What do we demand from our cements?

Biocompatible: From reading the previous sections and working on clinic, we know the oral environment is continuously changing and can test the materials we place. If they leach too much, we don't want them causing irritation to the surrounding tissues, or become toxic. We have previously mention lichenoid reactions, and so, not all our materials cover this fundamental principle. A good history also is required to note if patients have a known allergy. A bonus of the cement would be to achieve bactericidal properties when placed and in the GIC lesson we know they have the benefits of releasing fluoride.

Resilience: To serve its purpose, the cement needs to resist compressive forces, but also, in a sense, more importantly, resist tensile and sheer forces. The material needs to be insoluble as we know the mouth is very wet, and have the ability to resist fracture both cohesively (within the material) and at the tooth restorative interface. The ideal would be it is both mechanically bonded and chemically adhered to the tooth.

Aesthetics: This is emphasised with anterior ceramics and composites.

Easy to work with: A cement needs to be <25microns for an indirect restoration to seat fully. Therefore, we need to be able to mix accurately, and have ease of placement to achieve such a thin layer. Ideally we could command set the cements but the opacity of some restorations will prevent this occurring.

Cements:

The Zinc Oxide and Acid-Base Cements:

Zinc Oxide Eugenol:

• Zinc Oxide + Eugenol (the classic smell of the dentist). The addition of zinc acetate helps to speed up the setting reaction time.
  Hydrolysis reaction (acid-base) forming an amorphous gel and crystallisation occurs as the material matures.

Considerations:

• Incompatible with Resin - Eugenol reacts with camphorquinone (the light imitator in resin) and weakens the resin bond.
• Soluble - micro-leakage
• Direct pulp contact = cytotoxic and risk of necrosis.
  

The Good:

• Provisional cement (e.g. TempBond) - but not ideal if a resin is planned to be used as the definitive choice of cement. (seen as a soft cement)
• Reinforced with Ortho-ethoxybenzoic acid (EBA) , aluminium oxide and polymethacrylate = IRM
• Anti-inflammatory properties (dressing "sedative")
• Can be used in Endodontics
• Modified to be used as a impression material

Zinc Phosphate and Zinc Polycarboxylate set via an acid-base reaction which is exothermic. Mixing is carried out on a cool glass slab to aid working time. When reactions are exothermic, consideration of the pulp proximity should be in mind.

Zinc Phosphate:

Zinc Oxide + Orthophosphoric acid

Considerations:

• Acidic when setting (low pH 3.5) a consideration of pulp insult is required, as low pH occurs on set, potentially infiltration into the dentine tubules causing post cementation sensitivity. Dentine dissociation (over drying) can exacerbate this process.
• Vulnerable on set to solubility and therefore immediate micro leakage - requires good moisture control. When the material matures this becomes less. However, the material is vulnerable if exposed to pH < 4.5 (erodes). Does the patient suffer from a form of reflux e.g. GORD?
• No chemical bond = Friction Fit. This material requires your traditional preps with long axial walls and minimal taper (2-20^o) optimum being around 10^o. Note: the convergence on our burs are around 6^o. More detail about preps will be discussed in the indirect restorative lessons. Therefore, a more destructive prep is required. Long "traditional" preps helps to decrease the amount of tensile and shear stress through the material, which is good because zinc phosphate doesn't do well under these forces. BUT, again, think increased insult to the pulp.
• Opaque

The Good:

• Can produce a thin layer if mixed well - allows full seating of restoration. In addition, having a thin layer decreases the risk of cohesive failure. Note: continuous pressure required throughout setting phases due to hydrostatic pressure push back.
• Good working time
• Compressive strength adequate
• Fluoride can be added to the above cements, but it can weaken the cement matrixes, potentially more so than the cariostatic beneficial affects.

Zinc Polycarboxylate:

Zinc Oxide + Polyacrylic acid (e.g. Poly F)

Considerations:

• Lower compressive strength compared to Zinc Phosphate
• Vulnerable up to 24hrs during set - solubility decreases with time.
• Vulnerable to erosion in low pH
• Rubber phase on set. The pressure must be consistent and maintained to protect against distortion during the cements setting.
• Opaque

The Good:

• Higher molecular weight; therefore, won't infiltrate the dentine tubules.
• Bond very well to metal - Oxide bond. This is why we are reminded to wipe our instrument because once bonded it is extremely difficult to remove. Therefore, failure in the mouth is usually seen cohesively, and not at the cement restoration interface.
• Bonds to the tooth - chelating calcium like GIC. This is thought to seal the tubules and decrease the likelihood of sensitivity. Again failure seen within the cement not at wider interface
• Shear thinning is thought to occur, allowing the material to rebound
• Longer term provisional cements

Glass Ionomer Cements:

Glass Ionomers are also acid-base reactions. They contain polyacrylic acid but the major particle constituent is calcium fluoroaluminosilicate. Below is the considerations to take when using GIC as a cement. More detail regarding GIC is provided in the direct restoration lesson.

• Fluoroaluminosilcate glass + polyacrylic acid (e.g. AquaCem)

Considerations:

• Vulnerable to wash out on set - varnish protection.
• Crack if desiccated (over dry) - cohesive failure
• Water sorption at margins - loss of integrity particularly if not protected in the early phases.
• Low pH - hypersensitivity of the pulp - especially if dentine desiccation has occurred
• Allow sufficient time before finishing procedures (see manufacturers guidelines but will usually be around 3-6mins required)

The Good:

• Higher compressive strength compared to the Zinc based cements
• Chemical bond to the tooth
• Release fluoride
• More translucent than the zinc based cements.
• Conventional crown cementation

Hybrid - RMGIC

RMGIC (e.g. Rely X luting cement) were designed to take the benefits of both GIC materials and Resin composites to overcome their limitations. They incorporate acid-base reactions alongside polymerisation on setting; in areas where light can't reach, radox reactions occur (dark curing). However, many difficulties are faced with these materials and unfortunately they have incorporated new limitations in their creation. As restorative materials, as you would have seen in the previous lesson, they may not offer a great deal of advantages compared to GIC; however, as cements they become more useful.

Considerations:

• HEMA - Expansion with water sorption - not advised to use with non reinforced ceramics (there is some data that shows this is more anecdotal than hard evidence when used with full ceramic crowns)
• HEMA is cytotoxic - think pulp!
• Care not to desiccate as the GIC constituents will crack.

The Good:

• Resist water solubility
• Greater resistance to acid erosion (compared to the zinc based cements)
• Increased initial strength
• Good bond to metal
• Bonds to the tooth structure but true adhesion may not be as effective as GIC. The hydrophilic nature of HEMA does allow for an adequate dentine bond.
• Better aesthetics than GIC
• Option for command set
• Seals dentine and provides a good marginal seal
• Less post -op sensitivity compared to GIC
• Fluoride release
• Conventional crown cementation - in those with a history of high caries rate.

Resin Cements:

Soft:
Trial and temporary options. These are usually created by using a hydrophilic resin e.g. HEMA. Care must be taken not to bond these materials too effectively if carried out a dentine bonding procedure (see below). This will create too strong a bond and difficulty in removal of the temporary.

Hard:
Resin cements differ from their restorative materials mainly in the filler used/ amount loaded. This allows a considerable decrease in the viscosity and handling of the material.

Reminder of Resin Constituents:

• Monomer: After polymerisation forms the resin matrix e.g. BisGMA, UDMA (MA: methacrylate)
• Diluent: decrease the viscosity for ease of handling e.g. TEGDMA
• Filler: Fine inorganic molecules
• Coupling Agent: Bonds the filler to the matrix e.g. Silane
• Radiopacifiers: allows to be viewed on a radiograph: Barium, Strontium - Radiopacity required = equvilency of 2mm aluminium
• Pigmentation: Alter shade e.g. metal oxides: Iron, Titanium.
  
  Note:
• Dentine bonding agents = DBA - (e.g. HEMA, 4-META, MDP)

Resin Cement Properties:

Considerations:

• Sub-gingival margins - initial reaction of resin needs meticulous moisture control
• Highly technique sensitive procedures
• Polymerisation at the margins can cause microleakage
• Increase sensitivity to pulp (infiltrate tubules)
• Colour stability of dual cured systems should be considered in aesthetic zone with translucent ceramics (change over time)
• Some materials produce too great a cement thickness.
• Flash - ensure this is cleared before curing.

The Good:

• Better tensile strength (resist the forces working against the path of insertion and those causing shear stresses)
• Good compressive strength
• Strong enamel bond
• Adequate dentine bond
• Less destructive tooth preparations required due to bonding systems
• Less soluble compared to other cements
• Seen to strengthen some indirect restorations e.g. ceramic veneers
• Seal dentine tubules effectively (infiltrate)

The systems in place to obtain a high quality resin bond and seal are discussed below.

Resin Bonding:

These systems have allowed huge advancements in the modern day dentist world. The advantages of bonding to a tooth means, less destructive preparations and the ability to restore a tooth with less favourable retentive features a convention cement would require. However, we cannot simply rely on these advancements and allows seek to maximise retention (within reason) and consider the occlusal dynamics our restoration may be exposed to.

Enamel:

Enamel is produced by ameloblasts by secreting the matrix via Tome's processes. This creates the prismatic structure of enamel rods and inter-rods. This is a highly mineralised material consisting of 96% hydroxyapatite with the remaining constituents being organic proteins and water. Enamel is full of irregularities and for successful bonding we need to maximise this surface area and infiltrate, locking our materials in place.

Explain the principles: Optimisation of Micromechanical bonding:

• Etch: Increase surface area by dissolution of the surface of enamel. Removal of saliva pellicle.
• Prime: Improves wettability and allows greater infiltration of the material.
• Bond: Resin applied and when cured, chemically bonds to the prime constituents and locks into the enamel irregularities.

Overview of Bonding techniques:

• Etch: Commonly used etch is 37% phosphoric acid (gel or liquid form). Decalcification of the enamel occurs maximising the irregularities. The etching process is affected by the quality and quantity of the enamel. Poor quality enamel will be seen in conditions such as MIH, where there is less mineral to the structure. The quality of the enamel is also different in those showing fluorosis- fluorapetite is more resistant to etchant methods. Deciduous teeth also have very dense enamel and more resistant to acid etch. Care must be taken not to over-etch or over-rinse the enamel during this process. Over etching can cause a detrimental effect to the structure creating larger defects resulting in enamel clefts.

• Adhesive primer: This agent infiltrates the now cleansed enamel surface and spreads the resin through the irregularities. This material has a low surface tension, therefore, as such is "pulled" towards the enamel and maximises resin penetration.

• Bond/ Cement: This is our composite resin which has fine particles and lower loads. The monomers on curing can now form chemical bonds to the infiltrated resin, and surface interlocking occurs. Remember polymerisation of resins is anaerobic and the oxygen inhibition layer can be cured though a glycerol gel, or oxygaurd is used at the margins if using a dual cured or chemical cured method.

Dentine:

Dentine is produced by odontoblasts. It is less mineralised (70%) and contains more organic material. It is a highly dynamic living tissue. It can react and repair, as the odontoblast processes within its tubules sense the hydrostatic pressure changes (e.g. thermal, osmolarity changes). Dentine can be variable with tertiary dentine and sclerotic areas, in addition to the continuous change seen with age (pulp retreats with secondary layering). The more we prepare our tooth, the greater the insult to the pulp, with increased dentine tubule exposure.

Bonding to a material that is living, more water dense, and the presence of collagen fibres is difficult with hydrophobic materials, some of which are directly toxic to the pulp tissue. But there are ways.

Optimise Micromechanical bonding - clearance of the smear layer creates the dentine resin hybrid layer.

• Conditioning: Smear layer removal
  Total etch technique - demineralise via acid removal leaving exposed dentine and collagen. Care not to over etch. Common conditions used are: 10% phosphoric acid, 17% EDTA (maleic, or citrus acid)

• Priming: These are Dentine Bonding agents (DBA)
  Amphiphilic or Bifunctional methacrylates are used; therefore, at one end they have hydrophilic groups that are able to interact with the dentine, and the other hydrophobic groups that will bond with the resin. Infiltration is assisted with acetone (solvents) that displace water. DBA are also seen to react with the calcium in dentine and with the addition of glutaraldehyde, interact with the collagen.
  Common agents use: HEMA, 4-META, MDP

• Bonding:A low viscosity resin which can be polymerised via light, chemical or dual. We now have a dentine resin complex known as the hybrid layer.

Bond Systems (Generations):

During my reading in preparation for this lesson I have personally found it easier to look at the systems as 3 stage, 2 stage and 1 stage; rather than listing generations 1 through to 8. If you wish, you can find more depth in the reading material listed below.

3 Stage System:

Etch + Prime + Bond (as mentioned above)

• Smear layer clearance
• Thought to produce optimal performance
• Etch differs for enamel and dentine. May require a separate enamel etch procedure.
• Highly demanding process increasing risk for clinician error.

2 Stage System:

Etch + (prime/bond) = "bottle bond" system

(etch/prime) + bond = "self etch primer" system

• Easier to use - less steps - quicker
• Self etch primers appear to obtain a better seal than the bottle bond system.
• Inclusion of the dissolved smear layer is thought to produce a more effective hybrid layer and less post op sensitivity.
• Self etch systems are not compatible with dual cured or chemical cured resins. The acidic monomers of the self etch primer interact with the amines of the chemical cured resins, disrupting the curing set.

1 Stage System:

Self etch and bond

• This system is not appropriate for load bearing areas due to the inferior polar bonds obtained.

Surface treatment, alongside bonding specifics surrounding the indirect restorative interface will be discussed in the future lessons.