Adhesion to tooth structure: a critical review of "macro" test methods.
OBJECTIVE
Bond strength between adhesive systems and the tooth structure is influenced by a large number of variables, which makes the comparison among studies virtually impossible. Also, failure often times propagates into the dental substrate or the composite, deeming the results questionable at best. In spite of the increased popularity gained by micro-tensile and micro-shear tests, in vitro evaluations using specimens with relatively large bonding areas remain frequent. This review focuses on aspects related to specimen geometry and test mechanics of "macro" shear and tensile bond strength tests.


METHODS
Besides information drawn from the literature, the effect of some parameters on stress distribution at the bonded interface was assessed using finite element analysis (FEA).


RESULTS
Bond strength tends to increase with smaller bonding areas and with the use of high elastic modulus composites. Stress concentration at the bonded interface is much more severe in shear compared to tension. Among shear methods, the use of the chisel shows the highest stress concentration. Within the limits suggested by the ISO/TS 11405, crosshead speed does not seem to influence bond strength values. Pooled data from currently available adhesives tested in either shear or tension showed 44% of adhesive failures, 31% mixed and 25% cohesive in the substrate (tooth or composite). A comparative bond strength study involving three adhesive systems revealed similarities between "macro" and "micro" counterparts regarding material ranking, whereas "macro" tests presented a higher incidence of cohesive failures.


CONCLUSIONS
Simplicity warrants "macro" bond strength tests an enduring popularity, in spite of their evident limitations. From a mechanical standpoint, knowing the stress distribution at the bonded interface and how it is affected by the materials and loading method used is key to explain the results.
Braga
Roberto R
RR
University of São Paulo School of Dentistry, Dept. of Dental Materials, Av. Prof. Lineu Prestes, 2227, São Paulo, SP 05508-000, Brazil. rrbraga@usp.br
Meira
Josete B C
JB
Boaro
Leticia C C
LC
Xavier
Tathy A
TA
D003188:Composite Resins / Q000737:chemistry; D001840:Dental Bonding; D003743:Dental Enamel; D019232:Dental Restoration Failure; D003799:Dental Stress Analysis / Q000379:methods*; D003804:Dentin; D017220:Dentin-Bonding Agents; D055119:Elastic Modulus; D020342:Finite Element Analysis; D006801:Humans; D008422:Materials Testing / Q000379:methods; D019279:Resin Cements; D033081:Shear Strength; D013314:Stress, Mechanical; D013718:Tensile Strength
D003188:Composite Resins; D017220:Dentin-Bonding Agents; D019279:Resin Cements