In case you haven't found it already, here's our page that explains post and cores.

Generally, a core is placed to idealize the shape of the tooth (replace portions of missing tooth structure) so the trimmed-down nub of tooth the crown fits over is ideal in shape (this aids with crown retention). The purpose of a post is to anchor the core.

Posts can be made out of flexible (fiber) or rigid (metal, ceramic) materials. Each type has its own advantages. Neither type is best for all considerations.

The important question here is why three attempts at anchoring a post have failed.

It could be a failure with the materials used (cement, bonding), kind of like you explain. But a post should not be held in just by the grip of its "cement." Its Resistance Form (resistance to being dislodged due to its shape and fit inside the tooth) is a more important factor.

"Adhesive" cements have only been available in dentistry over the last 30 years or so. For many many decades prior to that, dentists were able to place successful post and cores just with cements that acted as "fillers" between the cemented parts. They worked due to the mechanical design of the post/tooth relationship.

Having said that, for future attempts, probably your dentist will choose a cement known to bond to both tooth structure and metal. That can certainly be an aid.
Repeated failure could also be due to the fact that something is amiss with the "bite" of your crown(s). If it is too prominent in some aspect (like possibly just when you slide your teeth a certain way) and the tooth receives excessive forces, the post might get dislodged.

In your case, it seems you have had multiple crowns on your tooth. It seems unlikely that each one would have been crafted with the same "bite" discrepancy.
Another issue that may be playing a role is the lack of a crown/crown preparation ferrule effect. Go to Google images and type in "crown ferrule" and look at the pictures for an explanation.

A crown ferrule has to do with the way the edges of a crown rest on the tooth. This effect helps to direct forces to the tooth itself. A short, incomplete or nonexistent ferrule would allow a higher level of force to be directed to the post and core complex (thus dislodging it).

The cure in this case would be to replace the post and core, then re-prepare the tooth (reshape it so a more substantial ferrule design exists). A new impression would then need to be taken and a new crown made (to fit the new shape of the tooth).
Only your dentist can determine what is going on and what solution is needed. Good luck.
Other than being successful with the post and core, there's no other avenue for rebuilding your tooth as it exists.
Conceivable some type of design could be contrived where a neighboring tooth shares in receiving some of the force directed to the tooth (like making a 2-unit bridge or such) but that wouldn't typically be considered a first-choice approach like getting the post anchored successfully would.

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