The prepared tooth

For a dental crown, there is a single prepared tooth. In case of dental bridges all abutment teeth are prepared. The bridges retainer crowns are cemented on the abutment teeth to ensure proper support.

Not all clinical situations require the preparation of natural teeth. Sometimes, the crown of the tooth is heavily damaged or the tooth is missing altogether. In such circumstances, either the tooth will need a post and core or, if conditions are met, a dental implant is inserted into the jawbone.

Abutment types

As mentioned, the abutment can be a natural tooth, a post and core or a dental implant.

1. Natural tooth abutment

   When a natural tooth is prepared (to support either a dental crown or a dental bridge), a certain portion of the tooth original structure is preserved.

   Nevertheless, the operation involves permanently removing parts of the tooth's original structure, including portions that might still be healthy and structurally sound.

   
   
   

   prepared natural tooth

   
   

   Before starting the tooth preparation, the practitioner must ensure that several conditions are met :

   • The tooth must preserve enough structure to provide the necessary support for the future prosthetic restoration. If the tooth is heavily decayed or the remaining structure is too thin or too short, a post and core or a direct buildup is required.

   • Tooth mobility should be minimal. A slight level of mobility can be accepted if the dental radiography shows no excessive bone loss around the tooth.

   • The tooth must be healthy or properly treated.

   • Opposite teeth should not be overerupted. If opposite teeth are overerupted (see image), the artificial teeth cannot be properly built because of the lack of space.

     

   
   

2. Artificial abutment

   An artificial abutment is used when, for various reasons, the tooth is missing or shows advanced coronal destructions. There are two types of artificial abutments :

   • Post and core

     A post and core is a prosthetic device that is utilized when there is inadequate tooth structure remaining to support the restoration.

     A small rod (the post), usually metal, is inserted into the root space of the tooth and protrudes from the root a couple of millimetres. The post is then used to hold a filling, or a core, in place.

     
     

     

     post and core

     The post can be inserted at the dental office to hold a filling (direct tooth buildup). More often, the post and core is manufactured at the dental laboratory (normally as one single casted piece) after a dental impression (indirect buildup).

     To construct a post and core, a tooth must fulfil several requirements :

     • The tooth must first be endodontically treated.

     • The tooth's root has to be sufficiently long and strong to hold the post.

     • Tooth should not show apical tissues infections or advanced bone loss around it.

     • The tooth mobility should not be excessive

     more about post and core procedure

     
     

   • Dental implant

     A dental implant is a metal device that is surgically inserted into the jawbone in order to replace one or more missing teeth.

     After the surgical procedure, dental implants will fuse with the bone through a biologic process called osseointegration. The process usually takes 4 to 6 months.

     After this period, the dental implant is connected to an artificial abutment, specifically fabricated to fit the bone implant. Broadly, the artificial abutment has the shape and size of a prepared natural tooth.

     
     
     

     dental implant artificial abutment

     The dental prosthesis (crown, bridge, denture) is constructed over the artificial abutment.

Abutment characteristics

Depending on the position inside the mouth cavity, the prepared abutment teeth (both natural and artificial) must have specific dimensions that will allow the dental technician to manufacture a restoration that perfectly fits.

• Shape

  The shape differs depending on the position of the tooth inside the mouth cavity. Usually, after the preparation, abutments have a slightly conical shape with a 3 to 10 degree taper that allow for the restoration to be properly placed on the tooth.

  

  Too much taper will severely limit the grip that the crown has on the prepared tooth, thus contributing to failure of the restoration.

  
  

• Size

  Because of the size difference between posterior teeth (molars) and those located in the front (incisors), in the back part of the mouth, prepared abutments are bigger while in the front part smaller.

  After the preparation, abutments should have adequate dimensions especially when aesthetics is a big concern.

  
  
  

  molar preparation for
  a ceramic fused to zirconia crown

  

  incisor preparation for
  a ceramic fused to zirconia crown

  
  

  When ceramic reconstructions are designed, several layers of porcelain are fused on the metal (or zirconia) frame. If the abutment has excessive dimensions, the dental crown will appear big and unsightly.

  On the other hand, if the abutment is too small it might have a lower resistance or the grip that the crown has on the abutment might be inadequate.

What happens when a dental bridge is designed ?

We will have, of course, several abutments. The general characteristics are the same, with one particular exception: abutments must be prepared parallel to each other because the bridge must be able to fit onto the abutment teeth simultaneously.

when a dental bridge is designed
all abutment teeth are prepared parallel to each other

This condition is especially important when large restorations are designed. A single dental bridge may have both natural abutments and artificial abutments (post and cores).

A bridge designed on dental implants has special characteristics.