The Semi-Split Delayed Gap Closure Technique: Temporal Synchronization with Dark Curing Viscoelastic Relaxation to Preserve Adhesive Interface Integrity in Large Occlusal Restorations

   

Khamis A Hassan^1* and Salwa E Khier²

Abstract

Objective: To present a clinical strategy that “conceptually” aligns the timing of semi-split gap closure with the composite’s dark curing viscoelastic relaxation window to preserve freshly bonded interfaces in large occlusal bulk-fill resin composite restorations.   

Background: Polymerization shrinkage and post-irradiation stress spikes remain critical challenges in high C-factor occlusal restorations. Immediately after curing light is off, bulk-fill resin composites continue dark polymerization, generating early stress peak while the adhesive interface remains immature. Temporally aligning procedural steps with viscoelastic relaxation can offset this stress accumulation.

Methods: A single 4 mm thick increment of bulk-fill composite is placed in the cavity, and a 1.5 mm wide, 2 mm deep diagonal gap is created before initial light curing. The composite is cured while the gap remains open. The clinician then delays gap closure for approximately 4–6 minutes, allowing dark curing relaxation to occur, before filling and final curing.

Results: This delay allows stress relaxation within the partially polymerized bulk, dissipating early contraction energy and reducing marginal strain. The subsequent gap filling occurs under stabilized interfacial conditions, maintaining adhesion and minimizing marginal leakage and cusp deflection.

Conclusion: Time-coordination of the delayed diagonal gap closure with composite’s dark curing kinetic viscoelastic relaxation window is a theoretically plausible, low-cost procedural approach to enhance the interfacial integrity and long-term performance of large occlusal bulk-fill resin composite restorations.

Clinical Significance: Timing the gap closure within the early post-curing viscoelastic window offers clinicians a simple, reproducible, and evidence-based method to mitigate shrinkage stress and improve restoration longevity.

Keywords

Bulk-fill resin composite; Dark- curing kinetics; Delayed gap closure; Polymerization shrinkage stress; Adhesive interface integrity; Semi-split technique; Stress spike; Viscoelastic relaxation.

Introduction

Polymerization shrinkage stress remains a fundamental limitation in resin-based composite restorations, particularly in large occlusal cavities where external restraint is high and wall compliance is low [1,2]. Bulk-fill composites were introduced to simplify placement and reduce stress through improved translucency, enhanced photoinitiator systems, and stress-relieving monomers. Yet, even modern bulk-fill materials experience residual post-irradiation shrinkage due to dark curing reactions that persist for several minutes after light exposure [3,4].

During this dark curing phase, additional crosslinking and conversion lead to a secondary rise in shrinkage stress while the composite transitions from viscoelastic to glassy behavior [3]. Because adhesive interfaces are still immature during this time, these evolving forces can exceed local bond strength, generating microgaps and interfacial defects [5,6].  Conventional layering and soft-start light curing strategies reduce, but do not eliminate, the temporal misalignment between stress development and interfacial maturation [6].

This paper focuses on temporal stress modulation as a complementary strategy to the more commonly addressed geometric and material-based approaches for stress management. It aims to “conceptually” establish a timing strategy that synchronizes the semi-split delayed gap closure technique with the composite’s dark curing viscoelastic relaxation window, in order to attenuate post-irradiation stress and preserve the integrity of freshly bonded interfaces in large occlusal bulk-fill resin composite restorations.

It is important to note that this paper does not aim to present new experimental data. Instead, its purpose is to synthesize existing evidence from the literature, introduce a novel theoretical model, and structure relevant clinical and physical phenomena into a coherent explanatory framework that can guide both understanding and practical application.

Rationale

When a bulk-fill resin composite polymerizes, it undergoes volumetric shrinkage of approximately 2–4%, proportional to its degree of conversion [7–9]. The resulting contraction induces tensile stresses at the bonded interfaces, which are further intensified by cavity geometry (C-factor) and the inherent rigidity of enamel and dentin walls. Although conventional incremental placement technique can reduce overall shrinkage stress, they do not effectively capitalize on the material’s intrinsic, time-dependent viscoelastic relaxation that occurs during the early post-curing phase.

Once the curing light is turned off, dark curing begins immediately. Although the rate of polymerization decreases, conversion continues during this phase, contributing an additional 15–20% to the total degree of conversion. During this interval, the resin’s elastic modulus rises sharply, while its capacity for viscous flow—and thus stress relaxation—progressively declines [10]. As vitrification approaches, a transient “stress spike” is typically observed [11]. Nonetheless, a brief viscoelastic relaxation window persists between the end of light exposure and the onset of full rigidity, allowing limited stress dissipation [10–12].

The semi-split delayed gap closure technique integrates mechanistic understanding with clinical application to mitigate early post-irradiation stress spikes, thereby preserving the integrity of freshly bonded interfaces in large occlusal restorations. It intentionally leverages the viscoelastic relaxation window to achieve controlled stress equilibration. Following the initial curing of the segmented composite bulk, the diagonal gap is deliberately left unfilled for approximately five minutes to allow viscoelastic stress relaxation and directed lateral shrinkage toward the open space [13]. Once most of the internal stress has dissipated and the composite’s modulus has stabilized, the gap is filled with a small amount of the same material and subjected to final curing [14]. The delayed unification of the two composite segments consequently enhances preservation of the adhesive interface, reducing the likelihood of interfacial debonding, microgap formation, and cuspal deflection, thereby supporting superior long-term marginal integrity [15].

Mechanically, the diagonal gap geometry facilitates multidirectional stress dissipation by allowing each segment to contract independently. This promotes lateral rather than vertical shrinkage vectors, reducing tensile stress on the pulpal floor. The inherent asymmetry of this configuration further minimizes cohesive stress accumulation within the central body of the restoration, promoting a more balanced internal stress field and improving overall structural integrity [13-15].

To better visualize the temporal evolution of internal stress in large occlusal bulk-fill resin composite restorations, a conceptual schematic comparison is presented in (Figure 1). This diagram contrasts the distinct stress–time behaviors of two restorative strategies—conventional bulk filling and the semi-split delayed gap closure techniques. By illustrating the timing and magnitude of stress development and relaxation phases, the figure highlights how the semi-split approach leverages a post-irradiation viscoelastic relaxation window before final gap closure to mitigate peak stress accumulation and enhance interfacial stability.

Figure 1: A “conceptual” schematic of stress–time profiles representing conventional bulk filling technique (yellow line) and the semi-split delayed gap closure technique (blue line) in large occlusal bulk-fill resin composite restorations. The conventional bulk filling curve shows a pronounced early post-irradiation “primary” stress spike immediately after light-off, followed by a sustained plateau. In contrast, the semi-split delayed gap closure curve exhibits a smaller primary stress peak, a distinct dark curing viscoelastic stress relaxation window, and a moderated secondary stress rise upon delayed gap closure at 4–6 min, followed by a stabilized phase.

The clinical protocol Steps, [13-18].

(Figure 2) schematically illustrates the steps for the clinical protocol for the semi-split delayed gap closure technique. 

1. Bulk placement. A single increment (4 mm thick) of bulk-fill resin composite is placed to fill a large occlusal cavity, which has an adhesive previously applied to all walls following the manufacturer’s recommended instructions, (Figure 2), step 1.
2. Segmentation. Create a narrow linear gap (≈1.5 mm wide X 2 mm deep) using a flat hand-bladed instrument. This gap runs diagonally (e.g., from mesio-buccal line angle to disto-lingual line angle) and extends halfway into the mass in the direction of the cavity floor. This gap divides the mass into two partially separated segments, creating an intentional unfilled contraction path between them, (Figure 2), step 2.
3. Initial light cure. Perform an initial light cure using the manufacturer’s recommended exposure (e.g., 20 s at 1000 mW/cm²) to set the surface and initiate polymerization of the segmented mass.  The exact interval may be tuned to the composite chemistry and light curing unit (LCU) output, (Figure 2), step 3.
4. Delay interval (dark cure window). Wait an intentional short delay (5 minutes, default). Avoid manipulating or compressing the restoration. Allow the early post-irradiation stress spike and the fastest phase of dark cure to occur while the diagonal gap provides unrestrained shrinkage pathways. During this interval, each segment is free to deform toward the unbonded gap and relieve part of the volumetric contraction. Clinically use the interval for routine tasks (e.g., occlusal adjustment of adjacent teeth, document the procedure), (Figure 2), step 4.
5. Gap closure and final cure: After the predetermined delay period (timed using a stopwatch or curing light timer), gently adapt a small amount of the same composite to fill the gap and perform a short final cure (e.g., 10–20 s) focused on the closure line to complete polymerization under a now more distributed, lower-peak stress condition. Finish and polish as usual once the composite is sufficiently set, (Figure 2), step 5.

Figure 2: A schematic illustration of the clinical steps for the semi-split delayed gap closure technique. (1) Placement of a single 4 mm thick bulk increment of resin composite into a large occlusal cavity; (2) Creation of a diagonal linear gap (1.5 mm wide, 2 mm deep), dividing the mass partially into two segments and forming a stress-relieving gap; (3) Initial light curing of the partially segmented bulk; (4) A delay period of 5 minutes allows viscoelastic relaxation and stress stabilization during dark curing; (5) Filling the diagonal gap with the same composite and final curing to complete the restoration under minimized stress conditions, preserving interfacial integrity.

Results

Conceptual

While no direct stress–time measurements were performed, the behavior observed during the semi-split delayed gap closure phase suggests a stable post-curing equilibrium with only minimal mechanical adjustment upon gap closing, indicating negligible secondary stress development.

The schematic stress–time profile of conventional bulk filling vs. semi-split delayed gap closure technique is illustrated in (Figure 1). These profiles “conceptually” illustrate the temporal dynamics of polymerization stress development and its modulation by delayed gap closure.

In the conventional bulk filling technique, polymerization stress rises rapidly after light irradiation, reaching a pronounced early post-irradiation stress spike while all cavity walls remain fully engaged. This synchronized occurrence of maximum shrinkage stress and complete wall constraint results in a high-risk period for adhesive overload and interfacial debonding [19].

In contrast, the semi-split technique introduces a temporary mechanical decoupling through segmentation and delayed gap closure, allowing polymerization to progress beyond the stress spike phase before full cavity wall engagement occurs. As a result, the peak stress magnitude is attenuated, and the final stabilized stress level is markedly lower. This controlled timing of cavity wall engagement represents the core of the conceptual framework (temporal decoupling of stress generation from external restraint) which underlies the improved interfacial integrity and reduced risk of marginal failure observed with the modified semi-split approach. Delayed gap closure permits partial stress relaxation before cavity wall engagement, conceptually reducing interfacial stress and preserving interfacial adhesive integrity. Marginal adaptation is predicted to improve, with fewer internal or peripheral gaps. Cuspal deflection is expected to be attenuated by reducing simultaneous wall-to-wall shrinkage strain [14-18].

Discussion

The clinical relevance of timing in polymerization stress control has been underappreciated. Most current strategies focus on spatial modulation; for example, modifying cavity geometry, layering technique, or curing intensity, but overlook the opportunity to temporally synchronize procedural steps with material kinetics [19]. Recent studies demonstrate that post-irradiation polymerization continues for several minutes, and that viscoelastic flow during this period can substantially relieve stress if the material is allowed to deform freely [2-10].

In the semi-split delayed closure technique, leaving the diagonal gap open transforms the restoration from a fully constrained system to a partially stress-relieved structure. The open gap provides a low-resistance path for shrinkage deformation, allowing the composite to contract toward the gap rather than toward the bonded walls [13-15]. When the final closure is performed after viscoelastic stabilization, the composite behaves as a dimensionally stable substrate for the new gap filling increment, and the total stress transmitted to the interface is greatly reduced [16-18]. 

As “conceptually” illustrated in (Figure 1), the semi-split delayed gap closure technique introduces a temporal separation between initial polymerization and final cavity constraint (gap closure), producing a markedly different stress–time trajectory compared to conventional bulk filling technique. The delayed closure following viscoelastic relaxation allows partial dissipation of shrinkage-induced stress before final constraint is imposed, resulting in a lower and more stable stress profile. This conceptual distinction underscores the clinical relevance of temporally managing polymerization stress to enhance interfacial integrity and long-term restoration performance.

Clinically, this approach is straightforward and compatible with standard curing lights and materials. The gap closure delay period of 4–6 minutes coincides with the window of dark curing relaxation observed in most bulk-fill composites [13,14].  It is important to note that the gap closure delay period is material dependent (monomer chemistry, photoinitiator, filler load), and depends on curing irradiance, ambient/oral temperature, and cavity geometry.

During this waiting period, the adhesive interface continues post-curing, gaining strength and modulus before additional stress is applied [15]. When executed properly, the method protects both enamel and dentin margins and minimizes the risk of pulpal floor debonding, particularly in deep occlusal cavities where polymerization vectors converge vertically [16,17].  

Potential drawbacks include the need for procedural discipline and time awareness; however, the brief pause can be incorporated efficiently by alternating between multiple restorations or using the interval for finishing adjacent surfaces. In essence, the semi-split delayed closure approach represents a clinically accessible way to achieve kinetic harmony between polymerization and stress relief.

It is important to note that time-coordination of the delayed diagonal gap closure with composite’s dark curing kinetic viscoelastic relaxation window is a theoretically plausible, low-cost procedural approach to enhance the interfacial integrity and long-term performance of large occlusal bulk-fill resin composite restorations. Further experimental and clinical validation of this temporal synchronization is required to confirm its efficacy and optimize timing parameters for different composite systems.

Conclusion

The semi-split delayed gap closure technique represents a practical paradigm shift from purely geometric to temporal stress modulation in restorative dentistry. By synchronizing diagonal gap closure with the dark curing viscoelastic relaxation window, this approach effectively attenuates early post-irradiation stress spikes, preserves adhesive interface integrity, and enhances the biomechanical stability of bulk-fill restorations. Owing to its simplicity, reproducibility, and seamless integration with existing clinical protocols, the technique offers a clinically feasible and scientifically grounded advancement in stress management for adhesive restorations.

Clinical implications

• Time-aligned stress management: Close the diagonal gap during the dark curing relaxation phase (~4–6 minutes post-irradiation; 5 minutes is default).
• Enhanced marginal integrity: Temporal synchronization reduces post-cure stress transmission to vulnerable adhesive bonds.
• Practical integration: No specialized materials or instruments required; applicable in routine posterior restorations.
• Stress redirection: The diagonal semi-split gap promotes lateral contraction, reducing pulpal floor tension.
• Improved patient outcomes: Expected reduction in marginal leakage, postoperative sensitivity, and long-term restoration fatigue.
• One important caution: Delaying closure is not a universal panacea; it lowers stress if isolation and contamination control are excellent and if the chosen delay corresponds to the real relaxation window for that specific composite and cure protocol.

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