ENAMEL AXIOM #2
This is a watershed concept and pivotal to composite excellence. It is not widely understood in composite technique.
Enamel axiom #2 is “Always rod -ends”
The research article which establishes this fundamental is :
Munichika T Suzuki K Nishiyama M Ohasi M Horie K. A comparison of the tensile bond strengths of composite resins to longitudinal and transverse sections of enamel prisms in human teeth J Dent Res 1984: 63:1079-1082.
This article established that :
- “Crystals within the enamel rod cylinders run parallel to the length of the enamel rods, which are in turn approximately perpendicular to the external surface”
- “When the transverse section or face of the crystal, rather than its side, is exposed to acid, the central core of the crystal is most susceptible to acid dissolution
- ”Resin bond strengths are twice as high when adhering to the acid-etched ends of the crystals as compared to the sides of the crystals”
In the adjacent photo we see that the enamel crystals have been dissolved out of the center of the rods, leaving the circumference or sheath of the rod intact. It is the selective etchability of rod ends that determines this phenomenon. The etch-resistance of the rod sheath has a purpose in natural teeth: to limit the lateral spread of caries in enamel.
In composite restorations, we want to engage rod ends. Thish doubles the adhesion of bonding relative to the rod sides/sheaths.
Restoration not bonded to rod-ends are faulty and prone to marginal stain, leakage, de-bond, sensitivity, recurrent decay, and short lifespan.
HOW DOES THIS APPLY TO RESTORATIVE DENTISTRY?
A clear application of this concept:classic GV Black proximal box walls, prepared at 90 degrees to the cavosurface, expose only rod sides.Therefore GV Black box walls are subject to poor adhesion and leakage. Walls must be bevelled. See Outside-inwards Class II box Preparation
This Handbook takes you through all preparation designs and revises them for optimum adhesion.
It also provides a platform against which to evaluate methods proposed by other sources. Ask the question; does this prep design expose rod ends? If not, it is inferior.