ferrule effect

The term “ferrule effect” is something your dentist talks about while examining your tooth for restoration.

A dental ferrule is a band of cast metal that encircles around the coronal surface of the tooth.

Whether the structural loss of the tooth is due to trauma or caries, the remaining tooth structure must have certain dimensions to aid in the resistance form of the crown preparations.

The ferrule effect around your tooth protects it from fracture by providing support to the crown.

Continue reading it further to know more about the ferrule effect, its types, and significance in your restorations.

What is the ferrule effect?

The word ferrule is derived from a combination of two Latin words- Ferrum (iron) and Viriola (bracelets).

According to the Glossary of Prosthodontic Terms (GPT), A ferrule is “A band or ring used to encompass the root or crown of the tooth” (1)

One way to visualize this effect is by imagining a wine barrel. The metal hoops or bands, which encompass the wooden barrel, give support when the barrel is full.

Why is it important?

This encircled preparation on your tooth provides a protective effect against fracture of the tooth by reducing stresses within a tooth.

Several studies suggested that the ferrule effect strengthens the teeth against functional, wedging, and lateral forces. (2)

At one time dowel and core were thought to strengthen the remaining tooth structure and was referred to as ‘Intracoronal crutch’ by Rosen (3)

Use of a dental ferrule as part of the core or artificial crown may be of some benefit in reinforcing root- filed teeth.

A solid foundation of the tooth structure is necessary so that the crown has its finish margins on the natural tooth.

What is the procedure involved?

For the ferrule effect to work, a post is inserted into the root canal system of your tooth.

The core of the post and core unit extends off the post. This core is modified using a high-speed bur keeping in mind about the minimum width required for the ferrule effect.

After the preparations are complete, the crown gets cemented over the tooth, thus finishing the restoration of your tooth.

Can all cases benefit from the ferrule effect?

Ferrule effect doesn’t guarantee about preventing your tooth from fracturing, but it reduces the risk of fracture.

Sometimes, even after the surgical procedure, your tooth may not have enough structure present for crown placement. In these cases, extracting your tooth becomes the only possible option.

As per the research in the Balkan Journal of Dental Medicine, an amount of sound tooth structure should be above the alveolar bone and soft tissue (4).

It helps in leaving some space for the soft tissue to attach to the tooth and for the ferrule or crown. When there isn’t much of the tooth remaining, you may need a surgical procedure called crown lengthening.

During this procedure, the oral surgeon removes tissue and bone to expose enough tooth structure above the bone.

Advantages of ferrule effect

The ferrule effect has the following four benefits:

  • It helps in promoting hugging action between the tooth and the artificial crown.
  • It preserves the root from external forces.
  • It resists the lateral forces and functional lever forces exerted during insertion of the dowel.
  • It also helps to reduce the wedging effect of a tapered dowel.

Classification of the dental ferrule

Based on the risk assessment of the mechanical failure of the dental ferrule, this classification has five categories (5) –

Category A – No anticipated risk

The dentinal walls are sound and remain all around the tooth, with a height that is greater than 2 mm and a minimum thickness which is about 1 mm.

Such teeth do not present any risk for mechanical or structural failure.

Category B – Low risk

The height of the wall in this category is less than 2 mm.

The walls of the tooth are compromised, or no ferrule is present on either proximal surface or two compromised proximal walls on the tooth that undergoes light lateral loads.

Such teeth present a low risk for structural or mechanical failure of the dental ferrule.

Category C – Medium risk

A compromised buccal or lingual wall of a tooth that undergoes light lateral loads or two compromised proximal walls on a tooth which undergo heavy lateral loads are cases of this category.

Such teeth present a medium risk for structural or mechanical failure of the ferrule.

Category D – High risk

A compromised buccal or lingual wall on a tooth which undergoes heavy lateral loads, or that has only two adjacent walls or a tooth with a single wall remaining is considered as a high risk.

Such teeth present a high level of risk for structural or mechanical failure.

In such cases, your dentist should consider alternative treatment modalities for further treatment.

Category X

No ferrule placement can occur as the tooth is non-restorable. In these cases, your dentist might have to extract your tooth.

Factors affecting dental ferrule

Few factors affect the functionality of ferrule in your teeth that are as follows:

Ferrule height

  • The higher the height of the remaining tooth structure, the better will be the fracture resistance.
  • A ferrule height with dimensions of 1.5 to 2 mm of vertical tooth structure would be beneficial.
  • The artificial crown should encompass at least 2 mm past the connection of tooth and core to ensure a protective ferrule effect.

Ferrule width

  • The esthetic restorations often require preparations around the gingival margin and sometimes buccal defects, such as abfraction, may compromise the buccal dentin wall.
  • The walls of the tooth are too thin if they are less than 1 mm in thickness which would negate the ferrule effect.

No of walls and ferrule location

  • A circumferential ferrule would be optimal, but dental caries may affect the interproximal areas and cause abrasion or erosion on the buccal walls.
  • A crown preparation will further reduce the thickness of the wall, and only a partial ferrule will remain.
  • Ng et al said that a good palatal ferrule is as effective as having a complete “all around” ferrule. (6)
  • Al-Wahadni and Gutteridge55 found that having a 3mm ferrule placed on the buccal aspect was better than having no ferrule at all.

Type of tooth and extent of the lateral load

  • Two factors that distinguish anterior from posterior teeth: their relative size and the direction of loads they need to withstand.
  • Anterior teeth are loaded non-axially whereas posterior teeth are loaded occluso-gingivally.
  • Lateral forces have a larger potential in damaging the tooth-restoration interface when compared to vertical loads.
  • Before restoring the tooth, a thorough review of the occlusal pattern, the functional and parafunctional forces should be performed, as these will influence the success of the final restoration of the particular tooth. (7)

Type of post

  • Cast posts were in use for years to support the final restoration.
  • However, in recent times these restorations have been progressively replaced by composite cores with a metal post or glass fiber post.
  • Fiber-reinforced posts have shown positive results when compared to metal posts.
  • A ferrule of 1.5-2 mm sound tooth structure between the core and the finish line is more critical in fracture resistance than the post design or type.

Core materials

  • Composite resin with a dentin bonding agent is frequently in use as a material which strengthens the tooth as compared to amalgam.
  • It is true because amalgam does not bond to the tooth structure, and it requires undercuts for retention, which weakens the remaining walls of the tooth.
  • Alternatively, multiple studies have shown improved resistance to fracture with MOD cavity preparations restored with composite or fiber reinforced resin. (8)

Takeaway

A dental ferrule is a band that encircles the external dimension of the residual tooth structure.

It is an essential aspect in deciding the success or failure of the artificial crown placed on your restored tooth.

It is a must in the post-endodontic reconstruction of a severely degraded tooth. The hugging effect between your artificial crown and the natural tooth helps in improving the resistance of the restoration.

Your teeth or crown may flex which can lead to fracture of the tooth structure. A ferrule helps to reduce the risk of fracture to the crown.