HPB Bolhuis, PhD and P Pallav, PhD

It is important to produce a sound adhesive bond with the dentin in order to supply reliable grip for the build up material, and to create an extra adhesive seal over the (often mechanical) endodontical seal.

Slowly setting materials have an advantage because the anaerobe conditions strongly accelerate the setting of composite materials. Lentulo paste carriers are much to slow; the post may not fit in with all consequences!
Syringed application with needle tubes is reliable and convenient.

Chemical cure (2-part materials) and dual cure materials (light-sensitive 2-part materials) have an advantage, because the shrinkage stresses arise considerably slower in the critical area, which is the (low-light) pulp chamber and root canal. This is much less demanding on the adhesive bond with the dentin. On the other hand the chemical curing reaction is incompatible with several especially acidic adhesive systems, as the acid has much more time than with light-curing materials to diffuse into the composite and impede the chemistry of the setting process (below; Bolhuis et al., 2006).

Composites cements bond well to metal, carbon fiber, and fiberglass posts (Bachicha et al., 1998). The problem is the adhesion to the root dentin. Magnifying glasses or a microscope (addictive!) may be useful to clear the walls from remnants of the root canal sealant, but also washing with 30% phosphoric acid will provide a more reliable substrate for adhesion. This will also neutralize any adverse effects of the endodontical washing fluid (NaOCl) (Morris et al., 2001).

The subsequently applied adhesive forms a thin elastic layer between cement and dentin. Although this is more complex than with cements with only a primer, the separate adhesive impregnates the dentin better than the more viscous cement (Carvalho et al., 2004) and the elastic layer mitigates shrinkage stresses (Aussiello et al., 2004).

In particular in root canals, the setting of composite cements creates considerable shrinkage stresses (Bouillaguet et al., 2004), which is the main reason why the adhesive strength is much less than with ordinary composite restorations.

Shrinkage stresses mainly occur in the pulp chamber and deeper, because of the magnitude of the C-factor in this area. To some extent these stresses may be ‘controlled’ by layered applications of the light-curing composite. Shrinkage stresses hardly play a role at the coronal part, as the material can shrink freely here (Feilzer et al., 1993).

Composite cements may increase the bond strength of root posts, compared to regular cements and block micro leakage (Bachicha et al., 1998; Nissan et al., 2001). Nevertheless the root canal is an awkward and critical region, where it is hard to produce a reliable and reproducible bond between the post and the dentin (Bolhuis et al., 2004).

Compatibility issue

Do Etching Adhesives Impede Chemical Cure of Cements?

Self-etching acidic adhesive systems cannot be used with chemically or dually curing composites, as the acid of the adhesive interferes with the chemical (part of the) polymerization near the adhesive interface (Sanares et al., 2001). This decreases the bond strength and increases the chances on leakage and separation. 
The erroneous conclusion with separation in such a case is that a post would have been better…

A point of concern is that with certain composite cements, which must be either dually or chemically curing in order to achieve complete setting, acidic primers are supplied, which impede the chemical setting. Some manufacturers are adapting the composition of the primer for this reason.

The chemical (part of the) polymerization requires less acidic adhesive systems or systems that have otherwise been adapted to meet this problem adequately.

Light-curing composite materials don’t have this problem, because these are polymerized immediately after application with light-initiation, before too much acid has diffused into the composite (Bolhuis et al., 2006). 
Moreover, light-curing build up composites tend to be stronger and are more similar to dentin in mechanical properties.