Resin Bonded Retainers Overview

Resin Bonded Retainers

• Definition: Single pontic supported by thin metallic retainers placed lingually and proximally on abutment teeth.
• Advantages:
  • Conservative preparation: Confined to enamel, leaving labial surface intact.
  • Tissue tolerant: Supragingival margins minimize trauma.
  • Less traumatic preparation: Reduces pulpal trauma.
  • Aesthetic advantages: Labial surface remains intact.
  • Easy impression taking.
  • Anesthesia often unnecessary.
  • Provisional restorations not required.
  • Re-bonding possible.
  • Reduced cost.
  • Reduced chair-side time.
• Disadvantages:
  • Questionable longevity: Debonding rates increase with time, influenced by preparation design, luting agent, and arch location.
  • Higher dislodgement rates in posterior and mandibular areas due to occlusal forces and isolation difficulty.
  • Cannot restore badly broken down teeth.
  • Greyish appearance in thin, translucent teeth (with metallic retainers, not ceramic).
  • No space and contour correction.
  • Enamel modifications required.
  • No alignment correction.
  • Usually limited to single pontic (except for missing lower central incisors).
• Indications:
  • Replacement of congenitally missing teeth or teeth lost through trauma, especially in young patients.
  • Replacing missing anterior teeth in children and adolescents, especially when conventional prosthesis is contraindicated due to management problems, plaque control, and large pulp size.
  • Replacing one or two maxillary incisors in an open-bite, edge-to-edge occlusion.
  • Short spans and open bite situations.
  • Replacing a single posterior tooth.
  • Significant crown length (2-3 mm occluso-gingival height) for adequate bonding surface area.
  • Unrestored and caries-free abutment teeth.
  • Sufficient enamel on abutment teeth for etching.
  • Medically compromised and adolescent patients.
  • Splinting periodontally compromised teeth.
  • Stabilizing dentition after orthodontic treatment.
  • Excellent moisture control.
• Contraindications:
  • Insufficient enamel (hypoplasia, demineralization).
  • Very thin or translucent anterior teeth (causing grayness).
  • Unaesthetic abutment teeth.
  • Abutments with short clinical crowns, extensively restored or damaged teeth.
  • Long edentulous spans, as they produce greater stresses on the casting and adhesive bond.
  • Edentulous spaces requiring adjustment.
  • Deep vertical overlap, as removing enamel from maxillary incisors for clearance might compromise retention.
  • Nickel sensitivity (as most restorations are Ni-Cr).
  • Space problems like diastema.
  • Parafunctional habits (bruxism, deep-bite). These habits place excessive forces on the bridge, and resin-retained bridges have lower displacement resistance than conventional FDPs.

Fabrication

• General principles of successful resin-bonded retainer design:
  • Creating a distinct path of insertion by modifying enamel contours of abutment teeth.
  • Resistance to displacement in any direction by occlusal forces.
  • Framework design should limit stresses on luting cement and bonded interface.
  • Axial reduction and guide planes on the proximal surface with a faciolingual lock.
  • Maximum coverage of virginal enamel (180° wrap-around) for retention and bonding area.
• Posterior tooth preparation:
  • Retainer design with occlusal rests, retentive surfaces, and proximal wrap to resist occlusal and torquing forces.
  • Mandibular molars require a single-plane lingual wall reduction.
  • Maxillary molars require two-plane reductions due to occlusal function and cusp taper.
• Finish line:
  • Chamfer finish line (0.3-0.5 mm), 1 mm away from gingival margin (supra-gingivally).
  • Knife-edge interproximal finish line to avoid enamel penetration.
  • 180° wrap-around extending buccally beyond the distobuccal line angle in the mesial abutment and mesiobuccal line angle in the distal abutment for resistance to lingual displacement.
  • Proximal walls should be parallel or have slight taper.
• Proximal grooves:
  • 0.5 mm depth to resist torquing forces.
  • Two parallel grooves: One near the facioproximal angle adjacent to the edentulous space and one at the opposite lingoproximal corner.
• Occlusal seat:
  • Extend on the cuspal slope of the lingual cusp.
• Mandibular first premolar preparation:
  • Placement of a rest seat would leave insufficient solid tooth structure in the lingual cusp.

All-Ceramic RBB

• High-strength, non-etchable ceramics like zirconia are preferred.
• Surface treatment of ceramics:
  • Zirconia requires sandblasting or Rocatec system due to its un-etchable nature.
  • Manufacturers have developed primers for bonding resin cement to zirconia, un-etchable ceramics, and metals.
  • Z-Prime Plus enhances bond strength with unique MDP phosphate monomer composition.
  • Research supports the long-term bonding of MDP phosphate monomers to zirconia, whereas silanes do not contribute to adhesion.
• Surface conditioning:
  • Increases the critical surface energy for adhesion.
  • Uses silanes or primers.
• Silane coupling agents:
  • Polymerize with hydrophobic resin-composite monomers.
  • Bond with hydrophilic inorganic hydroxyl-rich (-OH) surfaces (silica and silica-coated surfaces).
  • Phosphate-based primers are effective in bonding to zirconia.
• Surface Treatment of Tooth Structure:
  • Acid Etching technique: Tooth is cleaned with pumice, washed, and dried. Etched with 37% phosphoric acid for 30 seconds, washed, and dried.
  • Laser Etching technique: Tooth is cleaned with pumice, isolated, and dried. Laser initiator is applied. Etched with laser for 60 seconds. Excess initiator is removed, and the tooth is washed and dried.
• Bonding Resin:
  • Conventional resin cement: Modified unfilled/filled composite resin with thin film thickness. Sets within 60-90 seconds under the casting, but not at the margins exposed to air.
  • Light-cure the margins or apply Oxyguard II to exclude air. Rinse away Oxyguard II after 2 minutes and remove residual cement.
  • Major finishing, polishing, and occlusal adjustments should be performed before bonding.

Causes of Failure of Resin Bonded Retainers

• Improper patient selection:
  • Gender: Masticatory forces are stronger in men, but gender doesn't seem to affect longevity.
  • Parafunctional habits: Parafunctional habits and occlusal interference can lead to higher debonding rates. Bruxism is a significant stress factor.
  • Span length: Resin-bonded bridges are limited to single tooth missing. Increasing the number of pontics significantly decreases longevity.
  • Existing old restorations or caries: Class III composite restorations should be replaced to promote adhesion. Large class IV caries or old composite restorations have questionable prognosis with resin-bonded restorations. Full coverage restoration is more advisable for long-term performance.
  • Improper tooth alignment (poor path of insertion).
  • Insufficient vertical length of the abutment.
  • Insufficient enamel for bonding.
• Improper alloy selection: Base metal alloys like Nickel Chromium or Cobalt-Chromium are preferred over gold alloys due to their higher modulus of elasticity.
• Improper tooth preparation:
  • Insufficient lingual and proximal reduction.
  • Incomplete 180° wrap-around extension.
  • Lack of proximal grooves.
  • Lack of accommodation to mandibular excursion (protrusion).
• Failure of Bonding Procedure:
  • Contamination.
  • Inappropriate luting cement.
  • Incorrect manipulation of cement.