Rubber Dam During Post Space Preparation: A Question No One Has Properly Measured

A colleague asked me: during root canal treatment we place a rubber dam and keep the field isolated. Should we do the same while preparing the post space? In practice almost nobody does, but the question is a serious one and it has no straightforward answer.

What the Evidence Says

When I looked for a paper that had measured exactly this, I found only one: Goldfein's retrospective study from 2013, which concluded that placing a post under rubber dam gives a higher success rate. But the study has several serious weaknesses. Its criterion for identifying rubber dam use was seeing a clamp on the radiograph, even though a rubber dam can be placed without a metal clamp. The rubber dam group had only 30 teeth against 174 teeth without one. And most importantly, the operator effect was not controlled; someone who places a rubber dam probably performs the other steps more carefully as well. So the difference cannot be attributed to the rubber dam.

The rest of the sources are either laboratory or animal studies, and all of them model a different scenario: a post space left open for weeks or months. That is not the same as a few minutes of work without a rubber dam followed by cementing the post. So in practice we have no reliable data for this question and have to rely on reasoning.

My Reasoning

Endodontic sources describe a concept called the apical seal: when 4 to 5 mm of gutta-percha remains at the end of the canal after post space preparation, the apical seal is in place. The coronal seal is what the final restoration, that is, the post and core and the crown, provides. In a successful treatment, the system is closed from both ends.

The question is what happens if saliva enters the post space in the short interval between preparation and cementing the post. Whether that interval falls in the post fabrication appointment or the delivery appointment, it is a matter of minutes. My answer is that when the apical seal is left intact and the coronal seal is established by the restoration, the small number of bacteria trapped between the two barriers has neither a food source nor a way out.

This is the same logic we rely on in biomimetic dentistry. There the rule is that the margin of the cavity, the roughly 2 mm perimeter called the peripheral seal zone, must be completely free of caries so that the bond and the peripheral seal are reliable. But in the center of the cavity, close to the pulp, if complete caries removal would lead to an exposure, we deliberately leave some carious dentin and seal over it. Sealed caries is arrested, because bacteria beneath the seal lose their activity. In other words, what matters is that the peripheral seal is sound, not that the inside of the system is sterile. The post space is in exactly the same situation: the apical seal on one side, the coronal seal on the other, and a negligible amount of salivary flora trapped between them.

Summary

This reasoning has not been tested yet, because no study has followed this chain all the way through. But it rests on concepts that endodontic sources themselves define, and it is consistent with the logic of sealing in biomimetics. Until better data arrive, this is my position: a sound apical seal plus a proper coronal seal neutralizes the short-term contamination that occurs during preparation.

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