Material Selection for Direct Posterior Restoratives

Citation: John O. Burgess, Deniz Cakir (2011) Material Selection for Direct Posterior Restoratives. Dental CE Digest (PennWell Publications) (RSS)
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Download: http://www.ineedce.com/courses/2067/PDF/1108cei dentsply Restoratives.pdf
Tagged: Chemistry (RSS)

Summary

Abstract

• type of material selected for posterior restoration (filling of molars) depends on patient/individual situation – new technological advances have been made
  • types of fillings – amalgam (alloy of silver and mercury) or resin (many different varieties of polymers that vary in chemical composition)
  • amalgam – used for long time; successful; however not esthetically pleasing, which has become a high demand
  • esthetic restorations – glass ionomers, compomers, composite resin (all types of resins that just vary in chemical composition)
  • desirable attributes in material - fluoride release (fluoride – strengthens tooth); wear resistance; low polymerization shrinkage (sometimes after filling is completed, the material shrinks allowing bacteria to enter the cavity again); low polymerization stress (the more a material is resistant to stress, the less likely polymerization shrinkage will occur

Introduction

• material selection for posterior teeth restoration depends on: patient’s age, caries (cavity) risk, esthetic requirements, how well the tooth can be isolated, and functional demands of the restoration – each material has certain pros and cons in their usage
  • compomers, glass ionomers, composite resins – pros: bond to tooth structure chemically; may reinforce tooth; long-lasting; non-invasive procedure; esthetic; good thermal insulators; have fluoride release – cons: clinical limitations (requires more attention to detail during adhesive placement; longer time; more difficult procedure in comparison to an amalgam filling); postoperatively – polymerization shrinkage possible as a result of difficult procedure where there can be poor adhesive placement, all of which lead to possible leakages at the tooth surface

Posterior Amalgam Restorations

• history of clinical success
  • good moisture tolerance since it does not bond to tooth structure chemically – not necessary to keep the tooth isolated and in dry conditions like in resin restorations
  • wear resistance – metal alloy – malleable so easily formed into shape of tooth but also strong and durable
  • limitations: galvanism (battery effect occurs because of amalgams composition of two metals, usually silver and mercury, in a liquid medium, saliva – produces electric current which leads to break down of amalgam and corrosion) high thermal conductivity, poor esthetics

• in resin fillings – bonding material must be applied before the resin in order to form a chemical bond between the tooth surface and the resin
  • amalgam does not require a bonding material, but one has been developed called the bonded amalgam technique using adhesives (most successful called “4-META-based Amalgambond Plus (Parkell)”
  • a bonding agent bonds to dentin (second layer of the tooth from outside in; after the enamel which forms the crown visible on the outside) with a hybrid layer
  • bonding resin to amalgam attachment is still mostly mechanical, not chemical
  • amalgam use criticized especially in children and decreased in popularity because it contains mercury – however, after many studies and tests, there have been no significant signs of mercury having a negative affect on health

Fluoride-releasing Materials

• types:
  • glass ionomers – useful as liner/base so for deep cavities
    • release high levels of fluoride
    • low bond strength
    • conditioner or primer is needed to improve bond of ionomer to tooth surface – usually weak inorganic acids – clean tooth surface before bonding
    • low overall strength – paste/paste resin easier to mix and place but these lower the strength of the system (weaker bonding than powder/liquid resin)
    • low wear resistance
    • medium fluoride recharge (ability of tooth to uptake fluoride from the environment and incorporate in the tooth structure)
  • high-viscosity glass ionomers
    • release high levels of fluoride
    • medium bond strength
    • medium overall strength
    • medium wear resistance
    • medium fluoride recharge
  • resin-modified glass ionomers -> nanofillers added – reduce particle size – smoother, more esthetic appearance
    • release high levels of fluoride – increases long-term survival; good for high caries risk patient
    • medium bond strength
    • low overall strength – not ideal for posterior restorative (on molars)
    • low-medium wear resistance – cannot be used if cavity is located at the occlusal surface of the tooth (top surface) because it receives the most stress
    • high fluoride recharge – increases long-term survival; good for high caries risk patient
  • compomers – blends of resin composite and glass ionomer
    • release medium levels of fluoride – between resin composites and glass ionomers – successful on use for children’s teeth – bonding system uses adhesive which blocks fluoride uptake in dentin, thus only releasing fluoride onto the outer tooth surface
    • high bond strength
    • medium overall strength
    • medium wear resistance – ideal for children’s teeth
    • medium fluoride recharge
  • fluoride releasing composites
    • release low levels of fluoride – not good for high caries risk patients
    • high bond strength
    • high overall strength
    • high wear resistance – best of any fluoride-releasing material
    • low fluoride recharge – not good for high caries risk patients

Composite Resin

• pro: improved wear resistance – gaining popularity in usage for posterior restorations as opposed to solely bicuspids (frontal teeth)
• con: composite resin shrinkage during polymerization – causes eventual breakdown and thermal sensitivity
  • visible light cured composite is placed in prepared cavity and light cured in 2mm incredments– photoinitiators in the resin (camphoroquinone, usually in the presence of an amine accelerator/catalyst) are activated (more chemistry detail to analyze here)
  • combo of photoinitiator types may cause problems because they need to absorb different wavelengths of light for their reactivity
  • usually LED lights are used but quartz-tungsten-halogen or plasma arc curing lights polymerize all photoinitiators
  • often the wrong type of light is used leading to low wear resistance in the final cured product
• soft-curing lights – decrease polymerization stress – unproven results
  • slows rate of polymerization with initial low intensity or short pulses of light – allows for polymer chain movement – provides stress relief

Flowable Composites

• composite resins but with a lower viscosity because of lower filler load which allows them to better adhere to the cavity surface – may reduce polymerization stress (no clear consensus based on numerous studies has been reached)
  • con: lower filler load may reduce wear resistance - however percentage of filler may be chosen relative to type of tooth in concern
• even though they have higher polymerization shrinkage than composite resins – flowables are more elastic – provide stress relief - debate is still continued
  • main use: cavity adaptation

Composite Resin Shrinkage and Stress

• in composite resins, polymerization shrinkage 3.7%-0.9%
  • new resin monomers developed to reduce polymerization shrinkage stress
    • Filtek LS –> silorane ring-opening chemistry 0.7%-0.9%
    • N’Durance -> dimer chemistry 1.2%
    • Thiolene polymers (thiolene/thiol epoxy hybrid networks) – not available commercially – 90% less stress than in control dimethacrylate resin
    • C-factor (shape of preparation) – ratio of bonded surfaces to unbounded surface in final restoration – more stress at the margins (cavities located on frontal teeth or near gums) – no ideal solutions

Low stress Composite

• Stress Decreasing Resin (SDR) Technology – reduces internal stress from polymerization shrinkage – instead of 2mm increments of polymerization it uses 4mm increments
  • Polymerization modulator embedded chemically in the resin backbone
    • interacts with the photoinitiator to regulate a slow modulus development while at the same time still allowing a steady rate of polymerization or conversion of the material
    • modulator allows more linear and branching chain propagations and conversions in the polymerization – slower modulus development without increasing cross-linking density – decreases stress
• SDR highly translucent – high light transmission allows for bulk polymerization
  • Used as a base or filler up until enamel layer of tooth - cannot be used on the surface of the tooth because of its low wear resistance (high shrinkage) – highly filled material should be placed on top surface

Case study

• Procedure with photographs of a restoration using SDR technology

Summary

• Each material has pros and cons to its usage - should be personalized to each clinical situation and needs of the patient; there have been many new developments in esthetic composite resins that cause low polymerization shrinkage and low stress, allowing for even more options and therefore accuracy in restorations