Fracture Load of Metal, Zirconia and Polyetheretherketone Posterior CAD-CAM Milled Fixed Partial Denture Frameworks

Improving accuracy and efficiency of the machined PEEK denture based on NSGA-II integrated GABP neural network

OBJECTIVES

The polymer polyetheretherketone (PEEK) is gradually being used in dental restorations because of its excellent mechanical properties, chemical resistance, fatigue resistance, thermal stability, radiation translucency and good biocompatibility. To process PEEK dentures with lower surface roughness as quickly as possible, the non-dominated sorting genetic algorithm-II (NSGA-II) integrated genetic algorithm back propagation (GABP) neural network was proposed, which can adjust the combination of process parameters for milling PEEK dentures.

METHODS

The PEEK machining was conducted using a four-axis dental milling machine at different process parameters. The surface roughness of PEEK dentures was characterized using surface roughness profiler and scanning electron microscopy (SEM). The optimum machining performance of milling PEEK dentures was investigated using a multi-objective optimization model named as NSGA-II integrated GABP neural network algorithm. The surface roughness (Ra) and material removal rate (MRR) were used as optimization objectives.

RESULTS

The multi-objective optimization model effectively improved surface roughness and machining efficiency for milling PEEK dentures. The validation experiments showed that the surface roughness of all PEEK dentures was less than 0.2μm, which was within the range of surface roughness set in this paper. The GABP surface roughness prediction model had an average error of 6 %. For the same surface roughness value, the optimized milling parameters all had a greater material removal rate.

SIGNIFICANCE

The research results can improve current PEEK denture CAD/CAM technology by providing appropriate milling parameters using NSGA-II integrated GABP algorithm.

Effect of cement spacer on fit accuracy and fracture strength of 3-unit and 4-unit zirconia frameworks

BACKGROUND

Cement spacer is essential for compensating deformation of zirconia restoration after sintering shrinkage, allowing proper seating and better fracture resistance of the restoration. Studies assessing the effect of cement spacer on fit accuracy and fracture strength of zirconia frameworks are missing in the literature. Therefore, the aim of this study was to evaluate the effect of different cement spacer settings on fit accuracy and fracture strength of 3-unit and 4-unit zirconia frameworks.

METHODS

Sixty standardized stainless-steel master dies were manufactured with 2 prepared abutments for fabricating 3-unit and 4-unit zirconia frameworks. The frameworks were assigned into 6 groups (n = 10) according to cement spacer setting (30 μm, 50 μm, and 80 μm) as follows: 3-unit frameworks; 3u-30, 3u-50, 3u-80, and 4-unit frameworks; 4u-30, 4u-50, and 4u-80. The frameworks were assessed for fit accuracy with the replica method. The specimens were cemented to their corresponding dies, and the fracture strength was measured in a universal testing machine. The Weibull parameters were calculated for the study groups and fractured specimens were inspected for failure mode. Two-Way ANOVA followed by Tukey test for pairwise comparison between study groups (α = 0.05).

RESULTS

The cement spacer had a significant effect on both fit accuracy and fracture strength for 3-unit and 4-unit frameworks. The 50 μm spacer had significantly better fit accuracy followed by 80 μm, and 30 μm spacers. Both 50 μm and 80 μm spacers had similar fracture strength, and both had significantly better strength than 30 μm spacer.

CONCLUSIONS

For both 3-unit and 4-unit zirconia frameworks, 50 μm cement spacer can be recommended over 30 μm and 80 μm spacers for significantly better fit accuracy and adequate fracture strength.

Clinical performance of polymer frameworks in dental prostheses: A systematic review

STATEMENT OF PROBLEM

High-performance polymers including polyetheretherketone (PEEK) and polyetherketoneketone (PEKK) have been used as substitutes for metal frameworks in dental prostheses. However, the clinical performance of polymer-based frameworks is still uncertain.

PURPOSE

The purpose of this systematic review was to compare the clinical performance of PEEK and PEKK with that of metal frameworks for different dental prostheses.

MATERIAL AND METHODS

This review follows the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Six databases and non-peer-reviewed literature (without language or follow-up restrictions) were searched for studies conducted before February 2022. Only clinical studies, either randomized clinical trials (RCTs) or nonrandomized clinical trials (N-RCTs), comparing the clinical performance of polymer and metal frameworks were included. The risk of bias and certainty of the evidence were assessed with the RoB 2.0, ROBINS-I, and GRADE. Biologic (plaque and gingival indices, probing depth, bleeding scores, implant stability quotient, marginal bone loss) and mechanical outcomes (ridge base relation, prosthetic marginal gap, and fracture) were assessed.

RESULTS

Only 9 studies (7 RCTs and 2 N-RCTs) were included, all with moderate to serious risk of bias and low to very low certainty of evidence. No meta-analysis was possible, but qualitative analysis revealed lower plaque and gingival indices, probing depth, and marginal bone loss, with higher survival rates for implant-supported fixed prostheses and overdentures fabricated with PEEK than for metal frameworks. No significant differences were found between groups for removable partial dentures. The marginal fit of PEEK frameworks was also better for single crowns. Three fractures were reported in the 3 PEKK fixed dental prostheses with cantilevers.

CONCLUSIONS

PEEK and PEKK seem to be promising materials for dental prostheses, with acceptable response from the periodontal tissue. However, further well-designed studies are necessary to better understand their clinical and long-term limitations.

Effects of combined modification of sulfonation, oxygen plasma and silane on the bond strength of PEEK to resin

OBJECTIVE

This study aimed to evaluate the combined effects of sulfonation, non-thermal oxygen plasma and silane on the shear bond strength (SBS) of PEEK to resin materials.

MATERIALS AND METHODS

Two hundred and eighty specimens were randomly divided into four groups: (A) untreated; (B) sulfonation for 60 s; (C) oxygen plasma for 20 min; (D) sulfonation for 60 s and oxygen plasma for 20 min. According to the instructions, 120 samples (N = 30) were coated with silane, adhesive, and resin composites. Each group of bonding specimens was divided into two subgroups (n = 15) to measure immediate and post-aging SBS. The surface morphology and the interface between the samples and adhesive were analyzed through SEM. Physicochemical characteristics of the surface and mechanical properties were determined through XPS, FTIR, light interferometry, contact angle measurement, and three-point bending tests.

RESULTS

Sulfonation produced a porous layer of approximately 20 µm thickness on the surface, and the oxygen plasma increased the O/C ratio and oxygen-containing groups of the sample surface. After coating with silane, the SBS values of sulfonated PEEK and plasma-treated PEEK increased (9.96 and 10.72 MPa, respectively), and dual-modified PEEK exhibited the highest SBS value (20.99 MPa), which was significantly higher than that of blank group (p > 0.01). After 10,000 thermal cycles, the dual-modified PEEK still displayed a favorable SBS (18.68 MPa).

SIGNIFICANCE

Sulfonation strengthened the mechanical interlocking between PEEK and the resin while oxygen plasma established a chemical bonding between silane and PEEK. This dual modification of the surface microstructure and chemical state synergistically improved the bond strength of PEEK to resin and resulted in considerable long-term effects.

Relation of Crown Failure Load to Flexural Strength for Three Contemporary Dental Polymers

Polymeric materials show great promise for use in a variety of dental applications. Manufacturers generally provide flexural strength information based on standardized (ISO and ASTM) specimen dimensions and loading conditions. It is not clear, however, if flexural strength data are predictive of the clinical performance of dental crowns. The objectives of this study were, therefore, to determine whether flexural strengths, as measured via three-point bending (3PB), would be predictive of failure loads assessed via crunch-the-crown (CTC) tests. Three brands of polymers (Trilor, Juvora, and Pekkton) were fabricated into rectangular bars and fully contoured crowns (10 specimens of each polymer brand, 30 specimens of each shape). Differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), and burn off tests were used to characterize/confirm the materials. Bars were tested blindly in 3PB to determine flexural strength, and crowns were CTC-tested to determine failure load after luting to resin abutments. The statistical significance of the test results was evaluated via one-way ANOVA (α = 0.05) and Pearson's correlation coefficient, while regression analysis was used to test for a correlation between 3PB and CTC results. The fracture mechanisms and failure surface characteristics were characterized using scanning electron microscopy (SEM). There were significant differences (p < 0.05) in the mean crown failure loads (Trilor (7033 N) > Juvora (5217 N) > Pekkton (3023 N)) and mean flexural strengths of the bars (Trilor (468 MPa) > Juvora (197 MPa) = Pekkton (192 MPa)). The mode of crown fracture was different between the materials and included deformation (Juvora), ductile-to-brittle fracture (Pekkton), and a combination of cracks and deformation (Trilor). Flexural strengths did not correlate with the corresponding crown failure loads for any of the materials tested. These results suggest that dental practitioners should not rely on the flexural strengths reported from three-point bending tests, as advertised by the manufacturer, to predict the performance of polymeric crowns.

Effect of veneering material type and thickness ratio on flexural strength of bi-layered PEEK restorations before and after thermal cycling

OBJECTIVE

The purpose of this study was evaluating the biaxial strength of bi-layered PEEK restorations before and after aging using different veneering materials in different thickness ratios.

MATERIAL AND METHODS

Ninety specimens of thickness 1.5 mm were divided into three groups according to their veneering material. Group (CAD LD): BioHPP discs veneered with CAD milled lithium disilicate (n=30), group (CAD C): BioHPP discs veneered with CAD milled composite (n=30), and group (LC): BioHPP discs veneered with conventionally layered composite (n=30). Each group was subdivided into 3 subgroups (n=10) according to the different thickness ratios between the core and the veneering material (TC:TV). Subgroup 1: TC:TV=1:0.5, subgroup 2: TC:TV=0.7:0.8, and subgroup 3: TC:TV=0.5:1. Half of the specimens of each subgroup were subjected to thermocycling, and the bi-axial flexural strength of all specimens was tested before and after aging. Three-way ANOVA followed by Bonferroni's post hoc test were used for data analysis. The significance level was set at P ≤ 0.05.

RESULTS

Material, thickness ratio, and aging all had a significant effect on biaxial flexural strength. (LC) group had the highest biaxial flexural strength. TC:TV=0.5:1 showed the lowest biaxial flexural strength. All groups showed significant decrease in biaxial flexural strength after aging.

CONCLUSIONS

Veneering material for PEEK together with the thickness ratio between the core and veneering material greatly affect the flexural strength of bi-layered restorations. Thermocycling negatively impacts the flexural strength of PEEK bi-layered restorations.

CLINICAL SIGNIFICANCE

According to the results of that study, PEEK cores are best veneered with conventionally layered composite with core to veneering thickness ratio being 1:0.5.

Clinical Outcomes of Monolithic Zirconia Crowns on Posterior Natural Abutments Performed by Final Year Dental Medicine Students: A Prospective Study with a 5-Year Follow-Up

The conventional metal-ceramic is still considered the gold standard in fixed prosthetics especially in terms of longevity. Among alternative materials used, Monolithic Zirconia has shown the capability to reconcile excellent biomechanical properties with acceptable aesthetic performance and to overcome several inconveniences related to veneer restorations. This study aims to clinically evaluate Monolithic Zirconia prosthetic crowns on natural abutments in the posterior sectors, performed by final-year dental medicine students (undoubtedly with less experience in the management of such material) by the standardized California Dental Association score system evaluation, to better understand the viability of Monolithic Zirconia. This prospective study was carried out at the Dental School of the University of Bari "Aldo Moro", Italy. Prosthetic rehabilitation included single crowns or a short pontic prosthesis with maximum one intermediate. Final-year dental students performed tooth reduction under the supervision of three expert tutors. The California Dental Association systematics (based on color, surface, anatomical shape, and marginal integrity) were adopted to evaluate the prosthetic maintenance status over time. Annual follow-up visits were re-evaluated by the same parameters each year. Univariate logistic regression analysis was performed to evaluate outcomes and the Kaplan-Meier plot to report survival. The sample consists of 40 crowns performed on 31 patients, 15 males (48.4%) and 16 females (51.6%) with an average age of 59.3 years. The clinical cases subjected to experimental study were found to be "Excellent" (1a/2a/3a/4a) in 34 cases (85%), "Acceptable" in 4 cases (10%), and "To be re-done" in 2 cases (failures) (5%). Our conclusive data support the predictability of Monolithic Zirconia restorations on natural posterior abutments at a long-term follow-up of five years, even when performed by less-experienced clinicians.

Comparative in vitro study on biomechanical behavior of zirconia and polyetheretherketone biomaterials exposed to experimental loading conditions in a prototypal simulator

Zirconia and polyetheretherketone (PEEK) are two biomaterials widely investigated as substitute for metals in oral prosthetic rehabilitation. To achieve a proper biomechanical behavior, the prosthetic biomaterials must ensure a good resistance to loads, as this is a crucial characteristic enabling their use in dental applications. The aim of this study was to investigate differences in the fracture resistance of different biomaterials in an experimental environment: fixed partial dentures (FPDs) screwed in a prototype of biomimetic mandible. 10 Samples of FPDs were allocated in 2 groups (A and B): Group A (n=5) involved FPDs in zirconia-ceramic, and Group B (n=5) involved FPDs in PEEK-composite. The samples were loaded by means of a three-point bending mechanical test, and the load to fracture has been evaluated generating a point-by-point graphics (speed/load and time/deformation). The samples were further analyzed by micro-computed tomography (micro-CT) and described under experimental loading conditions. Zirconia-ceramic FDPs were the samples reporting the worst results, showing a lower value of vertical displacement with respect to PEEK-based samples. The micro-CT results have further confirmed the preliminary results previously described. This in vitro study aims to give analytic data on the reliability of PEEK as a reliable and strong biomaterial for prosthetic treatments.

Comparison of marginal and internal adaptation of three-unit fixed dental prostheses made using CAD/CAM metal-free materials

The aim of this study was to compare the marginal and internal adaptation of three-unit fixed dental prostheses (FDPs) fabricated from different metal-free materials using CAD/CAM methods. A total of 100 three-unit FDPs were produced from a cubic zirconia, a fiber-reinforced resin composite, a polyetheretherketone (PEEK), a polyetherketoneketone (PEKK), and a polymer composite material by the CAD/CAM method (n = 20 per material). The zirconia group was considered the control/reference material. Marginal and internal gap values of the produced FDPs were measured using the silicone replica method at ×40 magnification under a stereomicroscope. The obtained data were analyzed using one-way ANOVA and Tukey's HSD tests. The marginal and internal gap values for the cubic zirconia material were found to be statistically significantly lower than those seen for the PEEK, polymer composite, PEKK, and fiber-reinforced resin composite materials. While the marginal and internal adaptation of the cubic zirconia material was found to be better than the others, it should be noted that the marginal and internal gap values for all other materials tested were found to be within the clinically acceptable range.

Shear Bond Strength of Two Repair Systems to Zirconia Ceramic by Different Surface Treatments

Introduction: Intraoral repair has been suggested as a treatment option to repair the chipping or fracture of veneered zirconia; the success of the procedure is mainly determined by the bonding between zirconia and composite resin. The present study aimed to investigate and compare the shear bond strength (SBS) of two intraoral repair systems to zirconia ceramic treated with a laser or air-abrasion surface modification. Methods: Ninety tube-shaped samples (diameter of 10 mm and height of 4 mm) were divided into three main groups: Group I (zirconia 100%, n=30); Group II (veneer ceramic 100% n=30); Group III (zirconia with a veneer ceramic n=30). Each main group was subdivided into two subgroups (n=15): Subgroup A: samples repaired with Ceramic Repair N; Subgroup B: samples repaired with the Cimara Repair System. The subgroup samples were further subdivided based on the treated surface (n=5 samples): Control (no surface modification), Er,Cr:YSGG laser surface modification, and air-abrasion surface modification. The SBS was employed using a universal testing machine. The mode of failure was observed using a stereomicroscope. Results: Significant differences were observed in the mean SBS values between the different surface modifications (P˂0.05). Tukey's post hoc test showed that the air-abrasion surface modification of the veneer ceramic repaired with the Ceramic repair N system had the highest mean value (13.74 MPa) among the different groups, while no surface modification of zirconia repaired with the Cimara repair system had the lowest mean value (2.84 MPa). The control group (no surface modification) had the lowest mean value among all the treated groups. Conclusion: The SBS is surface modification-dependent, and higher SBS is obtained by air-abrasion than Er, Cr:YSGG laser surface modifications with the selected parameters. The Ceramic repair N system had significantly higher SBS for all surface-treated substrates than the Cimara repair system.

PEEK in Fixed Dental Prostheses: Application and Adhesion Improvement

Polyetheretherketone (PEEK) has been widely applied in fixed dental prostheses, comprising crowns, fixed partial dentures, and post-and-core. PEEK’s excellent mechanical properties facilitate better stress distribution than conventional materials, protecting the abutment teeth. However, the stiffness of PEEK is not sufficient, which can be improved via fiber reinforcement. PEEK is biocompatible. It is nonmutagenic, noncytotoxic, and nonallergenic. However, the chemical stability of PEEK is a double-edged sword. On the one hand, PEEK is nondegradable and intraoral corrosion is minimized. On the other hand, the inert surface makes adhesive bonding difficult. Numerous strategies for improving the adhesive properties of PEEK have been explored, including acid etching, plasma treatment, airborne particle abrasion, laser treatment, and adhesive systems.

PEEK Biomaterial in Long-Term Provisional Implant Restorations: A Review

Polyetheretherketone (PEEK) has become a useful polymeric biomaterial due to its superior properties and has been increasingly used in dentistry, especially in prosthetic dentistry and dental implantology. Promising applications of PEEK in dentistry are dental implants, temporary abutment, implant-supported provisional crowns, fixed prosthesis, removable denture framework, and finger prosthesis. PEEK as a long-term provisional implant restoration has not been studied much. Hence, this review article aims to review PEEK as a long-term provisional implant restoration for applications focusing on implant dentistry. Articles published in English on PEEK biomaterial for long-term provisional implant restoration were searched in Google Scholar, ScienceDirect, PubMed/MEDLINE, and Scopus. Then, relevant articles were selected and included in this literature review. PEEK presents suitable properties for various implant components in implant dentistry, including temporary and long-term provisional restorations. The modifications of PEEK result in wider applications in clinical dentistry. The PEEK reinforced by 30-50% carbon fibers can be a suitable material for the various implant components in dentistry.

Influence of Digital Technologies and Framework Design on the Load to Fracture of Co-Cr Posterior Fixed Partial Denture Frameworks

PURPOSE

To compare the load to fracture of cobalt-chromium (Co-Cr) 3-unit posterior fixed partial denture (FPD) frameworks manufactured by conventional and digital techniques and to evaluate the influence of the framework design on the fracture load.

MATERIAL AND METHODS

Eighty 3-unit Co-Cr posterior FPD frameworks were fabricated with two designs: intermediate pontic (n = 40) and cantilever (n = 40). Each design was randomly divided into four groups (n = 10): casting, direct metal laser sintering, soft metal milling, and hard metal milling. After thermal cycling, all specimens were subjected to a 3-point bending test until fracture. Data were statistically analyzed using one-way ANOVA, Welch and Brown-Forsythe test, Ryan-Einot-Gabriel-Welsch F and Tamhane T2 post hoc test, Student's t test, and Weibull statistics (α = 0.05).

RESULTS

Significant differences (p < 0.001; F = 39.59) were found among intermediate pontic frameworks (except between laser sintering and hard metal milling), and cantilevered frameworks (F = 36.75) (except between laser sintering and hard metal milling, and casting and soft metal milling). The cantilever groups showed load to fracture values significantly lower than those of the intermediate pontic (p < 0.001; F = 28.29). The Weibull statistics corroborated the results.

CONCLUSIONS

Hard metal milling and laser sintered frameworks exhibited the highest load to fracture values. However, all tested frameworks demonstrated clinically acceptable load to fracture values. The framework design directly affected the fracture load, with drastically lower values in cantilevered frameworks.