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Bond strength of three chairside crown reline materials to milled polymethyl methacrylate resin

Bond strength of three chairside crown reline materials to milled polymethyl methacrylate resin



Bond strength of three chairside crown reline materials to milled polymethyl methacrylate resin




Journal of Prosthetic Dentistry, 2021-03-01, Volume 125, Issue 3, Pages 544.e1-544.e8, Copyright © 2020 Editorial Council for the Journal of Prosthetic Dentistry


Abstract

Statement of problem

Information on the bond strength of milled polymethyl methacrylate interim restorations when relined with chairside reline materials is lacking.

Purpose

The purpose of this in vitro study was to measure the shear bond strength of various combinations of 3 different chairside reline materials bonded to milled polymethyl methacrylate blocks with 3 different types of surface treatments.

Materials and methods

Uniform blocks (10×10×22 mm) were milled from tooth-colored polymethyl methacrylate disks (Vivid PMMA; Pearson Dental Supply Co). The surface treatments tested were airborne-particle abrasion with 50-μm particle size aluminosilicate, application of acrylic resin monomer (Jet Liquid; Lang Dental Manufacturing Co) for 180 seconds, and airborne-particle abrasion with monomer application. The control groups were blocks with no surface treatment. The chairside reline materials tested were Jet acrylic resin (Jet Powder; Lang Dental Manufacturing Co), bis-acryl resin (Integrity; Dentsply Sirona), and flowable composite resin (Reveal; Bisco). All materials were applied through a Ø1.5×3-mm bonding ring. Ten specimens for each of the 12 groups were tested in a universal testing machine. Load was applied at a crosshead speed of 1 mm/min. Fracture surfaces were then analyzed for cohesive versus adhesive or mixed failure. Data were analyzed using 2-way ANOVA and Tukey-Kramer post hoc analysis (α=.05).

Results

The mean shear bond strength values ranged from 1.77 ±0.79 MPa to 28.49 ±5.75 MPa. ANOVA revealed that reline material ( P <.05), surface treatment ( P <.05), and their interactions ( P <.05) significantly affected the shear bond strength among the experimental groups. The strongest combination was Jet acrylic resin applied on specimens treated with airborne-particle abrasion and monomer. All 3 failure modalities (adhesive, cohesive, and mixed modes) were observed.

Conclusions

Of the materials tested, the most reliable material to bond to milled polymethyl methacrylate was Jet acrylic resin, and the bond strength values were increased substantially when the milled polymethyl methacrylate surface was airborne-particle abraded and monomer was applied.

Clinical Implications

Surface treatments to the intaglio surface of milled polymethyl methacrylate interim restorations are essential in ensuring adequate bonding to chairside reline materials.

Interim restorations commonly require relining during long-term fixed prosthodontic treatment, and a reliable bond is necessary between the reline material and interim base to provide pulpal protection, positional stability, occlusal function, nonimpinging margins, strength, retention, and esthetics. The bonding characteristics between autopolymerizing acrylic resin, dual-polymerizing resin, and light-polymerizing resin-based materials have been reported. Factors such as type of interim restorative base, chemical composition of reline resin, and bonding surface all contribute to the overall bond strength.

Traditionally, polymethyl methacrylate (PMMA) has been used for interim restorations in the form of liquid methyl methacrylate monomer hand mixed with prepolymerized PMMA polymer beads. In conjunction with computer-aided design and computer-aided manufacturing (CAD-CAM), manufacturers have developed industrial processes to condense the autopolymerizing mixed monomer and polymer under high heat and pressure to fabricate highly dense PMMA disks. The most widely documented applications of this material are milled denture bases, with enhanced mechanical properties and esthetics contributing to overall success. The bonding characteristics of milled PMMA have been reported for bonding of silicone-based or acrylic-based denture reline materials. Knowledge relating to the application of milled PMMA for fixed interim restorations and bonding characteristics to chairside reline materials is sparse.

Using a reline resin that is chemically similar to the interim base will provide the most durable result. The most chemically similar material to milled PMMA is autopolymerizing acrylic resin ; however, dual-polymerized or light-polymerized resin reline materials are popular. In spite of being more expensive, these materials provide the advantage of ease of use, minimal odor, patient comfort, and lower polymerization shrinkage. Given the minimal porosity, limited surface roughness, and hydrophobicity of milled PMMA, methods in which to reliably enhance shear bond strength to various reline resins should be determined. Attempts to improve shear bond strength include mechanical surface roughening with tungsten carbide burs or airborne-particle abrasion, and chemical surface treatments include the application of methacrylate monomer, acetone, and chloroform. These surface treatments may improve the accessibility of polymer chains on the milled PMMA surface for enhanced interlocking with polymer chains in the reline resin.

The purpose of this in vitro study was to measure the shear bond strength of combinations of 3 different chairside reline materials bonded to milled PMMA with 3 different surface treatments. The null hypotheses were that no difference would be found in the shear bond strength of different reline materials or with or without surface treatment to the milled PMMA base.


Material and methods

All specimens for this experiment were designed in a design software program (Meshmixer; Autodesk Inc) and were of uniform dimension (10×10×22 mm). The specimens were nested for milling by using a dental CAM software program (hyperDENT; Follow-Me Technology Group), and all specimens were milled with a 5-axis mill (VersaMill; Axsys Dental Solutions) by using chromium nitride–coated 2-flute helical end mills (Axsys Dental Solutions). A total of 120 specimens were milled from PMMA disks (Vivid PMMA; Pearson Dental Supply Co). Postprocessing of specimens involved a carbide cutting bur (Brasseler USA ) to remove the milled supports on the sides of the specimens. The surfaces intended for application of reline material were not altered.

The 120 specimens were then divided into 12 groups of 10 specimens each. Each group was assigned a surface treatment and reline material ( Table 1 ). A bonding ring template made with laboratory putty (Lab-Putty, LOT 140969; Coltène) was created to ensure that a uniform cylinder (Ø1.5×3 mm) of the reline material was applied to the milled surface ( Fig. 1 ). The putty bonding ring was designed such that after the reline material had set, the ring could be separated to prevent premature displacement or delamination of the reline material from the milled surface.

Table 1
Experimental groups (n=10) assigned to particular surface treatment and reline material
Experimental Group Surface Treatment Reline Material
1 (control) None Jet Acrylic Resin
2 (control) None Bis-acryl Resin
3 (control) None Flowable Composite Resin
4 Airborne-particle abrasion Jet Acrylic Resin
5 Airborne-particle abrasion Bis-acryl Resin
6 Airborne-particle abrasion Flowable Composite Resin
7 Monomer Jet Acrylic Resin
8 Monomer Bis-acryl Resin
9 Monomer Flowable Composite Resin
10 Airborne-particle abrasion + monomer Jet Acrylic Resin
11 Airborne-particle abrasion + monomer Bis-acryl Resin
12 Airborne-particle abrasion + monomer Flowable Composite Resin

Schematic outline of specimen preparation protocol. A, Bonding ring template with isolated Ø1.5-mm bonding surface area. B, Reline materials applied into bonding ring template. C, Fully prepared specimen. D, Specimen placed into universal testing machine for shear bond strength testing.
Figure 1
Schematic outline of specimen preparation protocol. A, Bonding ring template with isolated Ø1.5-mm bonding surface area. B, Reline materials applied into bonding ring template. C, Fully prepared specimen. D, Specimen placed into universal testing machine for shear bond strength testing.

Specimens in the control groups received no surface treatments. Specimens in the airborne-particle abrasion group were prepared with airborne-particle abrasion using 50-μm particle size aluminosilicate with a pressure of 0.2 MPa in a laboratory airborne-particle abrasion machine (Basic Classic; Renfert). Airborne-particle abrasion of the specimen was accomplished with multiple passes over the milled surface at 10 mm for 20 seconds. Specimens in the monomer group were treated with methyl methacrylate monomer (Jet Liquid; Lang Dental Manufacturing Co) with continuous application by using a brush (Bend-a-brush; Plasdent Corp) for 180 seconds. After monomer application, the specimens remained on the bench top to allow for evaporation before application of the reline material. Blocks in the airborne-particle abrasion and monomer groups were first treated with airborne-particle abrasion, and then methyl methacrylate monomer was applied to the abraded surface.

The interim restoration chairside reline materials tested were acrylic resin (Jet Liquid; Lang Dental Manufacturing Co), bis-acryl resin (Integrity; Dentsply Sirona), and flowable composite resin (AeliteFlo; Bisco Inc) ( Table 2 ). The Jet acrylic resin material was prepared by mixing the PMMA powder with the methyl methacrylate liquid in a 3:1 ratio and then flowing the mixture into the bonding ring template. The bis-acryl resin material was applied directly into the bonding ring template by using an automix application syringe (Integrity Cartridge Mixing Tips; Dentsply Sirona). The material remained in the bonding ring template for 120 seconds.

Table 2
Base and repair materials tested
Polymerization Method Material Composition Lot Number Manufacturer
CAD-CAM resin block for milling Vivid PMMA Pucks Polymethyl methacrylate 2019/3 Pearson Dental Supply Co
Powder/liquid mix Jet Acrylic Resin Polymethyl methacrylate 144218CX/01AB (liquid)
600719AJ/01AJ (powder)
Lang Dental Manufacturing Co
Injection with automix application syringe Integrity Dimethacrylate (Bis-acryl resin) 00017248 Dentsply Sirona
Light-polymerized resin Aelite Flo Bis-EMA, TEGDMA, glass frit, amorphous silica 1900005987 Bisco

In the experimental groups in which flowable composite resin was the reline material, a pilot study was completed with surface-treated specimens without the use of a composite resin bonding agent before the application of flowable composite resin. In the pilot study, the flowable composite resin would continually delaminate from the milled surface. Therefore, a protocol was developed in which a composite resin bonding agent (OptiBond FL, LOT 7511930; Kerr Corp) was applied to the milled surface and polymerized with a light-polymerizing unit (Vector LED-P; Vector USA) at a distance of 2 mm with an irradiance of 1400 W/cm 2 for 20 seconds before isolation with the bonding ring template. A polymerization cycle using the same light polymerizing unit was completed for 20 seconds with the flowable composite resin in the bonding ring template, and another 20 second polymerization cycle was completed with the bonding ring template removed.

All prepared specimens were stored at room temperature with 30% humidity before testing in the same environment and by following the protocol for the evaluation of shear bonding reported in International Organization for Standardization (ISO) 20795. All tests were performed on a universal testing machine (Instron 3365; Instron) with a crosshead speed of 1 mm/min until failure, and output values were tabulated in MPa ( Fig. 2 ).

Schematic design of shear bond strength testing design.
Figure 2
Schematic design of shear bond strength testing design.

After failure, each specimen was visually inspected to determine the amount of reline material remaining on the milled surface, and the nature of the failure was recorded as adhesive or cohesive. Adhesive failure was deemed to have occurred if the reline resin was completely delaminated from the milled PMMA base. Cohesive failure was deemed to have occurred if any amount of reline material remained on the milled PMMA base.

To determine the morphologic changes to the milled PMMA surface after surface treatment, 3 additional specimens were prepared. One with the as milled surface and the other 2 specimens were treated with airborne-particle abrasion and airborne-particle abrasion plus methyl methacrylate monomer application. The specimens were examined with a scanning electron microscope (SEM) (Tescan Vega-3 XMU SEM; Tescan Analytics) at 3 kV. SEM photomicrographs at ×200 magnification were made for visual analysis.

The data were analyzed with a multivariate ANOVA to determine the interactions between the surface treatment and bonding of the reline material, as well as interactions between the reline material and milled PMMA base (α=.05). The Tukey-Kramer post hoc tests (α=.05) determined the significance between different reline materials for a specific surface treatment and between different surface treatment groups for a specific reline material.


Results

Significant differences in shear bond strength were found between the experimental groups depending on the type of reline material and the surface treatment ( Fig. 3 ). Therefore, the null hypotheses were rejected.

Mean shear bond strength values. Error bars indicate standard deviations. J, Jet acrylic resin; B, Bis-acryl resin; F, Flowable composite resin.
Figure 3
Mean shear bond strength values. Error bars indicate standard deviations. J, Jet acrylic resin; B, Bis-acryl resin; F, Flowable composite resin.

Jet acrylic resin had the highest shear bond strength to the milled base, increased by airborne-particle abrasion and monomer application. The bis-acryl resin exhibited the lowest shear bond strength, especially when there was no surface treatment to the milled base. The flowable composite resin groups demonstrated bond strength values only slightly weaker than those of Jet acrylic resin. In both the bis-acryl resin and flowable composite resin groups, the experimental groups with the lowest shear bond strength values were specimens that had been treated with monomer only.

The shear bond strength of specimens in all surface treatment groups varied significantly ( P <.001) ( Table 3 ). No statistically significant difference was found among the specimens treated with Jet acrylic resin, regardless of the surface treatment ( P =.073). No statistically significant difference was found between Jet acrylic resin and flowable composite resin when applied to milled surfaces with no surface treatment ( P =.895), with airborne-particle abrasion ( P= .895), or with airborne-particle abrasion plus monomer ( P =.05) ( Table 4 ).

Table 3
Mean ±standard deviation shear bond strength (MPa) and statistical significance of variables for each experimental combination
Surface Treatment Jet Acrylic Resin Bis-acryl Resin Flowable Composite Resin P
Control 17 ±4 3 ±1 17 ±4 <.001
Airborne-particle abrasion 24 ±4 6 ±2 24 ±7 <.001
Monomer 21 ±3 2 ±1 12 ±1 <.001
Airborne-particle abrasion + Monomer 29 ±5 5 ±2 20 ±8 <.001
P .073 <.001 <.001

Table 4
Tukey-Kramer significance testing between different reline materials within a surface treatment group
Surface Treatment of Milled Surface P
Control
Jet Acrylic Resin – Bis-acryl Resin <.001
Jet Acrylic Resin – Flowable Composite Resin .895
Bis-acryl Resin – Flowable Composite Resin <.001
Airborne-particle abrasion
Jet Acrylic Resin – Bis-acryl Resin <.001
Jet Acrylic Resin – Flowable Composite Resin .895
Bis-acryl Resin – Flowable Composite Resin <.001
Monomer
Jet Acrylic Resin – Bis-acryl Resin <.001
Jet Acrylic Resin – Flowable Composite Resin <.001
Bis-acryl Resin – Flowable Composite Resin <.001
Airborne-particle abrasion + Monomer
Jet Acrylic Resin – Bis-acryl Resin <.001
Jet Acrylic Resin – Flowable Composite Resin .05
Bis-acryl Resin – Flowable Composite Resin <.001

Adhesive failure was found in all specimens relined with Jet acrylic resin and all specimens surface treated with airborne-particle abrasion ( Fig. 4 ). A combination of cohesive and adhesive failures was observed for all other reline materials and surface treatments. The highest incidence of cohesive failure occurred in specimens relined with flowable composite resin. The SEM images demonstrate the change in topography between a specimen with no surface treatment, a specimen with airborne-particle abrasion, and a specimen with airborne-particle abrasion plus monomer application ( Fig. 5 ), with a notable increase in surface roughness.

Percentage of failure mode for each material in treatment. A, By reline material tested. B, By surface treatment category.
Figure 4
Percentage of failure mode for each material in treatment. A, By reline material tested. B, By surface treatment category.

Scanning electron microscopy of experimental surfaces (original magnification ×200). A, As milled surface. B, Milled surface with airborne-particle abrasion surface treatment protocol. C, Milled surface with airborne-particle abrasion plus monomer application surface treatment protocol.
Figure 5
Scanning electron microscopy of experimental surfaces (original magnification ×200). A, As milled surface. B, Milled surface with airborne-particle abrasion surface treatment protocol. C, Milled surface with airborne-particle abrasion plus monomer application surface treatment protocol.

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