Shear Bond Strength with Increasing Light-Guide Distance from Dentin

Howard E. Strassler, DMD

Xu X, Sandras DA, Burgess JO. J Esthet Restor Dent. 2006;18:19-27.

Abstract

BACKGROUND: In Class II composite restorations, the adhesive covering the gingival floor of the deep cavity preparation is 2 mm to 8 mm from the light guide and may not be adequately cured with a typical 10-second curing time. PURPOSE: The purpose of this study was to evaluate the dentin bond strengths of resin composite when the curing light guide (quartz-tungsten-halogen light) was placed at various distances and to investigate the relationships between radiant exposure, degree of conversion, and shear bond strength. MATERIALS AND METHODS: Single Bond (3M ESPE, St. Paul, MN, USA) was placed onto the dentin following the manufacturer's directions. Four groups of 10 teeth were cured for 20 seconds through a 0, 2.3, 4.6, or 6.9 mm spacer. Two other groups of 10 teeth were cured through a 4.6 mm spacer for 40 seconds and 60 seconds, respectively. Z100 resin composite (3M ESPE) was placed over the cured adhesive and polymerized at the same distance as the adhesive. After 24 hours of storage in water, the shear bond strengths were tested. The irradiance through each spacer was measured using a digital radiometer. The degree of conversion of the adhesive was determined by near infrared spectroscopy. The data were analyzed using analysis of variance and Tukey-B post hoc tests. RESULTS: Dentin shear bond strengths decreased significantly with increasing distance, but they increased significantly when the curing time increased from 20 to 40 or 60 seconds. There is a linear correlation between shear bond strength, degree of conversion, and logarithm (radiant exposure). CONCLUSION: Increasing curing time can compensate for the decreased bond strength owing to a decreased irradiance associated with increased curing distance. CLINICAL SIGNIFICANCE: Under the conditions of this study, when curing the adhesives in deep proximal boxes with a quartz-tungsten-halogen light, the curing time should be increased to 40 to 60 seconds to ensure optimal polymerization.

COMMENTARY

Recent clinical studies comparing posterior composite resins to amalgam Class 2 restorations have reported a significant increase in recurrent caries at the gingival margin for the composite restorations. Also, in recent years, a number of curing lights have been introduced that make claims of 5-second light-curing. Are increasing rates of recurrent caries in Class 2 composite restorations related to inadequate light-curing? For most studies evaluating the depth of light-curing, the research design uses a Teflon or metal mold with a hollow cylinder. One problem with this design is that when we light-cure posterior restorations, the light tip is usually 1 mm to 2 mm from the cavosurface margin because of the cusp height, and even further from the gingival margins of a Class 2 proximal box. For a Class 2 composite resin, the light probe may be as far as 5 mm to 6 mm away from the gingival margin. Are we really curing the adhesive and composite resin adequately in this critical caries high-risk area? Probably not adequately.

This study by Xu et al investigated the curing time that would be necessary to adequately polymerize an adhesive on the critical gingival marginal area of a Class 2 composite resin. Dentin shear bond strengths decreased significantly with increasing distance from the light tip with a 10-second curing time. Increasing curing time can compensate for the decreased bond strength owing to a decreased irradiance associated with increased curing distance. This study makes a significant point—when curing resin adhesives in deep proximal boxes with a quartz tungsten halogen light, the curing time should be increased to 40 to 60 seconds to ensure optimal polymerization.

About the Author

Howard E. Strassler, DMD Professor and Director of Operative Dentistry Department of Endodontics, Prosthodontics and Operative Dentistry University of Maryland Dental School, Baltimore, Maryland