What is crown-to-root ratio?

December 05, 2021

  

Let us begin by understanding the structure of the tooth. The tooth that we see is known as the crown and is only one part of the tooth. The rest is hidden within the gums and is attached to the jaw bone. This part is known as the root of the tooth. The point where these two are demarcated is the cementoenamel junction. The crown that we see is covered by enamel and the root is covered by cementum and thus this point gets its name. This point is covered about 1 mm by the gums.

What is crown to root ratio in dentistry?

The term crown to root ratio is commonly used in dentistry. The length of the crown as compared to that of the root is called the crown to root ratio. The ideal crown root ratio is considered to be 2:3, with the root being longer to fix and support the tooth. Various factors like loss of gingiva, loss of bone due to infection or ageing exposes more of the root and provides less support to the crown. To put it simply, more root exposure means that there is less support available and that more of the crown needs to be supported. It is also described as relative tooth and relative root. The crown to root ratio also gets skewed when a person has short dental roots due to genetic or development problems.

People who have several missing teeth require partial dentures. Unfortunately their remaining teeth are often not strong enough to take the load of fixed partial dentures. The crown to root ratio is a way to assess their suitability for abutment teeth. In such situations implant abutment is preferred.

Ante’s Law

The concept of using crown to root ratio as a means of selecting patient’s remaining teeth for abutment for fixed partial dentures was first postulated by Ante, a Canadian dentist. The prinicple is called Ante's law. Though his theory has not been accepted in totality as many other factors are taken into consideration. Many feel that factors like the condition of the pulp, the opposite teeth, angulation and so on are equally important criteria for taking a decision.

How is it calculated?

The crown to tooth ratio is a 2D linear measurement that estimates the height of the crown to that of the root to calculate the ratio. Radiographic assessment is used to find accurate measurement.

Importance of crown-to-root ratio

• The crown to tooth ratio determines how firmly the tooth is entrenched in the jawbone. If the ratio is good then the concerned tooth can tolerate physiologic stress such as chewing. The abutment tooth that supports partial dentures needs to have a good crown to tooth ratio.
• Assessment of this ratio helps the dentist to understand the extent of gum recession and bone loss. This, in turn, is used for treatment of periodontal disease.
• It is also important when the dentist plans some kind of dental restoration such as fillings, crown caps or implants. In case several teeth are missing, then the decision for partial dentures is also based on the crown to root ratio.
• If it goes down to 1:2, that is the length of the root is twice the length of the crown, the tooth can safely be used as an abutment for partial dentures.
• If it is at a 1:1 ratio, then the support that the given tooth can offer is much lower and may not be able to take the stress of a partial prostheses. But it is still acceptable if other factors allow for it.
• If the ratio goes below 1:1 then there are chances of tooth loss and an abutment implant may become necessary to support fixed or removable partial dentures.
• If the root is exposed then again the crown to root ratio will be poor.

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