The use of composite resin materials to restore posterior teeth esthetically and functionally have increased their popularity in restorative dentistry. These materials have the undesirable, inherent property of polymerization shrinkage, which generates detrimental stresses in the restored teeth.

Composite restorations restricted in prepared cavities by their adhesive bonding to enamel and dentin of surrounding walls generate polymerization stresses upon light curing. These stresses exert pressure on the bonding adhesive interface and the surrounding tooth structure during the polymerization process. If this pressure is higher than the adhesive bond strength, or the strength of composite/tooth, it causes several detrimental clinical consequences. These include enamel crack propagation, cusp deflection, and marginal and internal micro-gap formation [1,2].

Bulk-fill resin composite materials have gained in popularity since their introduction in the field of restorative dentistry because of their simplified restorative procedures and the decreased chairside time due to insertion of a single 4 mm mass in deep occlusal cavities [3-5]. The manufacturers claimed a decrease in polymerization shrinkage of such materials, as compared to the conventional composite materials placed incrementally.

Upon light curing of a 4 mm mass of bulk-fill resin composite which is adhesively bonded to the surrounding walls of a deep occlusal cavity, composite polymerization results in restrained shrinkage and generates stresses. These stresses are not uniformly distributed within the composite mass or along the interfaces. This non-uniform stress distribution is attributed to variations in bonding of the polymerizing composite mass to enamel and dentin of surrounding cavity walls, and results in developing of displacement/movement within the mass and interfaces of varying magnitude and direction. This results in several detrimental consequences, among which are postoperative sensitivity, persistent tooth pain, and internal debonding and micro-gap formation at the pulpal floor [6].

This paper aims to provide a detailed account on the shrinkage movement or displacement generated by light curing and polymerization of bulk-fill resin composites placed directly in deep occlusal cavities. It is also the objective of this paper to highlight the internal debonding and micro-gap formation at pulpal floors as detrimental consequences of such polymerization shrinkage. In addition, it aims to review the gap-modified bulk filling technique utilized for reducing the shrinkage stresses generated in direct large occlusal bulk?fill resin composite restorations.

Description of the magnitude and direction of the shrinkage displacement/movement developed, upon polymerization, in a 4 mm bulk-fill resin composite restoration was simplified by some researchers [7]. They looked at such restoration as consisting of two equal mirror-image restorations bonded together at the horizontal midplane and are referred to as the top and bottom 2 mm composite regions.

Research studies reported that the magnitude and direction of displacement differ in the top and bottom 2 mm composite regions and this difference is greatly influenced by two factors :1) the bonding capability of enamel and dentin substrates, 2) the volume of the polymerizing composite increment/mass [8,9].

The first factor is related to the mineralization degree of enamel and dentin substrates, and the time taken for full bond development. Enamel, being the most mineralized and dry substrate, forms the highest bond strength that reaches full maturation in shorter times. Whereas dentin, being the least mineralized and moist substrate, forms lower bond strength which fully matures in longer times [6,10,11]. The second factor is related to the thickness of the polymerizing composite increment/mass. It was reported that a composite increment (2 mm thick or more) placed on the cavity floor in deep occlusal cavity undergoes displacement, upon curing, in a direction away from the dentin/hybrid layer at the pulpal floor and forms a micro-gap [12,13]. It was also reported that the higher the volume of composite increment, the larger is the formed micro-gap [10].

A displacement pattern was detected by Optical Coherence Tomography (OCT) upon curing of a single 4 mm composite mass placed in a deep occlusal cavity, as shown in Fig. 1. A slight lateral displacement component was observed and oriented generally inwards to the cavity center, while the axial displacement component was clearly dominating [8,14-16]. The axial displacement occurred in a downward direction in the top 2 mm composite region, then it got reversed to an upward direction in the bottom 2 mm composite region [7]. The upward displacement exerted by the top 2 mm composite region on the bottom region was augmented by the following two factors: firstly; the development of a higher bond strength between composite and both enamel and dentin of adjacent cavity walls in the top region, as opposed to a lower bond strength of composite to dentin only in the bottom composite region [12,15,16]. secondly; the development of stiffness in the resin matrix of the top composite region in a rate faster than that in the bottom region, which was attributed to positioning the curing light tip closer to the surface of the top region [7,17,18]. These two factors enabled the top composite region to exert a stronger pull in an upward direction on the bottom 2 mm composite region, forcing it to move away from the hybrid layer/dentin and debonded from the pulpal floor, as disclosed by the SEM and the OCT imaging. This debonding behavior results in micro-gap formation at the pulpal floor [7,19-21].

Figure 1: The Conventional Bulk-Fill Resin Composite Filling Technique.

(a) Uncured Single Mass Of Bulk-Fill Resin Composite Placed In Large Occlusal Cavity Preparation.

(b) Cured Single Mass Of Bulk-Fill Resin Composite.

(c) Cross-Sectional View Of The Restored Tooth Shown In (b), Illustrating The Direction Of Polymerization Shrinkage Displacement In Arrows (Black, Blue And Red). Note: Composite Displacement at the Occlusal Free Surface in Pulpal Direction, And Formation of Internal Gap at Pulpal Floor.

The micro-gaps formed at deep pulpal floor are described as micron-size spaces that form, upon light curing, at bonding interfaces between adhesive resin and dentin in deep occlusal cavities restored with bulk-fill resin composites [13]. These micro-gaps may form as a single space or scattered areas of debonding in the same restoration [14].

It is worth mentioning that the problem of internal debonding and pulpal floor micro-gap formation is not new in direct restorative dentistry. This problem was first reported in the literature in association with the conventional composite used incrementally in direct posterior restoration of deep occlusal cavities [15-17]. It was attributed to quick curing of the first composite increment of 2 mm thickness or more before complete maturation of the developing dentin bond of the hybrid layer at the pulpal floor. This quick curing may cause the high polymerization shrinkage stress generated in the first increment to exert a pulling action onto the developing dentin bond in a direction away from the pulpal floor dentin, resulting in this debonding problem [22].

This problem was recently reported again by several researchers recently in association with the use of bulk-fill resin composites for direct restoration of deep occlusal cavities [5,8,9,21]. However, there is a difference in the debonding mechanism in the bulk filling. This difference is due to variation in the thickness of the first increment overlying the adhesive bond in the hybrid layer at deep pulpal floor: 2 mm in the incremental vs. 4 mm in the bulk filling. The difference in debonding mechanism is related to the pattern, magnitude, and direction of shrinkage displacement which are essential factors for forming the internal micro-gaps at deep pulpal floors. As such, one should expect the restorative techniques reported in the present paper for solving this problem to be therefore different form those used for fixing the problem in the incrementally placed conventional composite restorations.

Internal debonding and micro-gap formation at pulpal floors were disclosed by some researchers in deep occlusal cavities using micro-computer tomography (μCT) imaging [21,23]. Such debonding and micro-gap formation could be related to the difficulty of adhesion of bulk-fill resin composites to the pulpal floor dentin in deep occlusal cavities.  The difficult adhesion is augmented by the increased tubular density and diameter, as well as the proportional reduction in intertubular dentin [8,24,25]. Formation of micro-gaps at deep pulpal floors compromises the longevity of the restoration [7,26].

Formation of pulpal floor micro-gaps was reported to result in postoperative sensitivity and persistent tooth pain, which is manifested clinically as discomfort upon subjecting the restoration to occlusal forces. Experience of discomfort was attributed to accumulation of dentinal fluid in the micro-gaps, which undergoes movement upon contraction or expansion caused by cold or hot stimuli. However, the shift of dentinal fluid into the micro-gap does not occur immediately after curing, but rather after a period of time [27,28]. Over the time, the accumulated dentinal fluid in the micro-gap could result in hydrolysis of adjacent composite resin, leading to biodegradation of resin-dentin bond and restoration failure [28].

This problem was recently reported again by several researchers recently in association with the use of bulk-fill resin composites for direct restoration of deep occlusal cavities [5,8,9,21]. However, there is a difference in the deboning mechanism in the bulk filling. This difference is due to variation in the thickness of the first increment overlying the adhesive bond in the hybrid layer at deep pulpal floor: 2 mm in the incremental vs. 4 mm in the bulk filling. The difference in deboning mechanism is related to the pattern, magnitude, and direction of shrinkage displacement which are essential factors for forming the internal micro-gaps at deep pulpal floors. As such, one should expect the restorative techniques reported in the present paper for solving this problem to be therefore different from those used for fixing the problem in the incrementally placed conventional composite restorations.

The gap-modified bulk placement technique is used for direct restoration of large occlusal cavities with bulk?fill resin composites for the reduction of the generated shrinkage stresses and their detrimental consequences. (Fig. 2) It is accomplished by creating a gap (1.5 mm wide) in a 4 mm uncured mass of bulk-fill resin composite filling. This gap is cut diagonally using a Teflon-coated plastic filling instrument in a push stroke and extends halfway (2 mm) through the composite mass thickness. Thus, the created gap splits the top 2 mm composite region, prior to curing, into two equal segments: one bonded to adjacent buccal wall and the other bonded to the opposing lingual wall. This is followed by curing the semi-split mass. Then, the gap is filled with the same bulk-fill composite, and the completed restoration is cured again [29].

Figure 2: The Gap-Modified Bulk-Fill Resin Composite Placement Technique.

(a) Cured Single Mass Of Bulk-Fill Resin Composite In Large Occlusal Cavity Preparation, Following Modification By A Diagonal Gap Extending Halfway In Pulpal Direction.

(b) Cross-Sectional View Of The Cured Gap-Modified Restoration Shown In (a), Illustrating The Direction Of Polymerization Shrinkage Displacement In Arrows (Black And Blue).

(c) Completed Restoration By Filling The Diagonal Gap With The Same Composite, And Light Curing Again.

(d) Cross-Sectional View of the Completed Restoration Shown In (c), Illustrating Absence of Formation of Internal Gap at Pulpal Floor.

Crating a diagonal gap for splitting the top 2 mm composite region into two separate segments keeps them apart so that each bonded segment is connected, during curing, to either buccal or lingual adjacent wall. Thus, it prevents the mass of composite resin from connecting two opposing cavity walls simultaneously, and thereby minimizes the occurrence of postoperative sensitivity and persistent tooth pain, which is accompanied by discomfort upon subjecting the restoration to occlusal forces [29]. This way, the generated lateral and axial displacement stresses would not become augmented in this region by decreasing the combined effect of 1) the high strength of bonding to enamel and dentin, and 2) the stiffness in the resin matrix which is developed faster by the closer position of the curing light tip. Therefore, the created gap weakens the generated lateral and axial shrinkage stresses and render them incapable of displacing the polymerizing composite in a direction away from the cavity walls. Thus, the attenuated lateral stresses become incapable of displacing the composite away from the adjacent bonded buccal and lingual cavity walls, ensuring absence of internal micro-gaps. Similarly, the attenuated axial stresses in the top composite region become incapable of exerting an axial pull on the bottom composite, ensuring no displacement of it away from the hybrid layer/dentin at the pulpal floor.

It is estimated that cutting a diagonal gap for a depth of 2 mm in the uncured top composite region would be sufficient for attenuating the lateral and axial stresses generated in this region, upon curing, ensuring the absence of micro-gap formation, especially at the deep pulpal floor along with subsequent absence of postoperative sensitivity and persistent tooth pain [30].

When the conventional bulk filing technique is used for restoring a 4 mm deep occlusal cavity with bulk-fill resin composite, the 4 mm mass generates, upon curing, polymerization shrinkage stresses and undergoes displacement/movement. This displacement, if not controlled, results inevitably in micro-gap formation at the pulpal floor, leading to postoperative sensitivity and persistent tooth pain.  Over the time, biodegradation of resin-dentin bond takes place and ends with restoration failure.

Using the gap-modified bulk placement technique for direct restoration of large occlusal cavities with bulk?fill resin composites results in the reduction of the generated shrinkage stresses and their detrimental consequences. It is accomplished by modifying the conventional bulk filling technique and creating a diagonal gap in the top 2 mm region of uncured mass of bulk-fill resin composite. Then, the segmented composite mass is cured. The restoration is completed by filling the gap with the same bulk-fill resin composite and curing it.

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