When Is a Tooth Too Damaged for a Dental Crown?

There is a surprisingly specific answer to this question, and it is not about how much of the visible tooth has broken away. It is about whether a continuous collar of sound tooth structure remains above the gum for the crown to grip.

That collar has a name — the ferrule — and it is the single most reliable predictor of whether a heavily broken-down tooth will hold a crown or fail underneath one.

Think of a wooden barrel. The staves are held together by metal hoops. Remove the hoops and the staves splay apart under pressure.

A crown works the same way on a broken-down tooth, but only if it extends down over sound tooth beneath the level of the core build-up. When the crown's inner wall wraps around genuine tooth structure, forces applied to the crown are converted into compressive hoop stress around that collar, which the tooth resists well. When the crown sits only on a block of filling material, there is no hoop. Load goes straight into the core, the core rocks, the cement seal breaks, and the assembly loosens or the root splits.

This is why two teeth with identical-looking crowns can behave completely differently. The difference is not visible once the crown is fitted.

The working figures quoted in restorative dentistry are consistent: around 1.5 to 2 millimetres of sound tooth height above the finish line, with walls at least 1 millimetre thick, and — critically — present around the entire circumference.

That last point is where most teeth fail the test. It is common to have 3 millimetres of good tooth on the cheek side and nothing at all on the tongue side, where the decay originally started. The average is fine; the tooth is not. A partial ferrule behaves far more like no ferrule than like a complete one, because the missing segment is where the assembly hinges.

The other requirement is that the ferrule must be tooth, not core material. A build-up in composite or glass ionomer restores the shape of the tooth and gives the crown something to sit on, but it contributes nothing to the hoop effect. The relationship between the two is covered in our article on core build-up requirements for crowns, and the underlying mechanics in the importance of the ferrule effect.

If sound tooth is buried beneath the gum, it has not necessarily gone. It may simply be in the wrong place relative to the gum and bone. There are two established ways to change that relationship.

Surgical crown lengthening. The gum and a small amount of bone around the tooth are reshaped so that the margin of sound tooth sits above the new tissue level. It is a straightforward procedure with a healing period of several weeks before the crown is made, since the tissue position needs to stabilise first.

Its limitations are worth knowing. Removing bone around one tooth also removes it from the neighbours, which can expose their roots and affect appearance, particularly at the front. It also shortens the root length held within bone, worsening the crown-to-root ratio. And it lengthens the visible tooth, which may look conspicuous in a smile line.

Orthodontic extrusion. The root is gently pulled upwards over several weeks using a light orthodontic force, bringing the sound tooth structure up above the gum without cutting anything away. Because the bone follows the tooth, it does not thin the support around neighbours in the way surgery does.

Its limitations are time — typically two to three months of movement plus a stabilisation period — and the fact that the gum and bone often come up with the tooth, sometimes requiring a small amount of reshaping afterwards anyway.

Which is appropriate depends on how deep the deficiency is, where the tooth sits in the smile, and how much root length is available to spare.

Even where a ferrule can be created, there is a point at which doing so leaves too little root in bone.

The tooth acts as a lever. The part above the bone is the lever arm, the part below is the anchorage. As the ratio between them shifts, the forces transmitted to the remaining bone rise. A commonly quoted target is a crown-to-root ratio of 1:1 or better, with 2:3 considered comfortable.

Crown lengthening makes this ratio worse in two ways at once — it lengthens the visible crown and shortens the anchored root. On a tooth that already has some bone loss from gum disease, a procedure that would create an adequate ferrule can leave a tooth that is technically restorable but mechanically poor.

• The ferrule is a continuous collar of sound tooth, roughly 1.5 to 2 mm tall, that the crown grips.

• A partial ferrule behaves much more like none at all than like a complete one.

• Core build-up material does not contribute to the ferrule; only tooth does.

• Crown lengthening and orthodontic extrusion can create a ferrule where sound tooth is buried.

• Crown-to-root ratio sets a second limit, particularly where bone has already been lost.

A tooth is genuinely beyond crowning where sound structure extends so far below the bone that creating a ferrule would leave inadequate root anchored; where the root has a vertical fracture, which no restoration can seal; where the root has been perforated in a position that cannot be repaired; where root length is already short, either naturally or through resorption, so nothing can be spared; and where the tooth is so mobile from bone loss that it cannot carry a restoration at all.

The vertical root fracture case is the clearest of all. A crack running lengthways down the root splits the tooth into segments along its long axis, and there is no way to rejoin them. This category and the wider decision are covered in our article on when a tooth extraction is the only option, and the direct comparison in crown versus extraction.

Equally worth knowing is what does not disqualify a tooth.

A tooth broken off at the gum line is frequently restorable, provided sound structure remains just below and adequate root length is present. It looks alarming and often is not, as our article on root canal treatment for a broken tooth explains.

A large infection at the root tip is not a barrier either. It generally resolves after root canal treatment, and the bone regenerates over months.

A previously root-treated tooth with a new problem is often retreatable. A failed first attempt is not the end of the line.

And a tooth with a large old filling is usually a straightforward crown or onlay candidate, as covered in our article on when a crown is necessary for heavily filled teeth.

It is worth being explicit about the failure mode, because it is not a gradual decline.

A crown on an inadequate ferrule tends to work acceptably for a period — sometimes a year, sometimes several — and then fail suddenly. The two common presentations are the crown and core coming away together, which is recoverable and usually means the tooth can be reassessed, and a vertical root fracture, which is not.

The second outcome is the reason dentists are cautious about proceeding without adequate structure. A crown placed on a tooth with no ferrule may consume the opportunity to preserve the root for future use, since a fractured root has to be removed and often takes surrounding bone with it. The same concern underlies the choice of post, discussed in our article on why some crowned teeth need internal support.

If you have been told a tooth may be too damaged to crown, an assessment that specifically addresses ferrule, root length and bone support will give you a clear answer and identify whether crown lengthening or extrusion could change it. Arrange a consultation through our contact page.

You can read more on our dental crowns page, our root canal page, our dental implants page and our pricing page.

This article is for general information only and does not constitute dental or medical advice. Whether a particular tooth can support a crown can only be determined by clinical examination and radiographs. Always consult a registered dental professional about your own circumstances.

Next review due: 4 August 2027