Almost every day, dentists see resin-based composite (RBC) restorations that have signs of margin chipping or breakdown, bulk fracture, bulk and marginal discoloration, loss of anatomical form, lack of retention, or secondary caries. There is considerable evidence that delivering inadequate energy to the restoration will result in a restoration that has less than optimal properties and poor clinical performance. Thus, it is important to keep a few basic facts in mind.
The most common reasons cited for replacement of light-activated restorations are secondary caries and restoration fracture.9-12 Other reasons include marginal breakdown and staining, wear, discoloration, pulpal death, and tooth fracture. It is well established that reduced levels of resin polymerization caused by delivering an inadequate amount of light, or light at the wrong wavelengths, will adversely affect many RBC properties.13-26

Bargi N, Ernst CP, Ferracane JL, Price RBT, Rueggeberg FA, Shortall A, Strassler HE, Watts DC.Effective Use of Dental Curing Lights: A Guide for the Dental Practitioner. American Dental Association Professional Product Review. July 2013

A poorly polymerized restoration may result in premature clinical failure because of marginal defects, secondary caries, or restoration fracture. In addition, the biocompatibility of the restoration is adversely affected when the resin is undercured.10-13

Price RB, Rueggeberg FA, Labrie D, Felix CM.Irradiance uniformity and distribution from dental light curing units. J Esthet Restor Dent. 2010;22(2):86-101.

…the clinician is left wondering which suggested [curing] time is “correct”, and as a result, tends to over-expose restorations to be on the “safe side”. However, in so doing, the longer exposure results in generation of more heat within the tooth and surrounding, exposed tissues, leading to possible post-operative, iatrogenic complications.100,105,106

Rueggeberg FA.State-of-the-art: dental photocuring—a review. Dent Mater. 2011;27(1):39-52.

Restorations are all assumed to start out as equal in properties. A restoration that is under-cured will have a different set of properties from the rest of the group. This point cannot be emphasized enough.

Bayne SC.Correlation of clinical performance with ‘in vitro tests’ of restorative dental materials that use polymer-based matrices. Dent Mater. 2012;28(1):52-71

Undercuring the resin adversely affects its physical properties,10,21,24-29 reduces bond strength,10,21,27 increases marginal wear and breakdown,28,29 decreases biocompatibility,30-35 potentially increases DNA damage resulting from leachates32 and increases bacterial colonization of the resin.35 Equally undesirable is the delivery of too much energy to the tooth, which may cause thermal damage to the pulp and exposed oral tissues.36-42

Price RB, Felix CM, Whalen JM.Factors affecting the energy delivered to simulated class I and class V preparations. J Can Dent Assoc. 2010;76:a94.

…hardness of the surface of the restoration does not indicate adequate curing of the entire restoration. This situation is of concern, given reports that inadequate polymerization adversely affects the resin’s physical properties,2-7 reduces bond strength,3-5,8,9 increases wear and breakdown at the margins,6,7 decreases biocompatibility of the resin restoration10-12 and increases bacterial colonization of the resin in the restoration.12

Seth S, Lee CJ, Ayer CD.Effect of instruction on dental students’ ability to light cure a simulated restoration. J Can Dent Assoc. 2012;78:c123.

Although time-saving, a fast cure with a high level of energy has a number of drawbacks, including increased polymerization shrinkage stress,5,6 which is generally related to several negative clinical effects, such as less integrity of the restoration cavity interface,7 marginal staining, cusp fractures,8 microleakage,9 secondary caries, postoperative sensitivity, or pain. Also, a rise in temperature and risk of pulp damage10 are associated with the use of curing units with very high intensity.

Ilie N, Bauer H, Draenert M, Hickel R. – Resin-based Composite light-cured properties assessed by laboratory standards and simulated clinical conditions. Oper Dent. 2013;38(2):159-167.