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DENTIN BIOMODIFICATION

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CONTENTS

1. Introduction and Definition
2. Dentin Structure Relevant to Biomodification
3. Need for Dentin Biomodification - Rationale
4. Mechanisms of Dentin Degradation
5. Classification of Biomodification Agents
6. Synthetic Chemical Agents
7. Naturally Occurring Agents (Polyphenols and Plant Extracts)
8. Physical Methods
9. MMP Inhibitors as Biomodification Agents
10. Effects of Biomodification on Dentin
11. Clinical Applications
12. Limitations and Future Perspectives
13. Summary Table
14. References

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1. INTRODUCTION AND DEFINITION

"A process by which the biomechanical and biochemical properties of dentin collagen are altered through the application of cross-linking agents or other physical/chemical modalities, in order to improve the biostability and longevity of the resin-dentin interface."

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2. DENTIN STRUCTURE RELEVANT TO BIOMODIFICATION

Composition of Dentin

• The organic matrix is predominantly Type I collagen (~90% of organic fraction)
• Non-collagenous proteins include osteopontin, dentin sialoprotein (DSP), dentin phosphoprotein (DPP), and dentin matrix protein-1 (DMP-1)
• These non-collagenous proteins serve as nucleation sites for hydroxyapatite and influence collagen cross-linking

The Hybrid Layer

• Formed after acid etching and resin infiltration
• Consists of resin-infiltrated demineralized dentin
• Contains exposed collagen fibrils partially or incompletely encapsulated by resin monomers
• The "incompletely infiltrated" zone of collagen-rich dentin is the primary site of degradation
• Over time, hydrolysis of resin and proteolytic degradation of exposed collagen lead to bond failure

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3. NEED FOR DENTIN BIOMODIFICATION - RATIONALE

Why Does Resin-Dentin Bond Fail Over Time?

• Water sorption at the hybrid layer plasticizes resin components
• Hydrolysis of methacrylate ester linkages occurs
• HEMA (hydroxyethyl methacrylate) is particularly susceptible

• Host-derived proteolytic enzymes - particularly matrix metalloproteinases (MMPs) and cysteine cathepsins - are activated by acid etching
• These enzymes hydrolyze the exposed collagen fibrils at the hybrid layer
• Results in progressive loss of the mechanical support for resin tags
• Eventually leads to restoration failure, marginal leakage, secondary caries, and postoperative sensitivity

• Studies have shown a significant decrease in microtensile bond strength (µTBS) after long-term aging (6 months to 2 years) compared to 24-hour values
• This degradation is the principal reason for investigating dentin biomodification strategies

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4. MECHANISMS OF DENTIN DEGRADATION

Matrix Metalloproteinases (MMPs)

• MMP-2 (gelatinase A) - degrades denatured collagen (gelatin) and fibronectin
• MMP-8 (collagenase-2) - cleaves native Type I, II, and III collagen
• MMP-9 (gelatinase B) - degrades denatured collagen
• MMP-3 (stromelysin-1) - broad proteolytic activity; activates other MMPs

Cysteine Cathepsins

• Lysosomal cysteine proteases (cathepsins B, K, L)
• Found in dentinal tubules
• Cleave denatured collagen and non-collagenous proteins
• Also activated by acid demineralization

Significance

1. Provide cross-linking of collagen fibrils to increase resistance to enzymatic cleavage
2. Inhibit MMP and cathepsin activity
3. Improve the mechanical properties of demineralized dentin matrix
4. Enhance resin penetration and adhesion

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5. CLASSIFICATION OF BIOMODIFICATION AGENTS

DENTIN BIOMODIFICATION AGENTS
├── A. PHYSICAL METHODS
│   ├── Laser treatment (Er:YAG, CO₂, Nd:YAG)
│   └── UV irradiation / Photochemical cross-linking
│
└── B. CHEMICAL AGENTS
    ├── I. SYNTHETIC AGENTS
    │   ├── Aldehydes (Glutaraldehyde, Formaldehyde, Glyceraldehyde)
    │   ├── Carbodiimide (EDC/NHS)
    │   ├── Epoxy compounds (Genipin - semi-natural)
    │   ├── Riboflavin + UV photoactivation
    │   ├── Oxidizing agents (Sodium periodate)
    │   └── Solvent-based: DMSO, Ethanol wet bonding
    │
    └── II. NATURALLY DERIVED AGENTS
        ├── Polyphenols
        │   ├── Proanthocyanidins (Grape Seed Extract - GSE)
        │   ├── Epigallocatechin-3-gallate (EGCG) - Green tea
        │   ├── Quercetin
        │   ├── Kaempferol
        │   └── Curcumin
        ├── Tannins (Sumac extract, Tannic acid)
        ├── Catechol derivatives
        ├── Hesperidin
        └── Cacao seed extract

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6. SYNTHETIC CHEMICAL AGENTS

A. Aldehydes

• A dialdehyde; most extensively studied synthetic crosslinker
• Mechanism: Reacts with ε-amino groups of peptidyl lysine and hydroxylysine residues of collagen through a Schiff base reaction, forming stable intermolecular crosslinks
• Effects: Reduces collagen degradation rate, improves mechanical stiffness of dentin matrix, decreases MMP activity
• Disadvantage: High cytotoxicity - limits clinical applicability in its pure form
• However, GLUMA (5% glutaraldehyde + 35% HEMA) is an established desensitizing and biomodifying agent

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B. Carbodiimide (EDC) + NHS

• A zero-length crosslinker - does not add mass to the collagen network
• Mechanism: Activates carboxyl groups (-COOH) of glutamic acid and aspartic acid residues; these then react with free amino groups (-NH₂) of lysine/hydroxylysine residues, forming amide bonds
• N-hydroxysuccinimide (NHS) enhances the reaction efficiency
• Advantages:
  • Non-toxic at working concentrations
  • Effectively inactivates MMPs (MMP-2, MMP-8, MMP-9)
  • Improves immediate and long-term bond strength
  • Can be used as a primer before adhesive application
• Clinical application: 0.3 M EDC applied to etched dentin for 60 seconds before adhesive
• One of the most promising agents for clinical translation

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C. Riboflavin (Vitamin B2) + UV Photoactivation

• Riboflavin is a photosensitizer
• Mechanism: UV light activates riboflavin to generate reactive oxygen species (ROS); these ROS create covalent cross-links between collagen fibrils (similar to corneal collagen cross-linking used in keratoconus treatment)
• Effects: Increases mechanical stiffness of dentin collagen, inhibits MMP activity, improves long-term bond strength
• Clinical application requires UV/blue light delivery into the cavity - a practical limitation
• Riboflavin incorporated into adhesive formulations has shown improved long-term bond performance (Hardan et al., 2022 - meta-analysis, PMID 35954261)

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D. Ethanol Wet Bonding

• Proposed by Sadek et al. as an alternative bonding strategy
• Principle: Ethanol dehydrates the demineralized collagen network differently than water, causing collagen fibrils to stiffen and partially prevent collapse
• Allows superior resin monomer penetration due to reduced water competition
• Does not cause collagen cross-linking per se, but biomodifies the collagen scaffold
• Studies confirm improved bond durability (Mehmood et al., 2021 - PMID 35035152)

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E. DMSO (Dimethyl Sulfoxide)

• An amphiphilic solvent
• Mechanism: Displaces water from the collagen fibril surface while also acting as a carrier to enhance penetration of adhesive monomers deep into demineralized dentin
• Does not chemically cross-link collagen but facilitates better resin infiltration
• Studies show highest bond strength values after 9-month storage when using DMSO wet bonding compared to other biomodification techniques (Mehmood et al., 2021 - PMID 35035152)

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7. NATURALLY DERIVED AGENTS

• Biocompatibility and low toxicity
• Multiple mechanisms of action (cross-linking + MMP inhibition)
• Availability from renewable resources
• Potential antibacterial and antioxidant effects

A. Proanthocyanidins (PA) - Grape Seed Extract (GSE)

• Source: Seeds and skins of grapes (Vitis vinifera)
• Oligomeric proanthocyanidins (OPCs) are condensed tannins - polymers of catechin and epicatechin units
• Mechanisms of action:
  1. Collagen cross-linking: PA interacts with collagen through hydrogen bonding and hydrophobic interactions; at higher concentrations, covalent cross-links form between proanthocyanidin hydroxyl groups and collagen amino acid residues
  2. MMP inhibition: PA directly inhibits MMP-2, -8, -9 through chelation of zinc ions at the active site and steric blockade
  3. Improvement of mechanical properties: Increases the modulus of elasticity of demineralized dentin matrix significantly (El Gindy et al., 2023 - PMID 36459707)
• Concentration used: 6.5% PA solution, 60-second application
• Evidence: PA is the only crosslinker that improves BOTH immediate AND long-term bond strength (Hardan et al., 2022 - meta-analysis)
• Limitation: Causes significant brownish-purple discoloration of dentin

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B. Epigallocatechin-3-Gallate (EGCG)

• Source: Green tea (Camellia sinensis)
• A catechin gallate; the most abundant catechin in green tea
• Mechanisms:
  1. Collagen cross-linking via hydrogen bonding with proline-rich sequences
  2. Potent MMP inhibitor (inhibits MMP-2, -9, -13 by chelating zinc)
  3. Antioxidant and anti-inflammatory properties
• Improves long-term bond strength as both a pretreatment and when incorporated into adhesive (Hardan et al., 2022)
• Advantage: Less discoloration than GSE; good biocompatibility

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C. Quercetin

• A flavonoid found in onions, apples, capers
• Mechanism: Collagen cross-linking + MMP inhibition through zinc chelation + antioxidant activity
• Quercetin with DMSO pretreatment showed significantly better bond durability at 9 months (Mehmood et al., 2021)

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D. Kaempferol

• A natural flavonoid found in tea, broccoli, spinach
• Mechanism: Dual mechanism - collagen cross-linking AND MMP inhibition (confirmed by MMP zymography and FTIR spectroscopy)
• FTIR analysis confirmed higher PO4 peak (cross-link indicator) with kaempferol
• Showed higher µTBS and eliminated nanoleakage at resin-dentin interface after thermocycling (Cho et al., 2023 - PMID 37404650)

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E. Curcumin

• Source: Turmeric (Curcuma longa)
• Anti-inflammatory, antioxidant, antibacterial, and collagen cross-linking properties
• Improves long-term resin-dentin bond durability when applied as 1-minute pretreatment (El Gindy et al., 2023)

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F. Sumac Extract (Tannic Acid)

• Source: Rhus coriaria
• Rich in hydrolysable tannins and flavonoids
• Improves µTBS after 6-month water storage
• Mechanism involves collagen cross-linking and MMP inhibition

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G. Genipin

• Source: Gardenia fruit extract (Gardenia jasminoides)
• A naturally occurring iridoid glycoside
• Mechanism: Reacts with primary amines of collagen to form fluorescent blue pigments and stable covalent crosslinks
• Significantly less cytotoxic than glutaraldehyde
• Improves stiffness and resistance of collagen to enzymatic attack
• Scoping review (Schestakow et al., 2025 - PMID 40324578) confirms genipin's role in facilitating dentin remineralization by improving collagen stability as scaffold

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H. Hesperidin

• A flavanone glycoside found in citrus fruits
• Cross-links collagen and inhibits MMP activity
• Preliminary studies show improved bond durability

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8. PHYSICAL METHODS

A. Laser Treatment

• Er:YAG laser: Modifies dentin surface morphology; may increase surface area for bonding
• CO₂ laser: Causes surface fusion and melting of hydroxyapatite, reducing acid sensitivity
• Nd:YAG laser: Promotes cross-linking of collagen and inhibits bacterial growth
• Results are variable; combination with chemical biomodification may enhance outcomes

B. UV Irradiation / Photochemical Cross-linking

• Ultraviolet light (UVA) activates endogenous or exogenous photosensitizers (e.g., riboflavin)
• Generates reactive oxygen species that form covalent cross-links between collagen fibrils
• Inspired by corneal collagen cross-linking (CXL) used in keratoconus
• Increases collagen stiffness and reduces MMP susceptibility

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9. MMP INHIBITORS AS BIOMODIFICATION AGENTS

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10. EFFECTS OF BIOMODIFICATION ON DENTIN

A. Biomechanical Effects

• Increased modulus of elasticity (stiffness) of demineralized dentin matrix
• Increased ultimate tensile strength
• Resistance to collagenase-induced degradation
• Prevention of collagen fibril collapse after acid etching

B. Effects on Resin-Dentin Bond

• Significant improvement in long-term µTBS (microtensile bond strength) after aging
• Reduction of nanoleakage at the hybrid layer
• More stable hybrid layer morphology (SEM confirmed)
• Reduced silver nitrate tracer penetration (indicator of nanoleakage)

C. Effects on MMP Activity

• Reduced activity of MMP-2, -8, -9 (zymography confirmed)
• Preservation of collagen fibril integrity over time

D. Effects on Dentin Remineralization

• Collagen cross-linkers improve the scaffold function of demineralized dentin
• Enhance wettability and introduce functional groups that attract calcium ions (nucleation sites)
• Reduce interfacial energy required for mineral deposition
• Support both passive (preserving collagen) and active (enhancing mineral uptake) remineralization (Schestakow et al., 2025 - PMID 40324578)

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11. CLINICAL APPLICATIONS

A. Pre-treatment Before Adhesive Application

• Etch dentin with 37% phosphoric acid (15-30 seconds)
• Rinse, blot dry (maintain slightly moist)
• Apply biomodification agent for 30-60 seconds
• Rinse and blot dry
• Apply adhesive as usual

B. Incorporation into Adhesive Systems

• Several crosslinkers (glutaraldehyde, riboflavin, EGCG) have been incorporated directly into adhesive formulations
• GLUMA conditioner (5% GA + 35% HEMA) - commercially available desensitizer with biomodifying action
• Simplifies the clinical procedure

C. Treatment of Deep Caries and Indirect Pulp Capping

• Carious dentin is rich in activated MMPs
• Biomodification of carious dentin before placement of liner/base improves interface stability

D. Cervical Lesion Bonding

• Class V restorations are prone to bond failure due to stress concentration
• Biomodification significantly improves marginal adaptation (ToF-SIMS analysis confirmed, Betancourt et al., 2021 - PMID 34443059)

E. Remineralization Therapies

• Used as adjunct to fluoride and casein phosphopeptide-amorphous calcium phosphate (CPP-ACP) treatments
• Cross-linker pre-treatment improves mineral deposition into the demineralized collagen scaffold

F. Clinical Trial Evidence

• Systematic review and meta-analysis (Silva et al., 2022 - PMID 35692222) evaluated proanthocyanidins and EGCG in clinical trials
• Results showed good retention rates and marginal adaptation scores
• No significant increase in postoperative sensitivity

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12. LIMITATIONS AND FUTURE PERSPECTIVES

Current Limitations

1. Discoloration: Most polyphenols (especially grape seed extract, genipin) cause brownish to blue discoloration of dentin - esthetically unacceptable in anterior teeth
2. Cytotoxicity: Glutaraldehyde and other synthetic agents have dose-dependent cytotoxicity on pulpal cells
3. Standardization: Natural agents (plant extracts) lack standardized concentration, extraction method, and bioactive compound content
4. Application time: Many agents require 60-second or longer application, adding to chair time
5. Long-term clinical data: Most evidence is laboratory-based; robust clinical trials are needed
6. Bioavailability at interface: Penetration of crosslinkers into the full depth of the hybrid layer is not always achieved

Future Directions

• Nanoparticle delivery systems: Nanoencapsulation of natural agents for sustained release
• Incorporation into adhesive primers: Single-step biomodification + bonding
• Biomimetic remineralization protocols: Combining biomodification with calcium phosphate-based remineralization
• Bioactive glass incorporation: Alkaline pH from bioactive glass may inhibit MMPs
• Dual-functional agents: Compounds with simultaneous antibacterial + MMP inhibition + cross-linking properties

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13. SUMMARY TABLE

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14. REFERENCES

1. Hardan L, Daood U, Bourgi R, et al. Effect of Collagen Crosslinkers on Dentin Bond Strength of Adhesive Systems: A Systematic Review and Meta-Analysis. Cells. 2022;11(15):2417. doi:10.3390/cells11152417. [PMID: 35954261] (Systematic Review & Meta-Analysis - Tier 1 Evidence)
2. Schestakow A, Huang CM, Yin IX, Chu CH. The role of collagen cross-linker in dentin mineralization: a scoping review. Journal of Dentistry. 2025;105799. doi:10.1016/j.jdent.2025.105799. [PMID: 40324578]
3. El Gindy AH, Sherief DI, El-Korashy DI. Effect of dentin biomodification using natural collagen cross-linkers on the durability of the resin-dentin bond and demineralized dentin stiffness. Journal of the Mechanical Behavior of Biomedical Materials. 2023;137:105551. doi:10.1016/j.jmbbm.2022.105551. [PMID: 36459707]
4. Cho J, Kim H, Yoo KH, et al. The effect of kaempferol on the dentin bonding stability through matrix metalloproteinases inhibition and collagen crosslink in dentin biomodification. Journal of Dental Sciences. 2023;18(3):1184-1191. doi:10.1016/j.jds.2022.12.002. [PMID: 37404650]
5. Mehmood N, Nagpal R, Singh UP, Agarwal M. Effect of dentin biomodification techniques on the stability of the bonded interface. Journal of Conservative Dentistry. 2021;24(4):326-331. doi:10.4103/jcd.jcd_106_21. [PMID: 35035152]
6. Silva JC, Cetira Filho EL, Silva PGB, et al. Is dentin biomodification with collagen cross-linking agents effective for improving dentin adhesion? A systematic review and meta-analysis. Restorative Dentistry & Endodontics. 2022;47(2):e23. doi:10.5395/rde.2022.47.e23. [PMID: 35692222]
7. Betancourt F, Kiss A, Krejci I. ToF-SIMS Analysis of Demineralized Dentin Biomodified with Calcium Phosphate and Collagen Crosslinking: Effect on Marginal Adaptation of Class V Adhesive Restorations. Materials (Basel). 2021;14(16):4535. doi:10.3390/ma14164535. [PMID: 34443059]
8. Nisar S, Hass V, Wang R. Effect of Different Crosslinkers on Denatured Dentin Collagen's Biostability, MMP Inhibition and Mechanical Properties. Polymers. 2023;15(18):3683. doi:10.3390/polym15183683. [PMID: 37765538]
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EXAMINER'S NOTE ON MARKING ALLOCATION (Suggested)