- 1- Introduction
A dental veneer is a layer of material placed on the front surface of a tooth to improve appearance, functionality, and protect the tooth from damage. The two main materials used for veneers are composite and dental porcelain. Ceramic veneers are either fabricated using CAD/CAM technology or indirectly by a dental technician, while composite veneers can be directly placed or indirectly fabricated. Veneers are adhesively bonded to the tooth with resin cement and are popular in esthetic dentistry due to minimal tooth preparation required.
Ultrathin veneers, as thin as 0.3 mm, preserve tooth structure without enamel preparation, unlike crowns. However, veneers have certain drawbacks, such as susceptibility to long-term color changes, fractures, and debonding due to lack of mechanical retention. Despite extensive literature on dental veneers, the effects of tooth preparation, aging, veneer type, and resin cement on veneer failures in laboratory and clinical settings remain unclear.
This narrative review aims to identify the main factors associated with dental veneer failures in laboratory tests and understand their implications for clinical success or failure.
- 2- Study Selection:
Articles on veneer survival and failure were identified using the keywords “dental veneer”, “complication”, “survival rate”, “failure”, and “success rate” in PubMed/Medline, Scopus, Google Scholar, and Science Direct. Inclusion criteria focused on articles published from January 1999 to January 2024 covering tooth preparation, aging processes of resin cement, veneer translucency, thickness, fabrication techniques, and shade and thickness of resin cement. Exclusion criteria omitted articles on marginal and internal fit, microhardness, water sorption, solubility, polishability, occlusal veneers, retention, surface treatments, and wear, as the study aims to correlate laboratory failures with clinical survival rates to enhance dental practitioners’ knowledge and reduce failures in practice.
- 3- Results:
The study selected laboratory studies to identify the causes of veneer failures and clinical studies to determine factors affecting survival and success rates. The types of veneer failures discussed include fractures, debonding, and color changes.
– Laboratory Failures
-Fracture Failures:
Fractures are significant complications with veneers, with risk increasing over time. The most common fracture location is the incisal edge, often occurring at the cervical one-third where enamel is thinner, leading to dentin exposure and compromised longevity. The causes of veneer fractures are summarized in Table 1.
Figure 1. Fracture failure of the laminate veneers. The fracture may start as a crack from the margin and move toward the incisal edge as a result of improper finishing and polishing procedures (A,B), sudden chipping of the incisal edge as an effect of improper adjustments of the occlusion during centric relation or protrusive movements (C), or involvement of the incisal edge with the labial surface (D). The photos were sourced from the School of Dentistry, University of Alabama, Birmingham, USA.
Table 1. The main factors affecting veneer fractures, with examples.
Die Spacer Thickness
Fracture failures in veneers have been linked to die spacer thickness. Magne et al. reported that veneers with a ceramic thickness/composite thickness ratio of less than 3 often experience catastrophic failure. Farag et al. found that while die spacer thickness (20 µm, 40 µm, 100 µm) did not affect the mean fracture resistance of CAD/CAM-generated lithium disilicate veneers, it significantly impacted the failure mode. Higher die spacer settings decreased the microshear bond strength of these veneers, with 100 µm having the lowest bond strength.
Fracture failures are linked to the stiffness (modulus of elasticity) of supporting structures, including resin cement and the tooth layer. A higher modulus of elasticity in supporting structures increases failure load. Dentin’s lower modulus (18 GPa) compared to enamel (70-80 GPa) decreases fracture resistance. Thinner cement layers are associated with higher fracture loads, so restoring cavities before the final impression is recommended to avoid thick cement layers. Using preheated restorative composite resins (PRCRs) for bonding can optimize mechanical properties.
Type of Veneer Ceramic Material
Fracture failures are also related to the type of ceramic material used. In cases requiring increased veneer thickness, such as lingually tilted teeth, peg lateral cases, and large diastema closures, materials with high flexural strength should be chosen. Glass-ceramic (450 MPa) and zirconia (1200 MPa) are recommended due to their high strength. Using feldspathic porcelain, which has a low flexural strength (60-70 MPa), in these cases can result in fractures.
Tooth Preparation
Tooth preparation design also impacts veneer fractures. Alghazzawi et al. reported that feldspathic porcelain veneers failed at lower loads compared to glass ceramic veneers and found no significant difference in failure loads between window preparation with incisal reduction and three-quarters veneer preparation designs. Jurado et al. found that complete coverage crowns and veneers with palatal chamfers had the highest fracture resistance values, with no significant difference between single crowns and veneers with palatal chamfers. Veneers with feather-edge and butt-joint designs had significantly lower fracture resistance. Ustun et al. reported that incisal bevel preparation provided better stress distribution than incisal overlap and feather-edge designs, with lateral forces causing more stress than vertical forces.
Veneer Thickness
Lithium disilicate has a flexural strength of 450 MPa. Veneer thickness does not affect failure load post-bonding. Mihali et al. found no failures in veneers of various thicknesses (0.5 mm to 2.5 mm) for both prepared and unprepared teeth. De Angelis et al. reported improved flexural properties when lithium disilicate was bonded to dentin, making the system behave like a single unit compared to conventional cementation. Adhesively luted 0.6 mm thick lithium disilicate had the same fracture load and flexural strength as 1.5 mm thick conventionally luted lithium disilicate. Maunula et al. found that 0.3 mm thick lithium disilicate veneers had comparable failure loads to 0.5 mm veneers. However, Blunck et al. noted increased fracture risk with thin veneers and preparations with medium to high dentin portions compared to thicker veneers. Sadighpour et al. found no significant difference in failure loads of veneers bonded to intact teeth and teeth with small class V composite fillings, but extensive composite fillings compromised bonding due to insufficient enamel structure (<40%).
3.1.2. Debonding Failures
Debonding is another significant complication, with causes summarized in Table 2. It can result from poor bond strength between the veneer and resin cement, the prepared tooth and resin cement, or within the resin cement itself. Adhesive failures between the tooth and resin cement are often linked to factors such as dentin exposure (>50%), large composite restorations, inadequate surface treatment of the tooth, and contamination.
Table 2. The main factors affecting veneer debonding, with examples.
Veneer Surface Treatment
Adhesive failure between the ceramic and resin cement is often due to poor surface treatment of the veneer internal surface and contamination. Zirconia veneers, which cannot be etched with hydrofluoric acid (HF) due to their polycrystalline structure, primarily fail due to debonding. Glass-ceramic and feldspathic porcelain veneers fail mainly due to fractures and, to a lesser degree, debonding. The debonding of glass-ceramics is closely related to improper HF etching—20 seconds for lithium disilicate and 60-90 seconds for feldspathic porcelain. Martins et al. found that HF followed by silanization is the most effective surface treatment for cementing lithium disilicate glass ceramics. Additionally, Alammar et al. suggested that airborne particle abrasion and the use of special phosphate monomer-containing primers or composite resin cements can ensure long-term durable resin bonds.
Tooth Preparation
Debonding can be caused by overpreparing the tooth and exposing more than 50% dentin, though the degree of dentin exposure does not affect the survival rate. Immediate dentin sealing (IDS) has been shown to improve the bond strength of dentin to resin-based restorations, regardless of the adhesive strategy used. Zhu et al. reported that the shear bond strength of veneers bonded to 100% enamel on finishing surfaces (around 20 MPa) was double that of veneers bonded to 0% enamel (around 10 MPa). There was no significant difference among the 40–100% enamel groups, but the 20% and 0% enamel groups had significantly lower shear bond strength than the 40% enamel group.
Tooth Contamination
Contamination of the tooth with sulcular fluids, blood, or saliva before cementation is a significant factor in debonding failures. This failure often results from bleeding due to gingivitis caused by poor provisional veneers (thick margins, rough surface), forgotten retraction cords from previous impressions, or residual resin cement from provisional veneers. It can manifest as red spots on the facial surface of the veneer. When veneers fail, part of the enamel needs to be removed to eliminate the previous resin cement and expose the dentin. Several authors recommend using an erbium, chromium: yttrium-scandium-gallium-garnet (Er,Cr:YSGG) laser to safely remove the veneer without damaging the underlying enamel layer.
Figure 2. Veneer contaminated with blood during cementation (A,B). If the contamination is low, it will appear late. However, if the contamination is high, it will immediately occur. A thinner veneer correlates to an increased chance of blood contamination. In this case, the solution was used to cut the veneer, and it was replaced with a new veneer. The bleeding needed to be controlled using astringent solutions, such as aluminum chloride. The photos were sourced from the School of Dentistry, University of Alabama, Birmingham, AL, USA.
3.1.3. Color Failures
The causes of color failures are summarized in Table 3 [16,17,30,41,44,45,46,47,48,49,50,51]. Color changes can be caused by the veneer thickness, material type, substrate color, and ceramic color [16,17,41].
Literature Concerning Color Changes
Optimal color matching and aesthetic outcomes for zirconia veneers depend on thickness and background shade. Thinner ceramics exhibit greater color changes. The final color of zirconia veneers is influenced by resin cement shades and veneer layers. Zirconia shows more color change with thermal aging than glass ceramics.
Survival and success rates of veneers are influenced by fractures, debonding, and color changes. Veneers bonded to enamel are stronger and more damage-tolerant than those bonded to dentin or mixed enamel and dentin. Exposed dentin is a risk factor for clinical failure.
Cemented veneers often change color over time, a phenomenon known as color aging. TEGDMA-based resins release more monomers into aqueous environments than Bis-GMA and UDMA materials, causing discoloration. Yellowing can result from higher camphorquinone (CQ) content and larger particle sizes. Natural tooth aging also darkens veneers, with lighter shades more affected over time. Dual-cure cement for veneers under 1 mm often leads to yellow discoloration. Amine-free cements with Ivocerin (IVO) and TPO show better color stability than CQ-amine cements. TPO alone has lower bond strength but Ivocerin + TPO are effective alternatives. Light-cure resin cements are recommended for full ceramic restorations due to better long-term color stability.
- 4- Discussion
This narrative review identifies primary causes of veneer failures and factors affecting their survival and success rates. Key points include:
– Fracture Failures: Most common due to thin veneers. Proper occlusion evaluation, use of high flexural strength materials, and night guards can mitigate this risk.
– Debonding: Influenced by the size of preexisting restorations and enamel preservation. Improved by immediate dentin sealing (IDS), preventing contamination, and proper bonding techniques.
– Color Changes: Affected by resin cement composition and veneer material. Using UDMA-based resins and appropriate veneer materials helps maintain long-term color stability.
Minimal or no-preparation veneers have the highest survival rates due to intact bonding within the enamel layer. Glass-ceramic veneers perform better than feldspathic porcelain. Effective communication between the dental laboratory and practitioner is crucial for selecting materials, especially for veneers on dark substrates. Long-term clinical studies on ultratranslucent zirconia veneers are recommended for patients with parafunctional activities and dark substrates.
- 5- Conclusions
Dental veneers generally have a high survival rate (>90% for more than 10 years). The survival rate can be optimized with minimal/no tooth preparation and by using glass-ceramic veneers. Fracture is considered the primary factor affecting the clinical survival rate, followed by debonding. While laboratory studies may reproduce clinical failures, long-term clinical studies are necessary to accurately predict the survival of veneers.
Author: By Tariq F. Alghazzawi
Published: 16 May 2024
1-Department of Substitutive Dental Sciences, Taibah University, Madinah 42353, Saudi Arabia
2-Department of Mechanical and Materials Engineering, The University of Alabama at Birmingham, Birmingham, AL 35294, USAArticles J. Funct. Biomater. 2024, 15(5), 131; https://doi.org/10.3390/jfb15050131Submission received: 23 March 2024 / Revised: 8 May 2024 / Accepted: 14 May 2024 / Published: 16 May 2024(This article belongs to the Section Dental Biomaterials)