Polyalkenoate Cements

-Glass ionomer cements and resin-modified glass ionomer cements

-water-based, plastic direct dental restorative cement formed from an acid-base reaction between a poly-alkenoic acid and ion-leachable calcium fluoro-aluminasilicate glass particles with strontium for radioopaqueness • Versatile materials • Good biocompatibility • Chemical cure, with no shrinkage stress • Light curing versions are available • Self adhesion to enamel and dentine

• Restorations (but not in load bearing areas unless using a sandwich technique) • Luting (dental cement connecting underlying tooth structure to a fixed prosthesis) • Bonding/conditioning • Protection • Suited for minimally invasive procedures • Deep carious lesions • Fissure sealing • Protection of root surfaces against caries

• Poly-alkenoic acid (such as polyacrylic acid, polyitaconic acid or polymaleic acid or copolymers) • This is the liquid phase (acid) • Glass powder: Ion leachable calcium fluoro-aluminosilicate glass particles • This is the base • Tartaric Acid • Water • Heavy metals such as strontium added for radioopacity

• Calcium fluoroalumino silicate glass particles • Acid soluble, ions are leached out • Silica affects transparency • Alumina affects opacity and setting time and can increase the compressive strength of the cement • Calcium fluoride- fluoride ions reduce fusion temperature, increase the strength of the cement and have a therapeutic effect -Structured ionically as a tetrahedral complex with centrally located aluminium ion and closely localized alkaline earth cations (sodium, potassium, calcium and strontium) to maintain electronegativity

• Poly-alkenoate acids • Polyacrylic acid • Polymaleic acid • Polyitaconic acid OR -Poly acrylic co maleic acid Poly acrylic co itaconic acid

• Improves handling • Extends the working time • Sharpens the set

-Acid-base neutralisation reaction

-Acids dont behave like acids in the absence of water

Dissolution • Outer layers of the glass particles are attacked by the acid • Release of aluminium, fluoride, calcium, sodium and strontium ions -Leads to diffusion based adhesion between the glass and matrix

Gelation and Hardening • Initial setting (4-10 mins) • Divalent Calcium ions bind to the caboxylate groups, forming a clinically hard surface • Calcium ions also able to chelate with carboxyl groups • Silicaeous hydrogel is formed, maturing over 24hs with further cross-linking with aluminium ions • Process can cause a volumetric shrinkage of up to 3% • Surface protection at this stage is advisable to ensure regulation of the movement of water in and out of the hydrogel, therefore permitting maturation to occur

Hydration • Hardening can take over a week • Aluminium ions become involved and aluminium polyacrylates are formed • 4-6 months • Improvement of properties

• Generally supplied as powder and liquid constituents • Ratio of powder to liquid produced on hand-mixing is susceptible to operator induced variability -Powder and liquid phase OR -Powder and acid (freeze dried) and water OR -In capsules: pre-dispensed amount of powder and liquid phase • Encapsulated systems marketed to overcome this variation • Allows accurate dispensation of powder and liquid • Also allows uniform proportioning and mixing resulting in a homogenous cement • Initial viscosity of the paste in the encapsulated systems can affect physical properties

• Provides ion transport • Acid base setting reaction • Fluoride release • Bound into the set cement • Affects ultimate stability

• GICs susceptible to hydration and dehydration during the early stages of setting • Early water contamination: Loss of polyacrylate chains, Al3+ ions may diffuse out, absorption of water, loss of translucency and loss of physical properties • Dehydration: Cracking and fissuring of the cement, softening of the surface and loss of matrix forming ion

• GIC polyacid displaces phosphate and divalent Calcium ions from the hydroxyapetite and these ions become incorporated into the GIC matrix, which sets • pH rises and re-precipitation of the minerals at GIC-tooth surface occurs forming an ion-enriched layer • Strongly attached to the tooth

-Hydrogen bond formation or metallic ion bridging between carboxyl groups of the polyacid and collagen

• Bulk placement possible • Does not require bonding agents • Appears as dense and opaque, due to phase separation of glass and mismatch of refractive index • Phase separation can be overcome by changing the Al, Ca and F content but this affects the strength adversely • Fluoride release: GICs release fluoride and it is believed that this increases caries resistance • Low solubility once set • Brittle with low fracture toughness

• Adheres to tooth enamel and dentine via an ion exchange mechanism • Therapeutic action due to fluoride release • Acceptable aesthetics • Low solubility after full maturation • Can be used in areas of poor moisture control

• Low fracture resistance • Susceptible to early water contamination • Short working time • Long setting time

-10% polyacrylic acid for 10 seconds • Removes smear layer and exposes calcium and phosphate ions on the mineralized surface • Increases the surface energy of the tooth surface to allow improved wettability of the GIC as it is placed on the tooth surface

-Setting via polymerisation and neutralisation -Same chemistry as a conventinal GIC with addition of HEMA (hydroxyethylmethacrylate- an ampiphilic molecule that can be photopolymerized) BisGMA and other photoinitiators -Command set achieved by the introduction of the resin matrix

• Primarily due to the acid base reaction • Rapid set due to polymerisation of 2-HEMA • Acid-base reaction continues after the preliminary hardening due to the polymerisation

• Similiar to GICs • Early stage desiccation (removal of water) is reduced • Compressive, tensile and shear bond strengths are higher in comparison to GICs -2-HEMA is cytotoxic, so unreacted HEMA can be cytotoxic to the pulp tissues and osteoblast • Adhesion of GIC allows conservative cavity prep • Isolation of GIC in early stages is advocated, use of varnishes such as Copal • In case of pulpal exposure or a very thin intervening dentine layer, a lining material should be used as it can cause necrosis and inhibit calcific repair