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    Why you need to rethink your bonding technique

    What you need to know about bonding and adhesives to ensure restorations are successful.

    The idea of bonding restorations into teeth has been around since at least the 1940s with silicate cements and the late 1950s with the adhesion of polymers to tooth structures facilitated by acid etching in the work of Dr. Buonocore.

    This etching process has been one of the most controversial and debated topics in dentistry. Various efforts have been directed at creating surfaces that contemporary monomers can penetrate into and harden in place: from selective etching of enamel only, to etching of dentin with low, 10-percent concentrations of phosphoric acid, to complete etching (total-etch) of all tooth structures involved with 35 percent phosphoric acid and finally to self-etch systems.

    Polymer durability and etched tooth structure (hybrid layer) should be of primary interest, so both the patient and clinician will be satisfied that the value is worth the effort.

    Fig. 1Fig. 1Sensitivity and early failure of adhesive/composite systems has been exhaustively discussed, and still to this day is not agreed upon by many experts. Much of the time, we are left with so little understanding as to why a system failed that we readily place blame on the product, not recognizing how our techniques may have contributed to the process.

    Without understanding the inherent strength of the tooth substrate, we are left with guesses and assumptions about how much bond strength is necessary. It seems appropriate to provide a short explanation on how we test the strength of dentin, as this information will serve us well for all future discussions about bonding.

    How strong is strong enough?

    In a method used in a 1999 IADR research project, whole human teeth were embedded in PPMA cylinders and then cut on a lathe to produce a 2.38 mm diameter button of dentin protruding out from the buccal surface (Fig. 1). Buttons were produced in shallow, middle and deep dentin.

    The specimens were then loaded into an Instron Universal Testing machine, and a precisely-fitted notched chisel exactly matching the diameter of the button was used to shear the button off. The force used to fracture the buttons ranged from 70 to 100 pounds.

    Shallow and middle dentin performed similarly, but the deep dentin became thin as it neared the pulp chamber and broke at lower values — not because dentin is weaker at the deep layers, but because it was too thin and flexed during the test.

    When we use a button diameter of 2.38 mm (about the diameter of the shank of a low-speed drill or polishing wheel) one pound equals 1 MPa. It stands to reason that if the tooth takes between 70 and 100 MPa to break, then we would want a composite bonded in place of the dentin to be able to withstand similar loads.

    Figure 2Figure 3

    Fig. 2                                                                                                     Fig. 3

    In order to accomplish this test, a notch shape that precisely fits the button must be used to load the specimen (Fig. 2). Interestingly, most of the shear research performed in the past has been performed using a straight chisel, which applies a point-load and falsely implies a lower value than what is actually present if tested correctly (Fig. 3).

    Up next: Technique, technique, technique

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