Why Overlays and Crowns Protect Cracked Teeth Better

For many years, the standard solution for a decayed tooth was simple:

“Drill the cavity and place a filling.”

But when a tooth has a very large filling, especially one that covers most of the chewing surface, the story changes. Instead of protecting the tooth, a big filling can sometimes leave the tooth:

Thin

Weak

And more likely to crack or break

Modern dentistry now offers a safer alternative for these teeth:

zirconium (zirconia) overlays and crowns.

In this article, you will learn:

Why large fillings increase the risk of tooth fracture

The difference between a filling, an overlay and a crown

How zirconium overlays and crowns help protect weakened teeth

When each option is recommended

What to expect from treatment

Why Large Fillings Can Be a Hidden Risk

A tooth is not just a piece of hard material. Its strength depends on its shape and internal structure.

When decay is small:

A simple composite filling can work very well

Most of the tooth’s natural structure remains intact

The tooth can handle normal chewing forces safely

When decay is large or has been treated several times:

Much more tooth structure has to be removed

The remaining walls become thin and unsupported

Chewing forces are no longer evenly distributed

Tiny cracks can start in the weakened enamel and dentine

Over time, this can lead to:

Cracks running along the tooth

Broken cusps (corners)

Sudden vertical fractures that may make the tooth unsalvageable

So even if the filling itself looks okay, the tooth around it may be at serious risk.

Warning Signs That a Large Filling Is in Trouble

Common symptoms that a big filling and the tooth around it are failing:

Sharp pain when biting on hard food

Discomfort when releasing the bite (pain on “unloading”)

Sensitivity to cold or sweet foods

A visible crack line or a missing corner of the tooth

Food getting stuck more often in that area

If you ignore these signs, a small crack can suddenly turn into a big fracture, sometimes in one single bite.

Restorative Options: Filling vs Overlay vs Crown

To protect weakened teeth, your dentist can choose from three main options:

1. Large Composite Filling

A composite filling:

Fills the hole where decay was removed

Bonds to the tooth structure

Can be done in a single visit

However, when too much tooth is missing:

The filling acts like a wedge inside the tooth

Chewing forces can flex the thin remaining walls

The risk of cracks and fracture increases over time

For small to medium cavities, composite is great.

For very large cavities, composite alone is often not enough support.

2. Zirconium Overlay (Onlay / Partial Crown)

An overlay (also called an onlay or partial crown):

Covers and protects the chewing surface and one or more cusps

Removes less tooth structure than a full crown

Works like a “protective helmet” over the weak parts of the tooth

With a zirconium overlay:

The weak cusps are covered with a strong, custom-made zirconium cap

Chewing forces are redirected along the long axis of the tooth

The tooth becomes much more resistant to fracture

Overlays are ideal when:

A large filling is already present or needed

One or more cusps are thin or cracked

There is still enough healthy tooth to avoid a full crown

3. Zirconium Crown (Full Crown)

A crown:

Covers the entire visible part of the tooth above the gum

Requires more tooth reduction than an overlay

Provides maximum protection and reinforcement

Zirconium crowns:

Are very strong and fracture-resistant

Have good aesthetics (tooth-coloured)

Are metal-free, suitable for many patients who want a more “biocompatible” look

Crowns are recommended when:

The tooth is severely weakened

There is a high risk of fracture

Large parts of the tooth are missing

The tooth has had root canal treatment

In these cases, a zirconium crown can save the tooth from splitting under daily chewing forces.

Why Zirconium (Zirconia) Is a Good Choice

Zirconium (more precisely zirconia) has become one of the most popular materials in modern restorative dentistry.

Key advantages:

High strength: zirconia is much stronger than composite fillings

Fracture resistance: ideal for molars and premolars exposed to heavy chewing

Metal-free: no grey lines near the gum as with some metal-ceramic crowns

Biocompatible: generally well accepted by gums and soft tissues

Aesthetic: can be layered or coloured to look very natural

When used as an overlay or crown, zirconium:

Wraps the tooth in a protective “shell”

Supports thin walls and weak cusps

Spreads chewing forces evenly

Greatly reduces the risk of the tooth breaking apart

In other words, instead of asking a large filling to hold a weak tooth together, we use a zirconium restoration to protect and reinforce the entire structure.

Overlay vs Crown: Which Is Better for Me?

The decision between a zirconium overlay and a zirconium crown depends on:

How much healthy tooth is left

Where the cracks and weak points are

Whether the tooth has had a root canal treatment

Your bite and chewing forces (for example, if you grind your teeth)

Choose a Zirconium Overlay When:

There is still enough strong tooth structure

Only some cusps are damaged or thin

You and your dentist want to be as conservative as possible

The nerve is healthy and the tooth responds normally

In this case, the overlay:

Preserves more of your natural tooth

Protects the weak areas

Decreases fracture risk while being minimally invasive

Choose a Zirconium Crown When:

Very little natural tooth remains

A large portion of the tooth has broken off

There are multiple cracks

The tooth has been root canal treated and needs extra protection

In this case, the crown:

Fully encases the tooth

Offers the highest level of mechanical protection

Is often the last step before extraction if the tooth is severely compromised

Your dentist will usually take X-rays, photos and clinical tests to decide which option is safer for the long term.

How the Treatment Works: From Large Filling to Zirconium Protection

Although details vary, most overlay or crown treatments follow these steps:

Diagnosis and Planning

Clinical examination

X-rays to check root, bone and existing fillings

Evaluate cracks, sensitivity and bite

Tooth Preparation

Remove any old leaking filling and decay

Shape the tooth for an overlay or crown

Protect the nerve if it is close to the surface

Impressions or Digital Scan

Traditional moulds or an intraoral scanner

Precise capture of your tooth shape and bite

Temporary Protection

A temporary filling or temporary crown is placed

You wear this while the lab makes your zirconium restoration

Laboratory Fabrication

The dental laboratory designs and mills the zirconium overlay/crown

Colour and shape are adjusted for a natural look

Final Cementation

The zirconium restoration is tried in, adjusted and polished

It is bonded or cemented to your tooth

Bite is checked and fine-tuned

Once fitted, the zirconium overlay or crown works as a strong armour for your weakened tooth.

Advantages of Choosing Zirconium Overlay or Crown Instead of Another Big Filling

Compared with simply placing another large composite filling, zirconium overlays and crowns offer:

Much lower fracture risk for the remaining tooth

Better protection for thin cusps and walls

More predictable long-term performance

Stronger resistance to chewing forces

A more stable shape and bite over time

Improved aesthetics, especially with modern zirconium ceramics

Put simply:

A large filling fills a gap;

a zirconium overlay or crown reinforces the whole tooth.

What About Root Canal Treatment?

Sometimes, a tooth with a large filling and cracks already has:

Persistent pain

Spontaneous aches

Prolonged sensitivity to hot or cold

In such cases, a root canal treatment may be needed before placing a zirconium overlay or crown.

The sequence is usually:

Root canal treatment

Core build-up (to replace missing internal tooth structure)

Zirconium crown or overlay to protect the tooth from breaking

Skipping the protective zirconium restoration after a root canal leaves the tooth fragile and at high risk of vertical fracture.

How Long Do Zirconium Overlays and Crowns Last?

Lifespan depends on:

Your oral hygiene

Your bite and habits (grinding, clenching, nail-biting, etc.)

Regular check-ups and professional cleaning

With good care, zirconium overlays and crowns can last many years, often 10–15 years or more, while providing:

Ongoing protection

Stable aesthetics

Reliable chewing function

No dental restoration is completely indestructible, but zirconium is one of the most durable materials available today.

Caring for Your Zirconium Overlays and Crowns

After treatment, you should:

Brush twice a day with a soft toothbrush

Clean between teeth with floss or interdental brushes

Use a night guard if you grind or clench your teeth

Visit your dentist regularly for check-ups and professional cleaning

Zirconium itself does not decay, but the tooth underneath and around it still can if plaque is not controlled.

Conclusion: Protect Weak Teeth Before They Break

Large fillings may look like they “fixed” the problem, but in many cases they leave the tooth mechanically vulnerable. The bigger the filling, the higher the risk that the tooth will eventually crack or break.

Zirconium overlays and crowns offer a smarter, safer solution:

They do not just fill a hole – they support and protect the entire tooth

They significantly reduce the risk of fracture in teeth with big fillings

They combine strength, aesthetics and long-term stability

If you have one or more teeth with very large fillings, especially in the back of your mouth, talk to your dentist about whether a zirconium overlay or crown would be a better long-term investment than simply placing “one more big filling”.