The Effect of Different Fiber Post Surface Preparations on Bond Strength with a Self-Adhesive Cement

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Introduction and Aim

Self-adhesive cements have become popular because they make the job simpler: they need neither etching nor a separate bonding step. But one point is usually overlooked. When we cement a fiber post there are two bonds: cement to root dentin, and cement to the post itself. If the second bond is weak, the post loosens or comes out, even when the cement has adhered well to dentin. This in-vitro study examined how much the bond of the post to RelyX U200 cement improves if the fiber post surface is prepared with different methods before cementation.

Study Method

Sixty fiber posts were divided into six groups of ten:

• Control: the post straight out of the package, with nothing done to it.
• Silane: a layer of silane applied to the post and dried with an air syringe.
• 24% hydrogen peroxide: one minute in the solution.
• Sandblasting: 30 seconds with 50-micron alumina particles.
• Ammonia plasma: 3 minutes.
• HMDSO plasma: 15 minutes. (Plasma is a laboratory method that changes the chemical structure of the surface with an ionized gas; it is not yet an everyday tool in the practice.)

Each post was then cemented inside a cylindrical mold filled with RelyX U200, so that the post sat in the middle of a block of cement. The block was sliced, like a salami, into one-millimeter discs. Each disc is now a ring of cement with a piece of post sitting in its center. Using a testing machine, the post was pushed out of the middle of this ring and the force needed to dislodge it was measured. The higher the force, the stronger the bond of post to cement.

Apart from this test, the post surfaces were also examined: once with an electron microscope, to see what the surface looked like after each method (for example, whether resin had been removed or fibers had become exposed), and once with a spectroscopic method that shows how the chemical composition of the surface had changed.

Key Findings

• Surface preparation had a clear effect on bond strength.
• Four methods, silane, sandblasting, ammonia plasma and HMDSO plasma, raised the bond significantly: from about 9.6 MPa in the control group to about 13 to 16 MPa. These four methods did not differ statistically from one another; none of them can be said to be better than the rest.
• 24% hydrogen peroxide gave no benefit at all (about 9.4 MPa). In practice it was as if nothing had been done to the post.
• The failure pattern was the same in every group: instead of the post coming cleanly out of the cement ring, both the post itself and the cement itself cracked. In other words, as the force rose it was the materials that broke, not the bond line between them. This usually indicates that the bond was not the weakest point of the assembly.

Why Were These Results Obtained?

• Silane: it acts as an intermediary. One end attaches to glass (the post's fibers) and the other end to resin (the cement). So it creates a genuine chemical bond between post and cement. The chemical analysis of the surface also showed that after silane these same silicon bonds had increased on the surface.
• Sandblasting: the alumina particles shave off a thin layer of resin from the post surface and expose the glass fibers beneath it, without damaging the fibers themselves. The surface becomes rougher and larger, and the cement grips it better. Here the bond is more mechanical than chemical.
• Hydrogen peroxide: in the microscope images, peroxide, like sandblasting, removed resin and exposed the fibers. So why did the bond not improve? The authors' probable explanation is that peroxide does not only eat into the surface, it also damages the bond that attaches the fibers to the resin inside the post. That is, the surface became rough, but what lies beneath it became weak. This explanation is, however, only the authors' conjecture.
• Plasmas: they improved the bond by changing the surface chemistry of the post, but since they cannot currently be used in the practice, they are mainly of research interest.

Clinical Conclusion

A cement being self-adhesive does not mean the post can be cemented without preparation. A layer of silane or a short sandblast, each on its own, clearly raises the bond of the post to the cement. Hydrogen peroxide has no value for this purpose.

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