The finite elements analysis took two cases into consideration:
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the post is sealed with a soft friction in the drilled canal: mechanical retention. |
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the post is bonded into the drilled canal and there is a space of 1/10 mm between the canal walls/post interfaces (i.e. 2/10 mm on the diameter): adhesion. |
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SHOULD THE POST BE BONDED OR SEALED ? |
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Sealed: oxyphosphate cement, glass-ionomere cement. |
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Bonded: adhesive composite cements. |
The answer is given by a mechanical argument based on the transmission of stress to the root.
The finite elements analysis demonstrates that in the areas further from the joined parts, there is a similar level of stress in the root. |
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Along the sealed length, the level of stress is significant . The stress dissipates rapidly through the dentin, but a part of the root is directly subjected to a high level of force. |
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In the case of a post bonded with 0.1 mm of cement, the bonded surface permits the distribution of stress all along the post. Instead of one large disturbed zone (which is the case with sealing), the forces are better distributed along the length of the bonding, over several small zones. The root is not directly subjected to the stress of the post. The bonding cement absorbs the high level of stress in the areas of contact with the post. It acts as a sort of protection for the root because it absorbs most of the stress in contact with the post. Beyond the bonding cement, the stress diminishes and no longer creates fragile zones in the root. |
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OTHER GOOD REASONS FOR BONDING |
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| When using bonding cements, a thickness of 1/10 mm between the root canal walls and the post is required. Besides the mechanical interest of this shock absorption cement layer, there are other points in favour of bonding. |
1. IT AVOIDS HYDRAULIC PRESSURE |
A very high hydraulic pressure can be created when sealing the post: if the two interfaces are very close together, a crack could happen, later inducing a fracture. This pressure can also push away the endodontic filling at the apex.
Note: in joinery, to assemble two wooden pieces with a wooden peg and a hole, it is said that the glue must be spread on the peg. The hole should never be filled with glue, otherwise the wooden piece might crack. |
2. IT MINIMIZES THE RISK OF AIR BUBBLES |
There are always air bubbles in cement. Between two close interfaces, these air bubbles are trapped and cannot escape; worst, a trapped air bubble can be smashed between the two interfaces: its surface increases and so does its internal pressure. This pushes away the cement and results on no cement at all being left on one third of the post surface (this is a well known phenomena in industries that use bonding).
When using bonding cements, as it is important to leave some space between the root canal walls/post interfaces, the trapped air bubbles can escape. |
3. THICKNESS OF THE JOINT |
In structural bonding used in industry (except with cyanoacrylate bonding), such as epoxy bonding, the thickness of the joint is of prime importance: for an optimum adhesion with a cohesive rupture, the thickness of the joint must be between 0,1 mm and 0,25 mm. For this reason, calibrated glass beads are mixed with the glue to create a regular space between the two pieces.
This is the reason why our finishing drills have a diameter of 0,2 mm larger than the matching post, thus leaving 0,1 mm of space between the post and the root canal dentin walls. We call our composite posts "floating posts".
Very often, on X-rays, you can see that the apex of the post does not exactly touch the end of the drilled canal, there is some cement between: this is excellent to protect the tooth apex from stress. |
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