Dental Cements for Bonding Application Dr. Waseem Bahjat Mushtaha Specialized in prosthodontics
Types of cements Zinc phosphate cement Zinc silicophosphate cement Zinc polycarboxylate cement Glass Ionomer cement Zinc Oxide- Eugenol cement Resin-based cement
Zinc phosphate cement General description: Zinc phosphate is the oldest of the cementation agents and thus is the one that has the longest track record. It consists of powder and liquid in two separate bottles.
Composition 1) Powder : Zinc oxide (90%) Magnesium oxide (10%). The ingredients of the powder are sintered at temperatures between 1000C and 1400 into a cake that is subsequently ground into fine powders. The powder particle size influences setting rate. Generally, the smaller the particles size, the faster the set of the cement.
b) Liquids : Phosphoric acid, water, aluminum phosphate, and in some instances, zinc phosphate. The water content of most liquids is 33% ± 5% Setting reaction: When the powder is mixed with the liquid, the phosphoric acid attacks the surface of the particles and releases zinc ions into the liquid. The aluminum, which already forms a complex with the phosphoric acid, reacts with the zinc and yields a zinc aluminophosphate cement is a core structure consisting primarily of unreacted zinc oxide particles embedded in a cohesive amorphous matrix of zinc aluminophosphate.
Factors Influencing Working and Setting Time 1) Powder : liquid ratio: Working and setting times can be increased by reducing the powder: liquid (P:L) ratio. This procedure, however, is not acceptable means of extending setting time because it impairs the physical properties and results in a lower initial PH of the cement. The reduction in compressive strength, along with the decrease in the P:L ratio. The initial PH of the mixture also decreases with increasing P:L ratio.
2) Rate of powder incorporation: Introduction of small quantity of the powder into the liquid for the first few increments increases working and setting times by reducing the amount of heat generated and permits more powder to be incorporated into the mix. Therefore, it is the recommended procedure for zinc phosphate cement.
3) Spatulation time: Operators who prolong the spatulation time are effectively destroying the matrix that was forming. Fragmentation of the matrix means extra time is needed to rebuild the bulk of the matrix.
4) Temperature of mixing slab: The most effecting method of controlling the working and setting times is to regulate the temperature of the mixing slab. Cooling the slab markedly retards the chemical reaction between the powder and the liquid so that matrix formation is retarded. This permits incorporation of the optimum amount of powder into the liquid without the mix developing an unduly high viscosity.
Physical and Biological properties Two physical properties of the cement that are relevant to the retention of fixed prostheses are the mechanical properties and the solubilities. The prosthesis can become dislodged if the underlying cement is stressed beyond its strength. High solubility can induce loss of the cement needed for retention and may create plaque retention sites.
Zinc phosphate cements, when properly manipulated, exhibit a compressive strength of MPa and a diametral tensile strength of 5.5MPa . Zinc phosphate cement has a modulus of elasticity approximately 13GPa. Thus, it is quite stiff and should be resistant to elastic deformation even when it is employed for cementation of restorations that are subjected to high masticatory stress.
The recommended P:L ratio for this zinc phosphate cement is about 1.4g to 0.5 ml. the increase in strength attained by addition of powder in excess of the recommended amount is modest as compared with the reduction incurred by decreasing the amount of powder in the mix. A reduction in P:L ratio of the mix produces a markedly weaker cement. A loss or gain in the water content of the liquid reduces the compressive and tensile strengths of the cement.
Zinc phosphate cements show relatively low solubility in water when they are tested in accordance with ADA specification. Retention : Setting of the zinc phosphate cement does not involve any reaction with surrounding hard tissue or other restorative materials. Therefore, primary bonding occurs by mechanical interlocking at interface and not by chemical interaction.
Biologic properties As might be expected from the presence of the phosphoric acid, the acidity of the cement is quite high at the time when a prosthesis is placed on a prepared tooth. Two minutes after the start of the mixing, the PH of zinc phosphate cement is approximately 2. The PH then increases rapidly but still is only about 5.5 at 24 hours. The PH is lower and remains lower for a longer period when thin mixes are employed.
Zinc phosphate cement probably occurs during the first few hours after insertion. However, studies of zinc phosphate cements prepared with liquids containing radioactive phosphoric acid indicate that in some teeth the acid from the cement can penetrate a dentin thickness as great as 1.5 mm. Thus, if the underlying dentin is not protected against the infiltration of acid via the dentinal tubules, pulpal injury may occur.
Manipulation 1) It is probably not necessary to use measuring device for proportioning the powder and liquid, because the desired consistency may vary to some degree with the clinical situation. However , the maximum amount of powder possible for the operation and should be used to insure minimum solubility and maximum strength.
2) A cool mixing slab should be employed. The cool slab prolongs the working and the setting times and permits the operator to incorporate the maximum amount of the powder before the matrix formation proceeds to the point at which the mixture stiffens. The liquid should not be dispensed onto the slab until mixing is to be initiated, because water will be lost to the air by evaporation.
3) Mixing is initiated by addition of a small amount of powder. Small quantities are incorporated initially with brisk spatulation. A considerable area of the mixing slab should be used. A good rule to follow is to spatulate each increment for 15 seconds before adding another increment. The mixing time is not unduly critical. Completion of the mix usually requires approximately 1 minute and 30 seconds. As stated previously, the appropriate consistency varies according to the purpose for which the cement is to be used. However, the desired consistency is always attained by adding more powder and never by allowing a thin mix to stiffen. For a fixed partial denture, additional time required to apply the cement. Therefore, a slightly decreased viscosity should be used.
4) The casting should be seated immediately with a vibratory action if possible, before matrix formation occurs. After the casting has been seated, it should be held under pressure until the cement sets to minimize the air spaces. The field of operation should be kept dry during the entire procedure. 5) Excessive cement can be removed after it has set. It is recommended that a layer of varnish or other nonpermeable coating should be applied to the margin. The purpose of the varnish coating is to allow the cement more time to mature and develop an increased resistance to dissolution in oral fluid.
Zinc silicophosphate cement Zinc silicophosphate cement (ZSP) cements consist of a mixture of silicate glass, a small percentage of zinc oxide powder, and phosphoric acid. The clinical indications for this cement are similar to those of zinc phosphate cement. Its strength is somewhat superior, the other major difference is that set ZSP cement appears somewhat translucent and releases fluoride by virtue of the silicate glass. Aesthetically, it is superior to the more opaque zinc phosphate cement for cementation of ceramic restorations. The use of ZSP cement is declining, as practitioners have choices of other more esthetically pleasing materials, such as resin and glass ionomer cements.
Zinc polycarboxylate cement a) Basic components: primary zinc oxide, small quantities of magnesium oxide. b) Acidic component: polyacrylic acid, which may be supplied: 1) As a viscous aqueous solution of concentration 30-40% 2) As a dry powder, blended with the basic components. c) additionally, some products contain stannous fluoride
Manipulation a) Polyacrylic acid solutions are more viscous than the liquidus of other cements, which affects the ease of mixing of the material. b) If the cement is being used to secure adhesion to enamel and dentin, it is important that the tooth surface should be clean and saliva-free
c) The cement should be applied to the tooth as soon as possible after mixing, otherwise poor adhesion may result. If a cement mix begins to “cobweb” on manipulation, it should be discarded. There is a continuous increase in cement viscosity during manipulation of the material d) Polycarboxylate cement will adhere to instruments, particularly those made of stainless steel.
Thus : 1) It is useful to use alcohol as a release agent for the mixing spatula. 2) Instruments should be cleaned before the cement sets on them. 3) If cement does inadvertently adhere to a spatula, most of it can be chipped off quite easily. The remaining material can be removed in boiling sodium hydroxide solution.
Setting reaction This involves the formation of a salt, zinc polyacrylate. The set material is a cored structure containing a considerable quantity of unreacted zinc oxide. Setting time: a) This depends on the composition and method of manufacture of the powder and liquid. b) A faster setting time is achieve at higher temperatures.
Properties a) These cements have very little irritant effect on the pulp. b) Low thermal diffusivity c) Chemical properties: these cements are more soluble than zinc phosphate materials. Some products may also absorb water, which can cause the material to become soft and gel-like.
d) Zinc polycarboxylate cements are almost as strong as phosphate materials in compressive and stronger in tension e) The set cement is very opaque because of the large quantity of unreacted zinc oxide that is present. f) Biological properties is similar to those of zinc phosphate cement
Adhesion properties: The polyacrylic acid is believed to react via the carboxyl groups with calcium of hydroxyapatite.
Zinc Oxide- Eugenol cement Composition : a) powder: Zinc oxide Magnesium oxide may be present in small quantities, it reacts with eugenol in a similar manner to zinc oxide Zinc acetate in quantities up to 1% as accelerators for the setting reaction
b) liquid: Eugenol, the major constituent of oil of cloves Olive oil, up to 15% Sometimes acetic acid, to act as an accelerator Manipulation : These cements are mixed by adding the powder in small increments to the liquid, until a thick consistency is obtained. A powder/liquid ratio of between 4/1 and 6/1 by weight will give a material of the required properties, with experience, a suitable consistency can be recognized without weighing the materials. As a rule, a thin glass slab and stainless steel spatula are used.
Setting reaction a) Chemical reaction, to form a compound called zinc eugenolate b) Absorption of the eugenol by the zinc oxide may also occur. Other factors to be noted: a) The setting reaction between pure zinc oxide and pure eugenol will not occur in the absence of water. Thus, a mixture of zinc oxide and eugenol, without added accelerators, can be kept in a desiccator for several days without undergoing much change. b) The set materials contains both some unreacted zinc oxide and eugenol.
Setting time This depends on: a) powder: Particles size a fine powder will have a greater surface area exposed to the eugenol so can react more quickly. b) Accelerating additive c) powder/liquid ratio: a thicker mix gives a faster setting material. d) Exposure to moisture on mixing or the addition of water will accelerate the reaction. e) Increase in temperature also causes faster setting
Types of Zinc Oxide- Eugenol cement Type 1: ZOE cement has a PH of 7 and is biocompatible with the pulp . The strength of temporary cement must be low to permit removal of the restoration without trauma to the teeth.
Type 2 On is based on the addition of alumina to the powder and ortho-ethoxybenzoic acid to the eugenol liquid, and the second based on the use of a polymer . The compressive strength improved ZOE cements but overall the mechanical properties are inferior to those of other cements.
Resin based cement General description: A varietly of resin-based cements have now become available because of the development of the direct-filling resins with improve properties, the acid-etch technique for attaching resins to enamel, and molecules with a potential to bond to dentin conditioned with organic or in organic acid. Some are designed for general used and other for specific uses such as attachment for orthodontic brackets or resin bonded bridge.
Glass-ionomer (Glass polyalkenoate) 1) Presentation : Traditional form : powder and liquid. Preproportioned capsules. Water settable cement : the polyacrylic acid is freeze-dried and added to the powder. In such a case the liquid may be distilled water or a diluted solution of tartaric acid.
Composition The powder is of the same composition as that of silicate cement N.B Barium is added to give radiopacity. The liquid is the same composition as that of the polycarboxylate cement.
Setting reaction The setting reaction is an acid base reaction that undergoes the following stages on mixing the powder with liquid: 1) Dissolution . 2) Migration 3) Reaction and precipitation
Dissolution Dissolution of the surface glass particles by the acid i.e H+ attack to release cations (ca++ , AL+++) and fluoride ions . Between 20% to 30% of the glass is decomposed by the acid attack. Migration Migration of the surface ions Ca++, Al+++ and fluoride ions complex into the liquid. The divalent Ca++ ions will migrate first followed by the trivalent Al++ ions. The sodium ions form silica gel on the surface of the particles.
Reaction and precipitation : The migrated Ca++ ions will react first with the carboxylic group of the acid to form the cross linked carboxylic salt gel leading to the initial set. This is followed by the reaction of the slowly migrate trivalent Al+++ ions. The later reaction takes longer time and results into a stronger cross linked cement. The precipitation process of the carboxylic gel salts is a continuous process and may take 24 hours. Therefore, the setting material should be protected against premature exposure to saliva as it affects the setting and the surface hardness.
Role of water in the setting process Water is an important constituent of the cement liquid. It serves initially as a reaction medium then it slowly hydrates the cross-linked matrix, thereby increasing the material strength. During the initial set, it is known as loosely bound water. As the reaction proceeds, it becomes tightly bound. Thus, if glass ionomer cement is subjected to dryness during the initial set, the reaction reaction will not go to completion and the surface will crack.
On the other hand, if glass ionomer cement is subjected to moisture contamination during the initial set, dissolution of the matrix will occur. Consequently, either the condition yield glass ionomer cement with reduced strength, increased solubility and poor aesthetics. Therefore, glass ionomer cement must be protected against water changes in the structure during setting process.
The microscopic of the set material The set material is a composite cored structure consisting of unreacted glass cores surrounded by silica gel embedded in a matrix of cross linked poly salt hydrogel of calcium and aluminum. Aluminum fluoracarboxylate salts constitute the main bulk of the matrix and provides the final strength.
Types of glass ionomer cement There are three types of GIC based on their formulations and their potential uses. These are designated as follows: Type I : for luting applications. Type II : as a restorative material. Type III : for use as a liner or based. Light-curable versions of GIC are also available.
Properties Biological properties: a) The glass ionomer cements have a mild effect on the pulp. In case of deep cavities, calcium hydroxide lining must be used under glass ionomer cement. b) Anticariogenic effect: these cements have the potential for inhibiting secondary caries due to the presence of fluoride.
Solubility and disintegration Glass ionomer cements are susceptible to attack by water during its setting. Therefore, it is necessary to coat the restoration immediately by varnish to protect the cement from premature exposure to the saliva. Value of solubility and disintegration of the glass ionomer cements in water after 24 hours immersion is about 1.5% by weight.
Film thickness The film thickness of the glass ionomer cement is about 25 microns which is similar to that of zinc phosphate cement.
Mechanical properties Compressive strength properties: The 24 hours compressive strength of glass ionomer cements ranges from 90-240 MPa. A glass ionomer cement as a filling material showed an increase in strength from 160 to 280 MPa between 24 hours and one year. The strength of the glass ionomer cements improves more rapidly when the cement is protected from moisture during the first 24 hours after filling.
Tensile strength: It is a brittle material. Its tensile strength ranges from 14-24 MPa. Bond strength : The glass ionomer cements bond chemically to tooth structure by the reaction of carboxylic group of polyacrlyic acid with the calcium and phosphate content of tooth structure. The bond strength of glass ionomer cement to tooth structure is lower than of the polycarboxylate cements because of the sensitivity of the glass ionomer cements to moisture during setting. To obtain a good bond to dentin, the surface must be treated with a conditioner to remove any smear layer which interfere with bonding.