Overcoming the Challenges of Light Curing Through Ceramics

The optical properties of ceramic materials are not exclusively determined by the chosen shade. An increase in a specific crystalline phase might alter the overall esthetic outcome of the restoration. Ceramic thickness also affects the optical properties—the thicker the restoration, the greater the color spectrum the ceramist can gain. When leveraging properties such as shade and thickness to optimize esthetics, it's important to understand that the darker the shade and the thicker the material used in an indirect ceramic restoration, the more light-curing power, irradiance, and energy are required to polymerize the resin cement used to place it.^1,2

The light-curing process, which is highly dependent on the device used, is a critical step that can have a profound effect on a restoration's lifespan. Many manufacturers of resin cements recommend a standardized curing time for their products that does not necessarily take into account the specific properties or thickness of the ceramic restorations being placed. The thickness of a restoration increases the distance between the curing light and the cement (Figure 1). Similarly, many curing light manufacturers generally promote that their devices can be used for a single output duration without taking into consideration factors such as the composition of the resin cement and the specific distance between the tip of the curing light and the restoration, both of which affect the degree of polymerization (Figure 2).

Both the restoration's bond to the resin cement and the resin cement's bond to the tooth structure are critical to the success of ceramic restorations. To realize the full bond strength of dual- and light-cure resin cements, they must be adequately polymerized. Failure to supply the recommended amount of light energy results in a low degree of conversion that can jeopardize the mechanical and adhesive characteristics of resin cements. According to the International Organization for Standardization's (ISO) standard on powered polymerization activators,³ most resin composites can be polymerized to a depth of 1.5 mm using light with an irradiance of 300 mW/cm² and energy of 10 to 12 J/cm².

Currently, the majority of resin luting cements use the Norrish type II photoinitiator camphorquinone, which is activated by light in the blue wavelength (470 nm). However, some modern resin cements incorporate a new Norrish type I germanium-based photoinitiator that is activated by light in the violet wavelength (412 nm). Dentists should ensure that they always deliver sufficient light energy at the correct wavelengths to fully polymerize the chosen resin cement. Although there are many curing light systems available with different mechanisms of delivering the light, the most common systems used are second-generation LED curing light systems. Their peak emission is uni-wave, which is designed to decrease beam divergence, enhance beam homogeneity, and increase average irradiance. Unfortunately, such curing light systems can only activate blue wavelength photoinitiators, leaving any other photoinitiators inactive. The need for radiant energy to activate Norrish type I photoinitiators prompted the development of third-generation LED poly-wave peak-emission curing light systems (eg, VALO^™ Grand LED Curing Light, Ultradent; Bluephase^® PowerCure, Ivoclar Vivadent; PinkWave^™ QuadWave^™, Vista Apex). These curing lights feature new chips that produce light in both the blue and violet wavelengths, and some produce light in other expanded wavelengths as well.

To overcome many of the complex variables involved in delivering sufficient energy through ceramic restorations to polymerize resin cements, regardless of the generation of curing light used, the highest irradiance, power, and energy can be delivered when the curing light is positioned as close as possible to the ceramic restoration.

According to the basic equation governing the use of curing lights, which is that the total energy delivered equals the irradiance multiplied by the amount of time that the light is applied, in order to increase the amount of energy, the clinician can either enhance the irradiance by utilizing a light-curing device with a greater irradiance or increase the curing time. Doubling the curing time recommended by the resin cement manufacturer (eg, from 20 to 30 seconds to 40 to 60 seconds) is often the most sufficient option to increase the likelihood of delivering the minimum necessary energy (10 J/cm²) through thicker ceramic restorations; however, the necessity of cooling the tooth with an air/water syringe to avoid overheating must be considered.

Ali Altak, DDS, MS
Adjunct Faculty
Research Associate
Division of Comprehensive Oral Health
Adams School of Dentistry
University of North Carolina at Chapel Hill
Chapel Hill, North Carolina