Luting agent


A luting agent is an application of a dental cement connecting the underlying tooth structure to a fixed prosthesis. To lute means to glue two different structures together. There are two major purposes of luting agents in dentistry – to secure a cast restoration in fixed prosthodontics, and to keep orthodontic bands and appliances in situ.
In a complex restoration procedure, the selection of an appropriate luting agent is crucial to its long-term success. In addition to preventing the fixed prosthesis from dislodging, it is also a seal, preventing bacteria from penetrating the tooth-restoration interface.
Zinc phosphate is the oldest material available and has been used in dentistry for more than a century. The introduction of adhesive resin systems made a wide range of dental materials available as luting agents. The choice of luting agent is dependent on clinical factors including dental occlusion, tooth preparation, adequate moisture control, core material, supporting tooth structure, tooth location, etc. Research has determined that no single luting agent is ideal for all applications.

Classification

There are many dental luting agents available. Recently introduced agents such as resins and resin-modified glass-ionomer cement are claimed to perform better clinically that some traditional ones due to their improved properties. Ultimately, the durability of restoration attached to the tooth surface using lutes depends on several factors, for instance strength of materials used, operator's skills, tooth type, and patient's behaviour.
Dental lutes can be classified in many ways, some of which based on:
user's knowledge and experience of use
type of setting mechanism
the expected duration of use of restoration
Zinc Phosphate
This is the luting cement that has been about for the longest and has become very firmly established. It is still routinely used by almost one third of UK practitioners today. It is usually made up of a powder and a liquid. Mixing of zinc phosphate is done by using a spatula to gradually incorporate the powder into the liquid. By using a chilled glass slab, the working time will be increased.
Clinical studies have been carried out and results show that over a ten-year period, zinc phosphate cemented restorations had a lower risk of failure compared to other conventional cements such as glass ionomer or resin-modified glass ionomer. However, it has some well-known clinical disadvantages including high clinical solubility, lack of adhesion, low setting pH and a low tensile strength.
Zinc Polycarboxylate
Zinc polycarboxylate was the first cement to bind to tooth structure. It is generally made up of the same powder as zinc phosphate but uses a different liquid – aqueous copolymer of polyacrylic acid.
It has a short working time which can make it difficult to use but this can be elongated by adding tartaric acid, mixing on a cold glass slab or using a lower powder: liquid ratio. In comparison with zinc phosphate, zinc polycarboxylate has been found to be distinctly superior in its adhesion to enamel and dentin under tensile loading.
Glass Ionomer
This is the first of the GI luting cements to appear in 1978. It consists of fluoroaluminosilicate glass and a liquid containing polyacrylic acid, itaconic acid and water. Alternatively, the acid may be freeze dried and added to powder with distilled water.
When in position it will release fluoride ions which could have a potential anti caries effect. It also binds physicochemically to tooth structure and has a low coefficient of thermal expansion, both of which are important to create a good seal and good retention. However, it has been linked with significant postoperative sensitivity. It is also very initially acidic which may cause pulpal inflammation and has a very slow setting reaction meaning hardening can take up to 7 days.

Resin cements

Resin cements are a type of polymerisable lutes. It consists of methacrylate and dimethacrylate monomers, urethane dimethacrylate, tri-ethylene glycol dimethacrylate ), filler particles and an initiator which can be either chemically or light activated.
Autopolymerisation occurs once all the constituents are mixed together. External source of energy such as light and heat is not needed to activate the setting reaction. Excess cement should be removed immediately after seating the restoration by using interproximal dental instruments such as dental floss. Autopolymerised cement is proved to be the most radiolucent among all resin cements, making it relatively difficult to be seen on radiographs.
Due to the presence of light-activated components, this type of resin cement requires an external light source to initiate the setting reaction. This characteristic allows command set at the periphery of the restoration where light can reach the cement. However, this type of cement is not suitable for thick restorations due to attenuation of light. Instead, a chemically-cured resin cement should be used.
It consists of a light-activated paste mixed with a chemical catalyst for resin polymerisation. It is widely used for luting dental restoration whereby the thickness allows penetration of light for partial curing only. On the other hand, the chemically-cured component is key in ensuring complete polymerisation and hence full strength acquisition. Discolouration may occur due to the presence of aromatic amine. Overall, the combination of its physical and chemical properties makes it the most favourable type.
Today resin cements are manufactured in different shades to accommodate the demanding aesthetic needs. It is also well known for its high flexural strength, which ranges from 64 to 97 MPa. Although it has the advantage of attaching restorations with minimal retentive capacity to tooth surfaces due to its high bond strength to dentine, its methacrylate constituent causes it to undergo polymerisation shrinkage when setting. The strain introduced by the shrinkage will tend to raise the tensile stresses significantly at areas where the cement is thick. However, the cement thickness usually used is sufficiently low to raise concern. Another way to look at the strain applied onto the tooth structure is to consider the configuration factor of the lute, especially in the case of inlay type restoration. The use of resin cements is considered technique sensitive as compared to conventional cements because it requires multiple steps for bonding and is difficult to clean up.

Resin-modified glass-ionomer cement (RMGIC)

RMGIC, also known as hybrid cements, was developed with the purpose of eliminating weaknesses of the traditional glass-ionomer to enhance its existing properties. The addition of polymerisable resins results in higher compressive and tensile strength, as well as lower solubility, all of which are ideal properties of a dental luting agent. The setting reaction takes place with the relatively quick polymerisation of resins and gradual acid-base reaction of GI. At the early stage of setting reaction, RMGIC has a certain degree of solubility at the margins. Therefore, it is important to keep the margin dry for around 10 minutes to minimise loss of marginal cement.
Theoretically, RMGIC benefits the teeth by releasing fluoride at the marginal area to reduce the risk of tooth decay. However, there is currently no clinical evidence to prove this since the cement film is very thin at the margin.

Provisional/Temporary cements

Provisional luting agents are used specifically for inter-appointment fixation of temporary restorations, prior to cementation of a permanent restoration. It is mainly provisional crowns and bridges that are cemented with eugenol-containing temporary cements, but sometimes they may be used for permanent restorations.
As these temporary restorations will require removal, their ideal properties should consist of poor physical properties, such as low tensile strength and high solubility; as well as no pulp irritability and easy handling. The main examples of temporary luting agents include zinc oxide-eugenol cements, non-eugenol containing zinc oxide cements and calcium hydroxide pastes.

Zinc oxide-eugenol

Eugenol is the principal constituent of clove oil, and when mixed with zinc oxide leads to a chelating reaction. All eugenol reacts to zinc oxide eugenol, meaning none is available to diffuse once setting is complete. Supposedly its therapeutic effects are supported by dentinal tubule fluid promoting the release of eugenol and its penetration towards the pulp.
Zinc oxide-eugenol is often found as a two-paste material when used for temporary cementation. The paste containing zinc oxide often includes mineral or vegetable oils, and the eugenol has fillers incorporated into it to form the other paste. A well-known product utilised in the two-paste form is .
Zinc oxide-eugenol may present as a powder that requires mixing with a liquid. The zinc oxide powder may contain up to 8% of other zinc salts as accelerators. The liquid containing eugenol has up to 2% of acetic acid added as an accelerator. A well-known product utilised in this powder-liquid form is .

Zinc oxide non-eugenol

If cementation of a definitive restoration would require a resin-based luting agent, there is evidence indicating the use of a zinc oxide non-eugenol containing cement. Non-eugenol materials use long chain aliphatic acids or aryl-substituted butyric acid to react with zinc oxide particles. Eugenol itself is known to be incompatible with resin polymers, as it is a radical scavenger and therefore inhibits polymerisation of resin materials.
Further evidence illustrated that the application of eugenol-containing cement to cured composite resin cores before final cementation with resin cement significantly reduced retention of the crowns. It is also worth bearing in mind that a temporary cement's incomplete removal from a cured resin composite core may affect the final restoration's cementation quality. A well-known product used in this instance is .

Clinical applications

Cements can either be permanent or temporary :

Definitive cements

Zinc phosphate

Zinc oxide eugenol

Selection of luting agent to be used for a given restoration should be based on a basic knowledge of the materials available, the type of restoration to be placed, the requirements of the patient and the expertise and experience of the clinician.

Summary properties of cements