The introduction of bulk-fill resin composites has simplified restorative procedures by enabling placement in thicker increments while maintaining acceptable polymerization depth and mechanical properties [1,2]. These materials were developed to overcome the time-consuming incremental placement techniques required for conventional resin composites, offering enhanced flowability, reduced polymerization shrinkage, and modified photoinitiator systems for deeper light penetration [3,4]. However, despite these advancements, polymerization shrinkage stress remains a critical concern in large occlusal cavities, particularly those with high cavity configuration factors (C-factors) [5,6].

Large occlusal cavities restored with bulk-fill resin composites are subjected to significant polymerization shrinkage stress, which interacts with the external restraint imposed by cavity walls [those aligned parallel to the long axis] and pulpal floors [those positioned perpendicular to the long axis] [5,7]. This stress originates from volumetric contraction, which ranges between 1-3% for most bulk-fill materials [1], and is amplified by high C-factors (3-5 in occlusal cavities), leading to tensile stresses at bonded interfaces that may exceed the early bond strength of adhesive systems [8,9]. The configuration of the cavity plays a pivotal role, as walls with low compliance impose greater external restraint, concentrating stress at the adhesive interface and promoting marginal gap formation [10].

Significant differences exist between bonding to cavity walls and pulpal floors in large occlusal bulk-fill resin composite restorations. These two substrates present distinct mechanical, adhesive, biological, and geometric challenges [11].

The cavity walls typically consist of dentin with a high density of peritubular dentin, presenting an oblique or longitudinal tubular orientation relative to the bonded surface, which provides better micromechanical interlocking and higher bond strengths [12]. In contrast, pulpal floors contain dentin with perpendicularly oriented tubules and higher tubular density, resulting in increased dentin permeability, lower mineral content, and higher susceptibility to hydrolytic degradation of the adhesive layer [13,14]. Stress transmission also differs: cavity walls, being aligned parallel to the long axis, tend to experience tensile and shear stresses, whereas pulpal floors, positioned perpendicular to the long axis, experience predominantly compressive and peel stresses, which are more detrimental to adhesive integrity [15].

A clear understanding of the mechanical, adhesive and histological, and geometric disparities is fundamental to preserving the adhesive interface, optimizing stress modulation strategies, and enhancing the long-term clinical performance of bulk-fill restorations [16].

The present paper, which forms the first part of a two-part series, focuses on the bonding disparities observed between cavity walls and pulpal floors in deep occlusal bulk-fill resin composite restorations.

In large occlusal bulk-fill resin composite restorations, the cavity walls’ and pulpal floors’ substrates present distinct mechanical, adhesive and biological, and geometric challenges. Significant disparities exist between these two substrates and are presented in Table 1.

Table 1: Mechanical, Adhesive and Histological, and Geometric Disparities between Cavity Walls and Pulpal Floors.

Bonding to cavity walls and pulpal floors presents distinct challenges due to differences in restraining ability, structural compliance, and polymerization stress response, as well as substrate composition, tubular density and permeability, and pulpal pressure. This is in addition to differences in innervation and sensitivity, bond strength, and hybrid layer, as well as configuration factor and gap formation pattern. These differences significantly influence the integrity and long-term performance of resin composite restorations [30-36].

Restraining Ability and Stress Modulation

The restraining potential of cavity walls differs markedly from that of pulpal floors in large occlusal cavities. Enamel and thick coronal dentin walls exhibit high stiffness and low compliance, effectively resisting polymerization shrinkage but simultaneously redirecting stress to the adhesive interface [5]. This stiff restraint increases the likelihood of marginal gap formation at enamel-dentin interfaces, especially under occlusal load [17]. Conversely, pulpal floors possess greater structural compliance due to thinner underlying dentin but, paradoxically, are more susceptible to internal stress concentration. Their flat geometry restricts stress redistribution, causing shrinkage forces to concentrate centrally at the adhesive interface [8,18].

Structural Compliance

The dentin in cavity walls exhibits moderate compliance, while the rigid enamel margins directly transmit stress to the bonded interface [5,17]. In contrast, pulpal floors, supported by thin underlying dentin, show low compliance and limited capacity to flex under polymerization shrinkage stress [8,18]. Biomechanically, the slight flexure of cavity walls can dissipate a portion of shrinkage stress, whereas the restricted flexure of pulpal floors promotes cohesive stress accumulation [17,18].

Polymerization Stress Response

Cavity walls can partially dissipate shrinkage stress through slight wall flexure, whereas the enamel largely resists deformation [5,17]. In contrast, pulpal floors exhibit greater stress accumulation due to their rigid, flat geometry and limited capacity for flexure [8,18].

Substrate Composition

Cavity walls are primarily composed of enamel or coronal dentin, with enamel at the margins providing more predictable bonding outcomes [19,20]. In contrast, pulpal floors consist mainly of deep dentin with a higher organic content, making adhesion more technique-sensitive and less predictable [21,22].

Dentin Permeability, Tubular Density, and Fluid Movement

Cavity walls, often containing enamel or coronal dentin, present lower tubular density and reduced permeability, which favor resin infiltration and hybrid layer formation [12,23]. Conversely, the pulpal floors are characterized by deep dentin with higher tubular density and larger tubule diameter that allow significant outward fluid movement under pulpal pressure, interfering with adhesive infiltration and increasing nanoleakage susceptibility [23,24].

Pulpal Pressure Influence

At cavity walls, pulpal pressure has minimal effect on fluid transudation due to the presence of thicker dentin [23]. Conversely, pulpal floors are significantly influenced by hydrostatic pulpal pressure, which hampers adhesive infiltration and increases the risk of microleakage [21,27] and internal gap formation [24].

Innervation, Hypersensitivity, and Post-operative Pain

Cavity walls have relatively sparse innervation, resulting in lower sensitivity during operative procedures [28]. In contrast, pulpal floors lie directly above pulp horns with dense neural innervation, making them more susceptible to postoperative hypersensitivity when adhesive sealing is inadequate [28,29]. Inadequate stress control and microleakage in these regions can further stimulate inflammatory mediator release, leading to pulpal hyperemia and patient discomfort [29].

Bond Strength Potential

Bond strength at cavity walls is generally higher, particularly along enamel margins and areas of sclerotic dentin [5,19]. In contrast, pulpal floors exhibit lower bond strength due to excessive moisture, reduced intertubular dentin, and limited resin infiltration [8,21,23].

Hybrid Layer Quality

The resulting hybrid layer quality differs significantly between cavity walls and pulpal floors. In cavity walls, the hybrid layer tends to be thicker and more uniform, stable, and well-polymerized [8,25]. In contrast, pulpal floors often develop heterogeneous resin infiltration zones, exhibiting a less uniform hybrid layer, making it more susceptible to nanoleakage and prone to degradation under functional and hydrolytic stresses [21,26].

Configuration Factor (C-Factor) Influence

The geometry of the bonded cavity further modulates stress development. High C-factor configurations in deep occlusal cavities inherently increase polymerization shrinkage stress; however, stress distribution differs between cavity walls and pulpal floors. Cavity walls allow some lateral stress redirection, especially if rounded internal line angles are used [17], whereas the flat pulpal floor concentrates stress within a confined adhesive interface, leading to internal gap formation [13,28]

Gap Formation Pattern

At cavity walls, marginal gaps may develop at the enamel-dentin interface, particularly under occlusal loading [20,25]. In pulpal floors, the risk of internal adhesive failure and subsequent gap formation is significantly higher [18,26]. Both cavity walls and pulpal floors can benefit from stress-relieving composite placement strategies, such as the spatial segmentation combined with the temporal diagonal gap closure delay within the semi-split bulk filling technique [30-36], which will be discussed in the second part of this series.

Clinical Implications

Distinct disparities exist between cavity walls and pulpal floors. Recognizing and addressing these disparities can significantly improve interfacial integrity, stress distribution, and long-term restoration performance.

Recognition of these mechanical, adhesive, and geometric disparities emphasizes the need for differentiated bonding protocols for cavity walls and pulpal floors. While cavity walls benefit from selective enamel etching, pulpal floors require careful adhesive infiltration and controlled curing protocols to mitigate shrinkage-induced interfacial failure.

This first part of a two-part series underscores the critical importance of recognizing the distinct mechanical, adhesive, and geometric disparities between cavity walls and pulpal floors when restoring deep occlusal cavities with bulk-fill resin composites. These differences, ranging from variations in dentin composition and tubular orientation to disparities in stress distribution, significantly influence adhesive performance and restoration longevity.

Optimizing clinical outcomes requires substrate-specific bonding protocols and strategic stress management. While cavity walls benefit from more predictable adhesion, pulpal floors demand meticulous moisture control, effective adhesive infiltration, and stress-relieving approaches. Composite placement strategies such as spatial segmentation and temporal gap closure delay within the semi-split bulk filling technique are particularly valuable for interrupting stress pathways, reducing interfacial failures, and improving the long-term durability of resin composite restorations.

The second part of this series will further elaborate on the practical application of these stress-mitigation strategies for enhancing adhesion in both cavity walls and pulpal floors.

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