Oxygen inhibition and incremental layer bond strengths of resin composites.
OBJECTIVE
When dentists light cure resin composite restorations in increments or after contouring the surface layer to shape with a hand instrument the surface layer is exposed to air during polymerization. The presence of an oxygen inhibited resin surface layer may impact on clinical performance. Conflicting data has been produced in vitro regarding this topic.


METHODS
To shed further light on this subject the current investigation assessed the thickness of the oxygen inhibited layer (OIL) and subsequent interfacial bond strength at various times post-cure of an "initial increment" for a range of experimental and commercial resin composites. The latter included conventional methacrylate-based composites and a novel low shrink Silorane resin chemistry product.


RESULTS
A decrease in composite viscosity brought about by an increase in diluent monomer content in the matrix for the experimental composite formulations led to increase in OIL thickness. The OIL surface layer thickness for the commercial methacrylate-based RBC specimens cured in air were 19.2+/-6.3 and 13.8+/-5.3 microm, respectively and 9.0+/-6.6 microm for Silorane. No test material exhibited a measurable OIL thickness polymerized in the nitrogen atmosphere.


CONCLUSIONS
From the current findings it may be concluded that incremental bond strength is not wholly reliant on surface inhibition since no differences in bond strength following immediate placement were observed between air and nitrogen atmosphere for any experimental or commercial material. For large restorations requiring multiple increments which are placed on fresh material, the bond strength between successive layers of Silorane should be no different to conventional methacrylate materials. Repair and bonding to aged Silorane restorations may be more problematic as inferior incremental bond strengths ensued when addition was delayed.
Shawkat
Essam S
ES
Biomaterials Unit, School of Dentistry, College of Medical and Dental Sciences, University of Birmingham, St Chads Queensway, Birmingham B46NN, UK.
Shortall
Adrian C
AC
Addison
Owen
O
Palin
William M
WM
D000388:Air; D017609:Barium Compounds / Q000737:chemistry; D017438:Bisphenol A-Glycidyl Methacrylate / Q000737:chemistry; D003188:Composite Resins / Q000737:chemistry*; D001840:Dental Bonding; D003764:Dental Materials / Q000737:chemistry*; D005027:Ethylene Oxide / Q000737:chemistry; D006801:Humans; D008422:Materials Testing; D009584:Nitrogen / Q000737:chemistry; D010100:Oxygen / Q000737:chemistry*; D011092:Polyethylene Glycols / Q000737:chemistry; D011108:Polymers / Q000737:chemistry; D011109:Polymethacrylic Acids / Q000737:chemistry; D011791:Quartz / Q000737:chemistry; D033081:Shear Strength; D012822:Silicon Dioxide / Q000737:chemistry; D012833:Siloxanes / Q000737:chemistry; D013314:Stress, Mechanical; D013499:Surface Properties; D013997:Time Factors; D014783:Viscosity; D015040:Zirconium / Q000737:chemistry
D017609:Barium Compounds; D003188:Composite Resins; D003764:Dental Materials; C543062:Filtek LS Low Shrink resin; C444701:Filtek Z250; D011108:Polymers; D011109:Polymethacrylic Acids; D012833:Siloxanes; C083568:Z100 composite resin; C102738:barium glass filler; D011791:Quartz; C020946:triethylene glycol dimethacrylate; D011092:Polyethylene Glycols; D017438:Bisphenol A-Glycidyl Methacrylate; D012822:Silicon Dioxide; D015040:Zirconium; D005027:Ethylene Oxide; D009584:Nitrogen; D010100:Oxygen