Photoinitiator Systems in Dental Resin-Based Composites: Mechanisms, Performance, and Clinical Implications

Introduction

Dental resin-based composites (RBCs) have become indispensable in contemporary restorative dentistry, offering a combination of esthetic appeal and functional performance. A critical component in the success of these materials is the photoinitiator system, which governs the initiation and efficiency of the light-activated polymerization process. Among the various photoinitiators, camphorquinone (CQ) has long been the standard due to its compatibility with most dental light curing unit (LCU) systems including the quartz-tungsten-halogen (QTH) and single-emission-peak and multiple-emission-peak light-emtitting-diode (LED) units that include blue spectrum and its well-documented clinical performance.¹

Nevertheless, CQ presents certain limitations, including its inherent yellow coloration, dependence on co-initiators, and reduced polymerization efficiency in acidic environments.² In response, alternative photoinitiators such as 1-phenyl-1,2-propanedione (PPD), diphenyl(2,4,6-trimethylbenzoyl) phosphine oxide (TPO), and phenylbis(2,4,6-trimethylbenzoyl) phosphine oxide (BAPO) have been developed to address these shortcomings.² These alternatives offer advantages such as enhanced esthetic outcomes, improved reactivity, and broader spectral sensitivity. However, their efficacy is highly dependent on the specific (LCU) employed, as each photoinitiator has a distinct absorption spectrum and activation profile.^3,4

Despite the increasing diversity of photoinitiator systems, a common oversight in clinical and laboratory settings is the assumption of their interchangeability. This oversimplification overlooks the fundamental differences in photochemical behavior, spectral requirements, and material compatibility. The inappropriate pairing of a photoinitiator with an incompatible resin system or curing light can adversely affect the degree of conversion, mechanical integrity, color stability, and overall clinical success of the restoration.^5,6

This review aims to provide a comprehensive overview of photoinitiator systems used in dental composites. It examines their chemical classifications, absorption characteristics, polymerization kinetics, and interactions with various curing technologies. By elucidating these relationships, the review underscores the importance of selecting appropriate photoinitiators tailored to the specific material formulation and clinical context, thereby optimizing restorative outcomes.

RBCs composition

The RBCs are composed of an organic resin matrix, inorganic filler particles, a silane coupling agent, photoinitiator system, pigments, inhibitors, and each component plays a critical role in light transmission and polymerization efficiency.⁷ The resin matrix, commonly based on Bis-GMA, UDMA, and TEGDMA, determines viscosity, refractive index, and mobility of reactive radicals; lower-viscosity monomers such as TEGDMA enhance monomer conversion but may increase shrinkage, while more rigid monomers like Bis-GMA can limit radical mobility and reduce degree of conversion at greater depths.⁸ Inorganic fillers—such as silica, quartz, or glass—improve mechanical properties but significantly affect light transmission; increased filler loading, larger particle size, and mismatch between the refractive indices of fillers and resin matrix lead to greater light scattering, reducing irradiance penetration and depth of cure. Conversely, nanofilled composites, although highly filled, may exhibit improved translucency if refractive indices are well matched, allowing better light propagation.⁹ The silane coupling agent ensures stress transfer between filler and matrix and indirectly supports polymerization efficiency by maintaining optical homogeneity. The photoinitiator system, most commonly camphorquinone combined with an amine co-initiator, strongly influences polymerization kinetics; camphorquinone absorbs blue light (≈468 nm), and insufficient light transmission due to composite opacity, shade, or thickness results in reduced free-radical generation and incomplete polymerization.¹⁰ Darker shades and opaque composites further attenuate light, necessitating higher irradiance or longer exposure times. Overall, the interaction between RBCs composition and optical properties governs light transmission, degree of conversion, depth of cure, and ultimately the clinical performance and longevity of resin-based restorations.¹⁰

Polymerization chemistry

Polymerization of the resin matrix in dental RBCs occurs predominantly through light-activated free-radical polymerization of methacrylate-based monomers such as Bis-GMA, UDMA, and TEGDMA, initiated by photoinitiator systems that generate reactive radicals upon light exposure; these radicals attack carbon–carbon double bonds (C=C), leading to chain propagation, crosslinking, and formation of a rigid polymer network.¹¹ The reaction proceeds rapidly during the pre-gel phase, followed by a diffusion-limited post-gel (vitrification) phase in which monomer mobility decreases, restricting further conversion and resulting in a degree of conversion (DC) that is typically incomplete.¹² DC is a critical determinant of mechanical properties, wear resistance, color stability, and biocompatibility, while insufficient conversion is associated with increased residual monomers and reduced clinical performance.¹³ Polymerization kinetics and final conversion are influenced by multiple factors, including monomer composition and viscosity, filler content and light scattering, photoinitiator type and concentration, spectral compatibility with the LCU, and curing protocol.¹⁴ An inherent consequence of polymerization is volumetric shrinkage, which generates shrinkage stress at the tooth–restoration interface and may compromise marginal integrity and bonding durability. Consequently, modern composite formulations aim to balance adequate conversion with controlled polymerization rates and reduced shrinkage stress through optimized resin chemistry, filler loading, and photoinitiator systems.¹⁵

Photoinitiators

Photoinitiators are essential components of resin composites, acting as light-activated molecules that initiate the polymerization of methacrylate monomers by generating free radicals.¹⁶ In accordance with the Grotthuss–Draper law, only absorbed light can induce a photochemical reaction, meaning that polymerization efficiency depends strongly on the spectral overlap between the initiator’s absorption band and the output wavelength of the curing light.¹⁷ The selection of photoinitiator directly affects the degree of conversion (DC), color stability, curing depth, and mechanical integrity of the restoration.¹⁴ Photoinitiators are broadly divided into Type I (unimolecular) and Type II (bimolecular) systems, distinguished by their mechanisms of free-radical formation.¹⁸

Type I photoinitiators, also known as Norrish Type I compounds, undergo α-cleavage when exposed to light, producing two radical fragments that can each initiate polymerization without requiring a co-initiator.¹⁸ Common examples include trimethylbenzoyl-diphenylphosphine oxide (TPO), bis-acylphosphine oxide (BAPO), and mono-acylphosphine oxide (MAPO).¹⁹ These phosphine-oxide derivatives are characterized by their absorption of shorter-wavelength violet light (≈380–420 nm) and high molar absorptivity.²⁰ This enables more efficient polymerization and reduced yellow discoloration compared with camphorquinone (CQ) systems, and leads to improved tissue shade matching and lower residual-monomer release due to higher cross-link density.^20,21

However, challenge arises because their absorption limits are near the ultraviolet region, leading to a spectral mismatch with single-emission-peak LED units, which are typically designed for CQ activation at 460–480 nm.²² To address this, multiple-emission-peak LED LCUs, such as Bluephase G2 and Valo emits multiple peaks, ensuring the simultaneous activation of both initiator types.²³

Type II photoinitiators, including camphorquinone (CQ), phenanthrenequinone (PQ), benzophenone (BP), and 1-phenyl-1,2-propanedione (PPD), require a co-initiator, usually a tertiary aromatic amine, to form free radicals via hydrogen-abstraction or electron-transfer reactions.²⁴ These initiators absorb light in the visible-blue spectrum (≈400–490 nm), aligns with most dental LCUs ( Table 1).^7,25

CQ remains the clinical standard due to its reliable curing depth and mechanical performance, although drawbacks include yellow coloration, amine oxidation, and reduced long-term color stability. Radical formation involves CQ excitation to a singlet state, intersystem crossing to a triplet state, and subsequent reaction with the co-initiator.²⁶ The efficiency of this process depends on initiator concentration, amine structure, and exposure energy.²⁷

To overcome the limitations of CQ-based systems, Type I photoinitiators, including trimethylbenzoyl-diphenylphosphine oxide (TPO) and bis-acylphosphine oxide (BAPO), have gained increasing attention.¹⁴ These initiators undergo α-cleavage upon light activation, generating radicals more efficiently without the need for amine co-initiators.²⁸ As a result, they demonstrate higher reactivity, faster polymerization, and improved color stability.²⁹ However, their absorption spectra lie primarily in the violet light range (approximately 350–420 nm), requiring multiple-emission-peak or violet-enhanced LED curing units to ensure adequate activation, particularly at greater depths.¹⁴

More recently, benzoyl germanium derivatives, such as Ivocerin^®, have been introduced as advanced photoinitiators for dental applications.²⁵ These materials exhibit broad absorption spectra extending into the visible range, high quantum efficiency, and excellent depth of cure while maintaining low intrinsic coloration.²⁵ Their improved reactivity allows for shorter curing times and enhanced mechanical properties, although long-term biological safety continues to be an area of active investigation.³⁰

Reactivity of photoinitators

The reactivity of a photoinitiator dictates its efficiency in generating free radicals upon light exposure. TPO, for instance, demonstrates higher reactivity and faster radical production than CQ, leading to more complete polymerization and higher degrees of conversion under similar curing conditions.¹⁹ Conversely, CQ requires the presence of a tertiary amine to act as an electron donor for radical formation, making its efficiency dependent on both amine concentration and oxygen inhibition levels.³¹

This reactivity difference explains the superior conversion rate and reduced residual monomer release observed in TPO-based composite systems compared with traditional CQ–amine formulations.⁷ Studies show that resins containing only TPO achieve comparable or higher degrees of conversion than CQ–amine systems, with superior color stability and reduced post-cure yellowing.^14,32 However, TPO-based materials sometimes exhibit a lower depth of cure, attributed to limited photon penetration at shorter wavelengths.²² In contrast, BAPO and PPD can activated under blue light activation, utilize both α-cleavage and hydrogen-abstraction mechanisms, thereby extending their curing compatibility with broader-spectrum LCUs.¹⁶

Overall, the development of alternative photoinitiators such as TPO, BAPO, and MAPO, together with multi-peak LED technology, has expanded the photopolymerization capabilities for dental RBCs. This has resulted in improving aesthetic outcomes and enabling optimized curing efficiency while minimizing residual stress and discoloration.⁷

Toxicity of photoinitiators

Despite their photophysical advantages, TPO and BAPO demonstrate significantly higher in vitro cytotoxicity and genotoxicity compared to CQ, some studies have reported their cytotoxicity to be 50 to 250 times greater in certain cell models.³³ This toxicity is further amplified when TPO is combined with monomers like Bis-GMA, as synergistic result in increased oxidative stress and reduced cell viability.³³

Nevertheless, fully polymerized composites release only minimal residual photoinitiator, and due to very low in vivo exposure levels, clinically significant toxicity has not been demonstrated. A separate concern arises from phosphine-oxide photoinitiators like TPO, which display poor biodegradability and environmental persistence, raising ecotoxicological concerns.³³ Consequently, researchers recommend ongoing monitoring and expanded biocompatibility assessments for TPO-containing RBCs, particularly in bulk-fill formulations and high-initiator-content systems.¹⁶

Influence of photoinitiator concentration and ratios

The concentration and ratio of photoinitiators and co-initiators play a critical role in determining the polymerization efficiency, optical properties, and clinical performance of resin-based composites.⁷ Increasing photoinitiator concentration generally enhances free-radical generation, leading to a higher polymerization rate and increased degree of conversion; however, beyond an optimal level, excessive photoinitiator can cause premature radical termination, increased light absorption near the surface, and reduced depth of cure due to attenuation of light transmission.³⁴ High concentrations of camphorquinone (CQ), the most commonly used photoinitiator, are also associated with increased yellow coloration, negatively affecting esthetic outcomes.³⁵ The ratio between CQ and the amine co-initiator is equally important, as an imbalance may limit radical formation or increase the presence of unreacted amines, which can compromise color stability and long-term biocompatibility.³⁶ Optimal CQ–amine ratios promote efficient electron transfer, maximizing polymerization kinetics while minimizing discoloration and cytotoxic by-products.³⁷ In alternative photoinitiator systems, such as phenyl-propanedione (PPD), bis-acylphosphine oxide (BAPO), and trimethylbenzoyl-diphenylphosphine oxide (TPO), concentration and ratio similarly influence curing behavior, with higher reactivity often improving depth of cure but increasing sensitivity to light wavelength compatibility.⁶

Overall, careful optimization of photoinitiator concentration and initiator–co-initiator ratios is essential to balance degree of conversion, curing depth, color stability, and mechanical performance of resin-based composites.¹⁴

Factors affecting photoinitiator efficiency

Photoinitiator efficiency in RBCs is influenced by multiple interrelated material- and curing-related factors that determine the extent of free-radical generation and polymerization.³⁸ The type of photoinitiator is fundamental, as different systems exhibit distinct absorption spectra and quantum yields; for example, camphorquinone (CQ) absorbs blue light (~468 nm), whereas alternative initiators such as TPO and BAPO absorb at shorter wavelengths and often generate radicals more efficiently.¹⁴ Photoinitiator concentration and initiator–co-initiator ratio critically affect polymerization kinetics, with insufficient levels leading to inadequate radical formation and excessive amounts causing light attenuation, increased surface curing, and reduced depth of cure.⁹ LCUs characteristics, including irradiance, wavelength distribution, exposure time, and beam homogeneity, must be compatible with the initiator’s absorption spectrum to ensure effective activation.²⁶ Composite optical properties, such as shade, translucency, filler content, particle size, and refractive index matching between filler and resin matrix, influence light transmission and thus initiator activation at greater depths.³⁹ The resin matrix composition also affects efficiency, as monomer viscosity and rigidity govern radical mobility and propagation; highly viscous systems may restrict conversion despite adequate initiation.³⁹ Oxygen inhibition at the surface can quench free radicals, reducing polymerization efficiency, particularly in low-initiator or low-irradiance conditions.⁴⁰ Additionally, temperature and increment thickness influence reaction kinetics, with thicker increments and lower temperatures reducing initiator activation and depth of cure.⁴¹ Collectively, these factors highlight the need for optimized material formulation and appropriate curing protocols to maximize photoinitiator efficiency and clinical performance.⁴²

Conclusion

Photoinitiator systems are critical to the clinical performance and longevity of resin-based composites, as they govern polymerization kinetics, depth of cure, degree of conversion, mechanical properties, color stability, and biocompatibility. Their effectiveness results from complex interactions among composite composition, optical properties, photoinitiator concentration and ratios, light-curing unit spectral output, and clinical curing protocols. Resin matrix chemistry and filler characteristics influence light transmission and initiator activation at depth, while inappropriate initiator concentrations may compromise polymerization efficiency and esthetic outcomes.

Advances in alternative photoinitiators such as trimethylbenzoyl-diphenylphosphine oxide (TPO) and bis-acylphosphine oxide (BAPO) have improved reactivity and reduced intrinsic discoloration compared with camphorquinone-based systems, although they require careful spectral compatibility with light-curing units and raise considerations regarding biological safety. As no single photoinitiator fulfills all clinical requirements, modern resin composites increasingly rely on multi-initiator systems that combine camphorquinone with Type I initiators to broaden absorption spectra, enhance curing efficiency, and improve esthetic and mechanical performance. Continued optimization of these systems, along with improved light-curing strategies and long-term biological evaluation, is essential for achieving predictable clinical outcomes and advancing resin-based restorative materials.