrasion was used as the control (CON). All samples were primed prior to acrylic resin polymerization. After the resin was cured, the SBS was tested. The obtained data were analyzed by using multivariate analysis of variance(α = 0.05). After the SBS test, the interface failure modes were observed by scanning electron microscopy. The MS exhibited significantly higher bond strength after thermal cycles (p < 0.05) than the CON. Nevertheless, statistically comparisons resulted in no significant effect of the differently sized microslits on SBS (p > 0.05). Additionally, MS (before thermal cycles: 34.8 ± 3.6 to 35.7 ± 4.0 MPa; after thermal cycles: 26.9 ± 3.1 to 32.6 ± 3.3 MPa) demonstrated greater SBS and bonding durability than that of CON (before thermal cycles: 17.3 ± 4.7 to 17.9 ± 5.8 MPa; after thermal cycles: 1.0 ± 0.3 to 1.7 ± 1.1 MPa), confirming that the micromechanical retention with laser-milled microslits was effective at enhancing the bonding strength and durability of the acrylic resin and zirconia. Polycrystalline zirconia-based ceramics are a newly accessible material for improving removable prosthodontic treatment, as the bond strength with acrylic resin can be greatly enhanced by laser milling.

Effect of different materials and undercut on the removal force and stress distribution in circumferential clasps during direct retainer action in removable partial dentures

OBJECTIVES

This study aimed to evaluate the effect of different materials and undercut on the removal force and stress distribution in the supporting tooth and in the circumferential clasp used in removable partial prosthesis.

METHODS

Upper molars prepared for Akers circumferential clasp with retention and opposing arm were modeled, scanned, elaborated with CAD software and the geometries imported in FEA and analyzed. Six different materials were selected for the clasp (Polyamide, Polyoxymethylene, Polyetheretherketone - PEEK, Gold alloy, Titanium and CoCr) and 3 different undercuts (0.25, 0.50 and 0.75mm), totaling 18 groups.

RESULTS

The clasps presented greater stress in their structure and potentially greater damage to the dental enamel when made with rigid materials and with more undercut; however, they presented greater ability to remain in position.

SIGNIFICANCE

Polyamide with a higher undercut is an esthetic alternative to rigid metallic clasps. It showed promising behavior because it strongly reduces the damage to the enamel, and even with an undercut of 0.75, the retention is lower than for CoCr with a 0.25 undercut, and this retention might still be sufficient. Polyoxymethylene and Polyetheretherketone (PEEK) are not suitable materials for the clasps, because the maximum stress occurring during removal with higher undercuts is higher than the material strength.