Introduction
Wear resistance remains one of the most significant determinants of the clinical longevity of dental restorative materials. As composite materials continue to expand their clinical applications from simple restorations to extensive rehabilitations, their ability to withstand complex intraoral wear mechanisms becomes increasingly critical. This article comprehensively examines the wear behavior of contemporary composite materials, analyzing the relationships between material composition, microstructure, and wear resistance. Through evaluation of current research evidence, clinical applications, and a detailed case study, this review aims to provide clinicians with evidence-based guidelines for material selection and optimization of clinical techniques to maximize wear resistance and restoration longevity.
Mechanisms of Wear in the Oral Environment
Fundamental Wear Processes
Dental restorative materials are subjected to multiple wear mechanisms operating simultaneously in the oral cavity:
- Abrasive wear: Results from harder materials or particles abrading softer surfaces. Two-body abrasion occurs during direct contact between opposing surfaces, while three-body abrasion involves an intermediate abrasive medium (food bolus, toothpaste).
- Adhesive wear: Occurs when asperities of opposing surfaces momentarily adhere together and material transfer takes place upon separation. This mechanism is particularly relevant during parafunctional activities.
- Fatigue wear: Results from repeated loading cycles that lead to subsurface crack propagation and eventual material loss. This is common in occlusal contact areas subjected to masticatory forces.
- Corrosive wear: Arises from chemical degradation of the material surface, making it more susceptible to mechanical wear processes. Acidic foods, beverages, and bacterial byproducts contribute to this mechanism.
- Impact wear: Caused by sudden contact between opposing surfaces, leading to localized damage. This can occur during mastication of hard foods or parafunctional habits
Tribological Factors Affecting Clinical Wear
Multiple factors influence the complex wear behavior of composite restorations in vivo:
- Contact force: Higher masticatory forces accelerate wear processes
- Contact area and pressure distribution: Concentrated forces increase localized wear
- Sliding distance and velocity: Related to masticatory patterns and habits
- Surface roughness: Rougher surfaces experience more rapid initial wear
- Lubrication: Saliva provides variable lubrication effects
- Temperature fluctuations: Thermal cycling affects material properties
- pH variations: Acidic environments accelerate degradation
- Duration and frequency of contact: Influenced by diet and parafunctional habit
Composition of Contemporary Composite Materials and Their Wear Behavior
Resin Matrix Developments
The organic matrix significantly influences wear resistance through its mechanical properties and hydrolytic stability:
- Traditional Bis-GMA-based systems: Prone to water sorption and hydrolytic degradation, limiting long-term wear resistance
- UDMA-based matrices: Generally demonstrate improved wear resistance due to higher flexibility and toughness
- Silorane-based systems: Ring-opening chemistry provides reduced polymerization shrinkage and improved hydrophobicity, potentially enhancing wear resistance
- Ormocer (organically modified ceramic) matrices: Combine organic and inorganic components at molecular level, showing promising wear resistance profiles
- High-viscosity bulk-fill matrices: Modified rheology and improved depth of cure, but variable wear resistance depending on filler incorporation
Recent innovations include:
- Stress-decreasing monomers (SDM): Reduce polymerization stress while maintaining mechanical properties
- Interpenetrating polymer networks (IPN): Enhance fracture toughness and resistance to fatigue
- Thiol-ene chemistry: Offers more homogeneous polymer networks with improved mechanical stability
Filler Technology and Its Impact on Wear Resistance
Filler characteristics profoundly affect wear behavior:
- Particle size: Evolution from macrofill (10-50 μm) to microfill (0.01-0.1 μm) to nanofill (0.005-0.01 μm) has generally improved wear resistance
- Filler morphology: Spherical particles typically result in better wear profiles than irregular particles
- Filler loading: Higher filler content generally correlates with improved wear resistance until an optimal threshold
- Filler composition: Zirconia, silica, barium glass, and other fillers demonstrate different wear behaviors
- Filler-matrix interface: Silane coupling agents significantly impact wear resistance through their effect on stress transfer
Contemporary Filler Configurations
- Nanofilled composites: Contain discrete nanoparticles (20-75 nm) and nanoclusters (0.6-1.4 μm), demonstrating excellent polish retention and wear resistance
- Nanohybrid composites: Combine nanoparticles with larger particles (up to 1 μm) for optimized mechanical properties
- Submicron hybrid composites: Utilize particles below 1 μm for improved polishability without compromising mechanical properties
- Microhybrid composites: Contain particles of various sizes (0.2-3 μm) for balanced mechanical properties and aesthetics
- Fiber-reinforced composites: Incorporate glass, polyethylene, or carbon fibers for enhanced fracture resistance and potentially modified wear behavior
CAD/CAM Composite Blocks
Industrial polymerization of CAD/CAM composite blocks under controlled pressure and temperature results in:
- Higher degree of conversion
- Reduced residual monomer content
- More homogeneous structure
- Improved mechanical properties
- Enhanced wear resistance compared to direct composites
Recent developments include polymer-infiltrated ceramic networks (PICN) that combine the advantages of ceramics and composites for potentially optimized wear behavior.
Methods for Evaluating Wear Resistance
In Vitro Testing Methodologies
Laboratory wear testing employs various approaches:
- Two-body wear simulators: Pin-on-disc, ball-on-disc configurations
- Three-body wear testing: Alabama OHSU, ACTA, and Zurich wear machines
- Toothbrush simulators: For evaluating abrasion resistance
- Chewing simulators: IVOCLAR, MTS, and Willytec systems that simulate complex masticatory cycles
Parameters measured include:
- Vertical substance loss
- Volume loss
- Weight loss
- Surface roughness changes
- Coefficient of friction
- Micromorphological surface alterations
Clinical Evaluation Methods
Clinical wear assessment techniques include:
- Direct methods:
- United States Public Health Service (USPHS) criteria
- Clinical evaluation parameters (FDI World Dental Federation)
- Qualitative visual assessment using loupes or microscopes
- Indirect methods:
- Serial impressions and cast comparisons
- 3D surface mapping with profilometry
- Digital scanning and superimposition
- Reference marking techniques
- Quantitative measurements:
- Vertical height loss at specific locations
- Volume loss calculations
- Surface roughness parameters (Ra, Rz)
Wear Resistance of Specific Contemporary Composite Categories
Conventional Composites
Microhybrid Composites
Microhybrid composites contain particles ranging from 0.2-3 μm and demonstrate balanced mechanical properties. Studies show:
- Moderate two-body wear resistance
- Good resistance to three-body abrasion
- Higher wear resistance than microfilled but lower than nanofilled composites
- Clinical annual wear rates of 5-10 μm in posterior areas
Nanohybrid Composites
these materials combine nanoparticles with microparticles and show:
- Improved wear resistance compared to traditional hybrids
- Better polish retention under abrasive conditions
- Reduced filler plucking during functional wear
- Clinical annual wear rates of 3-8 μm in posterior areas
Nanofilled Composites
With nanoscale primary particles and nanoclusters, these materials offer:
- Superior wear resistance in high-stress areas
- Excellent polish retention
- Fracture toughness comparable to microhybrids
- Clinical annual wear rates of 2-5 μm in posterior areas
Bulk-fill Composites
H igh-viscosity Bulk-fills
These materials allow placement in 4-5 mm increments and demonstrate:
- Wear resistance comparable to conventional composites
- Potential for increased wear at deeper parts of the restoration due to reduced degree of conversion
- Clinical performance heavily dependent on adequate curing protocols
Flowable Bulk-fills
Requiring a capping layer of conventional composite, these materials show:
- Lower wear resistance than high-viscosity bulk-fills
- Improved wear when properly covered with wear-resistant material
- Higher susceptibility to fatigue wear
Fiber-reinforced Composites
The incorporation of fibers (glass, polyethylene, or carbon) results in:
- Anisotropic wear behavior depending on fiber orientation
- Improved resistance to catastrophic fracture
- Potential fiber exposure with continued wear
- Need for specific placement techniques to optimize wear resistance
CAD/CAM Composites
Resin Nanoceramics
These materials contain approximately 80% nanoceramic particles embedded in a resin matrix and demonstrate:
- Wear behavior similar to natural enamel
- Good antagonist friendliness with minimal opposing enamel wear
- Clinical wear rates of 5-8 μm annually
Polymer-infiltrated Ceramic Networks
PICN materials (e.g., VITA Enamic) feature a dominant ceramic network infiltrated with polymer and show:
- Wear resistance intermediate between composites and ceramics
- Improved resistance to fatigue wear
- Better shock-absorbing capability than pure ceramics
- Clinical wear rates of 3-5 μm annually
Clinical Factors Affecting Wear Resistance
Patient-related Factors
- Masticatory forces: Bruxism and clenching accelerate wear
- Diet: Abrasive foods and acidic beverages impact wear patterns
- Parafunctional habits: Edge-to-edge contacts create severe wear conditions
- Salivary characteristics: Flow rate, buffering capacity, and composition affect lubrication
Restoration-related Factors
- Restoration size: Larger restorations experience more complex stress distributions
- Occlusal contacts: Nature and distribution of contacts influence wear patterns
- Cavity configuration: Restoration morphology affects stress concentration
- Marginal quality: Defective margins accelerate peripheral wear
Technique-related Factors
- Polymerization protocol: Insufficient curing compromises wear resistance
- Finishing and polishing: Surface roughness significantly impacts initial wear
- Incremental technique: Proper adaptation and polymerization of layers
- Moisture contamination: Compromises mechanical properties and wear resistance
Case Study: Management of Excessive Wear in Posterior Composite Restorations
Patient Profile
A 45-year-old male patient presented with concerns about the deterioration of multiple posterior composite restorations placed 5-7 years prior. The patient reported awareness of grinding his teeth during stressful periods and occasional jaw muscle fatigue upon waking.
Clinical Findings
Examination revealed:
- Moderate to severe occlusal wear on composite restorations in teeth #3, #14, #19, and #30
- Vertical dimension loss of approximately 0.8 mm in the posterior region
- Exposed dentin at the center of the #19 restoration
- Significant occlusal step formation between composite material and surrounding enamel
- Wear facets on opposing natural teeth
- Signs of parafunctional activity (masseter hypertrophy, scalloped tongue)
Material Selection Rationale
Based on the patient’s history of parafunctional habits and previous restoration wear, the following materials were selected for replacement:
- CAD/CAM composite overlay (LAVA Ultimate, 3M ESPE) for teeth #3 and #14
- Direct nanofilled composite (Filtek Supreme Ultra, 3M ESPE) for teeth #19 and #30
This selection allowed comparison of wear performance between indirect CAD/CAM and direct nanofilled materials under similar functional conditions.
Clinical Protocol
For CAD/CAM Overlays (teeth #3 and #14):
- Removal of failed restorations and minimal preparation with rounded internal line angles
- Digital impression using intraoral scanner
- Design and milling of composite overlays with anatomically accurate occlusal morphology
- Air abrasion of intaglio surfaces with 50 μm aluminum oxide particles
- Application of silane coupling agent followed by universal adhesive
- Bonding with dual-cure resin cement using selective etching protocol
- Occlusal adjustment and polishing with composite polishing system
For Direct Composites (teeth #19 and #30):
- Removal of failed restorations with preservation of remaining tooth structure
- Selective enamel etching and application of universal adhesive
- Incremental placement of nanofilled composite with attention to proper adaptation
- Creation of anatomically correct occlusal morphology using wax-up as reference
- Extended light-curing protocol (additional 20 seconds from buccal and lingual aspects)
- Meticulous finishing and polishing with multi-step system
Additional Interventions:
- Fabrication of a night guard for management of bruxism
- Occlusal equilibration to optimize force distribution
- Patient education regarding dietary factors and parafunctional habits
Follow-up Evaluation
The patient was evaluated at 6, 12, and 24 months post-treatment. Assessment included:
- Clinical evaluation using modified USPHS criteria
- Digital impressions for 3D superimposition analysis
- Standardized photographs for qualitative comparison
Results and Discussion
At the 24-month follow-up:
- CAD/CAM composite overlays demonstrated superior wear resistance with mean vertical loss of 35 μm
- Direct nanofilled composites showed mean vertical wear of 62 μm
- Both materials maintained acceptable occlusal contour and function
- Patient compliance with night guard usage was approximately 70%
This case illustrates the superior wear resistance of industrially polymerized CAD/CAM composites compared to direct materials, even when using advanced nanofilled formulations. The importance of addressing contributing factors (bruxism) alongside material selection is also highlighted.
Clinical Recommendations for Optimizing Wear Resistance
Material Selection Strategies
- Match material properties to functional demands of the specific clinical situation
- Consider indirect CAD/CAM composites for patients with severe bruxism
- Select nanofilled or nanohybrid composites for direct posterior restorations
- Evaluate antagonist material when selecting restorative material
- Consider fiber-reinforced composites for patients with high fracture risk
Clinical Technique Optimization
- Ensure complete polymerization through appropriate light-curing protocols
- Implement incremental placement technique for direct composites
- Create anatomically correct occlusal morphology with proper cusp-fossa relationships
- Establish optimal occlusal contacts with even force distribution
- Perform meticulous finishing and polishing
- Consider heat treatment of direct composites when possible
Maintenance and Monitoring
- Implement regular recall intervals for wear assessment
- Provide occlusal protection for patients with parafunctional habits
- Consider periodic repolishing of composite restorations
- Monitor and document wear progression with digital techniques
- Address early signs of excessive wear before catastrophic failure
Future Directions in Wear-resistant Composites
Novel Matrix Formulations
Research continues on advanced resin matrices:
- Self-healing polymers that can repair microcracks
- Biomimetic matrices inspired by natural dental tissues
- Stimuli-responsive polymers that adapt to changing oral conditions
Advanced Filler Technologies
Emerging filler technologies include:
- Functionalized nanofillers with improved matrix integration
- Bioactive particles that release ions to reinforce surrounding tissues
- Core-shell fillers with optimized mechanical and optical properties
- Graphene-reinforced composites with exceptional mechanical properties
Biomimetic Approaches
Biomimetic strategies aim to replicate natural tooth structure:
- Hierarchical structures mimicking dentin-enamel complex
- Gradient materials with site-specific mechanical properties
- Remineralizing composites that strengthen the restoration-tooth interface
Computational Design
Digital technologies are enabling advanced material development:
- Finite element analysis for optimizing restoration design
- Artificial intelligence for predicting wear behavior
- Computer-aided design of material microstructure
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