Dental cement

Dental cements have a wide range of dental and orthodontic applications. Common uses include temporary restoration of teeth, cavity linings to provide pulpal protection, sedation or insulation and cementing fixed prosthodontic appliances.^[1] Recent uses of dental cement also include two-photon calcium imaging of neuronal activity in brains of animal models in basic experimental neuroscience.^[2]

Traditionally cements have separate powder and liquid components which are manually mixed. Thus working time, amount and consistency can be individually adapted to the task at hand. Some cements, such as glass ionomer cement (GIC), can come in capsules and are mechanically mixed using rotating or oscillating mixing machines.^[3] Resin cements are not cements in a narrow sense, but rather polymer based composite materials. ISO 4049: 2019^[4] classifies these polymer-based luting materials according to curing mode as class 1 (self-cured), class 2 (light-cured), or class 3 (dual-cured). Most of the commercially available products are class 3 materials, combining chemical- and light-activation mechanisms.

• High biocompatibility – zinc phosphate cement is considered the most biocompatible material with a low allergy potential despite the occasional initial acid pain (as a consequence of inadequate powder/liquid ratio)
• Non-irritant – polycarboxylate cement is considered the most sensitive type due to the properties of polyacrylic acid (PAA).
• Antibacterial properties to prevent secondary caries
• Provide a good marginal (bacteria-tight) seal to prevent marginal leakage
• Resistant to dissolution in saliva, or other oral fluid – a primary cause of decementation is dissolution of the cement at the margins of a restoration
• High strength in tension, shear and compression to resist stress at the restoration–tooth interface.
• High compressive strength (minimum 50 microns acc. to ISO 9917-1)
• Adequate working and setting time
• Good aesthetics
• Good thermal insulation properties as a liner under metal restorations
• Opacity – for diagnostic purposes on radiographs.
• Low film thickness (maximum 25 microns acc. to ISO 9917-1).
• Low allergy potential
• Low shrinkage
• Retention – if an adhesive bond occurs between the cement and the restorative material, retention is greatly enhanced. Otherwise, the retention depends on the geometry of the tooth preparation.^{[5][page needed]}

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Dental cements can be utilised in a variety of ways depending on the composition and mixture of the material. The following categories outline the main uses of cements in dental procedures.

Unlike composite and amalgam restorations, cements are usually used as a temporary restorative material. This is generally due to their reduced mechanical properties which may not withstand long-term occlusal load.^[3]

• Glass ionomer cement (GIC)
• Zinc polycarboxylate cement
• Zinc oxide eugenol cement
• Resin-modified glass ionomer cement (RMGIC)

Amalgam does not bond to tooth tissue and therefore requires mechanical retention in the form of undercuts, slots and grooves. However, if insufficient tooth tissue remains after cavity preparation to provide such retentive features, a cement can be utilised to help retain the amalgam in the cavity.

Historically, zinc phosphate and polycarboxylate cements were used for this technique; however, since the mid-1980s composite resins have been the material of choice due to their adhesive properties. Common resin cements utilised for bonded amalgams are RMGIC and dual-cure resin based composite.^[3]

When a cavity reaches close proximity to the pulp chamber, it is advisable to protect the pulp from further insult by placing a base or liner as a means of insulation from the definitive restoration. Cements indicated for liners and bases include:

• Zinc oxide eugenol
• Zinc polycaroxylate
• Resin-modified glass ionomer cement (RMGIC)

Pulp capping is a method to protect the pulp chamber if the clinician suspects it may have been exposed by caries or cavity preparation. Indirect pulp caps are indicated for suspected micro-exposures whereas direct pulp caps are place on a visibly exposed pulp. In order to encourage pulpal recovery, it is important to use a sedative, non-cytotoxic material such as setting calcium hydroxide cement.

Luting materials are used to cement fixed prosthodontics such as crowns and bridges. Luting cements are often of similar composition to restorative cements; however, they usually have less filler, meaning the cement is less viscous.

• Resin-modified glass ionomer cement (RMGIC)
• Glass ionomer cement (GIC)
• Zinc polycarboxylate cement
• Zinc oxide eugenol luting cement

Cements are classified on the basis of their components. Generally, they can be classified into categories:

• Water-based acid-base cements: zinc phosphate (Zn₃(PO₄)₂), zinc polyacrylate (polycarboxylate), glass ionomer (GIC). These contain metal oxide or silicate fillers embedded in a salt matrix.
• Non-aqueous/oil base acid-base cements: zinc oxide eugenol and non-eugenol zinc oxide. These contain metal oxide fillers embedded in a metal salt matrix.
• Resin-based: acrylate or methacrylate resin cements, including the latest generation of self-adhesive resin cements that contain silicate or other types of fillers in an organic resin matrix.

Cements can be classified based on the type of their matrix:

• Phosphate (zinc phosphate, silicophosphate)
• Polycarboxylate (zinc polycarboxylate, glass ionomer)
• Phenolate (zinc oxide eugenol and ethoxybenzoic acid [EBA])
• Resin (polymeric)^[4]

Based on time of use:

• Conventional (zinc phosphate, zinc polycarboxylate, zinc oxide eugenol, glass ionomer cement)
• Contemporary (resin cements, resin-modified glass ionomers).

These cements are resin-based composites. They are commonly used to definitively cement indirect restorations, especially resin bonded bridges and ceramic or indirect composite restorations, to the tooth tissue. They are usually used in conjunction with a bonding agent as they have no ability to bond to the tooth, although there are some products that can be applied directly to the tooth (self-etching products).

There are three main resin-based cements:

• Light-cured – required a curing lamp to complete set
• Dual-cured – can be light cured at the restoration margins but chemically cure in areas that the curing lamp cannot penetrate
• Self-etch – these etch the tooth surface and do not require an intermediate bonding agent

Resin cements come in a range of shades to improve aesthetics.^[8]

1. Fracture toughness
   • Thermocycling significantly reduces the fracture toughness of all resin-based cements except RelyX Unicem 2 AND G-CEM LinkAce.
2. Compressive strength
   • All automixed resin-based cements have greater compressive strength than hand-mixed counterpart, except for Variolink II.^[9]

Zinc polycarboxylate was invented in 1968 and was revolutionary as it was the first cement to exhibit the ability to chemically bond to the tooth surface. Very little pulpal irritation is seen with its use due to the large size of the polyacrylic acid molecule. This cement is commonly used for the installation of crowns, bridges, inlays, onlays, and orthodontic appliances.^[10]

Composition:

• Powder + liquid reaction
• Zinc oxide (powder) + poly(acrylic) acid (liquid) = Zinc polycarboxylate
• Zinc polycarboxylate is also sometimes referred to as zinc polyacrylate or zinc polyalkenoate
• Components of the powder include zinc oxide, stannous fluoride, magnesium oxide, silica and also alumina
• Components of the liquid include poly(acrylic) acid, itaconic acid and maleic acid.

Adhesion:

• Zinc polycarboxylate cements adhere to enamel and dentine by means of chelation reaction.

Indications for use:

• Temporary restorations
• Inflamed pulp
• Bases
• Cementation of crowns^[8]

Zinc phosphate was the very first dental cement to appear on the dental marketplace and is seen as the “standard” for other dental cements to be compared to. The many uses of this cement include permanent cementation of crowns, orthodontic appliances, intraoral splints, inlays, post systems, and fixed partial dentures. Zinc phosphate exhibits a very high compressive strength, average tensile strength and appropriate film thickness when applies according to manufacturer guidelines. However, issues with the clinical use of zinc phosphate are its initially low pH when applied in an oral environment (linked to pulpal irritation) and the cement's inability to chemically bond to the tooth surface, although this has not affected the successful long-term use of the material.^[10]

Composition:

• Phosphoric acid liquid
• Zinc oxide powder

Formerly known as the most commonly used luting agent, zinc phosphate cement works successfully for permanent cementation. It does not possess anticariogenic effects, is not adherent to tooth structure, and acquires a moderate degree of intraoral solubility. However, zinc phosphate cement can irritate nerve pulp; hence, pulp protection is required but the use of polycarboxylate cement (zinc polycarboxylate or glass ionomer) is highly recommended since it is a more biologically compatible cement.^[11]

Dental materials such as filling and orthodontic instruments must satisfy biocompatibility requirements as they will be in the oral cavity for a long period of time. Some dental cements can contain chemicals that may induce allergic reactions on various tissues in the oral cavity. Common allergic reactions include stomatitis/dermatitis, urticaria, swelling, rash and rhinorrhea. These may predispose to life-threatening conditions such as anaphylaxis, oedema and cardiac arrhythmias.

Eugenol is widely used in dentistry for different applications including impression pastes, periodontal dressings, cements, filling materials, endodontic sealers and dry socket dressings. Zinc oxide eugenol is a cement commonly used for provisional restorations and root canal obturation. Although classified as non-cariogenic by the US Food and Drug Administration, eugenol is proven to be cytotoxic with the risk of anaphylactic reactions in certain patients.

Zinc oxide eugenol constituents a mixture of zinc oxide and eugenol to form a polymerised eugenol cement. The setting reaction produces an end product called zinc eugenolate, which readily hydrolyses, producing free eugenol that causes adverse effects on fibroblast and osteoclast-like cells. At high concentrations localised necrosis and reduced healing occurs whereas for low concentrations contact dermatitis is the common clinical manifestation.

Allergy contact dermatitis has been proven to be the highest clinical occurrence usually localised to soft tissues with buccal mucosa being the most prevalent. Normally a patch test done by dermatologists will be used to diagnose the condition. Glass ionomer cements have been used to substitute zinc oxide eugenol cements (thus removing the allergen), with positive outcome from patients.^[12]

• Acid-base Cements (1993) A. D. Wilson and J.W. Nicholson