Crown and Bridge: Types and Crown Preparation features

Introduction

A crown is provided to protect the weakened natural tooth, regain its shape, maintain or enhance the esthetics and regain the functional ability of a person. A bridge is a prosthesis that is constructed to replace the one or more missing teeth and is supported by the neighbouring teeth.

Tooth showing a tooth preparation for full crown
A posterior tooth prepared for full crown (pink area)
B-Buccal aspect, L-Lingual aspect

Types

They can be constructed in variety of the materials as follows:

  1. All Metal
  2. Porcelain fused to metal
  3. All ceramic material
  4. Pressed ceramic
  5. All acrylic

There are various types of crown based on the surfaces they cover.

  1. Full coverage crown
  2. 3/4 crown
  3. 4/5 crown
  4. Inlay
  5. Onlay
All metal includes precious metals like gold alloys, semi-precious that contains lesser amount of gold and non-precious alloys that are basically chrome- cobalt or nickel- chrome alloys.

Porcelain fused to metal crowns and bridges are made in any one of the above mentioned alloys. This metal crown, in the later phase of construction, is sintered with ceramic powders and chemo-mechanically fuses with metals in ceramic furnace.

“All ceramic” crown is made in Ceramic only. Aesthetically they are the best. Their con is that they are not as strong as metal ceramic or pressed ceramic.

A variety of factors have to be considered for the construction of a bridge.

  1. Span of the edentulous area
  2. Type of the teeth being replaced
  3. Quality of supporting teeth
  4. Conditions of the teeth in the opposing arch
  5. Age of the patient
  6. Patients’ ability to bear the treatment cost

Span of the edentulous area means how many teeth have to be replaced by bridge. The more number of teeth to be replaced, the longer will be the bridge; that in turn, will cause more stress in the bridge itself as well as onto the supporting teeth known as abutments. More stressed bridge will get more fractured ceramic from the metal part.

Type of the teeth decides as to how much masticatory load will have to be borne by the abutments. In case of the lateral incisor pontic, supported by one central incision and one canine, the masticatory load over abutments will be minimum. As we move from the Canines through Premolars to Molars, the quantity of masticatory load increases. Accordingly, we need to increase the number of abutments.

Quality of supporting teeth means how healthy the attachment apparatus of the supporting teeth is present. The healthier the apparatus, the better will be the masticatory load bearing ability of the abutments. It will result in a relatively longer life span of the bridge with minimum number of abutments. If the available healthy root surface area of the abutments will be less than the root surface area of the tooth or teeth being replaced, then more number of teeth should be used as abutment to prevent the overloading of any particular abutment tooth and ultimately bridge failure.

Conditions of the teeth present in the opposite dental arch includes the considerations of occlusal surface of the tooth and whether it is extruded. Is it causing mandible deviation during chewing?

Age wise a very young patient cannot be given crown and bridge as their pulp chambers are usually large and the trauma caused by heat during crown preparation or acids of luring agents may render the pulp tissue dead. This may lead to pulpal or periapical pathology ultimately leading to the failure of the bridge.

Crown Preparation Features

Preparation features for anterior crowns [1]

Treatment cost of the PFM bridges is very high. Therefore, the patient must understand the importance of the maintenance of the oral hygiene for the longer life of the prosthesis. The patient should be able to bear the cost of this treatment, otherwise a less expensive treatment needing easy maintenance should be chosen.

Preparation features of the posterior crowns [1]





Further Reading: Read BDJ article by clicking at the link

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