Light-activated resin composites are complex reacting networks where polymerization kinetics, viscoelastic property development, and boundary conditions (cavity geometry, adhesive bond, and tooth compliance) jointly determine the shrinkage stress imposed on the bonded interface. Although clinicians commonly equate “curing” with the period of light exposure, a substantial portion of conversion, modulus development, and stress accumulation can occur after irradiation in the so-called dark cure phase [1,2]. The timing and magnitude of this post-irradiation reaction are sensitive to material formulation and light-delivery parameters, and-critically-can produce a rapid early post-irradiation rise in stress (an early “stress spike”) that is poorly appreciated in routine clinical practice [3]. It should be emphasized that while the two modulated light-curing strategies-soft-start and ramped curing-are related, they are not exactly the same. Both are designed to reduce polymerization shrinkage stress, but the irradiance profiles differ. The soft-start curing begins with low light intensity, then increases to higher intensity after a short interval. The goal is to allow more pre-gel flow before rapid polymerization sets in. It starts by 10 seconds at 200 mW/cm², followed by 20 seconds at 600 mW/cm². While in the ramped curing, intensity continuously increases (ramps up) from low to high during the exposure, rather than in discrete steps. The ramped curing produces a more gradual transition from pre-gel to post-gel, potentially reducing stress even more smoothly. It starts at 100 mW/cm² and steadily rising to 1000 mW/cm² over 10 seconds. In short, soft-start curing is stepwise intensity increase, whereas ramped curing is continuous intensity increase Schematic comparison of curing protocols is presented in Figure 1 [2].

Figure 1: Schematic Comparison of Curing Protocols: (A) Conventional Curing Delivers Constant High Irradiance Throughout Exposure. (B) Soft-Start Curing Begins with a Low-Intensity Phase Followed by a Stepwise Increase to High Intensity. (C) Ramped Curing Employs a Continuous, Gradual Increase in Irradiance from Low to High, Providing a Smoother Transition. Both Soft-Start and Ramped Protocols Are Designed to Extend the Pre-Gel Phase and Reduce Polymerization Shrinkage Stress Compared With Conventional Curing.

Rationale

Polymerization shrinkage stress remains one of the most critical challenges in adhesive dentistry. Despite decades of investigation, the emphasis of both research and clinical teaching has been disproportionately placed on the behavior of resin composites during active light exposure. In contrast, the kinetics of post-irradiation polymerization (commonly referred to as dark curing) and the associated early stress spike have received comparatively little attention.

The rationale for this focused analysis is threefold:

• Scientific gap. Laboratory studies consistently demonstrate that stress continues to evolve for minutes to hours after light termination, often with an abrupt rise during the early post-irradiation window [1-3]. Yet, this critical period is not typically considered in clinical guidelines, bonding strategies, or material development.
• Clinical consequences. Adhesive interfaces are most vulnerable in the minutes immediately following curing, when the adhesive layer is still maturing and tooth structures are undergoing elastic deformation. Stress generated during this window may exceed bond strength, induce cuspal deflection, or initiate microgaps that compromise long-term marginal integrity [4-6].
• Opportunity for intervention. Awareness of dark curing and the associated stress spike enables clinicians to tailor photoactivation protocols, adopt placement strategies that modulate boundary conditions (e.g., incremental placement), and allow quiet time before finishing procedures. Such interventions are simple, clinically feasible, and potentially impactful for restorative longevity [2,5,7].

By highlighting the underestimated importance of dark curing and the early stress spike, this review seeks to reframe polymerization shrinkage not as a momentary phenomenon confined to light exposure, but as a time-dependent process extending into the early minutes post-irradiation. Recognizing and addressing this overlooked window is essential for aligning laboratory knowledge with adhesive dentistry practice.

Mechanisms: Why Stress Grows After Light-Off and Why a Spike Can Occur

• Continuing conversion in the dark:Polymerization proceeds after light termination because free radicals continue chain growth until termination reactions reduce radical concentration or vitrification arrests mobility. This increases network crosslinking and volumetric contraction well after irradiation [1,4].
• Vitrification and sudden modulus increase:During polymerization, the system transitions from a viscous, stress-relaxing regime to a glassy, stiff network. If vitrification occurs while contraction is still ongoing, the ability to relieve shrinkage diminishes abruptly, causing a rapid stress rise (stress spike). High instantaneous irradiance and rapid conversion accelerate this transition [2,5].
• Interfacial and structural constraints:The tooth-restoration bonded interface and tooth structure impose restraint, where thin adhesive layers, high C-factor cavities, and rigid cuspal support convert volumetric contraction into interfacial stress. Continued contraction after vitrification is translated into interface tensile/shear stress and cuspal deflection [6].

Laboratory Evidence: Stress Development Continues After Irradiation

Multiple experimental approaches (real-time shrinkage stress analyzers, simultaneous kinetics/stress measurements, and post-irradiation hardness) demonstrate that shrinkage stress and mechanical properties evolve for minutes to hours after light exposure:

• Real-time stress traces frequently show a continuing rise after light-off, with substantial increases in the first minutes and continuing evolution for hours [3,7].
• Simultaneous kinetic/stress work confirms that degree of conversion (DC) and stress do not end at light-off; high irradiance protocols can leave unreacted monomer and radicals that convert in the dark, driving post-irradiation stress [4,8].
• Post-irradiation increases in microhardness and modulus further confirm ongoing network formation [9,10].

These findings, together indicate that the most acute period for stress evolution is the first minutes after light-off - the early dark-cure window where a stress spike may occur [2,3,7].

Clinical Consequences on Adhesive Interface Integrity

The early post-irradiation stress spike is clinically relevant because stress applied to a freshly bonded, still-maturing adhesive layer or to thin tooth walls can produce:

• Interfacial failure or microgap formation: Which undermine marginal integrity and promoting microleakage [6].
• Cuspal deflection or microfracture: In high C-factor cavities and thin walls, especially in large occlusal bulk restorations [6].
• Postoperative sensitivity: Through gap formation or internal stresses transmitted to dentin and pulp.

These phenomena occur despite apparently “adequate” immediate conversion or hardness at light-off because the critical interfacial forces arise in the dark-cure phase [1,3].

Practical Mitigation Strategies

• Modulate irradiance and exposure strategy:Soft-start or ramped curing slows early conversion and delays vitrification, allowing more stress relaxation. Photoactivation mode significantly influences stress and DC kinetics [2,11].
• Incremental placement:Using smaller increments reduces effective C-factor and distributes contraction, decreasing instantaneous restraint [6].
• Adhesive system optimization:Early bond strength and compliance of the adhesive layer are critical. Thicker or more elastic adhesives can absorb stress during dark cure [6].
• Quiet-time before finishing:Deferring occlusal loading or finishing procedures for several minutes after curing allows much of the stress evolution to pass [3,7].
• Material selection:Bulk-fill materials engineered for lower shrinkage stress may reduce but not eliminate post-irradiation stress [12].

Research Priorities

• Clinical-scale stress mappingin anatomically realistic cavities [8].
• Systematic studiesintegrating irradiance, resin chemistry, and adhesive compliance [2,8].
• Time-targeted clinical protocolsvalidated in vivo to test strategies such as soft-start curing [3].

Clinical Relevance

• Polymerization shrinkage stress continues to develop after light exposure, with an early post-irradiation stress spike that can challenge freshly bonded interfaces.
• This stress spike can induce marginal gaps, cuspal deflection, and postoperative sensitivity, even when immediate conversion appears adequate.
• Awareness of dark curing allows clinicians to optimize curing protocols, use incremental placement strategies, and defer occlusal loading or finishing to protect the adhesive interface.
• Integrating these insights into restorative practice can improve marginal integrity, reduce early failures, and enhance the long-term performance of bonded restorations.

Limitations

• Most current evidence on dark curing and early stress spikes derives from laboratory studies using simplified cavity geometries, which may not fully replicate the complexity of clinical restorations.
• Variability in material formulations, adhesive systems, and photoactivation protocolslimits generalizability across all resin composites and curing units.
• Direct clinical correlation between early post-irradiation stress and long-term restoration failure remains incompletely quantified, as few prospective in vivo studies have addressed this phenomenon.

Dark curing and the associated early post-irradiation stress spike are mechanistically plausible, experimentally documented, and clinically relevant. An interplay exists between dark curing early stress spikes. Dark curing increases the degree of conversion, which is good for material strength, but it also coincides with the stress spike period that is bad for adhesive interfaces if not managed. In adhesive dentistry, this means that the critical window for stress buildup is not only during light exposure, but also in the first minutes following irradiation. These phenomena explain why restorations that appear well cured at light-off can still develop interfacial problems. Clinicians and researchers should consider polymerization kinetics over the minutes following irradiation - not only the immediate outcome - when selecting curing protocols and restorative strategies.

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