Abutment teeth selection will allow the practitioner to answer two questions :

• How many abutment teeth are needed to support the bridge ?
• Which are the abutment teeth that will support the dental bridge ?

General requirements

To be selected as an abutment, a tooth must meet certain requirements :

• Tooth should be healthy or properly treated

  The tooth should not manifest any signs of pulp or apical tissue infections (such as cysts or granuloma). If the tooth shows one of these conditions, it is advisable to perform the endodontic therapy before starting tooth preparation.

  If the tooth was endodontically treated in the past, it is wise to confirm on a dental radiography that the treatment was carried out correctly and there are no visible complications.

  If conditions are too advanced for any conservatory treatments, the tooth must be extracted and the design of the bridge revised.

  Dental X-rays :

  

  healthy tooth
  ok as abutment

  

  tooth with chronic infection:
  dental cyst
  not ok as abutment

  

  tooth with a proper endodontic treatment
  ok as abutment

  

  tooth with improper endodontic therapy ; reoccurring apical infection
  not ok as abutment

  
  
  
  
  

• Tooth should not show advanced destructions

  Sometimes, if tooth structure is inadequate, a post and core is needed.

  If destructions are very advanced, especially if they go beneath the gumline, it is prudent not to use the particular tooth as an abutment.

  
  

  

  tooth with coronal destruction ;
  post and core indicated

  

  post and cores definitively cemented

  
  
  

• Tooth should not show a pronounced degree of mobility or advanced gingival recession

  A certain degree of mobility is accepted especially if the bridge will be supported by many abutments.

  It is advisable to evaluate the amount of bone loss around the tooth on a dental radiography. This should not exceed 1/3 of the total length of the roots.

  
  

  

Specific requirements

Each tooth will play a different part in supporting the dental bridge. For example, back teeth will have a more important part in supporting the bridge because of their larger size and the higher number of roots.

Some teeth are "very powerful abutments" (molars) while others are "extremely week abutments" (incisors).

"The degree of support" that each tooth will give to the dental bridge depends on many factors. These factors may be influenced by pathological conditions or age.

• Number of roots : the higher it is, the greater the tooth stability and strength

• The length and thickness of the roots : the greater it is, the higher the tooth stability inside the bone

• Root divergence : the more divergent the roots, the stronger the tooth is anchored inside the bone

• Bone quality : refers to the amount and quality of the bone surrounding the tooth

According to these criteria, each tooth receives an index number that shows how much we can rely on it to support the dental bridge.

Molars

Molars are very strong teeth that have 2,3 or more roots. There are 12 molars in total: 6 upper (3 on each side, left and right) and 6 lower.

Molars have the biggest crown of all teeth. Molars are located in the back of the mouth and are extremely important abutment teeth. In their absence, it is difficult to design a reliable dental bridge (especially if the bridge is extended).

Tooth	Index	Description	Image
First upper molar	6	It is the most powerful upper tooth. It has 3 roots with very strong bone implantation.
Second upper molar	6	The tooth is almost as strong as the first molar. It has 2 or, more often, 3 roots.
Third upper molar	4,5 or 6	It is also called the wisdom tooth. It may have 2 or more roots. It should always be checked on a dental X-ray to verify the number of roots and the degree of bone implantation. It is used as an abutment only if there is no other possibility.
First lower molar	6	It has 2 very strong and extremely well anchored roots.
Second lower molar	6	It resembles the first lower molar. It usually has 2 strong roots.
Third lower molar	4,5 or 6	It may have 1, 2 or more roots of various sizes. It is used as an abutment only if there is no other possibility.

Premolars

Premolars are relatively strong teeth that can well be used as abutments, but they don't reach the strength and power of molars or canines. There are 8 premolars: 4 upper (2 on each side) and 4 lower.

They have a smaller crown than the molars and the roots are fewer and thinner. They are positioned immediately before the molars, hence the name.

Tooth	Index	Description	Image
First upper premolar	4	It has two thin roots and average bone stability.
Second upper premolar	4	Usually, it has a single root but there are times when it has two. Otherwise, it resembles the first upper premolar.
First lower premolar	5	It has a much stronger root and a better bone implantation than the upper premolars.
Second lower premolar	5	It is very similar to the first lower premolar. Sometimes the root is thinner.

Canines

Canines are strong teeth that have a long root with a very powerful bone implantation. They make the transition between front teeth (incisors) and lateral teeth (premolars and molars) and are located in the place where the dental arch curves. There are 4 canines: 2 upper and 2 lower.

Canines, along with the molars, are the most important abutment teeth.

Tooth	Index	Description	Image
Upper canine	5	It has the longest root of all teeth. Because of its position, it plays an important aesthetic role and it is a strong abutment for the bridge as well.
Lower canine	5	The root is shorter but the tooth is still very strong and well anchored inside the bone.

Lateral incisors

Lateral incisors are the smallest teeth in the mouth. They have a single root, thin and short, with a very weak bone implantation. They are 4 in number: 2 upper and 2 lower.

Regarding prosthetics, they are the weakest abutments in the mouth. Therefore, supplementary abutments are required when a lateral incisor is included in a dental bridge. They have a very important aesthetic position.

Tooth	Index	Description	Image
Upper lateral incisor	1	Tooth root is thin and short. Stability is very weak.
Lower lateral incisor	1	Strength and stability are weak. The root can be even thinner.

Central incisors

Central incisors are larger than the laterals and they have a longer root. Apart from that, they are weaker (as abutments) than any other class of teeth. There are 4 central incisors: two upper and two lower.

Central incisors (especially the uppers) have a very pronounced aesthetic impact. Therefore, when designing a bridge that will include central incisors, aesthetics will be the most important factor to be considered.

The upper incisors have a role in the phonation process in some consonant pronunciation.

Tooth	Index	Description	Image
Upper central incisor	2	It has a longer and better-anchored root compared to the lateral incisor.
Lower central incisor	1	It is weaker than the upper and the root is smaller.

Note : Similar teeth located on the same dental arch have similar features. (E.g. right upper canine and left upper canine).

Index calculation

As a general rule, a dental bridge is properly designed when the combined index of all abutment teeth is greater (or at least equal) than the combined index of missing teeth. If this happens, it is considered that the dental bridge is "properly supported".

To determine whether or not the abutment teeth can support a bridge, many dentists employ Ante's rule, which states that "the root surface area of the abutment teeth has to equal or surpass that of the teeth being replaced with pontics".

Basically, Ante's rule and the index calculation (and it is possible that many dentists employ other rules) claim the same important thing : abutment teeth must be stronger than the teeth they aim to support.

Let's take an example: in the image bellow a single tooth is missing : the first lower premolar.

The abutments used : second lower premolar and the lower canine.

Index calculation

Abutment teeth = 10 : second lower premolar (index 5) + the lower canine (index 5)

Missing teeth index = 5 : first lower premolar (index 5)

Abutment teeth index is greater than the index of the teeth to be replaced. That's why this particular bridge has an adequate support.

!!!All abutment teeth should meet the general requirements.

When selecting the abutment teeth, apart from the index calculation, other important factors are involved.