Class II – Depth of Cure

Contents:

• Why depth of cure matters
• Light-curing composite restorations
• Degree of conversion and depth of cure
• Curing light variables and depth of cure
• Positioning of the curing light tip
• Curing light beam characteristics
  
• Clinical technique and depth of cure
  
• Reliably obtain depth of cure with help from Dentsply Sirona

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.

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.

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!

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!