Commentary by Howard E. Strassler, DMD
Quantifying light energy delivered to a Class I restoration.
Price RB, McLeod ME, Felix CM. J Can Dent Assoc. 2010;76:a23.
PURPOSE: To measure the amount of light energy that dental students actually deliver to a Class I preparation in a dental mannequin.
MATERIALS AND METHODS: Approval for the study was obtained from the Dalhousie University Health Sciences Research Ethics Board. Each of 20 third-year dental students light-cured a Class I preparation in tooth 27 in a mannequin head. A photo-detector located at the bottom of the cavity preparation measured how much light would be received by a restoration. Each student cured the simulated restoration for 20 seconds using a quartz-tungsten-halogen curing light (Optilux 401). The irradiance received (mW/cm2) was recorded in real time, and the energy per unit area (J/cm2) delivered to the detector by each student was calculated. The students were then given detailed instructions on how to effectively use the curing light, and the experiment was repeated.
RESULTS: When the curing light was fixed directly over the tooth, the greatest amount of light energy delivered to the detector in 20 seconds was 13.9 +/- 0.4 J/cm2. Before instruction, the students delivered between 2.0 and 12.0 J/cm2 (mean +/- standard deviation [SD]: 7.9 +/- 2.7 J/cm2). After receiving detailed instructions, the same students delivered between 7.7 and 13.4 J/cm2 (mean +/- SD: 10.0 +/- 1.4 J/cm2). A paired Student's t-test showed that instruction resulted in a significant improvement (P < .001).
CONCLUSIONS: Although instruction yielded improvements, the mean energy delivered was much less (7.9 J/cm2 before instruction and 10.0 J/cm2 after instruction) than the expected 13.9 J/cm2. To maximize the energy delivered, the operator should wear eye protection, should watch what he or she is doing and should hold the light both close to and perpendicular to the restoration.
Much of the focus for light curing and its influence on the clinical success of the restorations we place has been the duration of light-curing and the light energy the light is delivering. Unfortunately, the perception of light-curing is that it is so easy that anyone could do it. Is this really the case? The goal of manufacturers has been the development of lights that offer higher-power/shorter curing times combined with the introduction of improved LED technology for light curing in the 420-nm to 480-nm wavelength ranges. Different composite resin brands and shades require variations in the energy needed to achieve adequate photopolymerization. Research has shown that the energy required to adequately cure composite resin can range from as little as 12 J/cm2 to 18 J/cm2. One paper reported values as high as 36 J/cm2. In other "Research and Application" columns I have abstracted clinical research articles that have shown the Class II composite resins have significantly more recurrent caries at the gingival margins when compared to amalgam, much of this due to inadequate light energy delivered to the cavity preparation and composite resin in the proximal box. The mantra was increase the light-curing time. What if there is more to this story? In fact there is.
These two research abstracts authoredby two premier researchers in the field of light-curing provide significant insight on quantifying the amount of light energy that is delivered to cavity preparations. This research demonstrates the challenges of achieving adequate photopolymerization of composite resin beyond higher energy curing lights and increasing the time of light-curing. In both abstracts, the results are startling in the variations of light energy delivered using the MARC device. In most cases the light energy measured was inadequate to ensure polymerization of the composite resin.
As clinicians we are not very good at light-curing composite resins. We are under-curing the composite resins we are placing. This research reinforces the need to better understand that to provide high-quality, long-lasting restorations, we need to improve the techniques used for light-curing. The MARC device allows for operator training to improve the techniques for light-curing. It should be part of any continuing dental education course that teaches direct placement of anterior and posterior composite resins by having a training area to teach us how to light-cure better. While higher-power curing lights with broader spectrum light energy will help, better techniques should also be our focus. To optimize the light energy delivered when light-curing, the operator should watch how they are holding and orienting the light to be at right angles to the tooth surface being light-cured and be as close as possible to the tooth being light-cured (use of barriers over the light guide allows for touching the tooth for posterior restorations.). Using these techniques will provide our patients with better composite resin restorations.
Howard E. Strassler, DMD
Professor and Director of Operative Dentistry
Department of Endodontics, Prosthodontics and Operative Dentistry
University of Maryland Dental School