Using an incremental layering technique and bulk fill composites can help address potentially high levels of polymerization stress​. The Class II direct composite restoration is the most common procedure in general dental practice. A broad base of evidence now suggests that direct composite can be used for restorations in the posterior dentition with low failure rates,1 comparable to those of silver amalgam.

The absence of dentin remaining beneath the cusps can be a factor in Class 2 composite restoration failure
Figure 1: The absence of dentin remaining beneath the cusps can be a factor in Class II composite restoration failure.

A review of available clinical literature suggests that failure can be related to several factors, including:

  • tooth-related factors, such as the presence or absence of dentin remaining beneath the cusps (so-called “residual dentin thickness,” Fig. 1)2

  • patient risk factors, including caries risk3 and socioeconomic aspects4

  • factors relating to the dentist performing the procedure, such as experience and skill

  • factors related to the placement, polymerization, and finishing of the restoration.

This article will focus on reducing failure via correct placement and polymerization of the resin.

Polymerization and Shrinkage Stress

The polymerization reaction of composite resin is exothermic and involves breaking down the carbon-carbon double bonds in monomer molecules, which then reform as carbon-carbon single bonds and create polymer chains. These covalent bonds result in decreased bond lengths5 and polymerization shrinkage.

 

The polymerization reaction proceeds in two main steps:

  1. In the pre-gel phase, the resin converts from a viscous liquid to an elastic gel. At this “gel point,” the free radical concentration and the reaction speed dramatically increase, and the elastic modulus (stiffness) of the resin remains comparatively low. This means the resin can de-form internally and release stress, thereby preventing stress transfer to the restoration margins.

  2. In the post-gel phase, the gel converts to a glassy polymer. Here, the viscosity of the resin substantially increases, making it more difficult for the monomer molecules to diffuse within the resin; there is a significant decrease in the reaction speed. During this “vitrification” stage, the elastic modulus of the resin increases, hindering plastic de-formation, which results in stress development.6 This stage occurs as early as a few seconds into the photoactivation period.
Polymerization shrinkage during the post-gel phase of a composite restoration can generate stress within the composite material at the tooth/restoration interface, leading to margin breakdown
Figure 2: Polymerization shrinkage during the post-gel phase of a composite restoration can generate stress within the composite material at the tooth/restoration interface, leading to margin breakdown.

In the post-gel phase, the continued polymerization shrinkage and the increasing elastic modulus generate stresses within the material at both the tooth/restoration interface and within the tooth structure.7 This can lead to:

  • margin breakdown (Fig. 2), resulting in microleakage, marginal discoloration, sensitivity, and secondary caries, and

  • fracture of tooth structure.

To summarize, polymerization contraction stress within the composite is governed by:

  • the rate and degree of volumetric shrinkage, and

  • the viscoelastic behavior of the resin — specifically, its rate of development of elastic modulus and ability to flow (this is also related to cavity configuration; see later).

In rough terms, an increased volumetric shrinkage rate and a fast modulus gain will result in increased stresses. This is why high-output curing lights can be an issue. Further, with some composite resins, consideration of the amount of resin placed per increment is important because a higher volume of resin results in a greater degree of stress.

Conversely, composites with a high percentage of filler content have a low matrix content. During polymerization, the filler remains unchanged while the matrix polymerizes and contracts; therefore, a higher filler content reduces the degree of polymerization shrinkage.

However, heavily filled resins generate high modulus, which reduces the material’s capacity to de-form, resulting in high stress levels.

Cavity Configuration and Polymerization Stress

The cavity configuration factor (“C-factor”) is a ratio that assesses the geometry of a cavity for bonding to predict the potential stress generated.8

C-factor is defined as the ratio of the bonded surface area in a cavity (the number of surfaces of the restoration that are in direct contact with the tooth structure) to the unbonded surface area (the surfaces that aren’t in contact with the structure). For example, the ratio of a Class I cavity can be 5:1 because the floor and mesial, distal, buccal, and lingual walls will be bonded, while the occlusal surface will not. In contrast, a resin veneer bonded to only the facial surface would have a ratio of 1:1.

The higher the C-factor, the greater the polymerization stress generated because high C-factor cavities are more bowl-shaped, while cavities with lower C-factors are flatter and shaped more like plates. Bowl-shaped cavities tend to constrain the composite more, allowing less de-formation and stress release within the resin during polymerization.

Clearly, a Class II cavity will have a high C-factor and, when filled with direct resin, will potentially generate high levels of stress.

The degree of stress generated by polymerization shrinkage depends on multiple factors:

  • Curing light intensity

  • Photoactivation time

  • Mechanical properties of the resin

  • Tooth structure

  • Restorative placement technique

  • Geometry and extent of the cavity.

What steps can be taken to minimize the stress?

Managing Polymerization Stress in the Class II Box

Cavity Design

Several authors (including Clark9) have suggested a very wide flare when preparing the Class II proximal box to reduce C-factor and polymerization stress. Nordbo10 demonstrated good clinical success with a saucer-type cavity design: 70% of an initial group of 51 restorations were deemed acceptable after 7.2 years.

The negative of this approach is that the preparation is more aggressive and destructive of sound tooth tissue.

Incremental Layering

The incremental or layering technique is an accepted and common approach that can reduce polymerization stress and eliminate gap formation between the margins and the resin.11 This was confirmed by Bicalho, who demonstrated an increase in bond strength when incremental layering was employed over bulk filling with conventional paste composite.12

The layering technique, which involves placing and curing multiple thin increments of 2 mm or less, can reduce polymerization stress and eliminate gap formation between the margins and the resin
Figures 3-6: The layering technique, which involves placing and curing multiple thin increments of 2 mm or less, can reduce polymerization stress and eliminate gap formation between the margins and the resin.

The technique involves placing multiple thin increments — horizontal, vertical (Figs. 3–6), or oblique, each 2 mm or less and polymerized separately — and reduces the C-factor, the volume of composite resin, or both.

Some downsides include significantly increased chair time and the possible inclusion of voids between increment layers.11 In my experience, voids were almost ubiquitous in the Class II box when employing a vertical or oblique layering approach.

Nikolaenko’s group demonstrated that horizontal incremental layering resulted in higher bond strengths to dentin at the cavity base.13 This is a commonly cited reference; however, it should be remembered that this was an in vitro study (lab-based) on Class I cavities.

Pulse Activation

In the pulse-activation approach originally described by Kanca and Suh,14 the resin is initially photopolymerized for a short time (two seconds or less) with low power output, then allowed to “relax” for several minutes before finally being light-cured for the recommended time. The concept is that the resin releases stress between the pre-gel and post-gel phases; variables related to the stress at the margin include the intensity of the light and the formulation of the composite resin.

There is good evidence to suggest that pulse activation reduces contraction stress.15 However, the literature lacks agreement about the initial photo-polymerization time (although it’s believed to be less than three seconds) and the relaxation time. As a result, there is no standard protocol. Anecdotally, I have experienced good outcomes in my clinical practice when using a two-second initial pulse and a three-minute relaxation period.

Use of Fibers

Fibers such as those made by Ribbond can be placed between the hybrid layer and the composite resin at the pulpal and axial walls; this is the “wallpapering” technique of Deliperi.16

It is critical to place the fibers as close to the residual tooth substrate as possible, creating a thin bond line between the fibers and the tooth structure. Within this “bond zone,” polymerization stress is decreased because the resin volume between the tooth structure and the fibers is reduced, and the fibers absorb energy and stress.

The problem with this approach is that it’s time-consuming, expensive, and challenging to closely adapt the fibers to the cavity in vivo.

Bulk Fills

Bulk fills are low-shrinkage composite resins, either paste or flowable, that allow placement of 4 mm increments.17 This is possible because their chemistry results in lower post-gel shrinkage and increased translucency, which allows improved light penetration and curing depth.18

Bulk fill resins can be placed in 4 mm increments, leading to faster layering and reduced chair time
Figure 7: Bulk fill resins can be placed in 4 mm increments, leading to faster layering and reduced chair time.

The advantages of bulk fills over conventional paste composites include reduced polymerization stress,19 faster layering and reduced chair time. Disadvantages include reduced aesthetics because of the translucency and low value (gray).

I tend to use bulk fills for most cases, with an incremental layering approach (Fig. 7) being employed in highly esthetic zones.

Prioritize Thoughtful Placement and Techniques

Correct placement and polymerization of composite resin can help ensure its successful use for restorations in the posterior dentition. By incorporating this information into their clinical workflow, dentists can increase the survival rate of their composite Class II restorations to match that of amalgam.

References

  1. Da Rosa Rodolpho, P.A., Donassollo, T.A., Cenci, M.S., Loguércio, A.D., Moraes, R.R., Bronkhorst, E.M., Opdam, N.J.M., & Demarco, F.F. (2011). 22-year clinical evaluation of the performance of two posterior composites with different filler characteristics. Dental Materials, 27(10); 955–963.

  2. Lempel, E., Lovász, B.V., Bihari, E., Krajczár, K., Jeges, S., Tóth, Á., & Szalma, J. (2019). Long-term clinical evaluation of direct resin composite restorations in vital vs. endodontically treated posterior teeth — retrospective study up to 13 years. Dental Materials, 35(9); 1308–1318.

  3. Wong, C., Blum, I.R., Louca, C., Sparrius, M., & Wanyonyi, K. (2021). A retrospective clinical study on the survival of posterior composite restorations in a primary care dental outreach setting over 11 years. Journal of Dentistry, 106, 103586.

  4. Worthington, H.V., Khangura, S., Seal, K., Mierzwinski-Urban, M., Veitz-Keenan, A., Sahrmann, P., Schmidlin, P.R., Davis, D., Iheozor-Ejiofor, Z., & Rasines Alcaraz, M.G. (2021). Direct composite resin fillings versus amalgam fillings for permanent posterior teeth. The Cochrane Database of Systematic Reviews, 8(8), CD005620.

  5. Leprince, J.G., Palin, W.M., Hadis, M.A., Devaux, J., & Leloup, G. (2013). Progress in dimethacrylate-based dental composite technology and curing efficiency. Dental Materials, 29(2), 139–156.

  6. Braga, R.R., Ferracane, J.L., & Condon, J.R. (2002). Polymerization contraction stress in dual-cure cements and its effect on interfacial integrity of bonded inlays. Journal of Dentistry, 30(7-8), 333–340.

  7. Calheiros, F.C., Sadek, F.T., Braga, R.R., & Cardoso, P.E. (2004). Polymerization contraction stress of low-shrinkage composites and its correlation with microleakage in Class V restorations. Journal of Dentistry, 32(5), 407–412.

  8. Feilzer, A.J., De Gee, A.J., & Davidson, C.L. (1990). Quantitative determination of stress reduction by flow in composite restorations. Dental Materials, 6(3), 167–171.

  9. Clark, D. (2009, February 1). Introducing the Clark Class I and II restoration. Oral Health Group. https://www.oralhealthgroup.com/features/introducing-the-clark-class-i-and-ii-restoration/

  10. Nordbø, H., Leirskar, J., & von der Fehr, F.R. (1998). Saucer-shaped cavity preparations for posterior approximal resin composite restorations: observations up to 10 years. Quintessence International (Berlin, Germany: 1985), 29(1), 5–11.

  11. Park, J., Chang, J., Ferracane, J., & Lee, I.B. (2008). How should composite be layered to reduce shrinkage stress: Incremental or bulk filling? Dental Materials, 24(11), 1501–1505.

  12. Bicalho, A.A., Pereira, R.D., Zanatta, R.F., Franco, S.D., Tantbirojn, D., Versluis, A., & Soares, C J. (2014). Incremental filling technique and composite material—Part I: cuspal deformation, bond strength, and physical properties. Operative Dentistry, 39(2), E71–E82.

  13. Nikolaenko, S.A., Lohbauer, U., Roggendorf, M., Petschelt, A., Dasch, W., & Frankenberger, R. (2004). Influence of C-factor and layering technique on microtensile bond strength to dentin. Dental Materials, 20(6), 579–585.

  14. Kanca, J. 3rd, & Suh, B.I. (1999). Pulse activation: reducing resin-based composite contraction stresses at the enamel cavosurface margins. American Journal of Dentistry, 12(3), 107–112.

  15. Krejci, I., Planinic, M., Stavridakis, M., & Bouillaguet, S. (2005). Resin composite shrinkage and marginal adaptation with different pulse-delay light curing protocols. European Journal of Oral Sciences, 113(6), 531–536.

  16. Deliperi, S., Alleman, D., & Rudo, D. (2017). Stress-reduced direct composites for the restoration of structurally compromised teeth: fiber design according to the “wallpapering” technique. Operative Dentistry, 42(3), 233–243.

  17. Zorzin, J., Maier, E., Harre, S., Fey, T., Belli, R., Lohbauer, U., Petschelt, A., & Taschner, M. (2015). Bulk-fill resin composites: polymerization properties and extended light curing. Dental Materials, 31(3), 293–301.

  18. van Dijken, J.W., & Pallesen, U. (2014). A randomized controlled three-year evaluation of “bulk-filled” posterior resin restorations based on stress-decreasing resin technology. Dental Materials, 30(9), e245–e251.

  19. El-Damanhoury, H., & Platt, J. (2014). Polymerization shrinkage stress kinetics and related properties of bulk-fill resin composites. Operative Dentistry, 39(4), 374–382.

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