Retainer Classification And Partial Veneer Crown Preparation

Retainer Classification And Partial Veneer Crown Preparation

Define and classify retainers. List the advantages and disadvantages of partial veneer crowns over other types of retainers. What are the factors that come into play in the selection of retainers?

Retainer

Definition

Any type of device used for stabilization or retention of prosthesis – GPT.

Function

Is to support and connect the body of the bridge with the abutment. It also restores the form, function, and esthetics of the abutment.

Read And Learn More: Fixed Partial Denture Short Essay Question And Answers

Classification of Retainers for Fixed Partial Denture

Class 1 – Extracoronal restorations

• Full-coverage crowns
  • All-metal crowns.
  • All-ceramic crown.
  • Metal–ceramic crown.
  • Acrylic-fused-to-metal crown.
  • Metal-free composites.
• Partial-coverage crowns
  • Posterior tooth
    • Mesial one-half crown
    • Three-quarter crown
    • Reverse three-quarter crown
    • Seven-eight crown.
  • Anterior tooth
    • Three – quarter crown
    • Variations of three-quarter crown (Selberg crown).

Class 2 – Intracoronal restorations

• Inlays.
• Onlays.
• Pin ledge.
• Combinations.

Class 3 – Radicular retainers

• Cast core.
• Blue Island posts.
• Para post techniques.
• Kurer technique.

Class 1 – Extracoronal restorations

• The preparation of the tooth and its cast retainer lies externally to the body of the coronal portion of the prepared tooth and restores a tissue-compatible contour for the crown
• The retention and resistance to displacement are developed between the inner walls of the casting and the external walls of the prepared tooth (complete crowns).

Class 2 – Intracoronal restorations

• The prepared cavity and its cast retainer lie within the body of the coronal portion of the tooth
• Retention and resistance are developed between the casting and the internal walls of the prepared cavity.

Class 3 – Radicular retainers

• The dowel type of retention is confined to the root portion
• Retention and resistance to displacement are developed by the extension of an attached metal dowel into the radicular portion of the tooth.

Partial Veneer Crowns

• A partial veneer crown is a restoration covering two or more surfaces of a tooth
• The surfaces usually covered are the lingual, proximal, occlusal, or incisal
• The retention and resistance to displacement depend on the internal surfaces and the auxiliary retentive means, such as grooves, boxing, and pins.

Types of partial veneer

Three-quarter crowns

Covers three-fourths of the gingival circumference of the tooth, leaving the facial surface intact.

Partial Veneer Advantages

• Esthetically pleasing
• Can serve as a single unit or as an FPD retainer
• Can be used on anterior and posterior teeth as retainers.

Reverse three-quarter crown

• This is when the preparation is done on the buccal aspect leaving the lingual surface of a mandibular posterior tooth intact
• Commonly done on mandibular molars with severe lingual inclination.

Seven-eighths partial veneer crown:
Covers seven-eighths of the gingival circumference of the tooth leaving the mesial aspect of the buccal surface intact.

Partial Veneer Indications

• For maxillary molars and premolars with extensive carious involvement or restoration on the distal surface with intact mesial surface
• Used on a mandibular premolar with the preparation extending from the distal finish line to the mid-facial surface.

Mesial half crown
Is a three-quarter crown rotated 90 degrees, preserving the distal surface of the tooth while covering the remaining surfaces.

Mesial Half Crown Indications

• In tilted mandibular molar abutment
• Used as a single retainer in drifted or tipped mandibular molar to alter the occlusal plane.

Mesial Half Crown Contraindication

• If the distal surface of the tooth is damaged by caries or large restoration.

Advantages of partial veneer crowns over complete crowns

• Conservative tooth reduction
• Good esthetics compared to a complete veneer cast crown
• Better biocompatibility with supportive tissues as there is less gingival involvement than with complete coverage
• Due to the supragingival finish line margin, accessibility for finishing and cleaning is good compared to complete crowns
• Easy verification of complete seating of the casting
• Seating is complete compared to complete crowns as the luting agent has space to flow out (less hydraulic pressure)
• Electric pulp testing can be conveniently accomplished on the intact enamel surface
• Reduced pulpal and periodontal trauma during tooth preparation
• Access to supragingival margins is easy and hence, better for oral hygiene as compared to full crowns
• Due to direct visibility, cement removal is easy.

Disadvantages of partial veneer crowns over complete crowns

• Less retentive than a complete veneer crown
• If good preparation is not carried out, there will be a display of metal with the partial veneer crown
• The partial veneer crown preparation can be done only on intact teeth with average crown lengths
• Less resistance forms as compared to complete crowns
• The placement of grooves, boxes, and pinholes requires good skill otherwise difficulty occurs in the seating of casting
• Not recommended in caries-prone patients with poor oral hygiene
• Unseating is common if excessive loads are applied.

Indications of partial veneer crowns over complete crowns

• Intact or minimally restored teeth with intact buccal wall
• To re-establish anterior guidance
• Used as retainers for a fixed partial denture when alteration of the occlusal plane is needed
• To splint teeth
• Can be prepared on teeth with average crown length
• Prepared when teeth have a normal anatomic crown form
• Anterior teeth need to have adequate labiolingual thickness.

Contraindications of partial veneer crowns over complete crowns

• High caries rate (More for partial veneers where both the un-veneered surface and the margin to finish line interface are susceptible to decay
• Teeth with extensive core restorations
• Deep cervical abrasion
• Short teeth
• Teeth that are severely constricted at the cervical require more axial reduction to provide adequate groove length
• In long-span FPD
• Should not be given on endodontically treated posterior teeth if the buccal cusps are weakened by the access cavity, or on teeth with an extensively damaged crown
• Should not be placed on teeth that are proximally bulbous
• Not recommended in thin teeth of restricted faciolingual dimension
• Not recommended in poorly-aligned teeth

In all of the above situations, complete veneer crowns need to be given. When there is difficulty in establishing adequate retention and resistance form, complete veneer restorations are more suitable.

Selection of Types of Retainers

• The selection of the types of retainers depends on the oral hygiene habits or the DMF rate of the patient, or both
• If the caries index is low any type of retainer is safe provided other factors such as periodontal status crown length are all present
• The length of the bridge span and the type of bridge determines the type of retainer selected
• Retainer selection for terminal abutments is a crucial decision.

Retainer selection is affected by the following factors

• Age
• DMF rate
• Edentulous span
• Periodontal support
• Arch position of the teeth
• Skeletal relationships
• Interocclusal and interocclusal conditions, such as crown length
• Existing and projected oral hygiene of the patient
• Vitality of the potential abutment.

Ideal Retainers

• An ideal retainer needs to support and connect the body of the bridge with the abutment
• It should restore the form, function, and esthetics of the abutment
• Should be biocompatible with the tooth and its adjacent tissues
• Retainer should be constructed without injury to pulp and supporting structures
• Should protect and maintain the pulp against thermal and galvanic shock
• Should provide safety for the tooth during the lifetime of the restoration
• Should provide self-cleaning property
• Should be resistant to corrosion and tarnish
• There should be a conservative preparation with uniformity of reduction of the abutment tooth
• Load should be dispersed to the more receptive areas of the abutment
• There should be large surface contact between the abutment and a retainer
• The selection of an adequate luting agent is important.