REVIEW ARTICLE |
https://doi.org/10.5005/jp-journals-10019-1410 |
Comparison of 3-D Printed Complete Denture Repair Methods to Conventional and CAD-CAM Complete Dentures: A Systematic Review
1-4Faculty of Dentistry, Sir John Walsh Research Institute, University of Otago, Dunedin, New Zealand
Corresponding Author: Sunyoung Ma, Faculty of Dentistry, Sir John Walsh Research Institute, University of Otago, Dunedin, New Zealand, Phone: +64 34797044, e-mail: sunyoung.ma@otago.ac.nz
Received on: 10 May 2023; Accepted on: 30 May 2023; Published on: 28 June 2023
ABSTRACT
Purpose: To perform a systematic review that provides an overview of the current literature on the most efficient surface treatment method for repairing three-dimensional (3D) printed dentures compared to conventional dentures and computer-aided design/computer-aided manufacturing (CAD/CAM) milled dentures.
Materials and methods: The review followed the Preferred Reporting Items for Systematic Review and Meta-analyses (PRISMA) guidelines. Electronic searches were done via Ovid, PubMed, ScienceDirect, Scopus, and Web of Science for studies reporting on denture base repair with surface treatment. Inclusion criteria were the English language with full text and studies that included surface treatment. Exclusion criteria were studies that did not assess the performance of surface treatment in denture repair. A quality assessment and selection of full-text articles were performed according to Consolidated Standards of Reporting Trials (CONSORT).
Results: A total of 531 articles were initially identified and screened. After applying inclusion and exclusion criteria, 14 articles were selected for the final analysis. Around 10 of the included studies utilized three-point flexural bending tests to test the flexural strength (FS) of the repaired samples. Four studies used the shear test. Around 12 studies used conventional resins and two studies used 3D-printed denture base resin specimens. Nearly 11 studies performed posttest analysis.
Conclusion: Studies have shown various promising enhancements in repair strength with both mechanical and chemical surface treatment. However, more research is required specifically on 3D printed and CAD/CAM dentures and to compare mechanical and chemical surface treatments or combinations of both, for it to become a more established treatment option.
How to cite this article: Seow J, Won G, Tawse-Smith A, et al. Comparison of 3-D Printed Complete Denture Repair Methods to Conventional and CAD-CAM Complete Dentures: A Systematic Review. Int J Prosthodont Restor Dent 2023;13(2):104-113.
Source of support: Nil
Conflict of interest: None
Keywords: Acrylic resins, Computer-aided design, Denture bases, Denture repair, Three-dimensional printing
INTRODUCTION
Complete dentures are an economical way to rehabilitate edentulous patients. Until recently, the most common fabrication method has been the conventional method using poly (methyl methacrylate) (PMMA) material. The recent fabrication of the first 3D-printed denture by Dentca in 2015, has added a new fabrication option.1 Continuous technology enhancement over the years and focus on efficient treatment have led to increased popularity and focus on these dentures. They are competitive in the market, posing strengths such as manufacturing efficiency, reasonable financial cost, and improved tissue adaptation.2
Despite their promising short-term clinical performance, 3D-printed complete dentures are also prone to common maintenance or complication issues such as denture base fracture with almost one-third of denture samples reporting at least one fracture.3 It may occur due to faults in denture fabrication, poor fit and lack of balanced occlusion, and low resistance to fracture. The most frequent fracture is in the median line followed by the incision area.4 Repair can be expensive and time-consuming and therefore, efficient repair methods are crucial to determine 3D printed dentures as viable long-term treatment options.
Surface treatment is crucial for repair as it facilitates better repair bond strength and decreases stress accumulation.5 Few studies have been carried out on repair methods of 3D dentures.6-9 One study6 proved all laser, airborne particle abrasion (APA), laser plus APA, and bur grinding lead to higher FS. However, a very common surface treatment with MMA monomer was not included. Another study7 used surface treatments of MMA, sanding and APA and tested the shear bond strength (SBS) and showed that the latter two showed significantly increased SBS compared to MMA.
The current research has provided some good evidence but there is a limit to the conclusions that can be reached and shows the need for future research. Therefore, the aim of this study was to perform a systematic review that provides an overview of the current literature on the most efficient surface treatment method for repairing 3D printed dentures compared to conventional dentures and CAD/CAM milled dentures.
MATERIALS AND METHODS
This systematic review was performed according to the PRISMA statement.10 It was conducted to answer the following question: What is the most efficient surface treatment method for 3D printed denture repair and how does this compare to conventional dentures and CAD/CAM dentures?
A systematic electronic literature search was conducted on Ovid, PubMed, ScienceDirect, Scopus, and Web of Science (Fig. 1). The searches were limited to the English language only, but with no publication year limit. The last search was done on 11 July 2022. The questions for this systematic review are the population, intervention, comparison, and outcomes (PICO), which was defined in Table 1. The search strategy consisted of MeSH terms (see “search combination” in Table 1) which were used for PubMed. The syntax was modified according to the other databases searched. Duplicate studies found between each database were removed. The titles and abstracts of the remaining studies were then screened for their suitability based on the focus question and PICO search strategy. Following this, the full texts were assessed based on the inclusion and exclusion criteria in Table 1. References of the studies were also assessed for additional studies.
Fig. 1: PRISMA 2020 flow diagram
| Focus question: what is the FS of repaired 3D-printed denture bases? | |
|---|---|
| Search strategy | |
| Population | Studies on repaired denture bases |
| Intervention | Surface treatment using methods such as chemical or mechanical means |
| Comparison | The unprepared surface of the denture resin |
| Outcome | Flexural or SBS of repaired denture base |
| Study type | Quantitative study |
| Search combination | “Denture bases” or “denture, complete” or “denture repair” and “Acrylic resins” or “CAD” or “PMMA” or “printing, 3D” and “Dental stress analysis” or “flexural strength” “stress, mechanical” or “pliability” or “shear strength” and “Materials testing” and “Surface properties” |
| Database search | |
| Electronic database | Ovid PubMed ScienceDirect Scopus Web of Science |
| Inclusion criteria | English language only Full text available Reported on the outcomes of repair of heat-cured acrylic, milled, or 3D-printed complete dentures Used autopolymerizing resin as a repair resin Published in and before 11th of July 2022 |
| Exclusion criteria | Non-English language Irrelevant to focus question or did not provide sufficient data Only abstract available Review articles |
| Outcome measures | Primary: flexural or SBS with respect to surface treatment methodSecondary: flexural or SBS in terms of artificial aging |
To minimize the potential for reviewer bias, two reviewers (JS and GW) independently conducted electronic literature searches and performed the study selection using the inclusion and exclusion criteria (Table 1). Any disagreement was resolved by discussion.
Data were extracted by one reviewer (JS) and examined by two reviewers (JS and GW). The following data were collected from the included articles—study information (e.g., authors, year, journal, and title), denture base material(s), number of specimens per group, artificial aging method(s), repair joint type, surface treatments, repair resin(s), testing method, main findings (surface treatment strength), mode of failure, and scanning electron microscope (SEM) appearance.
The risk of bias assessment in this systematic review was based on the modified CONSORT checklist.10 Each parameter was evaluated and judged with yes, no, or no information. Judging a result to be at a particular level of risk of bias for an individual domain implies that the result has an overall risk of bias at least this severe. The overall risk of bias judgment was made as follows—low-risk of bias, some concerns, high-risk of bias, or unclear.
RESULTS
The initial database search produced a total of 1,016 studies. A total of 500 and 31 studies remained after the duplicates were removed. These studies were further screened by reading the titles and abstracts. The full texts of the 25 remaining studies were analyzed and no additional relevant studies were found from reading the references of these studies. After discussion between the two reviewers (JS and GW), 14 studies were included in the systematic review (Fig. 1) with Cohen’s κ interrater agreement of 0.77. The characteristics of these studies are summarized in Table 2.
| Author (year) | Denture base material(s) N = total no. samples n = no. samples per group |
Artificial aging method(s) | Repair joint type | Surface treatments | Repair resin(s) | Testing method | Mode of failure | SEM surface appearance | Main findings (repair strength) |
|---|---|---|---|---|---|---|---|---|---|
| Alkurt et al. (2014)11 | Heat-polymerized acrylic resin (DeTrey QC-20; Dentsply Ltd) N = 96 n = 8 |
Stored in distilled water at 37°C for 1 week after repair | Butt | Control—P600 sandpaper MMA 120 seconds APA 250 μm, Al2O3, 10 seconds, 0.2 MPa, 10 mm distance Er: YAG laser, 2940 nm, spot size 0.8 mm, 10 Hz pulse frequency, 150 mJ pulse energy, 100 microseconds pulse duration, applied for 60 seconds under water irrigation, 10 mm |
Autopolymerizing resin (Takilon; Rodent) | Three-point bend test | Adhesive | Surface treatment resulted in irregularities and many small pits on the surface of the denture base resin | APA is significantly better than control All surface treatments improved the FS compared to the control |
| Bural et al. (2010)12 | Heat-polymerized acrylic resin (Selectaplus H/Trevalon + Universal Denture Liquid, Dentsply Caulk; Dentsply International) N = 54 n = 6 |
Storage in distilled water at 37°C for 28 days after sample fabrication | Butt | Control—600-grit sandpaper Heat polymerizing MMA-based monomer 180 seconds Auto-polymerizing MMA-based monomer 180 seconds Acetone 180 seconds |
Autopolymerizing acrylic resin | Three-point bend test | Adhesive | None | Heat polymerizing MMA-based monomer showed significantly higher FS than control and acetone |
| Gad et al. (2020)13 | Heat-polymerized acrylic resin N = 320 n = 20 |
Half stored in distilled water at 37°C for 48 hours ± 2 hours after repair Half stored in distilled water at 37°C for 4 weeks, then thermocycler for 5,000 cycles at 5 and 55°C, 30 seconds dwell time after repair |
Bevel | Control—MMA 120 seconds APA, Al2O3, 50 μm, 380 kPa, 15 seconds, 10 mm distance APA + silane coupling agent APA + methacrylate-based composite bonding agent applied and cured for 30 seconds |
PMMA | Three-point bend test | Before thermocycling - mainly cohesive and mixed (mainly adhesive in control) After thermocycling - mainly adhesive (mainly cohesive in APA) |
Morphological features observed | All groups that used APA had significantly higher FS than the control APA + silane coupling agent performed the best but was not significantly different from APA in the thermocycler group All groups except for APA showed a significant decrease in FS after thermocycling |
| Li et al. (2021)7 | 3D printed resin (FREEPRINT denture, Detax, Ettlingen, Germany) N = 224 n = 28 |
Half thermocycler for 5,000 cycles at 5 and 55°C in distilled water, 30 seconds dwell time after sample fabrication Thermocycling for 10,000 cycles at 5 and 55°C, 30 seconds dwell time after repair |
Butt | Control—untreated MMA P600 sandpaper APA, 125 μm, Al2O3, 10 mm distance, 0.2 MPa, 10 seconds |
3D printed resin (FREEPRINT denture, Detax, Ettlingen, Germany) |
Shear test | Before thermocycling—mainly cohesive After thermocycling—mainly adhesive in control and MMA (mainly cohesive in mechanical groups) |
Control—relatively smooth. More tiny particles after aging Monomer—relatively smooth, no apparent difference to control. More tiny particles are found after aging P600—more homogenous surface, relatively regular waveforms. No difference after aging APA—irregular rough surface, deep cavities, irregular cracks. No difference after aging |
No significant difference in surface treatments among nonaged groups Among aged groups, sandpaper—abraded and APA groups had significantly higher SBS than control and MMA groups Significant decrease in SBS after aging for control and monomer groups |
| Neshandar et al. (2021)6 | 3D printed PMMA resin (DentaBase; Asiga) N = 120 n = 20 |
Immersion in water at 37°C after sample fabrication Incubation at 37 ± 1°C in an aqueous medium for 24 hours, then thermocycler for 5,000 cycles at 5 and 55°C, 30 seconds dwell time after repair |
Bevel | Control—1200-grit sandpaper Er:YAG laser APA 250 μm, Al2O3, 0.2 MPa, 10 mm distance, 10 seconds Laser + APA Bur grinding |
Autopolymerizing acrylic resin (ProBase Cold; Ivoclar Vivadent AG) | 3-point bend test | Cohesive mainly in bur and APA groups, 50% cohesive in APA + laser, adhesive mainly in others Effect of thermocycling is not stated |
Control—parallel grooves APA—several depressions and projections Bur grinding—several depressions and projections Laser—irregular fine grooves Laser + APA—reduction in surface irregularities |
All treatments had significantly higher FS than the control Bur grinding had significantly higher FS than APA + laser, laser, and APA |
| Nishigawa et al. (2004)14 | Heat-polymerizing acrylic resin (Acron, GC Corp, Tokyo, Japan; N = 44 n = 11 |
Stored in 37°C distilled water for 100 days after repair | Butt | Control—#1200 sandpaper Plasma irradiation, 15 seconds, 6 mm distance Adhesive primer Adhesive primer + plasma irradiation |
PMMA powder and MMA liquid of self-curing acrylic resin (Unifast II, GC Corp, Tokyo) | Shear test | Cohesive | Control—parallel grooves, not significantly different from plasma Plasma irradiation—parallel grooves, not significantly different to control Adhesive primer—no data Adhesive primer + plasma irradiation—no data |
No significant difference found between any groups |
| Pereira et al. (2012)15 | Heat-polymerized acrylic resin (Lucitone 550; Dentsply Ind. e Com Ltda, Petro polis, RJ, Brazil) N = 50 n = 10 |
Stored in distilled water at 37°C for 1 week after repair | Butt | Control—600-grit sandpaper MMA 180 seconds 400-grit sandpaper 400-grit sandpaper + MMA 180 seconds |
Autopolymerizing acrylic resin (Simplex, Cla ssico, Artigos Odontolo gicos, Sao Paulo, SP, Brazil) | Three-point bend test | Not stated | None | MMA and MMA + sandpaper abrasion had higher FS than abrasion alone and control |
| Sarac et al. (2005)16 | Conventionally molded heat-polymerized acrylic resin (Meliodent) N = 120 n = 10 |
Stored in distilled water at 37°C for 7 days before surface treatments | Butt | Control—600-grit sandpaper Acetone 30 seconds Methylene chloride 30 seconds MMA 180 seconds |
Autopolymerizing acrylic resin (Meliodent) | Shear test | Mainly mixed and adhesive | Control—scratches, valleys, and depressions Acetone - smoother surface than the controls, porous topography Methylene chloride—smooth surface with hollows Monomer—smoother surface texture than the control groups |
All treatments had significantly higher SBS than control |
| Shen et al. (1984)17 | Heat-polymerized denture resins (Permatone, Kerr Mfg. Co., Romulus, Mich.) (Lucitone, L. D. Caulk Co., Milford, Del.) N = 64 n = 8 |
Stored in distilled water at room temperature for 48 h before repair | Butt | Control—600-grit sandpaper Chloroform 5 seconds |
Cold-curing resin | Three-point bend test | Adhesive | Control—debris from grinding remained Chloroform (5, 30, 60, and 120 s application times analyzed)—Lucitone surfaces became porous as time increased; Permatone surfaces had a cleaner and smoother surface with various size pits |
No significant difference |
| Shimizu et al. (2006)18 | Heat-polymerized resin (Acron, GC Corp., Tokyo, Japan) N = 80 n = 10 |
Stored in 37°C distilled water for 2 days after sample fabrication Stored 37°C distilled water for 1 day after repair Ethyl acetate for 120 seconds and dichloromethane 5 seconds thermocycler for 10,000 cycles in water at 5 and 55°C, 1 minute dwell time |
Butt | Control—100-grit sandpaper Ethyl acetate 5 seconds Ethyl acetate 30 seconds Ethyl acetate 60 seconds Ethyl acetate 120 seconds Dichloromethane 5 seconds |
Autopolymerizing acrylic repair resin (Unifast II, GC Corp.) |
Three-point bend test | Before thermocycling—mainly mixed After thermocycling—mainly adhesive in ethyl acetate for 120 seconds, 50% adhesive in dichloromethane 5 seconds |
Control—not analyzed Ethyl acetate 5 seconds—irregular surface, not much porosity Ethyl acetate 30 seconds—almost homogenous surfaces with pores <1 mm diameter Ethyl acetate 60 seconds—almost homogenous surfaces with pores <1 mm diameter Ethyl acetate 120 seconds—dissolved surfaces with no pores Dichloromethane 5 seconds—no information |
120s ethyl acetate significantly higher FS than other durations and control. No difference between 120 seconds of ethyl acetate and 5 seconds of dichloromethane Significant decrease in FS after aging for both treatments |
| Shimizu et al. (2008)19 | Heat-polymerized resin N = 80 n = 10 |
Stored in 37°C distilled water for 24 hour Samples in groups I, III, and V thermocycler in water for 10,000 cycles at 5 and 55°C, 1 minute dwell time |
Butt | Control—100-grit sandpaper Ethyl acetate 60 seconds Ethyl acetate 120 seconds Ethyl acetate 180 seconds Dichloromethane 5 seconds |
Autopolymerizing resin | Shear test | Adhesive in all controls. Mostly mixed in all dichloromethane groups Before thermocycling—mostly mixed for 120 seconds ethyl acetate After thermocycling—mostly adhesive |
Control—uniform parallel scratches Ethyl acetate 60 seconds—not analyzed Ethyl acetate 120 seconds—dissolved surface with a few pores Ethyl acetate 180 seconds—flatter and smoother than 120 seconds Dichloromethane 5 seconds—highly porous |
120 seconds ethyl acetate and 5 seconds dichloromethane significantly higher SBS than other three groups Significant decrease in SBS after aging for control and 120 seconds ethyl acetate only |
| Thunyakitpisal et al. (2011)20 | Heat-polymerized acrylic resin (Meliodent, lot no. 08DP0017, Heraeus Kulzer, Senden, Germany) N = 90 n = 10 |
Stored in 37°C distilled water 48 hours before testing | Bevel | Control—1000-grit sandpaper MMA 180 seconds Rebase II adhesive Methyl formate 15 seconds Methyl formate–methyl acetate solution (75:25 v/v) 15 seconds Methyl formate–methyl acetate solution (50:50 v/v) 15 seconds Methyl formate–methyl acetate solution (25:75 v/v) 15 seconds Methyl acetate 15 seconds |
Autopolymerized acrylic resin (Unifast TRAD, lot no. 0710151, GC Dental Products Corp, Tokyo, Japan) |
Three-point bend test | Mostly adhesive in control, mostly cohesive in other groups 100% cohesive —methyl formate, methyl acetate, and the mixtures of methyl formate–methyl acetate 60% cohesive, 40% adhesive—MMA and Rebase II adhesive |
Control - smooth with few debris particles MMA 180 seconds—shallow pits Rebase II adhesive - small crest patterns Methyl formate 15 seconds—honeycomb Methyl formate–methyl acetate solution (75:25 v/v) 15 seconds—honeycomb Methyl formate–methyl acetate solution (50:50 v/v) 15 seconds—honeycomb Methyl formate–methyl acetate solution (25:75 v/v) 15 seconds—honeycomb |
All treatments significantly increased FS compared to control Methyl formate 15 seconds and mixture of methyl formate–methyl acetate solution (25:75 v/v), 15 seconds (highest) significantly higher than MMA 180 seconds (lowest) |
| Vallittu et al. (1994)21 | PMMA (ProBase, Ivoclar, Schaan, Liechtenstein) N = 72 n = 12 |
Stored in water at room temperature for 4 days before testing | Bevel | Control—120-grit sandpaper MMA 5 seconds MMA 30 seconds MMA 60 seconds MMA 180 seconds |
Autopolymerising acrylic resin pink Pro Base Cold (Ivoclar) PMMA | Three-point bend test | Mostly adhesive in MMA groups, except 180 seconds (mostly cohesive) | Control—more grainy appearance MMA 5 seconds—slightly smoother than control MMA 30 seconds—slightly smoother than control MMA 60 seconds—smoother than control MMA 180 seconds—smoother than control |
MMA 180 seconds only group to show significant increase in FS |
| Vojdani et al. (2008)22 | Heat-polymerized resin N = 80 n = 10 |
Stored in distilled water at 37°C for 7 days before the repair Stored in distilled water at 37°C for 7 days after repair |
Bevel | Control—320-grit sandpaper Acetone 30 seconds MMA 180 seconds Chloroform 5 seconds |
Autopolymerised acrylic resin | Three-point bend test | Mostly adhesive for control, 50% cohesive and mixed for acetone, 60% cohesive and 40% mixed for MMA and chloroform | None | All surface treatments had significantly higher FS than the control group |
Nine of the 14 studies included in the systematic review did not fulfill all the prerequisites for risk of bias (Table 3). All studies showed a low-risk of bias, but several studies presented no clear description of the abstract and limitations, as well as no clear statement about the project funding.
| Citation | Abstract (1) | Background and objectives (a) | Background and objectives (b) | Intervention (3) | Outcomes (4) | Statistical methods (10) | Outcomes and estimation (11) | Limitations (12) | Funding (13) | Overall risk of bias |
|---|---|---|---|---|---|---|---|---|---|---|
| Alkurt et al. (2014)11 | Yes | Yes | Yes | Yes | Yes | Yes | Yes | Yes | Yes | Low |
| Bural et al. (2010)12 | Yes | Yes | Yes | Yes | Yes | Yes | Yes | Yes | Yes | Low |
| Gad et al. (2020)13 | No | Yes | Yes | Yes | Yes | Yes | Yes | Yes | No | Low |
| Li et al. (2021)7 | Yes | Yes | Yes | Yes | Yes | Yes | Yes | Yes | Yes | Low |
| Neshandar et al. (2021)6 | Yes | Yes | Yes | Yes | Yes | Yes | Yes | Yes | Yes | Low |
| Nishigawa et al. (2004)14 | Yes | Yes | Yes | Yes | Yes | Yes | Yes | Yes | No | Low |
| Pereira et al. (2012)15 | Yes | Yes | Yes | Yes | Yes | Yes | Yes | Yes | No | Low |
| Sarac et al. (2005)16 | Yes | Yes | Yes | Yes | Yes | Yes | Yes | Yes | No | Low |
| Shen et al. (1984)17 | Yes | Yes | Yes | Yes | Yes | Yes | Yes | Yes | No | Low |
| Shimizu et al. (2006)18 | Yes | Yes | Yes | Yes | Yes | Yes | Yes | Yes | No | Low |
| Shimizu et al. (2008)19 | Yes | Yes | Yes | Yes | Yes | Yes | Yes | No | No | Low |
| Thunyakitpisal et al. (2011)20 | Yes | Yes | Yes | Yes | Yes | Yes | Yes | Yes | No | Low |
| Vallittu et al. (1994)21 | Yes | Yes | Yes | Yes | Yes | Yes | Yes | Yes | No | Low |
| Vojdani et al. (2008)22 | Yes | Yes | Yes | Yes | Yes | Yes | Yes | No | No | Low |
Study Characteristics
Around 12 of the included studies utilized heat-cured acrylic resin, while two studies used 3D-printed resin.6,7,11-22
Artificial Aging Protocol
Five of the studies tested the effects of artificial aging after repair,6,7,13,18,19 while the remaining nine11,12,14-17,20-22 simply stored the samples in water prior to testing (either before or after repair). The studies which investigated the effects of artificial aging all used temperatures of 5 and 55°C. The number of cycles was either 5,000 or 10,000 and the dwell time was either 30 seconds or 1 minute. Gad et al.13 compared samples stored in distilled water for 48 hours with those that were thermocycler. A significant decrease in FS was found after thermocycling among all groups, except for APA. Shimizu et al.18 found a significant decrease in strength after thermocycling for the samples that underwent artificial aging (ethyl acetate 120 seconds and Dichloromethane 5 seconds). The same author group in their later paper19 found that two groups that were thermocycler (control and ethyl acetate 120 seconds) had significantly lower SBS after thermocycling, but one group did not (Dichloromethane 5 seconds).
Repair Joint Type
There were only two repair joint types which were either “butt” or “bevel” designs. Nine7,11,12,14-19 studies used a butt joint, while five6,13,20-22 used a 45° bevel joint design.
Surface Treatment Prior to Repair
The surface treatment methods largely consisted of chemical, mechanical, laser, or plasma treatments. Chemical treatments included MMA, acetone, silane coupling agent, methacrylate-based composite bonding agent, methylene chloride (also known as dichloromethane), chloroform, ethyl acetate, methyl formate, and methyl acetate. Mechanical treatments included APA with aluminum oxide (Al2O3) particles, sandpaper abrasion and bur grinding. Additionally, one study11 utilized an erbium-doped yttrium aluminum garnet laser (Er:YAG) to etch the surface of the denture base material and another study14 tested a plasma surface treater.
Postrepair Testing and Analysis Methods
A total of 106,11-13,15,17,18,20-22 of the studies used three-point flexural bending tests to test the FS of the repaired samples. Four7,14,16,19 of the studies used a shear test to measure the SBS of the repaired samples. Analysis of the failure mode was often done by a microscope, although Neshandar et al.6 used a video measuring machine to do this. Microscopes and SEM were also used by multiple studies to analyze the fracture interface of the samples. Modes of failure for each of the test samples were either adhesive, cohesive, or mixed. Mixed failures showed signs of adhesive and cohesive failure occurring in the same sample. Adhesive failure was often found with the chemically treated samples, although this was not the case for all studies. Thunyakitpisal et al.20 found mostly cohesive failures despite mainly trialing chemical treatments. Mechanically treated samples often exhibited cohesive failure, although adhesive failure was not uncommon. Those studies which tested the effects of thermocycling showed a general trend of cohesive or mixed modes of failure prior to thermocycling, to adhesive failure after. Shear tested samples showed mainly cohesive or mixed failures but also tended towards adhesive failure after thermocycling. Flexural tested samples showed a mixture of failure types.
Most of the studies6,7,11,13,14,16-21 in this review carried out SEM analysis of the treated surfaces. Among the studies which analyzed the topography of the treated surfaces, similar characteristics were generally attributed to treatments of similar types. Surfaces treated with sandpaper exhibited fairly regular grooves. Mechanical treatments such as laser, APA, and bur grinding produced surfaces with relatively irregular and rough surfaces. Chemically treated surfaces tended to have smoother surfaces—with porosities becoming apparent as the time spent in contact with the chemical treatment increased.
Three studies6,7,14 also measured the surface roughness of the treated samples by means such as a profilometer. In the study by Li et al.,7 although APA produced a significantly rougher surface than the control, and sandpaper abrasion did not, both methods produced significantly higher SBS than the control. The study by Neshandar et al.,6 showed that all surface treatments were significantly rougher than control groups. The bur group was the highest and had significantly higher surface roughness than the laser and laser plus APA groups. Nishigawa et al.14 found no difference in roughness with plasma and control.
Meta-analysis
While the risk of bias for the studies included in this systematic review was generally low, due to the substantial heterogeneity of the intervention and analysis testing methods used in the studies, meta-analysis was not recommended.
DISCUSSION
In this systematic review, 14 studies were identified which provided an overview of the current literature on surface treatment methods for repairing conventional heat-cured and 3D-printed denture bases. Papers investigating the repairability of milled dentures were not identified through the current search strategy.
All the studies included in this review tested their samples via a shear test or flexural bending test. The strength of denture materials is often measured through flexural tests such as the three-point bending test.23 FS of a complete denture base is considered the primary mode of clinical failure while a SBS test is typically used for evaluating different dental adhesive systems.24,25 Both are well-documented methods of testing the strength of materials, but it is questionable whether shear tests produce similar results to flexural tests in terms of the comparative strength between the different types of repaired samples. Furthermore, the mode of failure seems to be influenced by the type of testing method used. This may result in inconclusive evidence as to the effectiveness of the bond strength of the repair resin to the denture base resin.26 A flexural test may also be more representative of the intraoral forces exerted on a complete denture compared to a shear test. Nonaccidental fractures of acrylic resin dentures often occur due to flexural fatigue.27
All of the five6,7,13,18,19 studies that tested the effects of artificial aging used similar thermocycling regimes with temperatures cycling between 5 and 55°C, a 30-second or 1-minute dwell time, for 5,000 or 10,000 cycles in water. At this stage, there are no universally accepted protocols. However, a sequence of temperatures at 35, 15, 35, and then 45°C at dwell times of 28, 2, 28, and 2 seconds, respectively is thought to be the most clinically relevant regime with approximately 10,000 cycles being estimated to simulate 1 year.28
This review determined an unprepared denture resin surface as the control. Only one study7 did this for their control group. The remainder of the studies reviewed abraded their samples, usually with sandpaper, prior to surface treatment. Abrasion at the repair site is a better representation of denture repair as a gap needs to be cut for the repair resin to fill. Generally, mechanical surface treatments seemed to produce higher repair strengths where studies compared mechanical and chemical treatments. However, Pereira et al.15 also showed that MMA alone and MMA application in combination with sandpaper abrasion was superior to sandpaper abrasion alone. APA performed well in the two recent studies due to the creation of microretentive surface textures which increased the surface area for bonding.11,13 On the contrary, Shen et al.17 found that washing away microdebris and smoothing roughened surfaces improved repair strength. By immersing the sample in chloroform for 5 seconds prior to repair, undesired overhangs known to hinder the flow of repair materials and reduce van der Waals force attraction were removed, thereby improving the quality of bonding sites. However, the use of chloroform is no longer recommended in dentistry due to it being carcinogenic.29
Among the chemical treatments, 180 seconds of MMA performed well which was the longest application time tested in the literature. For PMMA resins, exposure to MMA monomer softens the surface and enhances the spread of superficial fissures, and forms a pit in the bond surface.30 However, in the study by Thunyakitpisal et al.,20 other chemical surfaces treatments such as methyl formate, methyl acetate, and Rebase II adhesive produced higher FS than 180 seconds of MMA. The solubility of PMMA is high in organic solvents (such as ketones and esters) which may permit better wetting and penetration of the repair resin.31 Shimizu et al.18,19 found that 120 seconds of ethyl acetate and 5 seconds of dichloromethane were the most effective. Despite this, dichloromethane (methylene chloride) which is similar to chloroform, is not recommended for dental practice because of its potential carcinogenicity.18,32
Of the studies that analyzed the surface roughness of the treated samples, the SEM surface appearance correlated with the surface roughness values. However, this did not seem to directly correlate with the overall findings of repair strength, such as that seen in the study by Li et al.,7 where only APA produced a significantly roughened surface, but sandpaper abrasion also produced a significantly high SBS. These results appear to indicate that any level of roughened surface is an advantage in terms of SBS when repairing fractured denture bases, and therefore roughening the fractured area should be encouraged as part of the denture base repair protocol. The studies12,16-22 which tested mainly chemical treatments did not seem to show a consistent trend between the surface topography and the repair strength. However, surface roughness was not measured in these studies. It is likely that the effectiveness of the chemical treatments depends on factors such as their ability to permit penetration of repair resin, create porosities, and increase the surface energy for bonding.18
Among the studies,6,7,13,16,18-22 which observed a mixture of failure types, the samples that exhibited cohesive failure tended to have higher strength. This was influenced either by the surface treatment itself or by the effects of thermocycling. Where mechanical and chemical treatments were compared7,11,13-15 (not including the studies’ control groups), a higher proportion of mechanically treated samples experienced cohesive failure. Adhesive failure was generally seen more in chemically treated samples. Cohesive failure usually indicates the clinical reliability of the adhesion as the adhesive strength exceeds the cohesive strength of the repaired sample.33 It has been suggested that the tendency toward adhesive failures following thermocycling could be due to the formation of microcracks in the surfaces of the repair and denture resins at the interface after artificial aging.34
A few limitations were identified from the studies included in this review. Distilled water was consistently used as a medium for artificial aging but is not considered entirely adequate. Artificial saliva has been suggested as a more representative medium.28 Additionally, it is difficult to simulate intraoral mechanical wear on the denture base in vitro studies. The oral environment is complex to replicate, and the forces directed on a complete denture may differ from those in vitro studies. In terms of comparing different surface treatments, it would be beneficial for future studies to compare mechanical and chemical treatments, or combinations of both, as this would provide clearer evidence for the most effective surface treatment method. One of the surface treatments used by Li et al.7 was the application of MMA. However, wetting with MMA failed to generate obvious morphological changes in the surface. The surface roughness also did not statistically differ from the control group. This may be attributed to the 3D denture base resin (FREEPRINT denture, Detax, Ettlingen, Germany) being MMA-free. Therefore, future studies may benefit from trialing different chemical surface treatments that may have a more significant effect on the 3D-printed resin. Furthermore, the compatibility of conventional repair resins with 3D printed resins should be further studied as some 3D printed resins are made of PMMA and may be more receptive to MMA treatment and conventional repair resins than other 3D printed resins.6 The study by Li et al.7 tested the SBS of repaired 3D printed denture resins by curing the same 3D printed resin upon the treated surface of the sample. A light-emitting diode curing device (1200 mW/cm2, Bluephase, Ivoclar Vivadent, Germany) was used to cure the resin. The manufacturer’s instructions do not indicate whether this denture resin can be effectively added onto an existing sample and cured by means other than the approved DLP printers and thus ensuring that there is no uncured resin that could be detrimental to patients. Future research could investigate the compatibility of using 3D printed resin to repair 3D printed dentures, compared to conventional repair resins. No relevant studies were found on the repair of CAD/CAM dentures. This would be an interesting topic for future studies to pursue.
CONCLUSION
This systematic review concluded that a three-point bending test is a more realistic test of denture repair strength than a shear test. Further studies are required to compare mechanical and chemical surface treatments (or combinations of both) in the same denture resin, but overall, mechanical roughening and the application of chemical treatment for a suitable length of time (e.g., 180 seconds for MMA), seems to significantly improve the repair strength. From the limited evidence, surface roughness is not necessarily an indicator of repair strength, although further research needs to be conducted. Future studies should also be carried out to investigate surface treatments for repairing CAD/CAM dentures, the strength of repaired dentures in vivo, and further research into the repairability of 3D-printed dentures.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
ORCID
Sunyoung Ma https://orcid.org/0000-0003-4722-7766
ACKNOWLEDGMENT
Funding: This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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