Blog Post

Among all the steps involved in a direct composite restoration, light curing can appear to be the easiest one. In studies, however, more than 37% of composites were found to be inadequately light-cured.1 Depth of cure is key in this context and can be defined as the distance from the composite’s illuminated surface at which it can still be sufficiently polymerized by light curing. Here we’ll look at the factors affecting depth of cure and how to consistently achieve thorough light curing of composites.

Why depth of cure matters


Inadequate curing negatively affects the physical properties of composites resulting in reduced bond strengths, breakdown at the margins, increased potential for microleakage, and an increased propensity for surface staining and discoloration. Additionally, open margins and fracture of a composite restoration can result if the composite material closer to the tip of the curing light is completely cured and the composite further away is insufficiently cured.2 Class II restorations are at particular risk of undercuring, especially at the floor of the proximal box which is often as much as 8 mm away or more from the tip of the curing light.3

Adverse clinical outcomes


Inadequate curing with failure to achieve the depth of cure for the composite material can result in several adverse clinical outcomes. These include:4,5

  • Marginal leakage.
  • Post-operative sensitivity.
  • Pulpal irritation.
  • Recurrent caries.
  • Restoration flexure (and fracture).
  • Restoration failure.


Reasons for Class II restoration failure


Class II restorations can fail due to recurrent caries, marginal breakdown, discoloration, surface defects, fracture of the restoration, tooth fracture or the need for endodontic treatment.6 The main reason for Class II restoration failure is recurrent caries.6,7,8 An elevated caries risk in a given patient is a risk factor in common with primary caries, while risk factors unique to recurrent caries include marginal gaps and voids, marginal leakage, composite breakdown, wash-out of composite and overhangs.9,10 These are associated with one or more of the following - poor operator technique, poor selection of composite and inadequate light curing. Given that 50% of all direct restorations are Class IIs,11 it’s crucial to understand the factors affecting light curing and how to reliably cure composite resins and achieve adequate depth of cure.

Light-curing composite restorations


All light-cured composites contain photoinitiators that are needed to initiate polymerization – the process by which monomers in the composite join together to form polymers. When the photoinitiators are exposed to the energy emitted from the curing light, they react with a coinitiator to form free radicals. These connect the monomers in the composite to polymers through the crosslinking of double bonds. A network of polymer chains containing the composite’s filler particles develops.12 Characteristics that influence the light-induced polymerization of composites include their shade, translucency, and the type and amount of photoinitiators.

The most important variable in achieving adequate curing is the light source. As more energy is delivered, more radicals can be formed from the initiator until it is used up completely, and more  energy can reach deeper areas of the composite.

The light exposure times and increment thickness recommended by the composite resin manufacturer should of course be followed, taking into account the light curing unit being used.

Degree of conversion and depth of cure


While depth of cure is related to the thickness at which adequate curing can still occur, degree of conversion refers to the percentage of monomers in the composite that are converted into polymers during polymerization. The degree of conversion corresponds to the elasticity modulus of the cured material (Young’s modulus of elasticity).12 In addition to monomers, photoinitiators and coinitiators, composite resins are formulated with inhibitors that suppress the polymerization process to ensure sufficient working time under ambient light. Other components include coupling agents that bond the resin and filler particles, UV stabilizers, and small quantities of other chemicals. 

Curing light variables and depth of cure


Adequate light curing relies on using a suitable curing light and technique. It’s a myth that as long as the stated output value is high, you just switch on the power source, check that the blue light comes on, hold the curing light tip towards the area and cure for long enough to harden the composite.

Depth of cure relies on sufficient energy reaching the full depth of the composite increment/layer, including deep in the proximal box of Class II restorations. To achieve this, the curing light must maintain a suitable intensity of emitted light over distance. This can be challenging as it relies on the ability to properly position the curing light tip, as well as the light beam collimation and focus and exposure time. Let’s look more closely at each of these.

Positioning of the curing light tip

One of the challenges when curing composites in the posterior region is the limited space available to correctly position the tip of the curing light –particularly when treating children or elderly patients. In addition, angled light guides hinder proper positioning. While placing the tip at an angle may help with space issues, it also reduces the amount of energy reaching the depth and corners of the proximal box! Pen-style slim curing lights help to solve   the problem, improving intraoral access and making it possible to keep the tip close to and parallel to the occlusal surface of Class II restorations. 

Curing light beam characteristics

The following two characteristics influence the beam of light reaching the tooth:

1.   Beam collimation. A well-collimated beam of light has close-to-parallel rays which means less dispersion of the light and better delivery of energy at a distance.13  The dispersion of light influences both the depth of cure and degree of conversion of composites.12

2.   Beam uniformity. At first glance, it would seem to always make more sense to have a light beam with a wide diameter to capture a greater area of the restoration and evenly cure it. Unfortunately, that’s not necessarily the case – if the beam is not uniform,   some areas of composite can end up undercured. It’s not dissimilar to an oven that doesn’t bake evenly, leaving areas of a cake overdone or underdone.

Distance from the tip


For all curing lights, the distance from the curing light tip is a significant factor for the amount of energy reaching the full depth of the preparation – the further from the tip, the greater the energy drop-off. Some curing lights deliver as little as 35% of their stated output in the deepest area of the proximal box.14

Make sure you know how well your curing light performs over distance.


Exposure time  


For the sake of efficiency and patient comfort, it may seem expedient to use a higher-intensity light and shorter curing time than indicated. This temptation should be avoided since it increases the risk of poor composite polymerization and ultimately poor clinical outcomes! There’s also less tolerance of transient tip positioning errors – each second the tip is incorrectly positioned becomes a larger percentage of the total curing time, increasing the risk of uneven polymerization.

Conversely, adding extra curing time as ‘insurance’ to compensate for any potential reduction in the amount of energy reaching the restoration may seem like a good idea, but it isn’t! Exceeding the recommended curing time won’t damage the composite, but excess energy can cause tooth sensitivity, pulpal irritation and damage to oral soft tissues.15

Clinical technique and depth of cure


Clinical technique is key for long-term successful outcomes with Class II restorations. So much so, that among operators participating in a study on curing lights there was a tenfold difference in the amount of energy delivered to simulated composite restorations.16

Possible errors with curing lights and potential solutions

Errors that may occur when using curing lights include the following:

  • Inadvertently selecting a curing cycle with the wrong parameters.
  • Angling the tip to help gain intraoral access, which results in shadowing and uneven illumination of the composite being cured.
  • Slight movement of the tip due to hand fatigue during longer procedures or exposure times or looking away from the blue light to help protect the eyes.
  • Debris is present on the tip – this blocks the beam, reducing the curing light’s output (and compromises infection control).

Potential solutions to these issues include:


  • Use a lightweight, low profile, ergonomic curing light that helps to avoid the need to angle the tip and may help avoid inadvertent tip movement.
  • Reposition the patient for access to light curing and visibility.
  • Stabilize the light when curing the composite.
  • Use a light curing unit with a clearly structured, intuitive setting and limited number of settings/cycles.
  • Make sure the curing light is free of defects and debris before using it.
  • Protective "blue blocking" (orange colored) eyewear or shields should be worn when operating light curing units.
  • Place barrier protection over the curing light tip and handpiece area prior to use. Decontamination for infection control should follow the curing light manufacturer’s written instructions for use.

In summary, many factors affect the curing of composites and depth of cure. It’s clear that both the curing light itself and clinical technique are critical for the depth of cure and success of Class II restorations!

Reliably obtain depth of cure with help from Dentsply Sirona

Choosing a reliable and effective curing light and a suitable composite is crucial for the success of composite restorations. With Dentsply Sirona’s SmartLite® Pro modular LED curing light and SDR® flow+ Bulk Fill Flowable, you can rest easy. 

The SmartLite® Pro modular LED curing light is designed to be ergonomic and  offers leading quality of cure thanks to the following features:

  • A slim pen-style design that provides for excellent intraoral access and makes it easier to maintain the proper curing angle when space is an issue.
  • Quick-connect 360-degree rotational tips to gain perfect intraoral access.
  • State of the art optics with an optimized 4 LED design that provides for a homogeneous beam over the entire active curing area.
  • Excellent beam collimation that reduces light divergence to ensure reliable depth of cure all the way to the floor of the proximal box.
  • A large 10 mm active curing diameter to cover larger restorations.
  • A built-in radiometer to ensure the appropriate curing energy.

What’s more, the SmartLite® Pro modular LED curing light features a unique modular design with interchangeable tips for a variety of clinical indications, such as the transillumination tip.  

Dentsply Sirona’s SDR® flow+ Bulk Fill Flowable is formulated to optimize its properties for the reliable creation of high-quality restorations even in deep posterior cavities. This low-viscosity, self-leveling bulk fill flowable composite offers excellent flowability enabling placement of gap-free restorations with a 4mm depth of cure. Thanks to the patented SDR Technology that includes a patented urethane dimethacrylate structure (UDMA) and a polymerization modulator, energy is effectively dissipated during polymerization and a more relaxed polymer network is produced. This results in minimized polymerization stress, even with a 4 mm bulk placement. Other features include: 

  • Easy and controlled delivery with a precise application tip for access to the site, accurate and void-free placement.
  • Fast bulk fill placement of up to 4 mm in depth, resulting in fewer steps and up to 40% reduced placement time.11


SDR® flow+ Bulk Fill Flowable material, used for the bulk fill technique, is proven to be safe, effective and clinically durable compared to conventional layering techniques.17 What’s more, it’s the most researched bulk fill material available.18

With Dentsply Sirona’s SmartLite® Pro modular LED curing light and SDR® flow+ Bulk Fill Flowable, consistency, reliability, excellent depth of cure, ergonomics, simplicity, and versatility are all built in.

Our team at Dentsply Sirona is here to support you with our entire online dental academy complete with webinars, how-to videos, and real-world examples on how to create streamlined solutions with efficient procedures and even greater patient satisfaction. Contact us now and let’s get started!


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2 Fabiano F, Borsellino C, Bonaccorsi L et al. Influence of irradiation exposure time on the depth cure of restorative resin composite. AAPP Atti della Accademia Peloritana dei Pericolanti, Classe di Scienze Fisiche, Matematiche e Naturali.2014;92(51):A1-A18.

3 Durable Bonds at Adhesive/Dentin Interface. Braz Dent Sci. 2012 ; 15(1): 4–18.

4 Price R, Felix C. Factors Affecting the Energy Delivered to Simulated Class I and Class V Preparations. JCDA Applied Research, 2010.

5 Garcia-Godoy F, Krämer N, Feilzer AJ, Frankenberger R. Long-term degradation of enamel and dentin bonds: 6-year results in vitro vs. in vivo. Dent Mater. 2010;26(11):1113-1118.

6 Demarco FF, Cenci MS, Montagner AF et al. Longevity of composite restorations is definitely not only about materials. Dent Mater 2023;39(1):1-12. doi: 10.1016/

7 Al-Asmar AA, Al-Khatib KM, Al-Amad TZ, Sawair FA. Has the implementation of the Minamata convention had an impact on the practice of operative dentistry in Jordan? J Int Med Res. 2019 Jan;47(1):361-369. doi: 10.1177/0300060518802523. Epub 2018 Oct 3. PMID: 30282511; PMCID: PMC6384461.

8 Eltahlah D, Lynch CD, Chadwick BL, Blum IR, Wilson NHF. An update on the reasons for placement and replacement of direct restorations. J Dent. 2018 May;72:1-7. doi: 10.1016/j.jdent.2018.03.001. Epub 2018 Mar 6. PMID: 29522787.

9 Askar H, Krois J, Göstemeyer G. et al. Secondary caries: what is it, and how it can be controlled, detected, and managed?. Clin Oral Invest 2020;24:1869–1876.

10 Posterior Composite Resins – A Current Assessment. Current_Assessment.pdf.

11 Dental Market IQ 2022 Research Report -  Calendar Year 2021. For more information, contact

12 Schneider LF, Cavalcante LM, Silikas N. Shrinkage Stresses Generated during Resin-Composite Applications: A Review. J Dent Biomech. 2010;2010:131630. doi: 10.4061/2010/131630.

13 Collimated beam.

14 Irradiance Value Comparison among commercially available curing lights. BlueLight Analytics. (2012)

15 Ruggerberg F, Ferracane J, Price R. What is the latest thinking on fast-curing composites? Inside Dentistry. 2013;9(2). 

16 Price and Felix IADR 2010 Barcelona #467 Quantifying Light Energy Delivered to a Class I Restoration.

17 van Dijken JWV, Pallesen U, 2017: Bulk-filled posterior resin restorations based on stress-decreasing resin technology: a randomized, controlled 6-year evaluation.; Eur J Oral Sci. 2017 Aug;125(4):303-309.

18 Internal data on file. For more information, contact 

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